2011 08-19 flier-&-support-docs-on-ust-low-threat closure policy

BACKGROUNDOn July 19th, 2011, the nine‐member UST Low‐Threat Closure Policy Task

Force presented its recommendations to the SWRCB. The SWRCB

encouraged the stakeholder group to hold outreach meetings to discuss

technical and practical aspects of its recommend policy. At the request of

the SWRCB, we have arranged the following meeting schedule. All

interested parties are invited. We hope that you will attend one of these

sessions:

August 31, 2011, 1:30 P.M.

San Francisco RWQCB 1515 Clay Street, Suite 1400

Oakland, CA 94612

Contact: Steven Hill [email protected]

September 15, 2011, 9:00 A.M.

Los Angeles RWQCB 320 W. 4th Street, Suite 200

Los Angeles, CA 90013

Contact: Dr. Yue Rong [email protected]

September 15, 2011, 2:30 P.M.

Santa Ana RWQCB 3737 Main Street, Suite 500

Riverside, CA 92501‐3339

Contact: Kurt Berchtold

[email protected]

September 16, 2011, 9:00 A.M.

San Diego RWQCB 9174 Sky Park Court, Suite 100

San Diego, CA 92123

Contact: John Anderson

[email protected]

September 23, 2011, 1:30 P.M.

Central Valley RWQCB 11020 Sun Center Drive, Suite 200

Rancho Cordova, CA 95670

Contact: Brian Newman

[email protected]

For additional information, questions or comments, please contact:

Ravi Arulanantham, PhD.

Geosyntech Consultants (510) 285‐2793

[email protected]

Barry Marcus, P.G.

Sacramento County EMD (916) 875‐8506

[email protected]

Regulatory Outreach Proposed Petroleum

Low‐Threat Closure Policy

CALIFORNIA ENVIRONMENTAL PROTECTION AGENCY

STATE WATER RESOURCES CONTROL BOARD (SWRCB)

The complete proposed policy and technical justification documents are

available on the internet at the following website:

http://www.waterboards.ca.gov/water_issues/programs/ust/lt_cls_plcy

.shtml

CONTACT INFORMATION

Documents developed by the UST stakeholder group are listed below:

Draft Low Threat UST Closure Policy

- Final 7/14/11

Technical Justification for Direct Contact

- Final 7/16/11

Technical Justification for Groundwater Plume Lengths, etc

- Final 7/12/11

Technical Justification for VI Pathway

- Final 6/30/11

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DRAFT Low-Threat UST Closure Policy 7-14-11

Preamble The State Water Resources Control Board (State Water Board) administers the petroleum UST (Underground Storage Tank) Cleanup Program, which was enacted by the Legislature in 1984 to protect health, safety and the environment. The State Water Board also administers the petroleum UST Cleanup Fund (Fund), which was enacted by the Legislature in 1989 to assist UST owners and operators in meeting federal financial responsibility requirements and to provide reimbursement to those owners and operators for the high cost of cleaning up unauthorized releases caused by leaking USTs. The State Water Board believes it is in the best interest of the people of the State that unauthorized releases be prevented and cleaned up to the extent practicable in a manner that protects human health, safety and the environment. The State Water Board also recognizes that the technical and economic resources available for environmental restoration are limited, and that the highest priority for these resources must be the protection of human health and environmental receptors. Program experience has demonstrated the ability of remedial technologies to mitigate a substantial fraction of a petroleum contaminant mass with the investment of a reasonable level of effort. Experience has also shown that residual contaminant mass usually remains after the investment of reasonable effort, and that this mass is difficult to completely remove regardless of the level of additional effort and resources invested. It has been well-documented in the literature and through experience at individual UST release sites that petroleum fuels naturally attenuate in the environment through adsorption, dispersion, dilution, volatilization, and biological degradation. This natural attenuation slows and limits the migration of dissolved petroleum plumes in groundwater. The biodegradation of petroleum, in particular, distinguishes petroleum products from other hazardous substances commonly found at commercial and industrial sites. The characteristics of UST releases and the California UST Program have been studied extensively, with individual works including:

a. Lawrence Livermore National Laboratory report (1995) b. SB1764 Committee report (1996) c. UST Cleanup Program Task Force report (2010) d. Cleanup Fund Task Force report (2010) e. Cleanup Fund audit (2010)

In general, these studies have recommended establishing “low-threat case closure criteria” to maximize the benefits to the people of the State of California through judicious application of available resources. The purpose of this policy is the establishment of low-threat petroleum site closure criteria. The policy is consistent with existing statutes, regulations, State Board precedential decisions and resolutions, and is intended to provide clear direction to responsible parties, their service

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providers, and regulatory agencies. The policy seeks to increase UST cleanup process efficiency. A benefit of improved efficiency is the preservation of limited resources for mitigation of releases posing a greater threat to human and environmental health.

This policy is based in part upon the knowledge and experience gained from the last 25 years of investigating and remediating unauthorized releases of petroleum from USTs. While this policy does not specifically address other petroleum release scenarios such as pipelines or above ground storage tanks, if a particular site with a different release scenario exhibits attributes similar to those which this policy addresses, the criteria for closure evaluation of these non-UST sites should be similar to those in this policy. This policy is a state policy for water quality control and applies to all sites governed by Health and Safety Code section 25296.10. The term “regulatory agencies” in this policy means the State Water Board, regional water boards and local agencies authorized to implement Health and Safety Code section 25296.10. Definitions: Unless expressly provided in this policy, the terms in this policy shall have the same definitions provided in Chapter 6.7 of Division 20 of the Health and Safety Code and Chapter 16 of Division 3 of Title 23 of the California Code of Regulations. Criteria for Low-Threat Case Closure In the absence of site-specific conditions that demonstrably increase the risk associated with residual petroleum constituents, cases that meet the general and media-specific criteria described in this policy do not pose a threat to human health, safety or the environment and are appropriate for UST case closure pursuant to Health and Safety Code section 25296.10. Cases that meet the criteria in this policy do not require further corrective action and shall be issued a uniform closure letter consistent with Health and Safety Code section 25296.10. Periodically, or at the request of the responsible party or party conducting the corrective action, the regulatory agency shall conduct a review to determine whether the site meets the criteria contained in this policy. It is important to emphasize that the criteria described in this policy do not attempt to describe the conditions at all low-threat sites in the State. Regulatory agencies should issue a closure letter for a case that does not meet these criteria if the site is determined to be low-threat based upon a site specific analysis. This policy recognizes that some petroleum-release sites may possess unique attributes and that some site specific conditions may make the application of policy criteria inappropriate. It is impossible to completely capture those sets of attributes that may render a site ineligible for closure based on this low-threat policy. This policy relies on the regulatory agency’s use of the conceptual site model to identify the special attributes that would require specific attention prior to the application of low-threat criteria. In these cases, it is the regulatory agency’s responsibility to identify the conditions that make closure under the policy inappropriate.

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General Criteria General criteria that must be satisfied by all candidate sites are listed as follows:

a. The unauthorized release is located within the service area of a public water system; b. The unauthorized release consists only of petroleum; c. The unauthorized (“primary”) release from the UST system has been stopped; d. Free product has been removed to the maximum extent practicable; e. A conceptual site model has been developed; f. Secondary source removal has been addressed and g. Soil or groundwater has been tested for MTBE and results reported in accordance

with Health and Safety Code section 25296.15. a. The unauthorized release is located within the service area of a public water system This policy is protective of existing water supply wells. New water supply wells are unlikely to be installed in the shallow groundwater near former UST release sites. However, it is difficult to predict, on a statewide basis, where new wells will be installed, particularly in rural areas that are undergoing new development. This policy is limited to areas with available public drinking water supplies to reduce the likelihood that new wells in developing areas will be inadvertently impacted by residual petroleum in groundwater. Case closure outside of areas with a public water supply should be evaluated based upon this policy and a site specific evaluation of developing water supplies in the area. b. The unauthorized release consists only of petroleum For the purposes of this policy, petroleum is defined as crude oil, or any fraction thereof, which is liquid at standard conditions of temperature and pressure, which means 60 degrees Fahrenheit and 14.7 pounds per square inch absolute, including the following substances: motor fuels, jet fuels, distillate fuel oils, residual fuel oils, lubricants, petroleum solvents and used oils, including any additives and blending agents such as oxygenates contained in the formulation of the substances. c. The unauthorized release has been stopped The tank, pipe, or other appurtenant structure that released petroleum into the environment (i.e. the primary source) has been removed, repaired or replaced. It is not the intent of this policy to allow sites with ongoing leaks from the UST system to qualify for low-threat closure. d. Free product has been removed to the Maximum Extent Practicable At petroleum unauthorized release sites where investigations indicate the presence of free product, free product shall be removed to the maximum extent practicable. In meeting the requirements of this section:

(a) Free product shall be removed in a manner that minimizes the spread of the unauthorized release into previously uncontaminated zones by using recovery and disposal techniques appropriate to the hydrogeologic conditions at the site, and that properly treats, discharges or disposes of recovery byproducts in compliance with applicable laws; (b) Abatement of free product migration shall be used as a minimum objective for the design of any free product removal system; (c) Flammable products shall be stored for disposal in a safe and competent manner to prevent fires or explosions.

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e. A conceptual site model has been developed The Conceptual Site Model (CSM) is a fundamental element of a comprehensive site investigation. The CSM establishes the source and attributes of the unauthorized release, describes all affected media (including soil, groundwater, and soil vapor as appropriate), describes local geology, hydrogeology and other physical site characteristics that affect contaminant environmental transport and fate, and identifies all confirmed and potential contaminant receptors (including water supply wells, surface water bodies, structures and their inhabitants, etc.). The CSM is relied upon by practitioners as a guide for investigative design and data collection. Petroleum release sites in California occur in a wide variety of hydrogeologic settings. As a result, contaminant fate and transport and mechanisms by which receptors may be impacted by contaminants vary greatly from location to location. Therefore the CSM is dynamic and unique to each individual release site. All relevant site characteristics identified by the CSM should be assessed such that the nature, extent and mobility of the release have been established to determine conformance with applicable criteria in this policy. f. Secondary source removal has been addressed “Secondary source” is defined as petroleum-impacted soil or groundwater located at or immediately beneath the point of release from the primary source. Unless site attributes prevent secondary source removal (e.g. physical or infrastructural constraints exist whose removal or relocation would be technically or economically infeasible), petroleum-release sites are required to undergo secondary source removal to the extent practicable as described herein. “To the extent practicable” means implementing a cost-effective corrective action which removes or destroys-in-place the most readily recoverable fraction of source-area mass. It is expected that most secondary mass removal efforts will be completed in one year or less. Following removal/destruction of the secondary source, additional removal and/or active remedial actions shall not be required by regulatory agencies unless (1) necessary to abate a demonstrated threat to human health or (2) the groundwater plume does not meet the definition of low threat as described in this policy. g. Soil and groundwater have been tested for MTBE and results reported in accordance

with Health and Safety Code section 25296.15 Health and Safety Code section 25296.15 prohibits closing a UST case unless the soil, groundwater, or both, as applicable have been tested for MTBE and the results of that testing are known to the regional water board. The exception to this requirement is where a regulatory agency determines that the UST that leaked has only contained diesel or jet fuel. Before closing a UST case pursuant to this policy, the requirements of section 25296.15, if applicable, shall be satisfied.

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Media-Specific Criteria Releases from USTs can impact human health and the environment through contact with any or all of the following contaminated media: groundwater, surface water, soil, and soil vapor. Although this contact can occur through ingestion, dermal contact, or inhalation of the various media, the most common drivers of health risk are ingestion of groundwater from drinking water wells, inhalation of vapors accumulated in buildings, contact with near surface contaminated soil, and inhalation of vapors in the outdoor environment. To simplify implementation, these media and pathways have been evaluated and the most common exposure scenarios have been combined into three media-specific criteria:

1. Groundwater 2. Vapor Intrusion to Indoor Air 3. Direct Contact and Outdoor Air Exposure

Candidate sites must satisfy all three of these media-specific criteria as described below. 1. Groundwater This policy describes criteria on which to base a determination that risks to existing and anticipated future beneficial uses of groundwater have been mitigated or are de minimus, including cases that have not affected groundwater. State Water Board Resolution 92-49, Policies and Procedures for Investigation and Cleanup and Abatement of Discharges Under Water Code Section 13304 is a state policy for water quality control and applies to petroleum UST cases. Resolution 92-49 directs that water affected by an unauthorized release attain either background water quality or the best water quality that is reasonable if background water quality cannot be restored. Any alternative level of water quality less stringent than background must be consistent with the maximum benefit to the people of the state, not unreasonably affect current and anticipated beneficial use of affected water, and not result in water quality less than that prescribed in the water quality control plan for the basin within which the site is located. Resolution No. 92-49 does not require that the requisite level of water quality be met at the time of case closure; it specifies compliance with cleanup goals and objectives within a reasonable time frame. Water quality control plans (Basin Plans) generally establish “background” water quality as a restorative endpoint. This policy recognizes the regulatory authority of the Basin Plans but underscores the flexibility contained in Resolution 92-49. It is a fundamental tenet of this low-threat closure policy that if the closure criteria described in this policy are satisfied at a release site, water quality objectives will be attained through natural attenuation within a reasonable time, prior to the need for use of any affected groundwater. If groundwater with a designated beneficial use is affected by an unauthorized release, to satisfy the media-specific criteria for groundwater, the contaminant plume that exceeds water quality objectives must be stable or decreasing in areal extent, and meet all of the additional characteristics of one of the five classes of sites listed below. A plume that is “stable or decreasing” is a contaminant mass that has expanded to its maximum extent: the distance from the release where attenuation exceeds migration.

