Transcript Air Force Plant 4 Superfund Site Evaluation of SVE Combined with

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?