NATURAL ATTENUATION OF DISSOLVED BENZENE AND MTBE - TWO CASE STUDIES

摘要

This paper compares the natural attenuation of benzene and methyl tertiary butyl ether (MTBE) at two Midwestern U.S. retail gasoline marketing outlets. The two sites were chosen to compare a site where natural attenuation clearly could play a role in site management, with a site where concentration trends indicate additional engineered remediation may be necessary. Dissolved plumes of benzene and MTBE were characterized as stable, expanding or shrinking based on non-parametric Mann-Kendall trend evaluations. In order to demonstrate stable or decreasing contaminant plumes, time trend evaluations were performed to ascertain the behavior of benzene and MTBE at individual monitoring wells. Trend evaluation results from individual monitoring wells were then interpreted within the hydrogeologic context of each site to describe site-wide trends in plume behavior. Datasets that did not exhibit upward or downward trends were further characterized as stable or non-stable trends with the coefficient of variation (CV) test. The attenuation rates were quantified using Sen's non-parametric indicator of median slope. Dissolved oxygen (DO) and oxidation-reduction potential (ORP) data were evaluated to qualitatively assess the potential for biological degradation of benzene and MTBE. Geochemical parameters indicate biological degradation is occurring at both sites. Significant downward trends in benzene and MTBE suggest natural attenuation could play a role in site management at one site, while steady and increasing trends in benzene and MTBE suggest source control measures may be needed at the other site.