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?