- ©2001. AAPG/DEG
The potential of lead isotopes as tracers of methyl tert-butyl ether (MTBE) in groundwater was assessed in a study that included: an evaluation of organolead's chemical affinity with individual gasoline hydrocarbon constituents; gasoline/water partitioning experiments; and site-specific investigations of MTBEimpacted groundwater. Concentrations of organolead were correlated with concentrations of MTBE, pentane, benzene, toluene, xylenes, ethylbenzene, 1,2,4-trimethylbenzene, and methylcyclohexane. Significant correlations (R2 > 0.98) were observed between organolead and methylcyclohexane, MTBE, and benzene; the latter two being the most water-soluble gasoline constituents. To evaluate the possible role of MTBE in the transport of organolead from a product into the aqueous phase, two unleaded gasoline–aqueous partitioning experiments were performed. Each unleaded gasoline was sequentially extracted with fresh volumes of water over a period of ∼3 weeks; a strong systematic relationship (R2 = 0.998) was observed between lead and MTBE in the aqueous phase, indicating that the organolead–MTBE chemical affinity in unleaded gasoline was maintained as these gasoline constituents partitioned into water. The lead isotopic equilibrium between unleaded gasoline and aqueous-phase lead also supports this conclusion. Site-specific analyses of groundwater impacted by MTBE corroborate the laboratory results, showing that groundwater impacted by MTBE releases exhibit lead isotopic ratios identical to those of unleaded gasoline responsible for the release. Our results also suggest that the fate/transport of MTBE plumes can be monitored by integrated lead/MTBE data, possibly to distances exceeding 500 m from the source of the release.
The significant problems we face cannot be solved at the same level of thinking we were at when we created them.—Albert Einstein
Richard W. Hurst received his doctorate from the University of California, Los Angeles, in 1975. He is a professor of biogeochemistry at California State University, Los Angeles, and has been a consultant in environmental forensic geo-chemistry since 1978; he has served as an adjunct professor at the University of Maryland, Eastern Shore, and is currently serving as a coadvisor to undergraduates in environmental geochemistry at the Massachusetts Institute of Technology. His research interests include the integration of isotope geochemistry with hydrogeological and statistical data to resolve problems associated with environmental remediation and petroleum exploration.
Diana Barron is currently working toward her master's degree at California State University, Los Angeles. The MTBE study was performed as a directed research project in fulfillment of her master's of science course requirements. She is employed by the California Environmental Protection Agency, Los Angeles Regional Water Quality Control Board.
Mandissa Washington completed her Bachelor of Science Degree at MIT in 1999. The copartitioning experiments comprised her senior research project.
Samuel A. Bowring received a doctorate from the University of Kansas. He is currently a professor of geology at the Massachusetts Institute of Technology with research interests focusing on the use of radiogenic isotopes as geochronometers and investigation of the evolution of the Earth's lithosphere.