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(1) a. The contaminant plume that exceeds water quality objectives is less than 100

feet in length. b. There is no free product. c. The nearest existing water supply well and/or surface water body is greater

than 250 feet from the defined plume boundary. (2) a. The contaminant plume that exceeds water quality objectives is less than 250

feet in length. b. The nearest existing water supply well and /or surface water body is greater

than 1000 feet from the defined plume boundary. c. The dissolved concentration of benzene is less than 3000 μg/l and the

dissolved concentration of MTBE is less than 1000 μg/l. (3) a. The contaminant plume that exceeds water quality objectives is less than 250

feet in length. b. Free product may be present below the site but does not extend off-site. c. The plume has been stable or decreasing for a minimum of five years. d. The nearest existing water supply well and/or surface water body is greater

than 1000 feet from the defined plume boundary. e. The property owner is willing to accept a deed restriction if the regulatory

agency requires a deed restriction as a condition of closure. (4) a. The contaminant plume that exceeds water quality objectives is less than 1000

feet in length. b. The nearest existing water supply well and/or surface water body is greater

than 1000 feet from the defined plume boundary. c. The dissolved concentration of benzene is less than 1000 μg/l and the

dissolved concentration of MTBE is less than 1000 μg/l. (5) a. An analysis of site specific conditions determines that the site under current

and reasonably anticipated near-term future scenarios poses a low threat to human health and safety and to the environment and water quality objectives will be achieved within a reasonable time frame.

Sites with Releases That Have Not Affected Groundwater Sites with soil that does not contain sufficient mobile constituents (leachate, vapors, or LNAPL) to cause groundwater to exceed the groundwater criteria in this policy shall be considered low-threat sites for the groundwater medium. Provided the general criteria and criteria for other media are also met, those sites are eligible for case closure. For older releases, the absence of current groundwater impact is often a good indication that residual concentrations present in the soil are not a source for groundwater pollution. 2. Petroleum Vapor Intrusion to Indoor Air

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Exposure to petroleum vapors migrating from soil or groundwater to indoor air may pose unacceptable human health risks. This policy describes conditions, including bioattenuation zones, which if met will assure that exposure to petroleum vapors in indoor air will not pose unacceptable health risks. In many petroleum release cases, potential human exposures to vapors are mitigated by bioattenuation processes as vapors migrate toward the ground surface. For the purposes of this section, the term “bioattenuation zone” means an area of soil with conditions that support biodegradation of petroleum hydrocarbon vapors. The low-threat vapor-intrusion criteria described below apply to release sites and impacted or potentially impacted adjacent parcels when: (1) existing buildings are occupied or may be reasonably expected to be occupied in the future, or (2) buildings for human occupancy are reasonably expected to be constructed in the near future. Appendices 1 through 4 (attached) illustrate four potential exposure scenarios and describe characteristics and screening criteria associated with each scenario. Petroleum release sites shall satisfy the media-specific screening criteria for petroleum vapor intrusion to indoor air and be considered low-threat for the vapor-intrusion-to-indoor-air pathway if:

a. Site-specific conditions at the release site satisfy all of the characteristics and screening criteria of scenarios 1 through 3 as applicable, or all of the characteristics and screening criteria of scenario 4 as applicable; or

b. A site-specific risk assessment for the vapor intrusion pathway is conducted and demonstrates that human health is protected to the satisfaction of the regulatory agency.

Exception: Exposures to petroleum vapors associated with historical fuel system releases are comparatively insignificant relative to exposures from small surface spills and fugitive vapor releases that typically occur at active fueling facilities. Therefore, satisfaction of the media-specific criteria for petroleum vapor intrusion to indoor air is not required at active commercial petroleum fueling facilities, except in cases where release characteristics can be reasonably believed to pose an unacceptable health risk. 3. Direct Contact and Outdoor Air Exposure This policy describes conditions where direct contact with contaminated soil or inhalation of contaminants volatized to outdoor air poses an insignificant threat to human health. Release sites where human exposure may occur satisfy the media-specific criteria for direct contact and outdoor air exposure and shall be considered low-threat if they meet any of the following:

a. Maximum concentrations of petroleum constituents in soil are less than or equal to those

listed in Table 1 for the specified depth below ground surface; b. Maximum concentrations of petroleum constituents in soil are less than levels that a site

specific risk assessment demonstrates will have no significant risk of adversely affecting human health; or

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c. As a result of controlling exposure through the use of mitigation measures or through the use of institutional or engineering controls, the regulatory agency determines that the concentrations of petroleum constituents in soil will have no significant risk of adversely affecting human health.

Table 1

Concentrations of Petroleum Constituents In Soil That Will Have No Significant Risk Of Adversely Affecting Human Health

Depth (feet)

Benzene (mg/kg)

Naphthalene(mg/kg)

PAH* (mg/kg)

0 to 5 2.3 13 0.038

5 to 10 100 1500 7.5 *Notes: Based on the seven carcinogenic PAHs as

benzo(a)pyrene toxicity equivalent [BaPe]. The PAH screening level is only applicable where soil was affected by either waste oil and/or Bunker C fuel.

Low-Threat Case Closure Cases that meet the general and media-specific criteria established in this policy satisfy the case-closure requirements of Health and Safety Code section 25296.10, including the requirement in State Water Board Resolution 92-49 that requires that cleanup goals and objectives be met within a reasonable time frame. If the site has been determined by the regulatory agency to meet the criteria in this policy, the regulatory agency shall notify responsible parties that they are eligible for case closure and that the following items, if applicable, shall be completed prior to the issuance of a uniform closure letter specified in Health and Safety Code section 25296.10. After completion of these items, the regulatory agency shall issue a uniform closure letter within 30 days.

a. Notification Requirements – Public water supply agencies with jurisdiction over the water impacted by the petroleum release, permitting agencies with authority over the land affected by the petroleum release, owners of the property, and the owners and occupants of all adjacent parcels and all parcels that are impacted by the unauthorized release shall be notified of the proposed case closure and provided a 30 day period to comment. The regulatory agency shall consider any comments received when determining if the case should be closed or if site specific conditions warrant otherwise.

b. Monitoring Well Destruction – All wells and borings installed for the purpose of

investigating, remediating, or monitoring the unauthorized release shall be properly destroyed prior to case closure unless a property owner certifies that they will keep and maintain the wells or borings in accordance with applicable local or state requirements.

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c. Waste Removal – All waste piles, drums, debris and other investigation or remediation

derived materials shall be removed from the site and properly managed in accordance with regulatory agency requirements.

Closing Comments This concludes the Low-Threat UST Closure Policy. This policy is based on existing statutes, regulations and State Water Board resolutions. This policy clarifies aspects of prior guidance and establishes criteria to be used by technical practitioners and all regulatory agencies in California.

Building Foundation

Required Characteristics of the Bioattenuation Zone:1. The bioattenuation zone shall be a continuous zone that provides a separation of at least 30 feet vertically between the LNAPL in groundwater and the foundation of existing or potential buildings; and 2. Total TPH (TPH-g and TPH-d combined) are less than 100 mg/kg throughout the entire depth of the bioattenuation zone.

*As used in this context, unweathered LNAPL is generally understood to mean petroleum product that has not been subjected to significant volitalization or solubilization, and therefore has not lost a significant portion of its volatile or soluble constituents (e.g., comparable to recently dispensed fuel).

Appendix 1 Scenario 1: Unweathered* LNAPL in Groundwater

Required Characteristics of the Bioattenuation Zone

Existing Building or Potential Future Construction

30'TPH < 100 mg/kg

throughout 30' depth

Unweathered LNAPL

Version date: July 11, 2011

Appendix 2Scenario 2: Unweathered* LNAPL in Soil

Required Characteristics of the Bioattenuation Zone

Required Characteristics of the Bioattenuation Zone:1. The bioattenuation zone shall be a continuous zone that provides a separation of at least 30 feet both laterally and vertically between the LNAPL in soil and the foundation of existing or potential buildings, and 2. Total TPH (TPH-g and TPH-d combined) are less than 100 mg/kg throughout the entire depth of the bioattenuation zone.

*As used in this context, unweathered LNAPL is generally understood to mean petroleum product that has not been subjected to significant volitalization or solubilization, and therefore has not lost a significant portion of its volatile or soluble constituents (e.g., comparable to recently dispensed fuel).

Existing Building or Potential Future Construction

TPH < 100 mg/kg for 30' from foundation

UnweatheredLNAPL in soil

30'30'

30'

30'


Where benzene concentrations are less than 1000 ug/L, the bioattenuation zone:1. Shall be a continuous zone that provides a separation of least 5 feet vertically between the dissolved phase Benzene and the foundation of existing or potential buildings; and 2. Contain Total TPH (TPH-g and TPH-d combined) less than 100 mg/kg throughout the entire depth of the bioattenuation zone.

Required Characteristics of Bioattenuation Zone For Sites With Oxygen ≥ 4%

Appendix 3Scenario 3 - Dissolved Phase Benzene Concentrations Only in Groundwater

(Low concentration groundwater scenarios with or without O2 measurements)

Defining the Bioattenuation Zone Without Oxygen Measurements or Oxygen <4%

Required Characteristics of Bioattenuation Zone For Sites Without Oxygen Measurements

Defining the Bioattenuation Zone With Oxygen ≥ 4%

Existing Building or Future Construction

5'

Figure A: 1) Where benzene concentrations are less than 100 ug/L, the bioattenuation zone:a) Shall be a continuous zone that provides a separation of at least 5 feet vertically between the dissolved phase Benzene and the foundation of existing or potential buildings; andb) Contain Total TPH (TPH‐g and TPH‐d combined) less than 100 mg/kg throughout the entire depth of the bioattenuation zone.

Figure B: 1) Where benzene concentrations are greater than 100 ug/L but less than 1000 ug/L, the bioattenuation zone: a) Shall be a continuous zone that provides a separation of at least 10 feet vertically between the dissolved phase Benzene and the foundation of existing or potential buildings; and b) Contain Total TPH (TPH‐g and TPH‐d combined) less than 100 mg/kg throughout the entire depth of the bioattenuation zone

TPH < 100 mg/kg

No O2 dataor <4%

Benzene < 100 ug/L

10' TPH < 100 mg/kg

Benzene < 1000 ug/L

Existing Building or Future Construction

5'TPH < 100

mg/kg O2 ≥ 4%

Benzene < 1000 ug/L

With O2 data

Figure A

Figure B

Figure C


Residential Commercial Residential CommercialConstituentBenzene < 85,000 < 280,000 < 85 < 280Naphthalene < 93,000 < 310,000 < 93 < 310Notes:

With Bioattenuation Zone*

Soil Gas Concentration (µg/m3)

*In order to use the screening levels with the bioattenuation zone, there must be: 1) 5 feet of soil between the soil vapor measurement and the building (or future building), 2) TPH (TPHg + TPHd) is less than 100 ppm (measured in at least two depths within the 5 foot zone), and 3) oxygen ≥ 4% measured at the bottom of the 5 foot bioattenuation zone. A 1000-fold bioattenuation of petroleum vapors is assumed for the bioattenuation zone. For the no bioattenuation zone, the screening criteria are the same as the California Human Health Screening Levels (CHHSLs).

Appendix 4Scenario 4 - Direct Measurement of Soil Gas Concentrations

Description of Soil Gas Sample Locations

Soil Gas Sampling Locations – No Bioattenuation Zone

Soil Gas Sampling Locations – with Bioattenuation Zone

Required Characteristics of Bioattenuation Zone

Soil Gas Concentration (µg/m3)

Soil Gas Screening Levels (ug/m3)No Bioattenuation Zone

a - beneath or adjacent to building (soil gas sample shall be collected at least 5' deeper than the bottom of the building foundation)b - for future construction scenarios (soil gas sample shall be collected at least 5' below the ground surface)

Depth of Foundation

ab

5'5'

Existing Building Future Construction

Existing Building Future Construction

5'TPH < 100 mg/kg

O2 ≥ 4% at lower end of zone

Required data includes: petroleum concentrations in soil and soil gas, and oxygen concentrations.

Measured concentrations of soil gases must be less than the screening values indicated in the table below for the applicable scenarios.

5'

O2 ≥ 4% at lower end of zone

TPH < 100 mg/kg




- Final 7/14/11


- Final 7/16/11


- Final 7/12/11


- Final 6/30/11

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Technical Justification for Soil Screening Levels for Direct Contact and Outdoor Air Exposure Pathways

Table of Contents 1 EXECUTIVE SUMMARY ..........................................................................................................................1

2 INTRODUCTION.....................................................................................................................................2

3 CONCEPTUAL SITE MODEL....................................................................................................................3

4 DERIVATION OF SCREENING LEVELS .....................................................................................................5

5 RESULTS: SOIL SCREENING LEVELS.......................................................................................................7

6 DISCUSSION OF RESULTS ......................................................................................................................7

7 REFERENCES ..........................................................................................................................................8

Tables ............................................................................................................................................................9

Figures.........................................................................................................................................................16

1 EXECUTIVE SUMMARY

Soil Screening Levels have been proposed to be used in conjunction with vapor intrusion criteria and

groundwater criteria for identifying sites posing a low‐threat to human health and the environment.

That is, these Soil Screening Levels are just one of three sets of criteria that should be evaluated to

determine if a site is low‐threat.

The Soil Screening Levels discussed in this document have been proposed for benzene, naphthalene,

and polyaromatic hydrocarbon (PAH) to define sites that are low‐threat with respect to “direct contact”

with soil. The exposure pathways considered in the site conceptual model are: ingestion of soil, dermal

contact with soil and inhalation of dust and volatile emissions from soil. Note these exposure pathways

are assumed to occur simultaneously, i.e. the screening levels are protective of the cumulative exposure

from all four exposure pathways.

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These screening levels were derived using standard USEPA and Cal/EPA risk assessment equations. The

exposure parameter values, chemical toxicity values, and chemical fate and transport properties are

based on standard values used in California.

Different screening levels have been developed for two soil horizons, one from 0 to 5 feet below ground

surface (bgs), and one from 5 to 10 feet bgs. This document describes the technical background for the

development of the direct contact screening levels. Three exposure scenarios (types of receptors and

land use) were considered and the screening levels for each soil horizon were chosen to be the most

conservative of the three scenarios.

