Formatted Title
Evidence for Typical and Atypical Chlorinated Ethene Degradation Pathways at a Superfund Site
Background/Objectives
A large Midwest Superfund site was the subject of a Remedial Investigation (RI) that identified numerous groundwater plumes. Twenty-two plumes are associated with releases of tetrachloroethene (PCE) and/or trichloroethene (TCE) that mostly occurred over 50 years ago. These chlorinated volatile organic compound (CVOC) plumes vary in size and strength with historically measured maximums up to six acres, 95,000 ug/l PCE, and 693,000 ug/l TCE. The shallow water table is within the overburden comprised of silty clay glacial drift with a geometric mean hydraulic conductivity of 4.3E-05 cm/s. The initial bedrock is generally twenty to thirty feet deep and acts as a confining layer.
A primary objective of the current Feasibility Study (FS) is to evaluate the viability of monitored natural attenuation (MNA) as a remedial option at the site, either in part or stand-alone. Further goals include understanding the primary natural attenuation drivers and groundwater compliance timeframes.
Approach/Activities
A detailed study was performed to investigate natural attenuation processes. Data collected during the RI from over 500 monitoring wells was used to focus the effort at ten representative plumes. The study included analyses for VOCs, chloride, nitrate, ferrous iron, dissolved manganese, sulfate, sulfide, total ammonia, dissolved organic carbon, ethene, ethane, methane, volatile fatty acids, next generation sequencing (NGS), compound specific isotope analysis (CSIA), and quantitative polymerase chain reaction (qPCR). The latter measured Dehalococcoides (Dhc) and key reductases (tceA, BVC, VCR). In addition, tests using carbon-14 labeled TCE added to groundwater samples were run to evaluate TCE cometabolism.Results/Lessons Learned
Since the conclusion of site-wide groundwater monitoring for the RI in 2009, concentrations at most CVOC plumes have declined about an order of magnitude as of 2021. Geochemical data show plume conditions fluctuate between mildly oxidizing and mildly reducing. Degradation appears to be biotically driven as conditions appear unsupportive for meaningful abiotic processes.
Sequential partial reductive dechlorination of PCE/TCE was consistently confirmed by substantial cis-1,2-dichloroethene (cDCE) measured, but vinyl chloride was either non-detect or limited (mostly below 50 ug/l). In addition, microbial data verify the presence of bacteria capable of partial and/or full reductive dechlorination, at 1.2% to 7.1% of the microbe assemblages, averaging 2.9% (based on NGS total reads). Site conditions are generally insufficiently reducing for full reductive dechlorination.
Aerobic direct metabolism (ADM) and aerobic cometabolism (ACM) are increasingly relevant for the less chlorinated compounds, including cDCE and VC. NGS data show ADM/ACM bacteria capable of degrading relevant CVOCs present at 1.7% to 8.5% of the microbe assemblages, averaging 4.6%. Importantly, ADM bacteria were detected on average at over six times the frequency of Dhc, further supporting ADM as the dominant process for degrading DCE.
Results from three representative wells showed no significant net rate of TCE co-metabolism based on tests using carbon-14 labeled TCE added to groundwater samples. Also, four relevant genes (SMMO, PHE, RMO, TOD) were generally non-detect. Regarding common primary substrates for ACM, low to non-detect concentrations were found for methane, ethene, toluene, phenol, nitrate, and total ammonia.
A potential TCE degradation pathway that generates chloromethanes was recently published for a plume in Poland with similar contaminants and geochemistry (Pierri, 2021). At the Midwest site, trichloromethane (chloroform) and dichloromethane (methylene chloride) were likewise detected, respectively at ten and fourteen plumes with corresponding maximums of 1.9 ug/l and 1,200 ug/l. While circumstantial, the data suggests a previously unrecognized pathway may contribute to natural attenuation as these plumes had no known source for these chloromethanes (including no carbon tetrachloride).
Additional groundwater monitoring is currently being planned to further assess contaminant trends. Resampling soil at source areas will be used to evaluate if substantial groundwater concentration declines in the last fifteen years coincide with attenuation in soil.