Formatted Title
Insights into Source Delineation, Degradation Mechanisms, and Degradation Rates at the Field Scale Using Groundwater CSIA Data
Background/Objectives
Compound-specific isotope analysis (CSIA) is becoming a frequently utilized and defensible tool to assess sources and degradation of organic contaminants in groundwater and other environmental media. Often, CSIA field assessments rely upon detailed laboratory studies such as long-term microcosm studies using site-specific materials. This can present significant hurdles for projects limited by time and/or budget. Through two case studies, this presentation will demonstrate the value of field-collected CSIA groundwater data in assessments of contaminant sources and degradation, avoiding the need for supplemental laboratory studies.
Approach/Activities
In the first case study, a temporal groundwater CSIA dataset was utilized to evaluate degradation mechanisms and rates for multiple compounds simultaneously across a large complex site. Estimated carbon-13 enrichment factors on four chlorinated solvents were ground-truthed to extensive literature data to establish the extent of degradation, identify dominant degradation mechanisms, and estimate degradation rates and half-lives across the site. These data were subsequently used to evaluate the effectiveness of monitored natural attenuation (MNA) in and downgradient of multiple source zones subject to a pump and treat system. In the second case study, a dual carbon-chlorine isotope analysis on a small set of groundwater samples was used to investigate a potential additional source of tetrachloroethene (PCE) when PCE impacts were observed cross-gradient to a known source area.
Results/Lessons Learned
In the first case study, the field-collected CSIA data showed higher degradation rates downgradient of source zones, suggesting MNA is effective in these areas. Lower degradation rates were observed at the plume toe. The CSIA data confirmed that mass loss in the plumes was enhanced by biological activity, above and beyond the mass extraction achievable with the pump-and-treat system. For lower-solubility dense, non-aqueous phase liquid (DNAPL) constituents, the efficiency of mass removal with the pump-and-treat system is low, and biological degradation becomes increasingly important the lower the DNAPL solubility. In the second case study despite a limited number of wells and groundwater data, the available isotope data suggested a clear difference in PCE source signature in the cross-gradient location and demonstrated that PCE is degrading along the flow path. The results of these studies highlight CSIA as a complementary tool for field-scale source and remedy assessments. Field-collected groundwater CSIA data and the methods applied in these case studies can provide valuable insight into source delineation and remedy effectiveness and can complement routine analyses while being relatively low-cost and low-effort relative to other advanced characterization tools.