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110 Filbert, Stockton <br /> may discontinue EPA Method 8020 for benzene, ethylbenzene, toluene, and xylenes <br /> (BTEX) and continue to use EPA Method 8260 which quantifies and reports BTEX along <br /> with Tertiary Butanol, MTBE, DIPE, ETBE, and TAME for all existing monitor well <br /> samples until further written notice from this office. Otherwise, you may reduce the <br /> number of samples being analyzed for oxygenates using EPA method 8260 collecting <br /> samples for analyses and reporting from MWl, MW2, MW-9, NPI-90, NPI-80, NPI-70, <br /> SVE North, SVE South and continue using EPA Method 8020 for BTEX on all existing <br /> monitor well samples. <br /> In correspondence dated June 16, 1999, you requested bi-annual monitor well sampling. <br /> Monitoring well networks existing at this Underground Fuel Tank (UST) leak site are <br /> insufficient to properly locate and define the extent, existence, and migration of <br /> Petroleum Hydrocarbon/Fuel Additive plumes as evidenced by the vertical and horizontal <br /> variations of contamination observed to date. Any reduction in monitor well sampling <br /> frequency will provide less information on plume migration and groundwater flow nets at <br /> this site which is undefined laterally and vertically. Therefore, you should continue to <br /> monitor the groundwater contaminant plume on a quarterly basis. Additional wells are <br /> currently required and more wells may be required to define the down gradient and <br /> vertical extent of the plume if the plume continues to migrate or the next set of wells fail <br /> to define the plume(s). <br /> The responsible parties claim use of the former tank system was discontinued more than <br /> 15 years ago. In spite of soil vapor extraction and air sparging efforts the hydrocarbon <br /> groundwater plume and sources in the soil remain in significant concentrations, mobile, <br /> undefined and unpredictable. <br /> Please note PHS/EHD staff has discussed this site in detail with the Central Valley <br /> Regional Water Quality Control Board (CVRWQCB) staff. It is likely that active <br /> remediation will be required to achieve a no further action determination given the high <br /> concentrations of fuel hydrocarbons in groundwater and variable flow directions observed <br /> to date (probably caused by changes in groundwater pumping in the area) at this site. <br /> Should you have questions regarding this correspondence please contact Ron Rowe at <br /> (209) 468-0342 or Margaret Lagorio at(209) 468-3449. <br /> Donna Heran, RENS, Director <br /> Environmental Health Division <br /> Ronald Rowe, Sr. REHS Margaret Lagorio, REHS <br /> LOP/Site Mitigation Unit Supervisor <br /> C: CVRWQCB - Mark List <br /> SWRCB/USTCF —Jim Munch I1nflbW"YM( <br /> 2 <br />