The soil screening level for “PAH” is appropriate to be compared with site concentrations for the total

concentration of the seven carcinogenic PAHs. The carcinogenic PAHs are: benz[a]anthracene,

benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, dibenz(a,h)anthracene, and

indeno(1,2,3‐cd)pyrene.

The toxicity value used for the entire group of carcinogenic hydrocarbons is California’s Office of

Environmental Health Hazard Assessment (OEHHA) cancer potency value for benzo(a)pyrene. This is a

conservative assumption because the few PAHs that are more carcinogenic than benzo(a)pyrene are not

commonly found in petroleum mixtures.

2 INTRODUCTION

The equations used to develop the Soil Screening Levels came from the California Environmental

Protection Agency (Cal/EPA) OEHHA’s California Human Health Screening Levels (CHHSLs; OEHHA 2005).

Exposure parameters values were assumed to equal the defaults values used in OEHHA’s California

Human Health Screening Levels (CHHSLs; OEHHA 2005). The Soil Screening Levels presented in this

document are conservative because the assumptions used to calculate the values are based on worst

case exposure scenarios.

The CHHSLs for “direct contact with soil” pathways, do not include volatilization of chemicals from the

soil to outdoor air. For the Soil Screening Levels presented in this document a volatilization factor was

added to the CHHSL equations in order to be conservative and was obtained from the American Society

of Testing Material’s (ASTM’s) Standard Guide for Risk‐Based Corrective Action Applied at Petroleum

Release Sites (ASTM 1996). The ASTM volatilization factor used to calculate concentrations in outdoor

air considers mass balance. The volatilization algorithm commonly used in USEPA screening level

equations can greatly overestimate the amount of contaminant volatilizing into outdoor air for volatile

chemicals (OEHHA, 2005). In the ASTM volatilization algorithm, if the calculated volatilization rate

depletes the source before the end of the exposure duration, then the volatilization rate is adjusted so

that the total source mass is assumed to volatilize by the end of the exposure duration. By using this

mass‐balance check, it is ensured that the total amount volatilized does not exceed the total amount of

contaminant in soil (which can happen with the USEPA volatilization algorithm).

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For dermal contact with soil, ingestion of soil, and inhalation of dust pathways, the exposure

concentration in soil is assumed to be constant at the screening level for the entire exposure duration.

2.1 Screening Levels vs. Risk

These Soil Screening Levels represent concentrations that indicate that the site is a low‐threat risk for

human health; they cannot be used to estimate site‐specific risks. Multiple conservative assumptions

were made when developing these Soil Screening Levels. Actual site risk is expected to be lower than

the risk targets used to develop the screening levels. For example, for residential sites, the receptor is

assumed to come into contact with soil with concentrations at the screening level almost every day (350

days/year) for a total of 30 years. While most residential exposures would not be at the default levels

used in this analysis, the defaults used here are designed to be protective for this hypothetical

“reasonable worst case” scenario.

Site concentrations exceeding the screening levels do not indicate unacceptable human health risks with

regards to these pathways; rather, an exceedance may indicate that a site‐specific evaluation of human

health risk is warranted.

3 CONCEPTUAL SITE MODEL

This section describes the exposure scenarios and receptors considered in the development of the Soil

Screening Levels.

3.1 Exposure Pathways

The Screening Levels consider four exposure pathways simultaneously:

• ingestion of soil,

• dermal contact with soil,

• inhalation of volatile soil emissions, and

• inhalation of particulate emissions.

Ingestion of and dermal contact with soil are direct exposure pathways, i.e., the receptor is assumed to

contact the soil directly and, therefore, the exposure point concentration is the actual concentration in

soil. For the inhalation exposure pathways, the exposure medium is outdoor air; the outdoor air

concentrations must be estimated using volatilization and particulate emission factors.

3.2 Receptors Considered

Soil Screening levels were calculated for three exposure scenarios, and then the most conservative

screening level was chosen for the screening levels. The exposure scenarios considered were:

• residential,

4

• commercial/industrial, and

• workers in a utility trench or similar construction project.

It is assumed that all four of the exposure pathways (discussed in section 3.1) are potential exposure

pathways for each of the three types of receptors. The input parameter values are different for each

receptor, however.

For the residential exposure scenario, it is assumed that the receptor is a child for 6 years and then an

adult for 24 years. When calculating carcinogenic risk, the total intake of a chemical over a lifetime is

used; therefore, the carcinogenic residential screening levels are protective of the combined child plus

adult scenario. For non‐carcinogenic health effects, the intake is not added over the exposure period.

In that case, the child is the more sensitive receptor, therefore the non‐carcinogenic screening levels are

developed for a child receptor and are protective for the adult resident as well.

The commercial/industrial exposure scenario assumes that the receptor is an adult and works in an

office or outdoors at the site; however, the adult is not expected to be digging in the soil. In this

scenario, it is assumed that the receptor works for a total of 25 years at 250 days/year at the same

location. It is likely that the direct contact exposure assumptions are very conservative for this exposure

scenario.

For the utility or construction worker, it is assumed that the worker may be working directly with the

impacted soil. In this exposure scenario, the exposure duration is assumed to be much shorter than in

the other two scenarios; however, the chemical intake per day may be higher due to increased

incidental ingestion.

3.3 Depths to Which the Screening Levels Apply

Two sets of screening levels were developed, based on depth of impacted soil: one set applies to 0 to 5

feet below ground surface (bgs) and the other set applies to 5 to 10 feet bgs. The screening levels

applying to soil at 0 to 5 feet bgs represent the lowest of the screening levels calculated for the resident,

worker, and utility worker. Screening levels for soil from 5 to 10 feet bgs represent the lower value of

either a utility trench/construction worker or the volatilization to outdoor air pathway for all of the

receptors. That is, the full depth of 0 to 10 feet is assumed to contribute to outdoor air concentrations

for all scenarios. Therefore, the screening levels for both soil horizons are protective of inhalation of

volatile emissions.

When calculating the residential screening levels, it is assumed that residents may come into contact

with the soil between the ground surface and a depth of 5 feet (“surface soil”). For impacted soil at

depths from 5 to 10 feet (a “swimming pool” or “septic system installation” scenario), it is assumed that

the potential risk posed to residents by direct contact would be small, because excavations by the

homeowner to that depth would be rare (exposure frequency and duration are short), most of the

petroleum‐affected soil would likely be removed to create the swimming pool or septic system, and

5

petroleum constituents in soil would volatilize and biodegrade very quickly if the affected soil was

placed at the ground surface (i.e. the top few inches of soil).

For commercial/industrial receptors it is assumed that commercial workers could contact the soil at

depths between ground surface and 5 feet. In the case of a utility trench or construction worker, it was

assumed that direct contact (dermal and ingestion) with soils could occur at depths from 0 to 10 feet.

4 DERIVATION OF SCREENING LEVELS

This section describes how the Soil Screening Levels were calculated. Standard equations from the

OEHHA CHHSLs were used for everything except the volatilization term which was discussed in Section

2. A target risk level of 1 × 10‐6 risk for carcinogens and a target hazard index of 1.0 for non‐carcinogens

were assumed in all cases.

4.1 Equations Used

4.1.1 Exposure Equations

The equations used to develop the Soil Screening Levels are shown in Tables 1 through 3 and the

variable definitions are shown in Table 4.

4.1.2 Volatilization Factor

As mentioned previously, the CHHSLs do not include a volatilization factor (VF), i.e. they do not consider

volatile emissions to outdoor air. A VF was included in the Soil Screening Levels, however to be

conservative. The volatilization factor used to predict outdoor air concentrations due to volatilization

from the soil is based on the ASTM guidance (1996).

The assumptions in the ASTM volatilization factor algorithm (ASTM 1996) are:

• Dispersion in air is modeled from a ground‐level source. It is assumed that the air in the outdoor air “box” is well‐mixed.

• The receptor is located onsite, directly over the impacted soil, 24 hours/day for the entire exposure duration.

• A long‐term average exposure–point concentration is estimated for the entire exposure duration.

The conceptual model for volatile emissions and inhalation of outdoor air is shown in Figure 1. Note the

assumed receptor location at the edge of the downwind side of the source (for 24 hours/day for the

entire exposure duration) is the most conservative location that could be used. The dispersion of

contaminant in the air, or mixing, is limited to the height of the breathing zone; that is, vertical

dispersion upwards as the air blows towards the receptor is not considered by the model. This is one

exposure scenario where the actual exposure assumed in the risk calculations would be impossible to

achieve and the algorithm used to estimate the risk from volatile emission is very conservative.

6

The ASTM VF is actually composed of two equations shown in Table 5: one equation assumes an infinite

source, and the other one equation includes a mass balance check to limit the volatilization term so that

the amount volatilized cannot exceed the total amount of mass in the soil initially. The VF is calculated

using both equations and the lower of the two volatilization rates is used for the VF in the exposure

equations. The default input values are shown in Table 6.

4.1.3 Particulate Emission Factor

A particulate emission factor (PEF) is used to estimate the outdoor air concentrations due to chemicals

airborne on particulates (dust). The default value used for the PEF for the residential and

commercial/industrial scenarios is the default value used in the CHHSLs = (1.3 x 109) [(mg/kg)/(mg/m3)].

For the utility trench (construction) worker, a PEF value of 1 x 106 [(mg/kg)/(mg/m3)] was used (DTSC

2005).

4.2 Exposure Parameter Values Used

The CHHSLs do not have a utility trench/construction worker receptor, so the default exposure

parameters for this receptor were obtained from California Department of Toxic Substances Control

(DTSC) Human and Ecological Risk Division (HERD) “Human Health Risk Assessment (HHRA) Note

Number 1” (DTSC 2005). Table 4 shows the default values used for each parameter and provides the

reference document where the value was obtained.

4.2.1 Ingestion of Soil

Receptors working or playing outdoors may ingest soil through incidental contact of the mouth with

hands and clothing. For the residential and commercial exposure scenarios, one of the very

conservative assumptions made is that the chemical concentrations remain constant over time in the

soil. In reality, this would not be the case for especially for volatile chemicals in the top few feet of soil,

where most of the direct contact would occur. Benzene is highly fugitive in surface soil, quickly

depleting the upper soil depths.

4.2.2 Dermal Contact with Soil

Some soil contaminants may be absorbed across the skin into the bloodstream. Absorption will depend

upon the amount of soil in contact with the skin, the concentration of chemicals in soil, the skin surface

area exposed, and the potential for the chemical to be absorbed across the skin.

4.2.3 Inhalation of Volatile and Particulate Emissions in Outdoor Air

The inhalation exposure route includes the inhalation of both volatile and particulate emissions. The

inhalation slope factors and non‐carcinogenic inhalation reference doses are shown in Table 7.

7

5 RESULTS: SOIL SCREENING LEVELS

Table 8 (which is included here for convenience) shows the Soil Screening Levels.

Table 8: Soil Screening Levels

Depth Benzene Naphthalene PAH (feet) (mg/kg) (mg/kg) (mg/kg) 0 to 5 2.3 13 0.038

5 to 10 100 1500 7.5 *Notes: Based on the seven carcinogenic PAHs as benzo(a)pyrene toxicity equivalent [BaPe]. The PAH screening

level is only applicable where soil was affected by either waste oil and/or Bunker C fuel.

Table 9 shows the soil screening levels calculated for each exposure scenario. Note that the lowest

screening level was chosen for the two different soil depths to obtain the screening levels in Table 9.

Table 9: Summary of Soil Screening Levels for Each Receptor

Chemical Residential Commercial/ Industrial Utility

Subsurface Soil --

Volatilization only

(for 5 to 10’ bgs) Residential

Scenario mg/kg mg/kg mg/kg mg/kg

Benzene 2.3 120 100 130

Naphthalene 13 45 1500 33,000

PAH 0.038 2.3 7.5 1 x 106

6 DISCUSSION OF RESULTS

This document has presented Soil Screening Levels to be used to identify sites that are low threat to

human health risk for the direct contact pathways from impacted soil. These Soil Screening Levels are

designed to be used in conjunction with the Vapor Intrusion Criteria and Groundwater Criteria to

determine if the site is a low‐threat from all exposure pathways.

Three exposure scenarios were originally considered: residential, commercial/industrial, and a utility

trench/construction worker. The final Soil Screening Levels were chosen as the lowest values for each

receptor. The equations used were based on the equations used by OEHHA in the development of the

CHHSLs, with the exception of the volatilization rate. A volatilization rate term was added to the Soil

Screening Level equations to be conservative.

8

OEHHA has indicated that the residential exposure scenario is protective for other sensitive uses of a

site. This means that these screening levels are also appropriate for other sensitive uses of the property

(e.g., day‐care centers, hospitals, etc.) (Cal/EPA 2005).

7 REFERENCES

American Society for Testing and Materials (ASTM). 1996. Standard Guide to Risk‐Based Corrective Action Applied at Petroleum Release Sites, ASTM E1739‐95, Philadelphia, PA.

DTSC (Department of Toxic Substances Control). 2005. Human and Ecological Risk Division (HERD). Human Health Risk Assessment (HHRA) Note Number 1. Recommended DTSC Default Exposure Factors for Use in Risk Assessment at California Military Facilities.

OEHHA (Office of Environmental Health Hazard Assessment). 2005. Human‐Exposure‐Based Screening Numbers Developed to Aid Estimation of Cleanup Costs for Contaminated Soil, Integrated Risk Assessment Branch, Office of Environmental Health Hazard Assessment. (Cal/EPA), January 2005 Revision. Available at: http://www.oehha.ca.gov/risk/Sb32soils05.html

OEHHA (2009). OEHHA Cancer Potency Values as of July 21, 2009.

SF RWQCB ESLs. Regional Water Quality Control Board (RWQCB) Region 2 – San Francisco. 2008. Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater. Interim Final. May

USEPA. 1989. Risk Assessment Guide for Superfund (RAGS) Volume I Human Health Evaluation Manual (Part A) EPA/540/1‐89/002, Office of Emergency and Remedial Response. December.

