Incidental Occurrence of MTBE and Other Oxygenates in U.S. Gasolines after State-Level MTBE Bans, 2004-2020

Abstract

The rapid rise in the use of MTBE in automotive gasoline in the U.S. in the early 1990s was matched only by its rapid decline in the early 2000s, the latter stemming from its unsettling and detrimental impact on groundwater and drinking water supplies following gasoline leaks and spills. Its decline was promulgated through implementation of numerous legislated, state-level bans, which along with the Energy Policy Act of 2005’s elimination of a minimum oxygen requirement for reformulated gasoline, effectively ended the “MTBE era” by the Spring of 2006. Despite the bans, most of which only prohibited the intentional use of MTBE, opportunities remained for its incidental occurrence in gasoline. In this study, we report on the existence, concentration, and frequency of incidental MTBE and other oxygenates (TAME, ETBE, DIPE, and TBA) in an ad hoc suite of 76 “post-ban” gasolines collected from retail service station dispensers in multiple states between 2004 (in states with early bans) and 2020. These results are compared to those for 75 “pre-ban” (oxygenated, reformulated, and conventional) gasolines. Using GC/MS full scan data with a method detection limit of 10 mg/kg, the results confirm the incidental occurrence of MTBE in 34% of the “post-ban” gasolines at concentrations between 980 and 17 mg/kg (avg. ± σ; 209 ± 307 mg/kg), which generally decreased between 2004 and 2020. MTBE’s origin(s) in “post ban” gasolines is reasonably attributed to cross-contamination with neat MTBE or MTBE-laden gasoline destined for U.S. export, and/or its otherwise lingering presence within the gasoline distribution system. Although less frequently detected, low concentrations (mostly < 1,000 mg/kg) of TAME, ETBE, DIPE and TBA were also found in 17, 14, 13 and 4% of the “post-ban” gasolines studied, respectively. The forensic implications of these results on constraining the “age” of gasoline-derived NAPL containing MTBE or of MTBE-impacted groundwater, which are equally applicable to the other oxygenates measured, are discussed. Of overriding relevance is that any presumption that MTBE completely disappeared from the U.S. gasoline pool after the implementation of the state-level bans and Energy Policy Act is false.