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• Preliminary results of an ongoing study at Shell Global Solutions (US) Environmental Chemistry laboratory indicate that no significant hydrolysis of MtBE is observed, even when the samples are heated to 70°C at pH I in a purge and trap system This study varied the preheat time (0, 3, 10 and 30 minutes) prior to purging to mimic the static headspace procedure used by EPA/ORD Hydrolysis of MtBE to TBA (<6%) was only somewhat significant when the samples were preheated for 30 minutes at the lower than typical pH of 1 These studies were done at an MtBE concentration of 500 pg/L In summary, it is our opinion that hydrolysis of MtBE to TBA is not of concern during storage and analysis of acrd-preserved samples Samples are typically analyzed by purge and trap GC methods that minimize the time that MtBE is heated (usually at temperatures 45°C or lower), the pH is 1 to 2, and samples with significant MtBE concentrations are diluted Although it is possible that a small portion of TBA found in groundwater could be due to analytical artifacts, it is extremely unlikely that the significant concentrations of TBA found in groundwater samples collected from some UST remediation project sites are attributable to lab methodology problems 8. Summary and Conclusions This study was initiated to determine the potential source of TBA in groundwater at selected LUST sites in Orange County, California A number of potential TBA sources were identified and investigated The most plausible explanation is that the TBA at these sites is predominately an intermediate biological degradation product, produced from MTBE primarily in either low-oxygen or methanogenic environmental conditions This is supported by laboratory microcosm studies under methanogenic conditions, using soils from selected sites in Orange County These tests showed degradation of MTBE, and a commensurate increase in TBA while the MTBE was decreasing Field data indicates that at least a portion of groundwater at the sites under study is methanogenic, based on measured dissolved methane concentrations in groundwater MTBE-gasoline has also been investigated as a potential source of TBA in groundwater Estimated water concentrations of TBA in dissolution from MTBE-gasoline are consistent with measured soluble concentrations of TBA in groundwater at the investigated sites Dissolution from gasoline as a sole source of TBA in groundwater cannot, however, explain high measured levels of TBA with low (or non-detect) co- located measurement of either MTBE or BTEX constituents Dilution, partitioning, and transport alone cannot explain the observed relative concentration levels of these chemicals Transformation, (in this case laboratory confirmation of biological degradation and transformation), helps explain the measured site data 22