Formatted Title
Lessons Learned Applying Compound Specific Isotope Analysis in Large, Dilute, and Co-mingled Groundwater Plumes
Background/Objectives
Large, dilute, co-mingled plumes present distinct characterization and remediation challenges. In many cases, the source area(s) may be unknown and contaminants from multiple sources may be co-mingled. The large size and low concentrations of contaminants can increase the costs of remedial alternatives and limit applicable technologies. Applying compound specific isotope analysis (CSIA) forensic techniques can provide additional characterization to potentially reduce remedial costs, including the identification of source area(s), evaluation of contaminant plume co-mingling, determination of groundwater flow direction, and assessment of natural attenuation.
Approach/Activities
CSIA, including two-dimensional (2D) analysis, was applied at several complex sites with large, dilute groundwater plumes where contaminants were co-mingled and/or the source area(s) unknown. Lessons learned were compiled from these sites regarding the specific challenges in the application of CSIA to identify of source area(s), evaluate contaminant plume co-mingling, determine groundwater flow direction, and assess natural attenuation.
Results/Lessons Learned
Based on the lessons learned in the application of CSIA at complex sites with large, dilute, and co-mingled plumes, a stepwise framework was developed as follows:
- Qualitative Evaluation – use 2-D isotopic values and groundwater flow pathways to identify areas most closely resembling the TCE source (i.e., lightest isotopic composition).
- Semi-quantitative Evaluation – evaluate the data for fit to the Rayleigh model, determine if physical or chemical attenuation is occurring.
- Quantitative Evaluation – calculate enrichment factor, determine degradation process(es) that are occurring, and calculate degradation rate along a groundwater flow path.
At some of the complex sites with large, dilute, co-mingled plumes, the quantitative evaluation could not be completed. However, using qualitative and semi-quantitative steps of the framework can still provide valuable site characterization, including source identification, confirmation of groundwater flow pathways, evaluation of co-mingled contaminants, and confirmation that natural attenuation is occurring. These results can significantly change the conceptual site model and provide optimization of the remedial approach.