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Enhanced Reductive Dechlorination (ERD) with Emulsified Vegetable Oil (EVO) has been used at hundreds of Department of Defense (DoD) sites to remediate chlorinated solvents, chromate, uranium, perchlorate, and explosives. This process commonly involves injecting EVO, nutrients, pH buffer or base, and microbial cultures to adjust biogeochemical conditions in the immediate vicinity of the contaminant, so that:
The overall objective of this project was to identify the reason(s) why remediation systems using EVO failed to meet cleanup goals at some sites while the technology has been shown to be very successful at other sites. At some locations, there are obvious reasons for not meeting cleanup goals including injection system designs that do not follow published design guidelines. However, at other sites, there is no obvious reason for the remediation system not meeting cleanup goals. This project aimed to identify changes in site characterization, remediation system design, and/or field implementation that would have improved remediation system performance.
The two sites evaluated in this project were both located at the former Naval Training Center (NTC) Orlando: (1) Study Area 17 (SA17); and (2) Operable Unit 2 (OU2). At both sites, the remediation systems were initially successful, resulting in substantial reductions in trichloroethene (TCE) concentrations. However, concentrations of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) increased in some wells due to TCE degradation and remain elevated. Results from the project evaluations were used to: (a) identify the reason(s) why the ERD systems failed to meet cleanup goals; and (b) develop new approaches and/or procedures to improve performance.
SA17 ASSESSMENT RESULTS
At SA17, two different depth intervals of a TCE source area were treated with EVO to stimulate ERD; Zone B extending from 15 to 30 ft bgs and Zone C extending from 30 to 50 ft bgs. Bioremediation performance in Zone B at SA17 source area was good with 2.8 to 4.6 Order of Magnitude (OoM) reductions in TCE. While cDCE and VC removal were lower, the sum of organic chlorine (ΣCl) declined by 0.8 to 3 OoM indicating a substantial portion of the parent compound was reduced to non-toxic end-products. TCE removal was also good in Zone C source area at SA17. However, higher levels of cDCE and VC accumulated with ΣCl declining by only 0.5 to 1.5 OoM. EVO distribution in both Zones B and C at SA17 was limited by: (a) injection of too little EVO; and (b) development of stagnation zones during injection. cDCE and VC removal in Zone C was inhibited by the low pH due to injection of too little base to neutralize acidity produced during ERD and the background acidity of the aquifer. While Dhc populations were low at many locations, substantial populations of Dhc capable of growing on VC developed at locations with sufficient substrate and appropriate pH, indicating ERD was not limited by absence of required microorganisms. There was no evidence of significant lower permeability zones near the target treatment zone that would result in substantial back diffusion of contaminants, limiting treatment. In summary, the primary factors limiting bioremediation performance at SA17 were inadequate levels of fermentable substrate and low pH due to injection of too little substrate, too little base to increase pH, and limited distribution of these materials throughout the target treatment zone.
OU2 ASSESSMENT RESULTS
Bioremediation was less effective in reducing chlorinated solvent concentrations downgradient of the EVO Permeable Reactive Barrier (PRB) at OU2. TCE concentrations in individual monitoring wells declined by 0 to 3.2 OoM at OU2 (median reduction of 0.5 OoM) with production of large amounts of cDCE. ΣCl removal at OU2 varied from 0.1 to 0.7 OoM with a median reduction of 0.2 OoM, which is lower than reported for other ERD projects. Effective distribution of EVO at OU2 was limited by: (a) injection of too little EVO; and (b) the presence of high TCE concentrations with and/or immediately adjoining lower permeability zones. Conversion of cDCE to ethene was inhibited by the low pH due to injection of too little base to neutralize acidity produced during ERD and the background acidity of the aquifer. While Dhc populations were low at many locations, substantial populations of Dhc capable of growing on VC developed at locations with sufficient substrate and appropriate pH, indicating ERD was not limited by absence of required microorganisms. While back diffusion of contaminants out of the underlying low permeability unit does occur downgradient of the OU2 PRB, the short travel distance from the PRB to the discharge point would greatly limit the impact of this process. In summary, the primary factors limiting bioremediation performance at OU2 were inadequate levels of fermentable substrate and low pH. The low substrate concentrations and low pH were due to injection of too little substrate, too little base to increase pH, and challenges in distributing these materials within and adjoining lower permeability units.
Lessons Learned
ESTIMATING BASE REQUIREMENT FOR AQUIFER pH CONTROL
At both sites evaluated in this project, low pH inhibited ERD of TCE to non-toxic end-products with accumulation of cDCE and VC. The low pH was due to: (a) low background pH of the aquifer; (b) acidity produced during ERD; and (3) injection of too little base to raise the pH to appropriate levels. To aid in the design of ERD projects at other sites, an MS Excel based design tool is presented to provide preliminary estimates of the amount of base required to maintain a neutral pH during ERD. The design tool approach and calculations were presented in Appendix D. Required input for the design tool include: (1) treatment zone dimensions and design life; (2) site characteristics including K, porosity, hydraulic gradient, contaminant concentrations and electron acceptors produced or consumed during ERD; (3) background pH, total inorganic carbon, mineral acidity, and pH buffering capacity (pHBC); (5) mass of organic substrate and base; and (6) target pH. The design tool calculates the amount of base required to: a) raise the pH of the aquifer material and influent groundwater, and b) neutralize acidity produced during reductive dechlorination and substrate fermentation.
CONCEPTUAL MODEL OF ERD TREATMENT WITH EVO
Conclusions and Lessons Learned in this project were integrated with prior laboratory and field studies to generate a general conceptual model of ERD with EVO and pH buffer. This conceptual model provides a relatively consise summary of the current understanding of ERD with EVO including: (1) ERD microbiology and organohalide respiration; (2) environmental requirements for efficient dechlorination; (3) EVO properties, transport and retention in the subsurface; (4) EVO consumption during ERD; (5) aquifer pH and buffering; and (6) injection system design.