Air Force Plant 4 Superfund SiteEvaluation of SVE Combined with ERH

for the Remediation of TCE Source Material

Jeffrey Ragucci

SWS 6262 – Soil Contamination& Remediation

November 2014

Contaminant Overview• Trichloroethylene (TCE)

• Manmade chemical solvent

• Colorless liquid with chemical formula C2Cl3H

• Past uses: cosmetics, drugs, pesticides

• Current uses: metals degreaser, adhesives, paints, varnishes

Contaminant Overview

• When released to soil, TCE will exist in four phases:

• Dense Nonaqueous Phase Liquid (DNAPL)

• Dissolved phase in soil water

• Gas phase in soil vapor

• Sorbed phase on aquifer solids

TCE Remediation Technologies

• Pump and treat - groundwater

• Extraction of groundwater using pumps and conventional wells followed by ex situ treatment

• Advantages

• Easy to permit, design, operate

• Low startup costs

• Disadvantages

• Long-term operation resultsin high total cost

TCE Remediation Technologies

• Excavation - soil, groundwater

• Physical removal of soil and water, with ex situ treatment or offsite disposal

• Advantages

• Equipment readily available

• Effective for small releases

• Proven and reliable

• Disadvantages

• Potential for worker or offsite exposure

• Difficult and/or costly in unstable soils, below water table, or close to structures

• Moves contamination from one location to another rather than eliminating it

TCE Remediation Technologies

• In Situ Chemical Reduction (ISCR) - soil, groundwater

• Zero valent iron (ZVI) used to cause reductive dechlorination

• Advantages

• Simple to implement and equipment readily available

• Can achieve results similar to thermal but at lower cost

• Disadvantages

• Adding water and clay reduces compressive strength of soil, possibly requiring post-treatment capping and/or soil stabilization

• Sites must be free of surface or buried obstructions

TCE Remediation Technologies

• In Situ Chemical Oxidation (ISCO) - soil, groundwater

• Chemical oxidants injected to cause in situ degradation

• Advantages

• Simple to implement and equipment readily available

• Disadvantages

• Multiple rounds of injections often required

• Preferential flow paths preventing uniform reactant distribution

• High costs of oxidants

• Possible side effects such elevated levels of sulfate or trace metals

TCE Remediation Technologies

• In Situ Biological Treatment - soil, groundwater

• Addition of a soluble carbon source or electron donor promotes reductive dechlorination

• Advantages

• Simple to implement and equipment readily available

• Disadvantages

• Multiple rounds of injections often required

• Preferential flow paths prevent uniform distribution

• Long term implementation and monitoring often required

• Possible side effects such as elevated levels of arsenic, heavy metals and methane

TCE Remediation Technologies

• Soil vapor extraction (SVE) - soil

• Extraction of soil gas from the vadose zone using vacuum pumps and conventional wells followed by ex situ treatment

• Advantages

• Easy to permit, design, operate

• Low startup costs

• Disadvantages

• Long-term operation results in high total cost

TCE Remediation Technologies

• Thermal treatment - soil, groundwater

• Heating of subsurface causing in situ destruction by pyrolysis, and/or followed by recovery of vapor or liquid

• Advantages

• High levels of contaminant removal, includingDNAPL and from low permeability zones

• Disadvantages

• High technical skill required

• High cost, energy use, and carbon footprint

• Incomplete heating may result in untreated areas

• Large number of vertical borings needed

Air Force Plant 4 Site

• 760 acre property

• Operated by government contractors since World War II for production of military aircraft

• TCE used for metals degreasing

• TCE source area present below Building 181 from former disposal pit and spills

• Plume extending across site known as Eastern Parking Lot (EPL) plume

• Additional contaminants onsite, but this report focuses on Building 181 source area

• 1996 - EPA Record of Decision requiring remediation at the site

Air Force Plant 4

Source Area and EPL Plume

Building 181

EPL Plume

Cross Section of Site

Treatment Selection

• Pump and treat - groundwater only. Conclusion: eliminated.

• Excavation - not feasible due to buildings on active facility. Conclusion: eliminated.

• In Situ Chemical Reduction (ISCR) - cost estimate of $2,500,000 to $6,000,000. Unable to perform soil mixing for application. Decreases compressive strength of soil, risking surface structures. Conclusion: eliminated.

• In Situ Chemical Oxidation (ISCO) - cost estimate of $2,000,000. Less effective on DNAPL. Risk of non-uniform treatment. Conclusion: eliminated.

• In Situ Biological Treatment - cost estimate of $3,700,000 to $7,000,000. Ineffective on DNAPL. Conclusion: eliminated.

Treatment Selection

• Soil Vapor Extraction (SVE)

• Effective on highly permeable soil. Terrace Alluvium conductivity is 13 to 132 ft/day.

• Pilot test demonstrated effectiveness.

• Cost estimate based on pilot study: $612,000.

• Conclusion: selected due to demonstrated effectiveness and cost. However, unable to treat groundwater.

Treatment Selection

• Thermal Treatment - Electrical Resistive Heating (ERH)

• Installation of electrodes into subsurface. Electricity passing through soil generates heat, turning DNAPL and groundwater containing dissolved TCE into soil vapor.

• Soil vapor captured by SVE system and treated.

• Pilot test demonstrated effectiveness.

• Cost based on actual implementation: $2,500,000.

• Conclusion: selected due to demonstrated effectiveness. Cost comparable to other technologies considered.

Implementation

• 1993 - SVE system installed as an immediate response action

• 2000 - SVE system expanded

• 2002 concentrations:

• Source area soil: up to 2770 mg/kg (cleanup goal of 11.5 mg/kg)

• Dissolved phase: up to 129 mg/L (cleanup goal of 10 mg/L)

• DNAPL still present

• 2002 - ERH system implemented

ERH System

• 98 electrodes

• 14 temperature monitoring points (TMPs)

• 12 monitoring wells

• Additional SVE wells installed

• Linked to existing SVE system

ERH Example

ERH Example

ERH Example

Results

• Mean soil concentrations: 90% reduction to 0.184 mg/kg

• Mean groundwater concentrations: 88% reduction to 4.1 mg/L

• 2008 Five Year Review - concentrations in groundwater rebounding to 20-50 mg/L, exceeding cleanup goal

Lessons Learned

• Problem: DNAPL was not fully removed

• Reason: insufficient power (and thus heating) was applied to the subsurface

• Root cause: reliance upon conductivity assessment from 2001 pilot study. Prior to full implementation in 2002, a full conductivity assessment was not conducted.

• Lack of sufficient pre-design work

What Now?

• Not feasible to re-install $2,500,000 ERH system

• ISCO and in situ biological treatment previously eliminated due to inability to treat large volume of DNAPL and cost

• Likely effective in treating residual DNAPL

• Cost reduced due to smaller treatment volume

Questions?