ENGI 9621 – Soil Remediation Engineering

Spring 2012 Faculty of Engineering & Applied Science

Lecture 12: In Situ Air Sparging and

Vacuum Extraction

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A remediation technique has broad appeal since about 1985 due to its projected low costs relative to conventional approaches

For the remediation of volatile organic compounds (VOCs) dissolved in the groundwater, sorbed to the saturated zone soils, and trapped in soil pores of the saturated zone

Often in conjunction with vacuum extraction systems to remove the stripped contaminants

In situ air sparging12.1 Introduction

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Source: Hardisty, 2005

Schematic of Air Sparging with Vacuum Extraction3

12.2 Applicability

In order for air sparging to be effective the VOCsmust transfer from the groundwater into the injected air, and oxygen present in the injected air must transfer into the groundwater to stimulate biodegradation

The criterion for defining contaminant strippabilityHenry’s law constant being greater than 1×10–5 atm-m3/mol

Compounds with a vapor pressure greater than 0.5 to 1.0 mmHg can be volatilized easily

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Examples of Contaminant Applicability for In Situ Air Sparging

Removal of fuel oil aerobic biodegradation

Source: Suthersan, 1997

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Qualitative presentation of potential air spargingmass removal for petroleum compounds

Source: Suthersan, 1997

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12.3 Description of the process

(1) Air injection into water-saturated soils

Air injected into the saturated zone groundwater necessarily be displaced the displacement of groundwater will have both a vertical and lateral component

Water table mounding a local rise in the water table caused by the vertical component

Mounding an indicator of the “radius-of-influence” of the sparge well during the early stages of air sparging

The magnitude of mounding depends on site conditions and the location of the observation wells relative to the sparge well vary from a negligible amount to several feet in magnitude

(2) Mounding of water table

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Source: Suthersan, 1997

The first transient behavior after initiation of air injection into the saturated zone

The second transient behavior before reaching steady state during air sparging

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(3) Distribution of airflow pathways

Source: Suthersan, 1997 9

(4) Groundwater mixing

may significantly reduce the diffusionlimitation for mass transfer during air spargingwithout generating any changes in the bulk groundwater flow

is important during air sparging to effectively transport dissolved oxygen for in situBioremediation

can be effective if it occurs at the porescale as well as over site-scale distances

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12.4 Enhanced air sparging technologies

(1) Horizontal trench sparging

Trench sparging developed to apply air sparging under less permeable (the hydraulic conductivities (in the horizontal direction) are less than 10–3 cm/s) geologic conditions when depth of contamination is less than 30 ft

Generally applicable where there is a shallow depth to groundwater and the formation is fine grained

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Horizontal trench sparging (Plan view)

Horizontal trench sparging (Section view)

12Source: Suthersan, 1997

(2) In-well air sparging

to use air as the carrier of contaminants to overcome the difficulties of injecting air into “non-optimum” geologic formations

Injection of air into the inner casing induces an “air lifting effect” water column inside the inner casing lifted upward and overflow over the top contaminated water drawn into the lower screen and continuously “air lifted” in the inner tube strippable VOCs captured for treatment

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In-well air sparging

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Source: Suthersan, 1997

(3) Biosparging

To remediate a dissolved plume of contaminant, which is a nonstrippable but extremely biodegradable compound

Injection of air at very low flow rates (0.5 cfm to less than 2 to 3 cfm per injection point) into water-saturated formation to enhance biodegradation

(4) Vapor recovery via trenchesA minor modification to conventional air sparging

that involves the recovery of stripped vapors from fine-grained formations of a shallow depth to groundwater

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Air sparging with vapor recovery through trenches

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Source: Suthersan, 1997

(4) Pneumatic fracturing for vapor recovery

Using pneumatic fracturing to enhance vapor recovery

Applicable to sites with fine-grained formations that extend below the water table and depths to water that prohibit trenching

Increased hydraulic and vapor flow conductivity near the top of the water table and in the overlyingunsaturated zone allowing stripped contaminants to be collected without spreading out laterally

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Air sparging combined with pneumatic/hydraulic fracturing18

Source: Suthersan, 1997

Spring 2012 Faculty of Engineering & Applied Science

Lecture 13: Pump and Treat

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ENGI 9621 – Soil Remediation Engineering

Pumping groundwater to the surface removing the contaminants then either recharging the treated water back into the ground or discharging it to a surface water body or municipal sewage plant

Goals of pump and treat (P&T)

13.1 Introduction(1) Definition

Hydraulic containment of contaminated groundwater Prevent contamination from spreading to

uncontaminated areasTreatment of contaminated groundwater Reduce

concentrations in groundwater to below cleanup standards (MCLs)

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Source: Hardisty, 2005 Schematic of pump and treat

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Source: US EPA, 2002

Until the very recent past, almost all groundwater cleanup systems installed involved variations of the P&T technology

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(2) Applicability contaminants with high solubility, low sorption capability to aquifer materials(3) Advantages very effective at containing and removing the dissolved phase contaminants(4) Lmitations

long duration from years to even decadeshighly dependant on the chemical nature of the

contaminants and the subsurface geology(5) Costs typically $50,000 - $5 million per case