9

TABLES Table 1: Equations Used to Develop Soil Screening Levels for the Direct Contact Pathways

for a Residential Exposure Scenario

Carcinogenic – Residential

Age‐Adjusted Ingestion Rate

⎥⎦

⎤⎢⎣

⎡ ×+

×=

a

aa

c

cc

BWIRSED

BWIRSED

adjIFS

Age‐Adjusted Dermal Contact Rate

⎥⎦

⎤⎢⎣

⎡ ××+

××=

a

aaa

c

ccc

BWAFSASED

BWAFSASED

adjSFS

Age‐Adjusted Inhalation Rate

⎥⎦

⎤⎢⎣

⎡ ××+

×=

a

aaa

c

cc

BWAFInhRED

BWInhRED

adjInF

Total

⎥⎥⎦

⎤

⎢⎢⎣

⎡

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+×××⎥

⎦

⎤⎢⎣

⎡ ×××⎥

⎦

⎤⎢⎣

⎡ ××

××=−

rriadj

oadjadjr

riskres

PEFVFSFInF

kgmgESFABSSFS

kgmgEIFs

EF

yrdC

16161

SF365ATTR

o

Carc

Non‐Carcinogenic (Hazard) – Residential

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+×+⎟⎟

⎠

⎞⎜⎜⎝

⎛ ×××+⎟⎟

⎠

⎞⎜⎜⎝

⎛×××

××=

rrc

i

dccc

o PEFVFInhR

RfDABSAFSAS

RfDoIRS

RfDD

yrd

11kgmg10

1kgmg10

1EEF

365BWTHQC

66cr

chaz-res

10

Table 2: Equations Used to Develop Soil Screening Levels for the Direct Contact Pathways for a Commercial/Industrial Exposure Scenario

Carcinogenic – Commercial/Industrial (c/i)

⎥⎥⎦

⎤

⎢⎢⎣

⎡

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+×××⎟⎟

⎠

⎞⎜⎜⎝

⎛ ××××⎟⎟

⎠

⎞⎜⎜⎝

⎛ ××

×××=−

rrii/c

oi/cij/ci/cr

i/criski/c

PEFVFSFInR

kgmgESFABSAFSAS

kgmgEIRS

EF

yrdBWC

16161

SF

365ATTR

o

Carc

Non‐Carcinogenic – Commercial/Industrial

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+×+⎟⎟

⎠

⎞⎜⎜⎝

⎛ ×××+⎟⎟

⎠

⎞⎜⎜⎝

⎛×××

××=

rri/c

i

dic/i/ci/c

o PEFVFInhR

RfDABSAFSAS

RfDoIRS

RfDD

yrd

11kgmg10

1kgmg10

1EEF

365BWTHQC

66c/ic/ir

a/ihaz-res

Table 3: Equations Used to Develop Soil Screening Levels for the Direct Contact Pathways for a Utility Trench Worker or Construction Exposure Scenario

Carcinogenic – Utility Trench Worker (ut)

⎥⎥⎦

⎤

⎢⎢⎣

⎡

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+×××⎟⎟

⎠

⎞⎜⎜⎝

⎛ ××××⎟⎟⎠

⎞⎜⎜⎝

⎛ ××

×××=−

utrutiut

oututjutiutr

utriskuti

PEFVFSFInR

kgmgESFABSAFSAS

kgmgEIRS

EF

yrdBWC

16161

SF

365ATTR

o

Carc

Non‐Carcinogenic – Utility Trench Worker

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+×+⎟⎟

⎠

⎞⎜⎜⎝

⎛ ×××+⎟⎟

⎠

⎞⎜⎜⎝

⎛×××

××=

ututut

i

dututut

o PEFVFInhR

RfDABSAFSAS

RfDoIRS

RfDD

yrd

11kgmg10

1kgmg10

1EEF

365BWTHQC

66utiut

uthaz-res

Table 4: Default Exposure Parameters

Parameter Variable Name

Units Value Reference

Averaging time for carcinogens ATcarc years 70 70 years by definition (USEPA 1989)

Body weight, residential child BWc kg 15 OEHHA (2005)

Body weight, residential adult BWa kg 70 OEHHA (2005)

Body weight, commercial/industrial BWc/i kg 70 OEHHA (2005)

Body weight, utility worker BWut kg 70 DTSC HERD (2005)

Exposure duration, residential child EDc years 6 OEHHA (2005)

Exposure duration, residential adult EDa years 24 OEHHA (2005)

Exposure duration, commercial/industrial EDc/i years 25 OEHHA (2005)

Exposure duration, utility worker EDut years 1

DTSC HERD (2005) Assumption is 1 month at 20 d/month, therefore ED = 1

Exposure frequency, residential child EFc d/year 350 OEHHA (2005)

Exposure frequency, residential adult EFa d/year 350 OEHHA (2005)

Exposure frequency, commercial/industrial EFc/i d/year 250 OEHHA (2005)

Exposure frequency, utility worker EFut d/year 20 DTSC HERD (2005), assumption is 1 month at 20 d/month

Soil ingestion rate, residential child IRSc mg/d 200 OEHHA (2005)

Soil ingestion rate, residential adult IRSa mg/d 100 OEHHA (2005)

Soil ingestion rate, commercial/industrial IRSc/i mg/d 100 OEHHA (2005)

Soil ingestion rate, utility worker IRSut mg/d 330 DTSC HERD (2005)

Soil to skin adherence factor, residential child

AFc mg/cm2 0.2 OEHHA (2005)

Soil to skin adherence factor, residential adult

AFa mg/cm2 0.07 DTSC HERD (2005)

Soil to skin adherence factor, commercial/industrial

AFc/i mg/cm2 0.2 OEHHA (2005)

Soil to skin adherence factor, utility worker AFut mg/cm2 0.8 DTSC HERD (2005)

Skin surface area exposed to soil, residential child

SASc cm2 2800 OEHHA (2005)

Skin surface area exposed to soil, residential adult

SASa cm2 5700 DTSC HERD (2005)

Skin surface area exposed to soil, commercial/industrial

SASc/i cm2 5700 DTSC HERD (2005)

Skin surface area exposed to soil, utility worker

SASut cm2 5700 DTSC HERD (2005)

Inhalation rate, residential child InhRc m3/day 10 OEHHA (2005)

Inhalation rate, residential adult InhRa m3/day 20 OEHHA (2005)



Inhalation rate, commercial/industrial InhRc/i m3/day 14 OEHHA (2005)

Inhalation rate, utility worker InhRut m3/day 20 DTSC HERD (2005)

Averaging time for vapor flux tau sec See

reference

ASTM (1996) ‐ equals exposure duration in seconds

Particulate emission factor, residential and commercial/industrial

PEFa m3/kg 1.3 x 109 OEHHA (2005)

Particulate emission factor, utility worker PEFut m3/kg 1.0 x 106 DTSC HERD (2005)

Dermal absorption factor from soils ABSd unitless See Table 7

Oral cancer slope factor SFo unitless See Table 7

Inhalation cancer slope factor SFi unitless See Table 7

Oral reference dose RfDo unitless See Table 7

Inhalation reference dose RfDi unitless See Table 7 Target hazard quotient THQ unitless 1 OEHHA (2005)

Target individual excess lifetime cancer risk TR unitless 1 x 10‐6 OEHHA (2005)

References: ASTM (1996). American Society for Testing and Materials, Standard Guide to Risk‐Based Corrective Action Applied at Petroleum Release Sites, ASTM E1739‐95, Philadelphia, PA. DTSC HERD (2005). Department of Toxic Substances Control, Human and Ecological Risk Division (HERD). Human Health Risk Assessment (HHRA) Note Number 1. Recommended DTSC Default Exposure Factors for Use in Risk Assessment at California Military Facilities. OEHHA (2005). Human‐Exposure‐Based Screening Numbers Developed to Aid Estimation of Cleanup Costs for Contaminated Soil, Integrated Risk Assessment Branch, Office of Environmental Health Hazard Assessment. (Cal/EPA). USEPA. 1989. Risk Assessment Guide for Superfund (RAGS) Volume I Human Health Evaluation Manual (Part A) EPA/540/1‐89/002, Office of Emergency and Remedial Response. December 1989.

Table 5: Equations Used to Estimate Volatilization and Particulate Emission Factors

Volatilization and Particulate Emission Factors

Effective Diffusion Coefficient (Deff)

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛= 2

T

310W

water2T

310a

aireff H1DDD

θθ

θθ //

Volatilization Factor (VF)

Infinite source:

( )( ) gm

kgcmtau)HKFOC(

HDU

Wsoilkg/mgairm/mgVF

abocw

eff

airair

b3

33

3

102

×⋅+⋅⋅+

⋅⋅⋅⋅

=⎥⎦

⎤⎢⎣

⎡−−

θρθπδρ

Mass‐balance considered:

( )( ) gm

kgcmtauUdW

soilkg/mgairm/mgVF

airair

b3

33

3

10×⋅⋅⋅⋅

=⎥⎦

⎤⎢⎣

⎡−−

δρ

Calculate VF using both equations, then use the lower of the two values.

VFr : Use tau = tauc + taur

VFc/i : Use tau = tauc/i

VFut : Use tau = tauut

Table 6: Default Volatilization and Soil‐Specific Parameters



Fraction organic carbon in soil FOC g OC/g soil 0.01 ASTM (1996)

Thickness of impacted soil D cm 305 ASTM (1996) (10 feet)

Wind speed in outdoor air mixing zone Uair cm/s 225 ASTM (1996)

Width of source area parallel to wind, or groundwater flow direction

W cm 1500 ASTM (1996)

Outdoor air mixing zone height δair cm 200 ASTM (1996)

Volumetric air content in vadose‐zone soils ΘA (cm3)/(cm3) 0.26 ASTM (1996)

Total soil porosity θ T (cm3)/(cm3) 0.38 ASTM (1996)

Volumetric water content in vadose‐zone soils

ΘW (cm3)/(cm3) 0.12 ASTM (1996)

Soil bulk density ρb g/cm3 1.7 ASTM (1996)

Averaging time for vapor flux, residential adult

taur s 7.57E8 ASTM (1996) = EDr in sec

Averaging time for vapor flux, residential child

tauc s 1.89E8 ASTM (1996) = EDc in sec

Averaging time for vapor flux, commercial/industrial

tauc/i s 7.88E8 ASTM (1996) = EDc/i in sec

Averaging time for vapor flux, utility worker tauut s 3.15E7 ASTM (1996) = EDut in sec

Effective diffusion coefficient in soil Deff cm2/s Chem. specific calculated

Diffusion coefficient in air Dair cm2/s Chem. specific See Table 7.

Diffusion coefficient in water Dwater cm2/s Chem. specific See Table 7.

Organic carbon‐water sorption coefficient Koc mL/g Chem. specific See Table 7.

Henry’s Law coefficient H ‐ Chem. specific See Table 7.

References: ASTM. 1996. Standard Guide to Risk‐Based Corrective Action Applied at Petroleum Release Sites, ASTM

E1739‐95, Philadelphia, PA.

Table 7: Chemical Parameter Values

Chemical Parameters1 Units Benzene Naphthalene PAH1 Reference

Henry’s Law constant - 0.23 0.018 1.9E-5 SF RWQCB ESLs Organic carbon partition

coefficient mL/g 58.9 1500 5.9E+6 SF RWQCB ESLs

Diffusion coefficient in air cm2/s 0.090 0.060 ND SF RWQCB ESLs Diffusion coefficient in

water cm2/s 9.8E-6 8.4E-6 ND SF RWQCB ESLs

Toxicity Parameters

Oral slope factor (SFo) 1/(mg/kg-d) 0.1 ND 12 OEHHA (2009) Inhalation slope factor

(SFi) 1/(mg/kg-d) 0.1 0.12 3.9 OEHHA (2009)

Oral reference dose (RfDo)

mg/kg-d 0.004 0.020 0.030 SF RWQCB ESLs

Inhalation reference dose (RfDi)

mg/kg-d 0.0086 8.6E-4 0.030 SF RWQCB ESLs

Dermal absorption factor from soil - ND 0.13 0.13 SF RWQCB ESLs

ND = No Data SF RWQCB ESLs. Regional Water Quality Control Board (RWQCB) Region 2 – San Francisco. 2008. Screening for

Environmental Concerns at Sites with Contaminated Soil and Groundwater. Interim Final. May OEHHA (2009). OEHHA Cancer Potency Values as of July 21, 2009. 1 The chemical properties for benzo(a)pyrene were used as a surrogate in developing screening levels for the

“PAH” group.

Table 8: Soil Screening Levels

Depth Benzene Naphthalene PAH (feet) (mg/kg) (mg/kg) (mg/kg) 0 to 5 2.3 13 0.038

5 to 10 100 1500 7.5 *Notes: Based on the seven carcinogenic PAHs as benzo(a)pyrene toxicity equivalent [BaPe].

The PAH screening level is only applicable where soil is affected by either waste oil and/or Bunker C fuel.

Table 9: Summary of Soil Screening Levels for Each Receptor

Chemical Residential Commercial/ Industrial Utility

Subsurface Soil --

Volatilization only

(for 5 to 10’ bgs) Residential

Scenario mg/kg mg/kg mg/kg mg/kg

Benzene 2.3 120 100 130

Naphthalene 13 45 1500 33,000

PAH 0.038 2.3 7.5 1 x 106

FIGURES

Figure 1. Conceptual Site Model for the Soil Screening Levels.

Impacted Soil from 0 to 5

feet bgs

Exposure Media

Exposure Routes R

esid

entia

l

Com

mer

cial

Tren

ch/U

tility

W

orke

r

Volatilization

Dust Emissions

Surface Soil

(0 to 5’ bgs)

Ingestion

Dermal Contact

Outdoor Air Inhalation

Exposure route is considered potentially complete

Exposure pathway considered in the development of the Soil Screening Criteria


feet bgsSubsurface

Soil (5 to 10’ bgs)

Ingestion

Dermal Contact


feet bgs

Exposure Media

Exposure Routes R

esid

entia

l

Com

mer

cial

Tren

ch/U

tility

W

orke

r

Volatilization

Dust Emissions

Surface Soil

(0 to 5’ bgs)

Ingestion

Dermal Contact

Outdoor Air Inhalation

Exposure route is considered potentially complete

Exposure pathway considered in the development of the Soil Screening Criteria


feet bgsSubsurface

Soil (5 to 10’ bgs)

Ingestion

Dermal Contact

Figure 2. Schematic for the ASTM Volatilization Factor.

