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Mr Michael J Infurna,Jr —San Joaquin County Public Health Services—Environmental Health Division <br /> May 8,2002 <br /> GR has evaluated the trends in concentrations of methyl tertiary butyl ether (MtBE) and TBA <br /> in groundwater from wells MW-3 and MW-7 from January of 2001 to the present Analysis <br /> for TBA did not commence until GR Initiated the current quarterly monitoring and sampling <br /> program in November of 2001 Charts 1 and 2 (attached)present a comparison of MtBE and <br /> TBA concentrations and groundwater elevation over time It is apparent from the data trends <br /> that concentrations of MtBE have been dramatically reduced while concentrations of TBA , <br /> have increased Since TBA is known to be a product of the breakdown of MtBE, it appears <br /> as if the mass of MtBE in these two wells may have degraded to TBA The mechanism of <br /> MtBE degradation is poorly understood at this time, and it is unusual for MtBE to degrade so <br /> quickly and completely to TBA. However, since the initial MtBE and the resulting TBA <br /> masses are approximately equal, degradation is suspected and it is unlikely that a new release <br /> has occurred at the site. Since the timing of the apparent MtBE to TBA degradation <br /> correlates with to the Dual-Phase Extraction (DPE) event performed in December 2001, it <br /> appears that the DPE event may have accelerated the degradation in ways not completely <br /> understood Additionally, the changes in groundwater elevations at the site do not present <br /> any alternative explanations for the increase in the concentrations of TBA <br /> GR is also in the process of completing the updated Corrective Action Plan (CAP) for this <br /> site, as required by the PHS-EHD. Further evaluation of the MtBE and TBA concentration <br /> trends will be presented in the updated CAP GR will submit the updated CAP to the PHS- <br /> . EHD by May 31, 2002 <br /> r <br /> a <br /> 4 <br /> t <br /> 140245 08 2 <br />