Stable Isotope Analysis of MTBE to Evaluate the Source of TBA in Ground Water by John T. Wilson, Ravi Kolhatkar, Tomasz Kuder, Paul Philp, and Seth J. Daugherty Abstract Although tert-butyl alcohol (TBA) has not been used as a fuel oxygenate in Orange County, California, the concentrations of TBA in ground water at gasoline spill sites are high compared those of the conventional fuel oxygenate methyl tert-butyl ether (MTBE). In the year 2002, the geometric mean concentration of TBA was 839 lg/L compared to 627 lg/L for MTBE. There are two plausible sources of TBA, the TBA originally present in the gasoline and biodegradation of MTBE to TBA. Thirteen sites where the concentration of TBAwas >10 mg/L were selected for study. Biodegradation was evaluated by deter- mining the stable carbon isotope ratio (d 13 C) of MTBE in the ground water. If MTBE was biodegraded, the residual MTBE should be enriched in the heavier carbon isotope 13 C. The historical highest concentration of MTBE in each monitoring well and the d 13 C of MTBE in the ground water were used to estimate the concentration of TBA produced from biodegradation of MTBE. Then, the estimated concentration of TBA was compared to the measured concentration. At each site, comparisons were made for the well in the source area, and the well in the plume outside the source area, that had the highest concentra- tion of TBA. In the wells in the source areas, the estimate of TBA production accounted for the majority of TBA at 4 of the 13 sites. In the wells in the plume outside of the source area, the estimate accounted for the majority of TBA at six sites. Introduction Prior to 1999, EPA method 8020 or 8021 was used for routine monitoring of fuel-derived contaminants in ground water at gasoline spill sites managed by the Orange County (California) Local Oversight Program. Concentrations of methyl tert-butyl ether (MTBE) were determined and reported, but concentrations of tert-butyl alcohol (TBA) were not reported. In 2000 and 2001, routine monitoring transitioned to EPA method 8260 or 8260B (purge and trap (PT) with gas chromatography (GC) with a mass spectrom- eter (MS) detector), and concentrations of TBA were rou- tinely reported. The concentrations of TBA were higher than had been expected. Figure 1 compares the distribution of TBA and MTBE reported in 2002. In 429 gasoline spill sites that reported concentrations of both TBA and MTBE, the geo- metric mean concentration of TBA was 839 lg/L compared to 627 lg/L for MTBE. The high concentrations of TBA are not unique to Orange County, California. Recently, Shih et al. (2004) reported the distribution of TBA, MTBE, and other oxygenates at 530 gasoline spill sites in Los Angeles County, California. The geometric mean concen- tration of TBA was 1730 lg/L compared to 900 lg/L for MTBE. Wilson (2003) reported the distribution of TBA and MTBE at 74 gasoline spill sites in eight eastern states in 1999. The geometric mean concentration of TBA was 1303 lg/L compared to 1506 lg/L for MTBE. In June 2002, the Local Oversight Program requested that a number of major petroleum marketers in Orange County conduct or commission studies to explain the high concentrations of TBA in ground water contaminated with gasoline. Studies were requested to evaluate the likelihood of TBA being derived from gasoline product sold at service stations. The petroleum marketers searched their records on the content of TBA and MTBE in their products sold in the Orange County market and then used equilibrium parti- tioning theory to predict the concentrations of TBA and MTBE that would result in ground water from a spill of gasoline. Biological degradation of MTBE to TBA is also a plau- sible source of the TBA in ground water in Orange County. Kane et al. (2001) showed the transitory accumulation of TBA during aerobic biodegradation of MTBE by naturally occurring bacteria from a gasoline spill site in Palo Alto, California. Hunkeler et al. (2001) showed transitory accu- mulation of TBA in laboratory cultures during aerobic growth on MTBE and during aerobic cometabolism of MTBE supported by 3-methylpentane. Somsamak et al. (2001) derived an enrichment culture that degraded MTBE under sulfate-reducing conditions. The culture cleaved the Copyright ª 2005 National Ground Water Association. 108 Ground Water Monitoring & Remediation 25, no. 4/ Fall 2005/pages 108–116