Demonstrating natural attenuation of tert-amyl methyl ether (TAME) through a multiple lines-of-evidence approach: a compilation of case studies

Tert-amyl methyl ether (TAME) has use as a gasoline fuel additive for octane improvement and air emissions reduction. The aqueous solubility and apparent low relative biodegradation rate of TAME has led to groundwater impacts due to accidental releases of gasoline containing TAME at motor fuel retail sites. This study investigated the natural attenuation of TAME based on field -measured groundwater data at 14 globally distributed fuel retail sites with historical releases of gasoline. Using a multiple lines-of-evidence approach, the research evaluated TAME degradation through historical monitoring data, geochemical analysis, and compound-specific isotope analysis (CSIA). First-order TAME attenuation rates (kpoint) were calculated from decreasing concentration trends over time in 43 individual monitoring wells, having a mean and standard deviation of 0.0018 ± 0.0013 day-1, and a median rate of 0.0016 day-1 (first order half-life = 1.2 years), comparable to similarly estimated rates for methyl tert-butyl ether (MTBE), a more widely studied ether oxygenate. Shifts in CSIA measurements at five sites indicated that biodegradation was the dominant attenuation mechanism at one site, a secondary factor at three, and was inconclusive at one site. The study also measured stable carbon and hydrogen isotopes in TAME and its intermediate metabolite, tert-amyl alcohol (TAA), providing new insights into degradation pathways. Results showed that attenuation is influenced by site-specific hydrogeological conditions, with hydrogeologically simpler aquifers yielding clearer biodegradation signals, and aerobic conditions resulting in more rapid biodegradation. The findings support the viability of monitored natural attenuation (MNA) as a remediation strategy for TAME-impacted sites. The study recommends further research to refine isotope enrichment factors under varying redox conditions. Overall, this work enhances understanding of TAME behaviour in groundwater and demonstrates the utility of CSIA in distinguishing between physical and biological attenuation processes.