Air Force Plant 4 Superfund Site Evaluation 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 results in 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 posttreatment 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, including DNAPL 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 EPL Plume Building 181 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?