USING COMPOUND-SPECIFIC ISOTOPE ANALYSIS (CSIA) AND FATE AND TRANSPORT MODELING TO QUANTIFY THE MASS CONTRIBUTION OF COMINGLED CHLORINATED ETHENE PLUMES
Chlorinated solvents are present in soil and groundwater at thousands of sites across the United States. Resolving the remedial and fiscal responsibility of multiple potentially responsible parties at sites containing comingled chlorinated ethene plumes can be both scientifically and legally challenging. Presented herein is a case study describing the development of a framework by which compound-specific isotope analysis (CSIA) is combined with traditional investigative techniques within a three-dimensional (3D) reactive transport model.The study site, located in central Oklahoma, is composed of a 60m thick sequence of both fine-grained and coarse-grained matrices with three saturated zones separated by semi-confining and confining layers making up the six major hydrostratigraphic units, overlying the primary drinking water aquifer for this region. Work is focused on a comingled multi-species chlorinated ethene contaminant plume discovered throughout the hydrostratigraphic units in the late 1980’s, emanating from three primary source locations. The investigation included traditional techniques, such as evaluation and synthesis of historical data, various field measurements (e.g., hydraulic gradient using pressure transducers, dye traces, and in situ permeability), and aqueous sampling for the measurement of chlorinated solvent concentrations. Compound-specific isotope analysis (CSIA) data collection and analysis was also performed to provide insight into biotransformation characteristics of contaminant plume(s) by revealing how the chlorinated ethenes have changed (i.e., fractionated) during subsurface migration. The vast historical site data developed following traditional investigative techniques and CSIA data were integrated into a modern 3D reactive transport model. Iterative simulations of potential source concentrations and isotopic ratios at suspected source locations were performed using a calibrated "base case" model representing a best fit for observed concentration and isotopic data, which allowed for the approximation of quantitative contributions from multiple source areas, specifically that ~75% of the contaminant mass observed at a central point within the site domain originated from a source area to the east and ~25% of the contaminant mass originated from a source areas to northeast. The synergistic relationship of traditional investigation techniques and innovative CSIA provide valuable data necessary for understanding contaminant sites. Traditional techniques allow researchers to understand contaminant concentration history, source locations, and subsurface hydraulics. CSIA provides insight into biotransformation characteristics of contaminant plume(s) by revealing how the chlorinated ethenes have changed (i.e., fractionated) during subsurface migration. These two techniques are not often combined and neither, used independently, allows for the quantification of contaminant sources within comingled contaminant plumes. When these two techniques are integrated into a three-dimensional reactive transport model, it is believed contaminant characteristics of present-day and future conditions may be more clearly understood. Results of this study indicate that traditional characterization techniques combined with CSIA can be used to establish a framework for modeling comingled plumes, thus providing a fundamental approach for source differentiation. This represents a novel approach to determining contaminant remediation responsibility at sites with complex comingled contaminant plumes.