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_ f <br /> San Joaquin County Public Health Services <br /> March 6, 1997 <br /> Page 2 <br /> 3. from well MW-2, located to the north and adjacent to the underground storage tanks. <br /> Below find a graph of the benzene concentrations detected in well MW-2. <br /> MW-2 Benzene Concentration Data(ug/1) <br /> 6000- <br /> 5000- <br /> 4000-- <br /> 3000-- <br /> 2000-- <br /> 1000 <br /> 00050004000300020001000 <br /> 0 <br /> 12/21/92 10/5/93 4/20194 1'2/9194 4/12/95 11/2195 4/22/96 11/14/96 <br /> I believe you can agree that the aggregate trend of this data is downward. <br /> EE � <br /> MTBE concentrations reported in wells MW-1 and MW-2 are higher,than reported <br /> during previous sampling events, however, it is unclear if this is a result of sampling <br /> variation, or a new release at the site. At this point, it seems appropriate to continue <br /> quarterly monitoring to better establish the trend of the concentration-data. I will also <br /> ask the current operator to provide inventory reconciliation records and tank testing <br /> results. <br /> I share your concerns with respect to the potential for further offsite migration of MTBE, <br /> however, the MTBE chemical data obtained from well MW-6 does riot suggest that a <br /> further migration is actually occurring. For example, the mean-MW-6 MTBE <br /> concentration is about 68 ug/l, and standard deviation is about.12 ug./1 -- indicating that <br /> the release has stabilized. This trend is not consistent with the variable and increasing <br /> concentration data one should expect in cases where further offsite migration is <br /> occurring. <br /> With respect to the potential for"upgradient migration" I can agree that well MW=3 is <br /> located close enough to the dispenser islands to warrant monitoring on a semiannual <br /> El <br /> basis. I do not believe that the data obtained from wells MW-4 and MW-7 warrant <br /> monitoring on a quarterly basis as you have suggested. Please confirm how the cost of <br /> obtaining this additional data has been weighed against the benefit to' beobtained from <br /> this data and the effect the data will have an the certainty of corrective actions decisions <br /> to be made at this site. At this point, we plan to sample wells MW-4 and MW-7 on'an <br /> annual basis, and to sample MW-3 semiannually. <br /> t <br />