Concentrations and Stable Isotope Values of BTEX in Gasoline-Contaminated Groundwater

Abstract

This work was undertaken to investigate the usefulness of stable carbon isotopic analysis as a monitoring tool for contaminant remediation. Concentrations and δ13C 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 ≈ 0.05 μM C) to about 120 ppm (≈10 000 μM C) total BTEX, with toluene generally having the highest concentrations. For BTEX compounds at monitoring wells averaged across all sampling dates, δ13C values ranged from −23.8 to −26.6‰ (benzene), −22.9 to −25.2‰ (toluene), from −23.0 to −25.3‰ (p&m-xylenes), and from −22.4 to −25.0‰ (o-xylene). The data strongly suggest two sources of contamination: one with lighter (12C-enriched) δ13C 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 (13C-enriched) δ13C 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.