Concentrations and stable isotope values of BTEX in gasoline-contaminated groundwater

This work was undertaken to investigate the usefulness of stable carbon isotopic analysis as a monitoring tool for contaminant remediation. Concentrations and delta(13)C values of BTEX (benzene, toluene, ethylbenzene, and xylenes) were measured at a gasoline-contaminated site in southern California. The BTEX data were determined using a purge-and-trap connected to a gas chromatograph/ion trap mass spectrometer and to an isotope ratio mass spectrometer system. Concentrations ranged from below detection (<0.5 ppb approximate to 0.05 mu M C) to about 120 ppm (approximate to 10 000 mu M C) total BTEX, with toluene generally having the highest concentrations. For BTEX compounds at monitoring wells averaged across all sampling dates, delta(13)C values ranged from -23.8 to -26.6 parts per thousand (benzene), -22.9 to -25.2 parts per thousand (toluene), from -23.0 to -25.3 parts per thousand (p&m-xylenes), and from -22.4 to -25.0 parts per thousand (o-xylene). The data strongly suggest two sources of contamination: one with lighter (C-12-enriched) delta(13)C Values emanating from the area near monitoring wells 1 and 2, and the other containing higher MTBE concentrations, an additive of unleaded gasoline, with heavier (C-13-enriched) delta(13)C Values coming from the vicinity of monitoring wells 8 and 9. The isotope data suggest that the leaded and unleaded gasoline at this site are isotopically distinct.