Exposure pointlocation for volatile

and particulate emissions

volatile and particulate emissions in outdoor air.

Wind Direction(towards receptor 24 hours/day)

Overall thickness of source = 10

feet(for volatilization)

Impacted Soil: -- uniform concentration,-- from 0 to 10’ bgs-- 15’ wide by 15’ long (areally)

15 feet

15 feet

Surface soil (0 to 5 feet bgs)

Subsurface soil (5 to 10 feet bgs)



volatile and particulate emissions in outdoor air.





15 feet

15 feet





volatile and particulate emissions in outdoor air.volatile and particulate

emissions in outdoor air.






15 feet

15 feet





- Final 7/14/11


- Final 7/16/11


- Final 7/12/11


- Final 6/30/11

Pat

Rectangle

Pat

Rectangle

Pat

Highlight

Technical Justification for Groundwater Plume Lengths, Indicator Constituents, Concentrations, and Buffer Distances (Separation Distances) to

Receptors

The purpose of this document is to provide technical justification for the four classes of low-threat groundwater plumes that are described in the Groundwater section of the Low-Threat UST Closure Policy (the Policy). The fifth plume class is a site-specific evaluation.

The Policy Stakeholder Group chose benzene, MTBE, and TPHg as adequate indicator constituents for the groundwater plume lengths discussed in the Policy. The technical justification for using these three constituents, discussed in more detail below, relies heavily on the facts that (1) benzene has the highest toxicity of the soluble petroleum constituents, (2) MTBE typically has the longest plume lengths, and (3) TPHg represents the additional dissolved hydrocarbons that may be present resulting from a typical petroleum release. Although TPHd is not used to describe plume lengths (largely because the hydrocarbons in the TPHd carbon range are of low solubility), other technical considerations associated with the use of TPHd data are discussed below.

Benzene and MTBE are used in research studies as key indicator constituents for the threat (human health risk and nuisance) posed by groundwater plumes from petroleum releases because (1) benzene has the highest toxicity of the soluble petroleum constituents, and (2) MTBE typically has the longest plume lengths and has a low secondary MCL (taste and odor threshold of 5 micrograms/liter [ug/l]).

Several significant multi-site studies of groundwater plume lengths from petroleum release sites have been conducted across the U.S. since the mid-1990s. These studies included sites where remediation had been performed and sites where no active remediation had been performed. Most of these studies focused on benzene plumes (e.g., Rice, et al. 1995; Rice et al. 1997; Busheck et al. 1996; Mace, et al. 1997; Groundwater Services, Inc. 1997; API 1998); three studied benzene and oxygenate plumes (including MTBE) (Dahlen et al. 2004; Shih et al. 2004; Kamath et al. in press). Most of these plume studies are further discussed in detail in the Fate and Transport chapter of the California LUFT Manual.

In summary for all of these multi-site studies, the average benzene plume length was less than 200 feet and 90% of the benzene plumes were less than 400 feet long. The peer-reviewed study by Shih et al. (2004) of plume lengths at 500 UST sites in the Los Angeles area is widely relied upon as representative of current knowledge of plume lengths at UST sites in California. Results for benzene, MTBE and TPHg from Shih et al. (2004) are as follows:

Constituent (and plume limit concentration)

Average Plume Length (feet)

90th Percentile Plume Length (feet)

Maximum Plume Length (feet)

Benzene (5 ug/l) 198 350 554 MTBE (5 ug/l) 317 545 1,046

TPHg (100 ug/l) 248 413 855 Data are from Shih et al. (2004). Plume lengths were measured from the source area.

Although the California MCL for benzene is 1 ug/l, Shih et al. (2004) used a plume limit concentration of 5 ug/l because of statistical uncertainty with concentrations too close to the laboratory reporting limit. The benzene plume lengths at a 1 ug/l concentration limit would be expected to be slightly longer than those shown here.

Ruiz-Aguilar et al. (2003) studied UST sites in the Midwest with releases of ethanol-amended gasoline (10% ethanol by volume) and found that benzene plume lengths may increase by 40% to 70% due to the addition of ethanol in gasoline (replacing MTBE). Ethanol is preferentially biodegraded over the benzene, which results in a longer benzene plume. However, the Policy addresses this potential for expansion of the plume lengths by adding safety factors of 100% to 400%.

It is well documented that, due to effective solubility, the hydrocarbons that will dissolve at measurable amounts into groundwater from a petroleum fuel release (including gasoline, kerosene, jet fuel, diesel or heavier fuels) are limited to primarily the very small aliphatics (less than C7) and the C14 or smaller aromatics (e.g., Shiu et al. 1990; Coleman et al. 1984). The C15 and larger hydrocarbons have very low effective solubilities and are not found in the dissolved phase of a petroleum fuel release. The carbon range of the potential dissolved hydrocarbons (less than or equal to C14) is largely covered by the TPHg carbon range (approximately C5 to C12). Therefore, TPHg should be sufficient to represent the dissolved hydrocarbons that may be present in addition to benzene and MTBE from virtually any type of product release. TPHd was not included as an indicator constituent for groundwater plume length because the vast majority of the TPHd carbon range (approximately C12 to C22) is higher than the carbon range for the possible dissolved hydrocarbons (less than or equal to C14). Oxygenates other than MTBE were not included as indicator constituents because Shih et al. (2004) documented that MTBE had the longest plume length of any of the oxygenates (MTBE, TBA, DIPE, TAME, ETBE) at any percentile, and Kamath et al. (in press) found that TBA plumes were comparable in length to MTBE plumes. Therefore, MTBE can be used as a conservative indicator for the other oxygenates including TBA.

For groundwater samples analyzed for TPHd for comparison to Water Quality Objectives (WQOs), a silica gel cleanup (SGC) should be included for the following reasons. It is well known that the TPHd analysis (Method 8015B) is not specific to hydrocarbons unless a SGC is used; otherwise the reported TPHd concentration can include polar non-hydrocarbon compounds in addition to the hydrocarbons that may be present in a water sample (e.g., Zemo and Foote

2003). These polar compounds can be from various sources, including metabolites from biodegradation of petroleum (primarily alcohols and organic acids, with possible phenols, aldehydes and ketones). At sites with biodegrading petroleum, the majority of the organics being measured as “TPHd” (without SGC) can be polar compounds and not dissolved hydrocarbons. WQOs for diesel-range petroleum hydrocarbons for health risk or taste and odor concerns are based on the properties of the dissolved hydrocarbons assumed to be present and not on the properties of the polar compounds. For example, the health-based ESL for TPHd is based on the assumption that 100% of the TPH has a toxicity equivalent to the C11 to C22 aromatics, and the taste and odor value for TPHd is based on the dissolved phase of fresh diesel/kerosene (which would be primarily the C14 and smaller aromatics) (SFRWQCB 2008). The San Francisco Bay RWQCB recognized that reported TPHd concentrations may include polar compounds and issued a guidance memorandum recommending that SGC be routinely used so that “….. decisions could be made based on analytical data that represents dissolved petroleum.” (SFRWQCB 1999). Only the hydrocarbon component of the TPHd concentration should be compared to the TPHd WQOs, and thus SGC is necessary to separate the hydrocarbons from the polar compounds in a groundwater sample prior to analysis. It is well established that a SGC does not remove the dissolved hydrocarbons in a sample (e.g., Lundegard and Sweeney 2004). Further, the potential for removal of hydrocarbons by a SGC is always monitored as part of the routine laboratory quality assurance reporting where lab control samples are spiked with a hydrocarbon (surrogate), are subjected to a SGC, and recovery of the surrogate is measured and must be within acceptable ranges.

The four classes of stabilized plume lengths and buffer distances from the plume edge to the closest water supply well or surface water (receptors) that are defined as “low threat” in the Policy are initially based upon the plume lengths from the studies cited above, but also are based on additional safety factors that the Stakeholder Group considered applicable to be protective in a state-wide policy document. For example, based on the plume studies, a total separation distance from the source area to the receptor of about 500 feet should be protective for 90% of plumes from UST sites, and a total separation distance from the source area to the receptor of about 1,000 feet should be protective for virtually all plumes from UST sites. Conversely, the “low-threat classes” require a known maximum stabilized plume length (which reduces uncertainty as to how long the plume might become in the future), and include additional safety factors and concentration limits developed by the Stakeholder Group.

Stakeholder Group participants also recognize and acknowledge that this Policy is consistent with other State and local practices regarding impacts to groundwater caused by other anthropogenic releases. For example, State and local agencies establish required separation distances or “setbacks” between water supply wells and septic system leach fields (typically 100 feet), and sanitary sewers (typically 50 feet; [DWR 1981]).

The Stakeholder Group acknowledges that the biodegradation/natural attenuation of petroleum hydrocarbon and oxygenate plumes has been documented by many researchers since the 1990s.

All of this work shows that biodegradation/natural attenuation of petroleum hydrocarbons and MTBE occurs under both aerobic and anaerobic conditions, but the rate of degradation/attenuation depends on the individual constituent and the plume geochemical conditions. The maximum concentrations for benzene and MTBE specified in the low-threat classes below are expected to biodegrade/naturally attenuate to WQOs within approximately 10 to 30 years, based on commonly-accepted rate constants for typical plume conditions and calculations (e.g., Wilson 2003; USEPA 2002). A time period of multiple decades or longer to reach WQOs has been determined to be “reasonable” for plumes of limited extent in existing State Water Board closure orders for UST sites (e.g., Order WQ 98-04 [Matthew Walker]).

TBA is a byproduct of biodegradation of MTBE, and TBA concentrations can build up temporarily in the anaerobic portion of a plume. With respect to the natural attenuation of TBA, Kamath et al. (in press) recently studied benzene, MTBE and TBA plumes at 48 UST sites (30 sites in California) and found that (1) most (68%) of the TBA plumes were stable or decreasing in size, and (2) in the stabilized plumes, the median attenuation rate for TBA was similar to the rates for MTBE and benzene. These findings indicate that TBA should not pose a significant threat to groundwater resources, and are consistent with the finding from Williams (in press) that TBA and MTBE have been detected in only a very limited number of public drinking water supply wells in California between 1996 and 2010. The average annual detection frequencies at any concentration and at concentrations greater than the WQO (12 ug/l for TBA and 5 ug/l for MTBE), through 2010 are: 1.4% and 0.2% for TBA, respectively, and 1.6% and 0.8% for MTBE, respectively (Williams, in press).

The following paragraphs present and discuss the key rationales for low-threat plume lengths, maximum concentrations, and separation distances for each low-threat class. Note that the specified concentrations are maximums, and would likely occur in only a few wells; the average concentrations in the plume would be lower. Note also that these groundwater plume class criteria (concentrations, plume lengths and separation distances) are only one component of the overall evaluation of site conditions that must be satisfied to be considered for closure as a low-threat site under the Policy.

Class 1: The “short” stabilized plume length (<100 feet) is indicative of a small or depleted source and/or very high natural attenuation rate. The 250 feet distance to a receptor from the edge of the plume represents an additional 250% “plume length” safety factor in the event that some additional unanticipated plume migration was to occur.

Class 2: The “moderate” stabilized plume length (<250 feet) approximates the average benzene plume length from the cited studies. The maximum concentrations of benzene (3,000 ug/l) and MTBE (1,000 ug/l) are conservative indicators that a free product source is not present. These concentrations are approximately 10% and 0.02%, respectively, of the typical effective solubility of benzene and MTBE in unweathered gasoline. These concentrations are expected to biodegrade/naturally attenuate to WQOs within a reasonable time frame. The potential for vapor

intrusion from impacted groundwater must be evaluated separately as per the vapor intrusion section of the Policy. The 1,000 feet distance to the receptor from the edge of the plume is an additional 400% “plume length” safety factor in the event that some additional unanticipated plume migration was to occur. Also note that California Health and Safety Code §25292.5 requires that UST owners and operators implement enhanced leak detection for all USTs within 1,000 feet of a drinking water well. In establishing the 1,000 feet separation requirement the legislature acknowledged that 1,000 feet was a sufficient distance to establish a protective setback between operating petroleum USTs and drinking water wells in the event of an unauthorized release.

Class 3: The “moderate” stabilized plume length (<250 feet) approximates the average benzene plume length from the cited studies. The on-site free product and/or high dissolved concentrations in the plume remaining after source removal to the extent practicable (as per the General Criteria in the Policy) require five years of monitoring to validate plume stability/natural attenuation (i.e., to confirm that the rate of natural attenuation exceeds the rate of NAPL dissolution and dissolved-phase migration). The potential for vapor intrusion from free product or impacted groundwater must be evaluated separately as per the vapor intrusion section of the Policy. The 1,000 feet distance to the receptor from the edge of the plume is an additional 400% “plume length” safety factor in the event that some additional unanticipated plume migration was to occur, and is consistent with H&S Code §25292.5 as discussed above.

Class 4: The “long” stabilized plume length (<1,000 feet) approximates the maximum MTBE plume length from Shih et al. (2004). The maximum benzene and MTBE source area concentrations (1,000 ug/l each) in the stable plume are expected to biodegrade/naturally attenuate to WQOs within a reasonable time frame. The maximum benzene concentration would not pose a vapor intrusion risk over the extent of the plume (assuming that five feet of bioreactive vadose zone is available over the extent of the plume; see justification for vapor intrusion screening criteria for details). The 1,000 feet distance to the receptor from the edge of the plume is an additional 100% “plume length” safety factor in the event that some additional unanticipated plume migration was to occur, and is consistent with H&S Code §25292.5 as discussed above.

