Phased Sequential Soil Gas Investigations to Delineate a Soil VOC Source of Underlying Groundwater Contamination

David Springer, P.G., Joachim Eberharter, P.G., Brian Dow, P.G., Harry O’Neill, and Craig Nathe

Platform Presentation

Historical releases of trichloroethene (TCE) at a former missile launch facility known as Space Launch Complex 3 West (SLC-3W) at Vandenberg Air Force Base (AFB) have impacted underlying groundwater and continue to source a contaminant plume estimated to occupy 265 acres. Previous investigative work suggested the presence of two distinct groundwater source areas centered on monitoring wells with elevated TCE concentrations. Persistent increasing TCE concentration trends in several site wells resulted in additional study into the potential presence of a continuing soil source of contamination.

Due to the depth of groundwater (>200 feet below ground surface [bgs]) and the size of the investigation area, a conventional approach utilizing deep investigative soil borings was not cost-effective. Previous negative results from soil samples collected from borings installed in a suspected source area suggested that an alternative approach may prove more successful. Thus, a phased investigative approach was devised, which featured an initial shallow passive soil gas (PSG) survey using an 80-point grid with implants installed at 100-foot spacing to address the area known to overly TCE groundwater concentrations greater than 1,000 micrograms per liter (µg/L), and which included all suspected source areas. An active soil gas (ASG) survey was then implemented to confirm passive results and permit step-out sampling to further delineate TCE presence. Finally, based on these initial data sets, locations were targeted for installation of deep nested soil vapor monitoring wells, designed to evaluate vertical contamination profiles and assess the provenance of TCE-impacted groundwater at various site wells.

The conceptual site model (CSM) and groundwater fate and transport model were revised using these data sets. A significant savings in project costs and compression of the project schedule was realized through implementation of this phased soil gas investigation approach, which underscores its applica-bility and effectiveness in mapping VOC contamination in soil.

David Springer, P.G. Tetra Tech, Inc., 301 Mentor Drive, Suite A, Santa Barbara, CA 93111, USA, david.springer@tetratech.com, Telephone: (805) 681-3100, Fax: (805) 681-3108 Joachim Eberharter, P.G. Tetra Tech, Inc., 301 Mentor Drive, Suite A, Santa Barbara, CA 93111, USA, joa-chim.eberharter@tetratech.com, Telephone: (805) 681-3100, Fax: (805) 681-3108 Brian Dow, P.G. Tetra Tech, Inc., 301 Mentor Drive, Suite A, Santa Barbara, CA 93111, USA, brian.dow@tetratech.com, Tele-phone: (805) 681-3100, Fax: (805) 681-3108 Harry O’Neill Beacon Environmental Services, Inc., 323 Williams Street, Bel Air, MD 21014, USA harry.oneill@beacon-usa.com, Telephone: (410) 838-8780, Fax: (410) 838-8740 Craig Nathe Department of the United States Air Force, 30 CCEVR, 1028 Iceland Avenue, Vandenberg AFB, CA 93437, USA, Amena.Atta@vandenberg.af.mil, Telephone: (805) 605-7249 Presenting Author: David Springer

I n t e g r i t y - S e r v i c e - E x c e l l e n c e

17 March 2011

Joachim Eberharter, P.G.

Tetra Tech, Inc.

David S. Springer, P.G.

Tetra Tech, Inc.

Craig Nathe

U.S. Air Force

PHASED SEQUENTIAL SOIL GAS

INVESTIGATIONS TO DELINEATE A SOIL

VOC SOURCE OF UNDERLYING

GROUNDWATER CONTAMINATION,

VANDENBERG AFB, USA

I n t e g r i t y - S e r v i c e - E x c e l l e n c e

Overview

Site Location and Background

Overview of Project Objectives

Rationale Behind Phased Investigative Approach

Scope and Methods / Results

Findings & Conclusions

Questions?

2

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General Location Map

Source: Google Earth 2010

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Background – Site Map

Site 6

SLC-3W Site 5

SLC-3E

Site 7

Bear Creek

Pond

Collectively,

Sites 5, 6, and 7

are referred to as

IRP Site 5 Cluster

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Background

Typical launch facility infrastructure and operations.

TCE used to clean rocket engines.

Historical release resulted in 90 acre groundwater TCE plume.

Ph

oto

: U

.S.

Air

Fo

rce

Ph

oto

: U

.S.

Air

Fo

rce

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Photo showing SLC-3W, with Building 770, deluge channel, retention

basin, and drilling at location 6-NSG-3B, looking northwest.

Background

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Background – Hydrogeology

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Background – Previously Known Extent of Impacts

OBSOLETE

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Objectives of Phased Investigation

Confirm/refute presence of potential persistent soil impacts

sourcing groundwater contamination

Previous investigative work suggested presence of two distinct

groundwater source areas centered on monitoring wells with

elevated TCE concentrations (5-MW-15 and 5-MW-18)

Increasing TCE concentration trends in groundwater monitoring

wells suggested possible persistent soil sources

Delineate and assess soil impacts at source area and at

previously identified groundwater hot spots

Incorporate findings into updated HHERA, CSM, and numerical

groundwater model to support the FS

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Rationale for Phased Investigation

Considerations included: (1) large investigative area; (2) depth

to groundwater; and (3) negative results from previous soil

sampling in suspected source area

Initial passive soil gas survey designed to focus investigation

early on

Target area approximately 18 acres in size

Follow-up active soil gas investigation designed to confirm or

refute PSG survey results and further delineate TCE impacts

Provide soil gas concentration data

Areas identified during previous surveys targeted for deep

nested soil gas well installations and sampling (two phases)

Create vertical soil gas concentration profiles

Consistent with Triad approach featuring initial high data

density followed by high data quality

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Passive Soil Gas Survey – Scope and Methods

Photo: Beacon Environmental Services, Inc.

