The overall objective of this project is to develop a protocol that can be used to estimate the contribution and impact of productive abiotic transformation processes on chlorinated ethene contaminant degradation under intrinsic or enhanced conditions. This project will evaluate naturally-occurring abiotic transformation processes and biologically-mediated abiotic degradation in fractured bedrock sites. The specific objectives are to: (1) determine if geochemical modeling can be used for identification of sites that have a high potential for abiotic transformation of chlorinated ethenes; (2) determine if laboratory studies can accurately predict the likelihood of in situ abiotic transformation of chlorinated ethenes using a protocol involving intact rock core microcosms, under conditions that simulate natural attenuation and in response to amendments; (3) determine if geophysical techniques can assess the potential for abiotic degradation based on sensitivity to magnetite and iron sulfide minerals in the rock matrix, using a protocol involving correlation of geophysical measurements and results of microcosm experiments; and (4) determine if using in situ passive vapor diffusion (PVD) samplers can greatly enhance the detection of acetylene in groundwater, the key abiotic chlorinated ethene degradation product.
The project will be divided into five tasks. Task 1 will address the hypothesis that sites with the potential for abiotic degradation have specific geochemical characteristics that can be identified through geochemical measurements and modeling. This will be accomplished by using geochemical modeling of several sites to identify three with the highest potential for abiotic degradation. Task 2 will involve collection of intact rock cores from the identified sites. Task 3 will involve preparation of novel intact core microcosms with samples from the three sites. The microcosms will be used to evaluate if addition of amendments (e.g., lactate, sulfate and/or iron) over a simulated fracture surface will enhance the rate and extent of abiotic and/or biotic degradation of trichloroethene (TCE) within the rock cores. The cores will be flushed with groundwater spiked with 10-20 mg/L of TCE plus a low level of bromide (conservative tracer) and resazurin (redox indicator). For each site, one set of the cores will receive 14C-labeled TCE, so that formation of soluble as well as volatile products can be quantified. Another set will receive groundwater without 14C added; samples from these will be used to evaluate the extent of enrichment in δ13C. Bromide levels in the water flowing over the cores will be used to calibrate a transport model to predict diffusive losses from the cores. This will facilitate assessment of TCE losses by processes other than diffusion. Task 4 will evaluate if geophysical techniques are sensitive to minerals controlling abiotic degradation, with sufficient sensitivity to screen a site for abiotic degradation potential. Task 5 will determine how well the occurrence of acetylene in the field (measured by PVD devices) and how microcosms serve as predictors of abiotic transformation.
Correlation of abiotic degradation of TCE in the intact core microcosms with geophysical analyses and acetylene measurements will advance the prospect of identifying factors that may be used to predict the potential for in situ abiotic activity. The current lack of predictive tools makes it very difficult to evaluate the contribution of abiotic degradation processes to natural attenuation of TCE in low permeability environments. (Anticipated Project Completion - 2024)