Formatted Title
Enhanced In Situ Biodegradation Evaluation of Chlorinated Ethenes in Low Permeability Groundwater
Background/Objectives
Background/Objectives. An in situ bioremediation (ISB) evaluation of chlorinated volatile organic compounds (cVOCs) in groundwater was performed using in situ microcosms (ISMs). The major cVOC in groundwater was tetrachloroethene (PCE) and to a lesser extent trichloroethene (TCE). The ISM evaluation was performed to determine the efficacy of using ISB to effectively remediate cVOCs in site groundwater. The ISB evaluation included various biostimulation and bioaugmentation options in shallow and intermediate monitor wells. After the ISM evaluation was completed, a clean water injection was performed in the injection wells to determine potential injection rates in the aquifer.
Approach/Activities
Approach/Activities. Groundwater samples were collected and evaluated for various electron acceptors, total organic carbon (TOC), major nutrients and key microbial populations (Dehalococcoides) and genes. Based on results of groundwater testing, seven ISM units: 1. MVH‑9s - EOS PRO; 2. MVH-9s – EOS PRO + SDC-9®; 3. MVH-9s – EOS PRO + KB-1®; 4. MVH-9s – Wilclear Plus + SDC-9®; 5. MVH-3 – Wilclear Plus; 6. MVH-3 – Wilclear Plus + SDC‑9®; and 7. MVH-3 – Wilclear Plus + KB-1 were deployed in wells MVH-3 and MVH-9s. The ISMs remained in the wells for 78 days and were then evaluated for cVOCs, volatile fatty acids (VFAs), anions and key microbial groups and genes including: Dehalococcoides, Dehalobacter, Desulfitobacterium, and Desulfuromonas and tceA and vinyl chloride reductase genes.
Results/Lessons Learned
Results/Lessons Learned: Groundwater results indicated that dissolved oxygen ranged from 0.83 milligram per liter (mg/L) to 2.83 mg/L. Nitrate ranged from 3.23 mg/L to 8.21 mg/L. The pH was generally neutral at 7.2. The dissolved gasses: methane, ethane and ethene were non-detect at less than 2.8 micrograms per liter (mg/L). Major nutrients, total Kjeldahl nitrogen (TKN) and ammonia were below detection limits (140 μg/L and 117 μg/L), respectively.
In MVH-3, the combination of the carbon substrate Wilclear Plus and the microbial consortium KB-1® showed the highest level of overall reductive dechlorination of cVOCs. This ISM unit also displayed the highest VFA and ethene production. In MVH-9s, the combinations of EOS PRO and the microbial consortium SDC-9 demonstrated the highest level of overall reductive dechlorination of cVOCs. This ISM unit also showed the highest VFA and ethene production of all the ISM units deployed in this well.
The clean water injection test showed that the hydrogeology of this area was a very tight formation. The lithology of INJ-1 and INJ-2 (screened 27-47 and 27-42 ft bgs) contained silty sand followed by weathered granite. The observed drawdown in INJ-1 and INJ-2 was 0.47 feet per minute (ft/min) and 0.84 ft/min, respectively. The injection rate into INJ-1D was sustained at about 0.33 gallons per minute (gpm) with an observed drawdown of 3.74 ft/min. The injection rate for INJ-2D was as high as 0.45 gpm and dropped off to approximately 0.2 gpm. The observed drawdown in INJ-2D was 3.88 ft/min. Due to the limited injection rate, the carbon substrate being considered for pilot-scale test will be a more soluble substrate to promote distribution of the carbon amendment.