Notes on Free Product Removal

State regulation (CCR Title 23, Division 3, Chapter 16, Section 2655) requires that “responsible parties“…. remove free product to the maximum extent practicable, as determined by the local agency…” (Section 2655a) “…. in a manner that minimizes the spread of contamination into previously uncontaminated zones”… (Section 2655b), and that “[a]batement of free product migration shall be the predominant objective in the design of the free product removal system” (Section 2655c). Over the years there has been debate on the meaning of the terms “free product” and “maximum extent practicable”. Product (light non-aqueous phase liquid [LNAPL]) can exist in three conditions in the subsurface: residual or immobile LNAPL (LNAPL that is

trapped in the soil pore spaces by capillary forces and is not mobile), mobile LNAPL (enough LNAPL is present in the soil pore spaces to overcome capillary forces so that the LNAPL can move) and migrating LNAPL (mobile LNAPL that is migrating because of a driving head). “Residual LNAPL”, “mobile LNAPL” and “migrating LNAPL” are described in detail in several peer-reviewed technical documents, including the 2009 Interstate Technology Regulatory Council (ITRC) Technical/Regulatory Guidance “Evaluating LNAPL Remedial Technologies for Achieving Project Goals”. Given the predominant objective of abatement of migration, the term “free product” in the State regulation is primarily equivalent to “migrating LNAPL” (which is a subset of “mobile LNAPL”), and secondarily equivalent to “mobile LNAPL”. Whether LNAPL is mobile (and therefore could potentially migrate) or not is usually tested by observing recharge of LNAPL after removing LNAPL from a monitoring well. Whether LNAPL is migrating or not is tested by monitoring the extent of the LNAPL body (usually using the apparent product thickness in monitoring wells) at a certain water level elevation over time. If the extent at that water level elevation does not expand, then the LNAPL is not migrating. Therefore, LNAPL must be removed to the point that its migration is stopped, and the LNAPL extent is stable. Further removal of non-migrating but mobile LNAPL is required to the extent practicable at the discretion of the local agency. Removal of mobile LNAPL from the subsurface is technically complicated, and the definition of “extent practicable” is based on site-specific factors and includes a combination of objectives for the LNAPL removal (such as whether the mobile LNAPL is a significant “source” of dissolved constituents to groundwater or volatile constituents to soil vapor, or whether there is a high likelihood that hydrogeologic conditions would change significantly in the future which may allow the mobile LNAPL to migrate) and technical limitations. The typical objectives for LNAPL removal, technologies for LNAPL removal and technical limitations of LNAPL removal are discussed in several peer-reviewed technical documents including the 2009 ITRC Guidance (see especially Section 4 “Considerations/Factors Affecting LNAPL Remedial Objectives and Remedial Technology Selection”, Table 4.1 [Example Performance Metrics], Table 5-1 [Overview of LNAPL Remedial Technologies], and Table 6-1 [Preliminary Screening Matrix]).

References

American Petroleum Institute (API), 1998. Characteristics of dissolved petroleum hydrocarbon plumes, Results from four studies. API Soil/Groundwater Technical Task Force, Vers. 1.1. December.

Buscheck, T.E., D.C. Wickland, and L.L. Kuehne, 1996. Multiple lines of evidence to demonstrate natural attenuation of petroleum hydrocarbons. Proceedings of the 1996 Petroleum Hydrocarbon and Organic Chemicals in Groundwater Conference. NGWA/API. Westerville, OH.

Coleman, W.E., J.W. Munch, R.P. Streicher, P. Ringhand, and F. Kopfler, 1984. The identification and measurement of components in gasoline, kerosene and No. 2 fuel oil that partition into the aqueous phase after mixing. Arch. Environ. Contam. Toxicol. 13: 171-178.

Dahlen, P.R., M. Matsumura, E.J. Henry, and P.C. Johnson, 2004. Impacts to Groundwater Resources in Arizona from Leaking Underground Storage Tanks (LUSTs). http://www.eas.asu.edu/civil/Environmental/Groundwater.htm. Groundwater Services, Inc. 1997. Florida RBCA Planning Study. www.GSI-net.com

ITRC (Interstate Technology & Regulatory Council). 2009. Evaluating LNAPL Remedial Technologies for Achieving Project Goals. LNAPL-2. Washington, D.C.: Interstate Technology & Regulatory Council, LNAPLs Team. www.itrcweb.org.

Kamath, R., J.A. Connor, T.E. McHugh, A. Nemir, M.P. Lee and A.J. Ryan, in press. Use of long-term monitoring data to evaluate benzene, MTBE and TBA plume behavior in groundwater at retail gasoline sites. Journal of Environmental Engineering. (Accepted for publication on June 15, 2011)

Lundegard, P.D. and R.E. Sweeney, 2004. Total petroleum hydrocarbons in groundwater: Evaluation of nondissolved and nonhydrocarbon fractions. Environmental Forensics, Vol 5: 85-95.

Mace, R.E., R.S. Fisher, D.M. Welch, and S.P. Parra, 1997. Extent, mass, and duration of hydrocarbon plumes from leaking petroleum storage tank sites in Texas. Bureau of Economic Geology, Geological Circular 97-1.

Rice, D.W., R.D. Grose, J.C. Michaelsen, B.P. Dooher, D.H. MacQueen, S.J. Cullen, W.E. Kastenberg, L.G. Everett, M.A. Marino, 1995. California leaking underground fuel tank (LUFT) historical case analyses. Lawrence Livermore National Laboratory (LLNL). UCRL-AR-122207. November.

Rice, D.W., B.P. Dooher, S.J. Cullen, L.G. Everett, W.E. Kastenberg, and R.C. Ragaini, 1997. Response to USEPA comments on the LLNL/UC LUFT cleanup recommendations and California historical case analysis. LLNL. UCRL-AR-125912. January.

Ruiz-Aguilar, G.M.L., K. O’Reilly, and P.J.J. Alvarez, 2003. A comparison of benzene and toluene plume lengths for sites contaminated with regular vs. ethanol-amended gasoline. Ground Water Monitoring & Remediation, Vol. 23, No. 1: 48-53.

San Francisco Regional Water Quality Control Board (SFRWQCB), 2008. Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater. Interim Final, May.

SFRWQCB, 1999. Memorandum: Use of silica gel cleanup for extractable TPH analysis. February.

Shih, T., Y. Rong, T. Harmon, and M. Suffet, 2004. Evaluation of the impact of fuel hydrocarbons and oxygenates on groundwater resources. Environmental Science & Technology. Vol. 38, No. 1: 42-48.

Shiu, W.Y., M. Bobra, A.M. Bobra, A. Maijanen, L. Suntio, and D. Mackay, 1990. The water solubility of crude oils and petroleum products. Oil and Chem. Poll. Vol. 7, No. 1, 57-84.

USEPA, 2002. Ground Water Issue: Calculation and use of first-order rate constants for monitored natural attenuation studies. EPA/540/S-02/500. November.

Williams, P.R.D., in press. MTBE in California’s public drinking water wells: Have past predictions come true? Environmental Forensics. (Accepted for publication on June 4, 2011)

Wilson, J.T., 2003. Fate and Transport of MTBE and Other Gasoline Components. Chapter 3 in MTBE Remediation Handbook, E.E. Moyer and P.T. Kostecki (editors). Amherst Scientific Publishers, Amherst, MA.

Zemo, D.A. and G.R. Foote, 2003. The technical case for eliminating the use of the TPH analysis in assessing and regulating dissolved petroleum hydrocarbons in ground water. Ground Water Monitoring & Remediation, Vol. 23, No. 3: 95-104.



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Technical Justification for Low‐Threat Closure Scenarios for Petroleum Vapor Intrusion Pathway

1 EXECUTIVE SUMMARY

For petroleum‐related volatile organic compounds (VOCs) at retail sites, current risk‐based screening levels (such as the California Human Health Screening Levels [CHHSLs]) for evaluating risk from vapor intrusion at retail sites are extremely conservative. This conservatism is caused by excluding biodegradation in site screening and often drives further unnecessary site evaluation. Recent models and field studies show that bioattenuation of petroleum hydrocarbons at retail sites is significant. Petroleum VOCs (such as benzene, toluene, ethylbenzene and xylenes (BTEX)) concentrations can attenuate by 4 to 6 orders of magnitude within short vertical distances (e.g., < 2 m) in the unsaturated zone. The VOC attenuation increases by an additional order of magnitude (or more) if transport across the building foundation to indoor air is also considered. The sharp decrease in petroleum VOC concentrations within a short vertical distance of the unsaturated zone is amenable to use of exclusion distances as a site‐screening methodology for vapor intrusion. Exclusion distances are defined as source (VOCs in soil or groundwater)‐receptor (building) separation distances beyond which the risk of vapor intrusion is negligible. Exclusion distance criteria can be broadly defined for two types of sources: low‐concentration and high‐concentration sources which are defined below.

Recent modeling studies and evaluations of field (soil‐gas) data from numerous retail sites and sampling locations demonstrate that biodegradation is sufficient to limit the potential for vapor intrusion at sites with “low concentration” hydrocarbon sources. For example, there is less than a 5% probability that benzene concentrations in soil gas would exceed a conservative screening level of 100 ug/m3 at a distance of 5 feet above the source. (Note the CHHSL for benzene in soil gas is 83 ug/m3.) The attenuation is predicted to increase with lateral displacement of the source from the building foundation. Vapor intrusion risks are thus expected to be rare to non‐existent at sites with low‐concentration sources.

At sites with “high concentration” volatile sources (unweathered residual LNAPL in soil and/or unweathered free‐phase LNAPL on groundwater), transport modeling shows that hydrocarbons will attenuate in the unsaturated zone by approximately 6 orders of magnitude within 7 m (~20 ft) at sites. This result is achieved assuming reasonable approximations for source type and biodegradation rate. Analysis of soil‐gas data collected from many retail sites with LNAPL sources indicate that the distance required to attenuate soil vapor concentrations to below typical screening levels are on the order of 8 – 13 ft. As with “low‐concentration” sources (weathered residual LNAPL in soil and/or dissolved concentrations in groundwater), the bioattenuation is more significant for LNAPL sources separated laterally from building foundations (i.e. the soil gas concentrations would attenuate in even shorter distances).

The Stakeholder Group has proposed screening criteria for four basic scenarios that can be used to identify low‐threat closure scenarios for vapor intrusion (VI). The purpose of this technical document is


to outline the intent of the Stakeholder Group for use of these screening criteria and to provide justification for the four scenarios below. These scenarios are:

Scenario 1: Unweathered LNAPL on groundwater Scenario 2: Unweathered LNAPL in soil Scenario 3: Dissolved phase benzene concentrations in groundwater Scenario 4: Direct measurement of soil gas concentrations

For each of these scenarios, screening criteria have been proposed that, if met, would identify the site as a low‐threat to human health from the vapor intrusion pathway.

It is important that the current state of the science as described herein be used to develop rational, technically defensible, approaches to address these potential vapor intrusion risk scenarios. In addition, many of the cited exclusion criteria are based on analysis of soil‐gas data collected from retail sites. The screening criteria may therefore not be applicable for non‐retail (e.g., pipeline, manufacturing, and terminal) sites where significantly larger volume petroleum hydrocarbon releases may have occurred. If conditions at non‐retail sites are significantly different than would be encountered at a typical retail site, they should be evaluated on a site‐specific basis. The materials referenced in this technical justification are consistent with the technical material being used to develop guidance by US EPA’s Office of Underground Storage Tanks (OUST)’s Task Force on Petroleum Vapor Intrusion.

2 INTRODUCTION

Biodegradation is the most critical process governing the potential for vapor intrusion at petroleum release sites. The significance of biodegradation depends largely on the demand for oxygen (O2) and its availability. Key factors that affect the O2 demand/availability include source strength/type (e.g., LNAPL or dissolved phase), source location (i.e., above or below the capillary zone), soil type (DeVaull, 2007), variable and/or high soil‐moisture saturation, building foundation type/size (Patterson and Davis, 2009; DeVaull (in press) and surface cover.

At sites with “low‐concentration” sources (weathered residual in soil and/or dissolved concentrations in groundwater), the significance of biodegradation is most notable because biodegradation conditions in the unsaturated zone generally remain aerobic. At these sites, O2 availability in the unsaturated zone generally exceeds O2 demand resulting from biodegradation. Biodegradation under aerobic conditions has been shown to be rapid resulting in the development of sharp attenuation fronts where BTEX concentrations decrease by several orders of magnitude over relatively short (e.g. <1 m) vertical distances (Fischer et al., 1996; Lahvis et al., 1999; DeVaull, 2007; Davis, 2009; and Hartman, 2010). The hydrocarbon reaction fronts (the point at which most of the degradation is occurring) tend to develop very near the water table at sites with dissolved‐phase only sources in groundwater (e.g., benzene concentrations < 15 mg/L). At these sites, effects of soil type, building foundation and surface cover will tend to be limited. Evidence to support these assertions exists both in the theory (modeling) (DeVaull, 2007, Abreu et. al. 2009, API, 2009) and in the field (Lahvis and Baehr, 1996; API, 2009; Davis, 2009).


Further attenuation is predicted for dissolved‐phase sources displaced laterally from the building foundation (Abreu and Johnson, 2005).

