Integrated differentiation of multiple trichloroethylene and tetrachloroethylene groundwater impacts using spatial concentration, biodegradation indices, chemical fingerprinting and carbon/chlorine isotope patterns

Abstract

Conventional testing methods provide sufficient information to evaluate human or ecological risk. However, contaminant concentrations patterns alone provide only limited resolution of important liability issues, such as when and where did contaminant releases originate. Over the past few decades, scientists explored the isotope applications to better identify, delineate, and manage contaminants in the environment. Advanced chemical fingerprinting and isotope technologies revealed important linkages between isotope ratios and contaminant origins (e.g., chemical feedstock and manufacturing process). Studies of environmental weathering distinguished abiotic and biotic changes in the chemical composition and isotope patterns. The combined application of chemical and isotopic fingerprints offers powerful complementary lines of evidence for delineating contaminants, assessing risk, and identifying historical sources. This manuscript provides an integrated forensic approach that systematically links conventional environmental investigation data with specialized chemical fingerprinting and carbon/chlorine isotope methods for identifying the sources of groundwater impacts especially when multiple potential point and non-point sources exist. This paper focusses on chlorinated solvents. Specifically, it features the synoptic use of chemical concentration patterns and compound-specific isotope analysis (CSIA) as effective tools for confirming organic contaminant sources, characterizing environmental weathering, and answering a growing list of site-specific questions. Unlike conventional isotope methods, which can be both time-consuming and expensive, this paper presents an optimized analytical method for chlorine CSIA using gas chromatography-quadrupole mass spectrometry (GC-qMS). Chlorine isotopic composition for multiple analytes (e.g., tetrachloroethylene [PCE], trichloroethylene [TCE], dichloroethylene [DCE], and vinyl chloride [VC]) can be determined in one acquisition thus reducing analysis time and cost. Precise CSIA isotope values were achieved for chloroethylene concentrations between approximately 5 micrograms per liter (ug/l) and 100 ug/l for carbon and between approximately 30 ug/l to 1,000 ug/l for chlorine. The gradual improvement in CSIA methods better addresses the wide concentration range encountered in typical samples collected from groundwater aquifers with significant chlorinated solvent impacts. A case study is presented featuring a tiered forensic investigation using spatial chemistry and isotope patterns to evaluate commingled plumes of PCE and TCE.