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Cross section along the x axis showing the cone of depression for a single extraction well superimposed on the regional water table

help to determine the number of extraction wells

13.2 Design of the pumping system

Capture Zone

Source: Fetter, 199924

T = transmissivityi = hydraulic gradient

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Source: Fetter, 1999

BU = Ti and Q = pumping rate

Source: Shanahan, 2004

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Injection and withdrawal well pair

Source: Shanahan, 2004

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(1) Oil/water separation

13.3 Treatment options

LNAPLs are floating on top of the groundwater table recovery of such contamination can be accomplished in two ways: (1) recovery of the LNAPLs separately from the contaminatedgroundwater, or (2) recovery of the LNAPLs and contaminated water as total fluids

If the LNAPL thickness is small and the site hydrogeologicconditions are less permeable total fluids recovery will be the preferred technique for recovering the floating separate phasecontamination separation of the recovered oil and water becomes necessary prior to further treatment of the contaminatedgroundwater

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Gravity separation, the primary and most common treatment is based on the specific gravity difference between water

and immiscible oil globules is used to move free oil to the surface of a water body for subsequent skimming of oil

Schematic description of an oil/water separator

Source: Suthersan, 1997

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(2) Air stripping tower

Use air stripping technique to remove volatile organic compounds (VOCs) present in pumped contaminatedgroundwater

Source: Fetter, 1999

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(3) Carbon adsorptionGranular activated carbon (GAC) is a granular media, approximately the size of medium fine sand has high interstitial surface area, 800 to 1100 m2/g the surface activation allows organic molecules to adsorb to the interstitial surface remove dissolved organic compounds from water

Source: Shanahan, 200431

(4) Metals precipitationTransforming a soluble metallic ion into an

insoluble precipitate through the addition of chemicals the most common technique used for treatment of

metal-containing waters

Hydroxide Precipitation

Sulfide Precipitation

Carbonate Precipitation

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Spring 2012 Faculty of Engineering & Applied Science

Lecture 14: In Situ Reactive Walls

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ENGI 9621 – Soil Remediation Engineering

an emerging technology that have been evaluated, developed, and implemented only within the last few years

is also known as “funnel and gate systems” or “treatment walls”

involves the installation of impermeable barriers downgradient of the contaminated groundwater plume and hydraulic manipulation of impacted groundwater to be directed through porous reactive gates installed within the impermeable barrier

14.1 Introduction(1) Definition

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Slurry walls a means of placing a low-permeability, subsurface reactive wall

Sheet-pile walls an impermeable or low-permeable reactive wall installed using closely spaced steel sheet piles

(2) Types of reactive walls

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Source: Houlsby, 2006

Grout curtains thin, vertical, grout walls constructed by pressure-injecting grout directly into the subsurface at closely spaced intervals

(a) Crossover from one direction to the other (b) Transition smoothly from one direction

(a) (b) (b) Source: Houlsby, 2006

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Most common reactive wall technologyPossible materials include

14.2 Slurry walls

Soil and bentonite clay (SB) have lower K, are less expensive Typical K = 10-7 cm/sec and Reported K’s as low as 5 x 10-9 cm/sec

Cement-bentonite (CB) have greater shear strength, lower compressibility use on slopes where strength is important use in areas where appropriate soils (for SB) are not available

Additives to enhance CB and SB Fly ash to increase carbon for adsorption Liners or sheet pile installed within wall to decrease K

(1) Introduction

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Source: Grubb, D. G. and N. Sitar, 1994

Slurry wall construction

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(2) Structure of a slurry wall

Typical vertical section for slurry wallShanahan, Waste Containment and Remediation Technology, 2004 39

Alternative vertical section for “hanging” slurry wall for LNAPLS

Shanahan, Waste Containment and Remediation Technology, 2004 40

Alternative horizontal section for slurry wall

Shanahan, Waste Containment and Remediation Technology, 200441

During construction, wall stability maintained byhigher head in trench than in ground water

Slurry density should be 0.25 g/cm3 lighter than emplaced backfill

Shanahan, Waste Containment and Remediation Technology, 2004 42

Slurry walls may leakConstruction can be difficultWaste may compromise wallRequires long-term pumping in slurry wall enclosuresSlurry walls are good barriers to advection, but not to

diffusion

(3) Limitation of a slurry wall

EPA review of slurry wall success Reviewed 130 sites and 36 had adequate data8 of 36 met remedial objective4 met objective except not yet for long term13 appear to have met objective4 appear not to have met objective7 are uncertain4 of 36 leaked and required repairs (leaks most often at “key” with floor) 43

In situ reactive wall system can cooperated with other soil remediation technologies/system lead to advanced technologies and successful implementation

14.3 Applicable reactive processes

(1) + Air sparging

Horizontal trench spargingSource: Suthersan, 1997 44

(2) + Adsoprtion

Easily replaceable, porous reactive cassettes at gates of the reactive walls

Liquid-phase granular activated carbon (GAC) used to remove manyorganics, especially those not easily removable by air stripping or biodegradation

Ion exchange resins used to remove dissolved heavy metals

Source: Suthersan, 1997 45

Key cost drivers Economy of scaleQuantity of material treated has a large impact Width of the plume to be treatedChoice of supplemental amendmentsAdditional monitoring required by regulators

$1000-2000 per cubic yard of the reactive wall

14.4 Economic consideration

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