At sites with LNAPL on the groundwater, biodegradation can also be quite notable. Exclusion distances for benzene and total petroleum hydrocarbons (TPH) determined from analysis of soil‐gas data primarily collected at retail sites have been estimated to be in the range of 8 to 15 feet (Davis, 2009; Hartman, 2010; Lahvis, 2011)1. The greater exclusion distance for LNAPL sites compared to dissolved‐phase sites is in part related to the added demand for O2 (noted above) for LNAPL sources and the tendency for LNAPL sources to be distributed above the capillary zone. For dissolved phase sources in groundwater, the capillary zone has been documented as an active zone for VOC attenuation (Lahvis and Baehr, 1996). Results from the analysis of the Davis (2010) soil‐gas database are consistent with other large field studies (Lahvis, 2011). As noted above, the significance of bioattenuation is largely dependent on source type. Differentiation of residual‐phase LNAPL (high concentration) sources from dissolved‐phase (low concentration) sources can, however, be difficult. The following general rules of thumb could be used as indicators of residual‐phase LNAPL sources in groundwater or in soil:

Presence of LNAPL Direct evidence:

• sites with current or historical evidence of LNAPL in soil or LNAPL at the water table as evidenced in wells

Indirect evidence:

• chemicals of concern (COCs) approaching (> 0.2) effective solubilities (Bruce et al., 1991) in groundwater (e.g., benzene > 3 mg/L ; total benzene, toluene, ethylbenzene and xylenes (BTEX) > 20 mg/L; TPH diesel range organics (DRO) > 5 mg/L) and in soil (TPH gasoline range organics (GRO) > 100 ‐ 200 mg/kg(2); TPH DRO > 10 ‐ 50 mg/kg) (see ASTM, 2006, Alaska DEC, 2011)3

• TPH vapor readings from a photo‐ionization detector (PID) of > 1,000 ppm (recent gasoline release sites), > 100 ppm (recent diesel/historic gasoline release sites), and > 10 ppm (historic diesel sites) (Alaska DEC, 2011). Note weathered LNAPL typically has a significant reduction in the VOC content and therefore represents less of a concern for vapor intrusion.

The following rules‐of‐thumb for can be used to determine whether LNAPL is a concern for vapor intrusion risk:

Differentiating between Weathered and Unweathered LNAPL

• For groundwater impacted by LNAPL or where groundwater is in proximity to LNAPL, effective solubility is a key indicator for whether the LNAPL is depleted of VOCs. For

1 It is important to note, that the soil‐gas data were collected primarily at retail sites. Approximately 16% of the soil‐gas sampling locations were directly below building foundations (i.e., sub‐slab).

2 TPH (GRO) between 100 to 200 mg/kg may indicate there may be a slight amount of LNAPL. TPH (GRO) less than 100 mg/kg is a good indication that there is no LNAPL present.

3 The primary driver for vapor intrusion is benzene. For petroleum‐based fuels other than gasoline, benzene is not found at levels that would cause a vapor intrusion problem.


example, benzene’s effective solubility is approximately 18 mg/L, if it constitutes 1% of gasoline. Therefore benzene concentrations < 1 mg/L are reasonable indicators that the LNAPL is weathered (depleted of VOCs).

• For soil sources, TPH (GRO) < 100 mg/kg is a good indication that there is a small or low concentration (VOC) source.

Naphthalene is currently considered a carcinogen via the inhalation exposure route and since it is also volatile, it can be considered a potential risk driver. The exclusion criteria defined for benzene are assumed to be conservative for naphthalene, which is also highly susceptible to biodegradation (Anderson et al., 2008; GSI, 2010). Naphthalene also has a much lower solubility value and Henry’s Law coefficient (compared to benzene), thereby limiting the amount of naphthalene available to volatilize into the gas phase. For these reasons, the screening criteria described here, while developed for benzene, should also be protective of naphthalene vapor intrusion.

3 TECHNICAL BACKGROUND – Discussion of Biodegradation Effects

This section will present the results of model studies and field data that support the assumptions made in the vapor intrusion exclusion criteria. First, the results found at “low‐concentration source” cases will be discussed followed by “high‐concentration source” cases.

Lastly, it is important to note that once the groundwater concentrations are below effective solubility, the actual hydrocarbon concentrations in groundwater are not necessarily good predictors of vapor intrusion risk. Field site observations show that dissolved‐phase hydrocarbon concentrations in shallow groundwater and soil gas concentrations overlying the water table are poorly correlated (Lahvis, 2011). The poor correlation at dissolved‐phase only sites can be attributed to the inability to accurately measure hydrocarbon concentrations at the water table and to the considerable bioattenuation of hydrocarbon vapors between the water‐table source and the soil‐gas measurement location. At LNAPL (residual‐phase) sites, soil‐gas concentrations are also poorly correlated with groundwater concentrations because LNAPL sources are typically present above the water table. For these reasons, it is recommended to focus the development of screening criteria solely on the basis of source type (LNAPL and groundwater) rather than source (groundwater) concentration.

3.1 Low-Concentration Sources (weathered residual in soil and/or dissolved concentrations in groundwater)

For purposes of this technical justification, low concentration sources at hydrocarbon sites are defined as dissolved‐phase concentrations. Low concentration sources will therefore be composed primarily of the more soluble (aromatic) LNAPL constituents, benzene, toluene, ethylbenzene, xylenes, and naphthalene. Of these constituents, benzene is the primary risk driver for vapor intrusion because of its relatively higher toxicity and vapor migration potential. Note, weathered LNAPL can behave like low‐concentration sources because the LNAPL may be depleted of volatile chemicals.


3.1.1 Model Studies

Results from detailed numeric (3‐dimensional) models (see Figure 3 from API below) indicate that complete attenuation of the hydrocarbons (approximately 10 orders of magnitude) is predicted between a relatively low concentration source (< 10 mg/L total hydrocarbon in soil gas) and indoor air where the source is separated from the receptor by > 3 meters (see API, 2009; Abreu et al., 2009)4. Note, the “hydrocarbon” modeled in these studies was assumed to have the same fate and transport properties as benzene. In addition, the simulations are based on assuming biodegradation takes place only in the aerobic portion of the unsaturated zone (i.e., when O2 concentrations exceed 1%). An aerobic biodegradation rate of 0.79 hr‐1 is assumed for the hydrocarbon (benzene) based on a mean of published rates (DeVaull, 2007). Note, while a degradation rate of 0.75 hr‐1 may seem high, the model only allows degradation in the regions where there is enough O2 to support it. The model cutoff for allowing degradation was 1% O2. A 10 mg/L benzene vapor source is consistent with a dissolved‐phase source of benzene (or BTEX) of around 40 mg/L assuming equilibrium partitioning between soil gas and groundwater and a Henry’s law coefficient of 0.25 for benzene (or BTEX). The attenuation with distance is increased for the latter condition because diffusion of the hydrocarbons will tend to be vertically upwards (toward the soil surface) rather than laterally towards the receptor. Hence, there is little potential for vapor intrusion to occur at sites with dissolved‐phase sources separated laterally from building foundations.

The following two figures from API (2009) show hydrocarbon and O2 profiles predicted by transport modeling for low‐concentration vapor sources varying between 0.1 mg/L hydrocarbon (0.4 mg/L dissolved‐phase equivalent) and 10 mg/L hydrocarbon (40 mg/L dissolved‐phase equivalent) and two different foundation configurations, basement and slab, respectively. Note, the “hydrocarbon” modeled in these studies was again assumed to have the same fate and transport properties as benzene. The source concentration was assumed to be equal to the combined concentrations of all of the BTEX. This approach was used because it was conservative to consider the increased O2 demand from the additional VOCs present (all of the BTEX). Therefore, these modeling study results can be considered conservative for benzene.

4 A 10 mg/L hydrocarbon soil gas source would equate to a ~40 mg/L source of BTEX in groundwater assuming a vapor/aqueous phase partition coefficient of around 0.25 (Morrison, 1999) assuming the source were dissolved.


3.1.2 Field Data A soil‐gas database has been developed by Robin Davis (Utah Department of Environmental Quality ‐ DEQ). The database was compiled from numerous retail, distribution, and manufacturing sites across several states, including California. The soil‐gas data were collected from locations on and off‐site. Approximately 16% of the soil‐gas data were collected directly below building foundations (i.e., subslab). The data from retail sites are being used to support the development of new state (see http://www.swrcb.ca.gov/ust/luft_manual.shtml ) and federal (US EPA OUST) vapor intrusion guidance. Analyses of the soil‐gas data are described in Davis (2009) and Hartman (2010). The data analyses support the model results discussed in the previous section. The analyses indicate that “dissolved‐phase” sources < 6 mg/L benzene in groundwater (or ~24,000,000 ug/m3 vapor phase equivalent5) are completely attenuated within distances of 5 ft. or less (see figure below from Davis, 2009).

It is important to note, however, that the Davis (2009) analyses of thickness of clean overlying soil required to attenuate benzene vapors (or “exclusion distance”) did not rigorously screen out potential residual LNAPL sources above the water table. These sites pose a similar risk for vapor intrusion as sites

5 Assuming a Henry’s Law coefficient of 0.25 cm3/cm3 for benzene.


with free‐phase LNAPL on groundwater (i.e., sites where LNAPL is observed in monitoring wells). The analysis shown in Figure 5 also includes data from “non‐retail” locations. It is also important to note that the Davis (2009) results imply that the vapor intrusion risk is dependent on the source concentration in groundwater. Again, this dependence has not been observed at other sites and is not recommended to be used in developing groundwater concentration‐based exclusion distances.

A slightly different analysis of the “retail‐only” data from the Robin Davis database by Lahvis (2011) shows that benzene will be bioattenuated below a relatively conservative soil‐gas screening level of 100 ug/m3 within 5 ft of the water table6. The analysis focused on identified sources of benzene in groundwater and filtered out sites with either direct evidence of LNAPL (current, historical) or indirect evidence of LNAPL (soil‐gas measurements collected near potential sources (i.e., locations within 25 ft of USTs and dispensers), and also screened out sites with benzene concentrations in groundwater > 15 mg/L or BTEX > 75 mg/L). The vast majority (84%) of the soil‐gas measurements were taken from sites with source concentrations of benzene in groundwater ranging from 0.1 mg/L (100 ug/L) to 15 mg/L.

Figure from Lavis (2011)

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6 This value represents the attenuation between a benzene source in groundwater up to 15 mg/L (or 7,500,000 ug/m3 in soil-gas) and a conservative soil-gas screening level concentration of 100 ug/m3. This concentration is representative of a screening-level concentration in soil gas (assuming an indoor air risk-based concentration of 2 ug/m3 and a slab attenuation factor of 0.02).


From a probability standpoint, the results from the scatter plot can be defined as follows (Lahvis, 2011):

Figure from Lahvis (2011)

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The probability of having benzene vapor concentrations near the receptor that exceed a conservative screening level (i.e., 100 ug/m3) at dissolved‐phase (retail) sites is less than 5%. The water table would have to essentially be in contact with a building foundation for there to be a potential concern for vapor intrusion at low concentration sites.

3.1.3 Summary of Low Concentration Sources

In summary, field data from retail sites shows that for low concentration (e.g., dissolved‐phase only) sources, benzene will be attenuated to below screening levels within 5 ft above the water table. Vapor intrusion risks would be rare to non‐existent at these retail sites provided the water table does not come in contact with the building foundation.

3.2 High-Concentration Sources (unweathered residual in soil and/or free-phase LNAPL on groundwater)

3.2.1 Model Studies As shown in the attached figure from Abreu et al. (2009), hydrocarbons are predicted to completely attenuate in the unsaturated zone above an LNAPL source within ~ 7m of the source. Again, the model simulations use benzene as a surrogate for all of the TPH present. A mean biodegradation rate of 0.79 hr‐1 was again assumed (DeVaull, 2007) in model regions where the O2 level was sufficiently high enough to support aerobic biodegradation.


Again, the attenuation is expected to increase for NAPL sources displaced laterally from the basement foundation (see Abreu and Johnson, 2005).

3.2.2 Field Data

A more recent analysis of the soil‐gas database by Davis (2010) indicates that the model predicted bioattenuation is conservative. Exclusion distances of only 8 ft. were found to be sufficient to attenuate LNAPL sources. This analysis takes into account residual LNAPL sources in the unsaturated zone (see the following figure from Davis (2010)).


Lahvis (2011) has interpreted the soil‐gas database compiled by Davis slightly differently. The next figure shows benzene concentrations in soil gas from retail‐only sites plotted as a function of distance above the water table ) (see following figure):

Near-Slab Multi-Depth, Sub-Slab

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Benzene SV Sample Event over LNAPL & Soil Sources

Method 2 Results for LNAPL & Soil Sources

Benzene: 48 exterior/near-slab + 22 sub-slab = 70 total

TPH: 17 exterior/near-slab + 18 sub-slab = 35 total

~8 ft CLEAN overlying soil attenuates vapors associated with LNAPL/Soil Sources

Figure from Davis (2010)


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Figure from Lavis (2011)

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As shown, benzene concentrations in soil gas generally attenuate by more than 4 orders of magnitude with at a source separation distance of > 12 ft from the source at LNAPL sites. The attenuation is most significant at distances > 12 ft above the source. A statistical analysis of these data shows a > 95% probability of encountering benzene concentration below 100 ug/m3 in soil gas at distances >~ 13 ft above the source.


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The lateral separation exclusion distances would be expected to be less than the vertical exclusion distances for the reasons previously explained.

3.2.2 Summary

Most recent field data analyses indicate that 8 to 13 ft of clean soil (soil with no LNAPL present) are sufficient to limit the risk for vapor intrusion at sites with LNAPL sources in either soil or groundwater.

3.3 Technical Background Conclusions

Low‐concentration sources have been shown to attenuate up to 6 orders of magnitude in the unsaturated zone within short vertical distances (e.g., < 5 ft) due to biodegradation. Biodegradation is sufficient to essentially eliminate these sites from further vapor intrusion consideration.

At sites with unweathered LNAPL sources (“high‐concentration sources”), 8 to 13 ft of clean soil are required to fully attenuate the hydrocarbon vapors. The attenuation due to biodegradation would be equally or more significant for LNAPL sources separated laterally from building foundations (i.e. a shorter distance would be required for attenuation). It is important that the current state of the science as described here be used in the development of more rational, technically defensible, approaches to vapor intrusion risk assessment.

