Tracing Organic Contaminants in Groundwater:  A New Methodology Using Compound-Specific Isotopic Analysis

The low concentrations (ppm to ppb) typical of organic pollutants dissolved in groundwater have been a major factor limiting the application of stable isotope tracing techniques at contaminated field sites. In this study, pentane extraction of benzene, toluene, ethylbenzene, and the xylene isomers (BTEX) from water is shown to be an excellent means of resolving this problem and preparing low concentration samples for carbon isotope analysis using high sensitivity gas chromatograph/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Pentane extraction provides accurate, reproducible, and linear isotopic results for dissolved BTEX in water with concentrations ranging from 0.1 to 100 ppm a range typically found in contaminated field settings. It is a rapid and flexible technique for extracting the monoaromatics from water and can readily be adapted to extract other organic contaminants. Demonstration of the feasibility of this technique for sparingly soluble BTEX compounds paves the way for applications of δ 13 C analysis of dissolved contaminants in both laboratory and field scenarios. GC/C/IRMS analysis of pure phase BTEX obtained from different manufacturers shows that the δ 13 C values of these compounds are resolvably different for each producer. These results suggest that isotope analysis may be a valuable tool for identification of contaminant origin in well-constrained field situations.