PSG survey conducted within 18 acre area overlying known

extent of TCE in groundwater >1,000 µg/L

Target area included suspected source area (deluge channel,

retention basin, well 6-MW-3) and wells 5-MW-15 and 5-MW-18

80 passive samplers containing hydrophobic adsorbent

material

100-ft grid spacing

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Passive Soil Gas Survey – Scope and Methods

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Passive Soil Gas Survey - Results

Passive samplers used to evaluate

mass flux distribution

Results in units of nanograms

Prominent TCE hotspot centered

at inside elbow of deluge channel

Secondary hotspot identified

approximately 250 ft to the west

Results ruled out significant

contamination in remaining initial

18 acre target area

5-MW-18

5-MW-156-MW-3

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Active Soil Gas Survey – Scope and Methods

Confirm/refute results of PSG survey

Assess TCE concentrations at source area and distal locations

Installed 34 double-nested soil gas probes at 17 locations at

nominal depths of 10 and 20 ft bgs using DPT

On-site mobile laboratory provided real-time data

Photo showing active soil gas survey at SLC-3W, looking north.

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Active Soil Gas Survey – Results

10 ft bgs 20 ft bgs

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Deep Soil Gas Investigation – Scope and Methods

Advanced four borings through nominally 230 ft of vadose zone

to groundwater using HSA technology

Continuously logged and screened soils (PID), collected soil

samples for environmental and soil physical properties testing,

and collected grab groundwater samples

Installed four quintuple-nested soil gas monitoring arrays (20

probes total) in two phases

6-inch woven stainless steel vapor probe inlets attached to ¼-inch

Nylaflow tubing terminated by Luer valves

Array assembled around central 1” PVC pipe while being lowered

inside augers

Probes carefully emplaced with construction materials (bentonite

hydrated in lifts, sand, etc.) while augers meticulously raised to

prevent formation collapse and ensure proper seal between probes

Minimum 3 system volumes purged days prior to sampling

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Deep Soil Gas Investigation – Scope and Methods

Photo showing system purge

prior to sampling

Samples collected with 50-ml

glass syringe and analyzed

by on-site mobile laboratory

Purge volume tests

Confirmation samples

collected in Summa canisters

for TO-15 analysis

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Photo showing installation of well 6-NSG-1, as tubing is

unspooled and attached onto central PVC pipe, looking northwest.

Deep Soil Gas Investigation – Scope and Methods

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Deep Soil Gas Investigation – Results

Primary purpose of nested arrays was to delineate vertical TCE

impacts in source area and determine provenance of TCE

impacts to groundwater at wells 5-MW-15 and 5-MW-18

Hypothesized three possible profiles, each indicative of a

different scenario explaining cause of soil gas impacts

Off-gassing from

groundwater according

to Henry’s and Fick’s

laws

Shallow soil impacts

coupled with off-gassing

from groundwater

Mass loading from soil

impacts throughout

vertical profile of vadose

zone

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Deep Soil Gas Investigation – Results

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Deep Soil Gas Investigation – Results

Investigation found deep perched groundwater zone beneath source

area at deluge channel; high TCE impacts (8,000 µg/L); underlying

groundwater impacts approx. 5,000 µg/L, consistent with

concentrations in wells 5-MW-15 and 5-MW-18

Vertical soil gas profiles at 6-NSG-1 and 6-NSG-4 (collocated with

5-MW-18 and 5-MW-15, respectively) consistent with off-gassing from

groundwater; ruled out as soil source areas; impacted groundwater

from these locations interpreted to have originated from source area

beneath deluge channel (near 6-MW-3)

Vertical soil gas profile at 6-NSG-2 shows mixed source of soil gas

impacts an no mass loading to groundwater from shallow vadose zone

Vertical soil gas profile at 6-NSG-3B (source area) shows continuous

soil source throughout upper vadose zone and mass loading to

underlying perched groundwater; soil impacts to lower vadose zone

(beneath perching clay) likely due to off-gassing from underlying

groundwater

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Deep Soil Gas Investigation – Results

6-NSG-1 6-NSG-4

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Deep Soil Gas Investigation – Results

6-NSG-2 6-NSG-3B

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Deep Soil Gas Investigation – Results

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Deep Soil Gas Investigation – Results

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Findings & Conclusions – Updated CSM

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Initial PSG survey quickly focused investigation, ultimately providing

more robust data set from area of interest

Results of subsequent ASG survey demonstrated viability and utility of

PSG surveys for preliminary screening purposes

Successful installation of 200-ft, quintuple-nested soil gas monitoring

well arrays and resulting informative data set demonstrated feasibility

and utility of this approach to evaluate provenance of groundwater and

extent of soil contamination

Use of soil gas matrix demonstrated as cost-effective alternative to soil

matrix sampling when evaluating soil source area, due to (1) greater

spatial reach with fewer sample locations and (2) repeatability of

sampling regimen

Approach facilitated regulator buy-in, expedited project schedule, and

reduced project costs allowing for bonus SVE pilot study at no extra

cost to the client

Findings & Conclusions

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Questions?

Joachim Eberharter, P.G.

Tetra Tech, Inc.

joachim.eberharter@tetratech.com

David Springer, P.G.Tetra Tech, Inc.

david.springer@tetratech.com