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US <br /> Ms Lor*Duncan Page 5 <br /> San Joaquin County Environmental Health Department <br /> February 4, 2003 <br /> Low concentrations(less than 2 mg/kg)of solvents Including TCE and cis-1,2-dichloroethene(cis-1,2-DCE) <br /> were detected in samples from both MW-11M and MW-11D Bromoform and dibromochloromethane were <br /> detected in MW-11D at 2 land 067 ppb, respectively Analytical data for MW-11MID are presented in <br /> Tables 1 and 2 Certified analytical reports and chain of custody documentation are included in Attachment <br /> D A summary of groundwater analytical data for the fourth quarter 2002 sampling event is presented in <br /> Figure 5 Tables summarizing current and historical groundwater analytical data are included in the Fourth <br /> Quarter 2002 Groundwater Monitoring Report <br /> CONCLUSIONS <br /> Based on the data collected during this assessment,groundwater in the vicinity of the newly installed dual- <br /> completion well MW-11M/D site has been impacted by MtBE The maximum contaminant level (MCL) <br /> for MtBE in drinking water is 13 ppb Based on this guideline, the vertical extent of MtBE contamination <br /> has been defined by MW-11D, however the downgradient extent of MtBE concentrations above the MCL <br /> (13 ppb)has not been defined Additional groundwater sampling should be performed to determine if MtBE <br /> concentrations are increasing or decreasing in this portion of the plume <br /> If you have any questions or comments regarding this letter, please call us at(916) 679-2299 <br /> Sincerely, <br /> URS Corporation Americas <br /> J.Ai \SE D GF��O <br /> Scott Looking ill t�CS ON PAU(F1/ <br /> Project Geologist <br /> 0 � <br /> Y * Exp 1 310 <br /> Vernon Elarth, R G <br /> Project Manager OF CAV\\ <br /> Attachments Table 1 - Groundwater Elevation and Analytical Data, MW-11M/D <br /> Table 2 - Analytical Data for VOCs MW-11M/D <br /> Figure 1 - Site Location Map <br /> Figure 2 - Site Plan with Geologic Section <br /> Figure 3 - Generalized Geologic Cross-Section A-A' <br /> Figure 4 - Groundwater Elevation Contours (December S, 2002) <br /> K V5 BP ARCON044931Reguiatory Correspondence14493 WIR doc <br />