4 THE FOUR LOW-THREAT VAPOR INTRUSION SCREENING SCENARIOS

The Stakeholder Group that was assembled by the Cal‐EPA/SWRCB examined the available current and relevant scientific information and recommends the following low‐threat guideline to manage the petroleum vapor intrusion pathway incorporating additional safety factors to protect human health in a state‐wide policy document. The Stakeholder Group developed four basic scenarios for decision‐making purposes and they are respectively:

Scenario 1: Unweathered LNAPL on groundwater Scenario 2: Unweathered LNAPL in soil Scenario 3: Dissolved phase benzene concentrations in groundwater Scenario 4: Direct measurement of soil gas concentrations

Scenarios 1 and 2 are essentially “high‐concentration sources”, while scenarios 3 and 4 are “low‐concentration sources”. The following section details the specific justification(s) for each of the sets of exclusion criteria outlined in these four scenarios. Benzene is assumed to be the primary risk driver for vapor intrusion from petroleum hydrocarbon sites. Although naphthalene is not present in gasoline at levels as high as typical benzene levels, and is potentially present at very low concentrations (mass fraction of 0.0026) in diesel (TPHCWG, 1998), it is another volatile carcinogenic chemical, and could potentially be considered as an additional risk driver. Also, naphthalene has similar (if not, higher (GSI, 2010)) degradation rates as benzene and much lower aqueous solubility. The discussions below focus on benzene attenuation.


4.1 Scenario 1: Unweathered LNAPL on Groundwater

‐ 30 ft vertical bioattenuation zone between a unweathered LNAPL (residual or free‐phase) source and a building foundation.

The proposed 30 ft exclusion distance7 is conservative based on:

• Model theory shows full attenuation within 7 m (~ 21 ft) of the source assuming reasonable approximations of the biodegradation rate (see figures below from Abreu et al., 2009).

Figure from Abreu et al. (2009)

7 The top of the residual‐phase source can generally be assumed to be consistent with the historic high water‐table elevation.



• Field soil‐gas data show full attenuation within 8 ft of the source (see figure, below, from R. Davis (2010) – also published in Hartman (2010)).


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• Analysis of the same soil‐gas data by Lahvis (2011) that shows benzene is attenuated to concentrations in soil gas < 100 ug/m3 (a conservative risk‐based screening level) at distances more than 13 ft from a LNAPL (residual or free‐phase) source benzene (probability = 95%).



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4.2 Scenario 2: Unweathered LNAPL in Soil

‐ 30 ft lateral and vertical separation distance between a unweathered LNAPL (residual or free‐phase) source in soil and a building foundation.

The same technical justification provided for Scenario 1 applies to Scenario 2. The proposed 30 ft. lateral off‐set distance is even more conservative for sources displaced laterally as shown in the following figure from Abreu and Johnson (2005). For example, an additional order of magnitude of attenuation is predicted for plume centerlines displaced 10 m (~30 ft). The attenuation would be significantly greater (e.g., several orders of magnitude) in cases where the plume (dissolved‐phase) boundary was separated by 30 ft.

Figure from Abreu and Johnson (2005)

As discussed in the technical background section 3.2.2, 13 ft. is more than adequate to fully attenuate LNAPL sources in soil and groundwater, therefore assuming a 30’ separation is very conservative.

4.3 Scenario 3: Dissolved Phase Benzene Concentrations in Groundwater

‐ No Oxygen Measurements ‐ 5 ft. vertical separation distance between a dissolved‐phase source < 100 ug/L benzene and a building foundation; 10 ft. vertical exclusion distance for a dissolved‐phase source < 1,000 ug/L benzene.

‐ With Oxygen > 4% – 5 ft. vertical separation distance between a dissolved‐phase source < 1,000 ug/L and a building foundation.


These separation distances are conservative with respect to protecting human health based on the following:

• Model theory shows 9 orders of magnitude (i.e., complete) attenuation (for reasonable approximations of the biodegradation rate) within a source/building separation distance of L=3 m (10 ft) for benzene vapor sources < 10 mg/L (or 40 mg/L dissolved phase concentration in groundwater assuming Henry’s Law coefficient of 0.25) (see attached figure from Abreu et al., 2009). The attenuation is complete regardless of the dissolved‐phase concentration (up to ~ 40 mg/L benzene in groundwater) for sources located 3 meters or more from a building foundation. The dissolved phase concentrations (especially) and required bioattenuation zone thickness specified in this scenario are therefore very conservative.


• The attenuation is shown to be complete within 2 m (6 ft.) for a soil gas source of benzene < 10 mg/L (or 40 mg/L dissolved phase concentration in groundwater assuming Henry’s Law coefficient of 0.25) (see attached figure from API (2009)).


Figure from API (2009)

Figure 3 API Figure 4 API

• Field soil‐gas data from Robin Davis collected at retail sites (Lahvis, 2011) that show the proposed exclusion distances and groundwater concentrations are highly conservative. The data imply that the potential risk of vapor intrusion from dissolved‐phase sources (up to 15 mg/L benzene in groundwater) is minimal unless groundwater is essentially in contact with the building foundation.



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• From a probability standpoint, the soil‐gas data show a > 95% probability of detecting benzene in soil gas at concentrations < 100 ug/m3 @ dissolved‐phase sites; conversely, there is less than a 5% probability that benzene soil gas concentrations will exceed 100 ug/m3 (a conservative risk‐based screening number for soil gas, Lahvis (2011)).


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4.4 Scenario 4: Direct Measurement of Soil Gas Concentrations

‐ Application of a bioattenuation (additional attenuation) factor of 1000x to risk‐based soil‐gas criteria (i.e., vapor sources) located within 5 ft. of a building foundation.

• Model theory predicts that bioattenuation is significant for LNAPL sources provided vapor concentrations are < 0.1 (1/10th) of a TPH vapor source of 100,000 ug/L (or 10,000,000 ug/m3). Therefore the proposed vapor screening criteria of 5,000 ug/m3 for benzene is very conservative. (See the following figures from Abreu et al. 2009.)

Figure from Abreu et al. 2009


Figure from Abreu et al. 2009

• The 4% oxygen requirement in this scenario is also a very conservative level for biodegradation to occur. The numeric models used 1% as a conservative estimate.

5 REFERENCES

Abreu, L.D., Ettinger, R. and T. McAlary , 2009, Simulated soil vapor intrusion attenuation factors including biodegradation for petroleum hydrocarbons. Ground Water Mont. Rem. 29, 105–177.

Abreu, L.D. and P.C. Johnson, 2005, Effect of vapor source, building separation and building construction on soil vapor intrusion as studied with a three‐dimensional numerical model, Environ. Sci. and Technol., 39, 4550‐4561.

Abreu, L.D. and P.C. Johnson, 2006, Simulating the effect of aerobic biodegradation on soil vapor intrusion into buildings: Influence of degradation rate, source concentrations, Environ. Sci. and Technol., 40, 2304‐2315.

Alaska DEC, 2011, Hydrocarbon Risk Calculator User Manual, prepared for Alaska Department of Environmental Conservation by Lawrence Acomb Geosphere, Inc., January 4, 2011 (http://www.dec.state.ak.us/spar/csp/guidance/hrc/HRC%20User%20Manual.pdf)


Andersen, R. G., Booth, E. C., Marr, Widdowson, M.A., Novak, J.T., 2008, Volatilization and biodegradation of naphthalene in the vadose zone impacted by phytoremediation, Environ. Sci. Technol., 42, 2575–2581.

API, 2009, Simulating the Effect of Aerobic Biodegradation on Soil Vapor Intrusion into Buildings—Evaluation of Low Strength Sources Associated with Dissolved Gasoline Plumes, Publication No. 4775; American Petroleum Institute: Washington, D.C., April 2009, pp. 37.

ASTM E‐2531–06, 2006, Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous‐Phase Liquids Released to the Subsurface, ASTM International, West Conshohocken, PA, 19428‐2959 USA

Bruce, L., Miller, T., and B. Hockman, 1991, Solubility versus equilibrium saturation of gasoline compounds: A method to estimate fuel/water partition coefficient using solubility or Koc, proceedings of National Ground Water Association Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Restoration, November 20‐22, 1991, Houston, Tx, 571 ‐582.

Davis, R.V., 2009, Bioattenuation of petroleum hydrocarbon vapors in the subsurface update on recent studies and proposed screening criteria for the vapor‐intrusion pathway, LUSTLine Report 61, May 2009, New England Interstate Water Pollution Control Commission (NEIWPCC), pp. 11‐14. (http://www.neiwpcc.org).

Davis, R., 2010, Evaluating the vapor intrusion pathway: Subsurface petroleum hydrocarbons and recommended screening criteria, 22nd Annual US EPA National Tanks Conference, Boston, Massachusetts, September 20‐22, 2010.

DeVaull, 2007, Indoor vapor intrusion with oxygen‐limited biodegradation for a subsurface gasoline source, Environ. Sci. Technol., 41, 3241‐3248.

DeVaull, in press, Vapor intrusion from subsurface to indoor air: biodegradable petroleum vapors versus recalcitrant chemicals.

Fischer, D. and C. G. Uchrin, 1996, Laboratory simulation of VOC entry into residence basements from soil gas, Environ. Sci. Technol., 30, 2598‐2603.

GSI Environmental Inc., 2010, BioVapor, A 1‐D Vapor Intrusion Model with Oxygen‐Limited Aerobic Biodegradation, User’s Manual, Published by American Petroleum Institute: Washington, D.C., April 2010.

Hartman, B., 2010, The vapor‐intrusion pathway: Petroleum hydrocarbon issues, LUSTLine Report 66, December 2010, New England Interstate Water Pollution Control Commission (NEIWPCC), pp. 11‐14. (http://www.neiwpcc.org).

Interstate Technology and Regulatory Council, 2007, Vapor intrusion pathway: A practical guideline, Interstate Technology & Regulatory Council, Washington, D.C., January, 2007, pp. 74.

Lahvis, M.A., 2011, Significance of biodegradation at petroleum hydrocarbon sites: Implications for vapor intrusion guidance, Presentation to the Ministry of Environment British Columbia, June 1, 2011.


Lahvis, M.A., and G. E. DeVaull, 2010, Alternative screening methodology for vapor intrusion assessment at petroleum hydrocarbon release sites, 22nd Annual US EPA National Tanks Conference, Boston, Massachusetts, September 20‐22, 2010.

Lahvis, M.A., and A.L. Baehr, 1996, Estimating rates of aerobic hydrocarbon biodegradation by simulation of gas transport in the unsaturated zone: Water Resources Res., 32, 2231‐2249.

Lahvis, M.A., Baehr, A.L., and R.J. Baker, 1999, Quantification of aerobic‐biodegradation and volatilization rates of gasoline hydrocarbons near the water table during natural‐attenuation conditions: Water Resources Res., 35, 753‐765.

McHugh, T., Davis, R., DeVaull, G., Hopkins, H., Menatti, J., and T. Peargin, 2010, Evaluation of vapor attenuation at petroleum hydrocarbon sites: considerations for site screening and investigation, Soil and Sediment Contamination, 19:1–21, 2010.

Morrison, R.D., 1999, Environmental Forensics: Principles and Applications, CRC Press.

Potter, T. and K.E. Simmons. 1998. Total Petroleum Hydrocarbon Criteria Working Group Series, Volume 2: Composition of Petroleum Mixtures.


TASK GROUP DISSENTING OPINION LETTER

Jan 2010

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January 4, 2010 Members of the State Water Resources Board 1001 I Street P.O. Box 100 Sacramento, CA 95812-0100 Re: Resolution 2009-0042 - UST Cleanup Program Task Force, Minority Opinion Dear Water Board Members: Thank you for the opportunity to participate in the Underground Storage Tank Program Task Force. The Task Force members have all invested a great deal of time and resources in preparing this report and many points of view were well represented. As regulators who served on the task force we offer the comments below as a dissenting opinion to the UST Cleanup Program Task Force Report. The regulators who served on the Task Force are concerned that the Task Force membership was dominated by responsible parties (RPs), and consultants who work for RPs. Regulators comprised approximately ten percent to the Task Force. This imbalanced representation resulted in proposals that primarily reflect the views of the RPs and their consultants. Dialogue within the task force was not neutral and minority views received very little consideration. The group focused on closing cases over the protection of groundwater resources and human health. The following issues represent some, but not all of our concerns with the Report. Default site closure criteria are proposed which assume uniform hydrogeologic conditions. For example it is assumed that a safe distance from a source of contamination to a water protection well is 1,000 feet. This and any other default closure criteria assumptions should be peer reviewed to demonstrate that the criteria are protective in all cases. Protection of groundwater resources requires the consideration of site specific conditions and the application of scientific and engineering principles. Groundwater basins are a complex system of surface recharge areas, multiple aquifers, and discharge areas, all in hydraulic communication with each other and each requiring the full measure of protection mandated by State law. With the State’s water dependency based on an unstable supply of imported water, it is even more important to protect local aquifer systems, many of which are currently being developed to provide more of the State’s water supply. The State’s continued growth and uncertain water supply make the ability to project future land and water use uncertain. We are concerned with the assumption that aquifers will not be used for centuries and consequently, contaminated conditions will be allowed to persist. Closing cases based on an arbitrary age of a case should not be considered. To protect the resource and human health, cases should be closed when the site meets the required cleanup criteria. Low risk closures should be considered based on site specific conditions, including off-site impacts and planned changes in land-use. It is important to protect groundwater aquifers so that current and future groundwater needs are protected.

Pat

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The recommendations and findings provided in the report should be based on peer reviewed scientific principles. We are concerned with many of the recommendations of the report including the ones cited above. Implementing sweeping change based on anecdotal evidence could put human health and environmental quality at unnecessary risk. Prior to making sweeping changes to the UST cleanup approach we recommend that the Board direct a peer review process where evidence and experience is considered in a scientific manner. To do otherwise is to develop a scientifically indefensible environmental policy for California that compromises groundwater resources and human health. We agree with the goals of Task Force to revise and improve the UST Cleanup process. Going forward we support a similar process that involves balanced representation of all stake holder groups and utilizes independent experts. Brian Newman Ken Williams Gerald O’Regan

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2011 08-19 flier-&-support-docs-on-ust-low-threat closure policy

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Transcript of 2011 08-19 flier-&-support-docs-on-ust-low-threat closure policy