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r ERI 227903 WO1 Former Unocal Service Stanon 5098Stockton Califorma November 12 1998 Well MW22D and MW22S Wells MW22D and MW22S will be located on the west side of Pacific Avenue (approximately 80 feet south of proposed welts MW21),- and will be constructed in a discrete deep zone (MW22D) as well as in the shallow portion of the water bearing zone (MW22S) Based on hydrogeologic information available for this site, these wells are intended to provide additional evaluation of groundwater quality in the area between known groundwater contamination and the Delta College well We do not propose to replace wells existing groundwater monitoring wells MW-13 or MW-15 for the reasons given below It is Unocal's and ERI'S understanding that the County is concerned with submerged well screens because of the potential for hydrocarbons in the groundwater to migrate past the wells in that portion of the groundwater above the screen and, therefore, be undetected MW-15 is historically upgradient of the former Unocal site and is upgradient of MW-10, where a shallow replacement screen is proposed Additionally, historical analytical information from MW-15 has shown no detectable hydrocarbons, whether the groundwater was below or above the top of the screen For these reasons, a shallow replacement screen at MW-15 is not considered necessary and would provide no significant information beyond what will be obtained from the proposed deep screen installation at MW-10 MW-13 is seasonally crossgradient or downgradtent from the source area that was removed by excavation in 1988 Additionally, historical analytical information from MW-13 has shown no detectable hydrocarbons, whether the groundwater was below or above the top of the screen For these reasons, a shallow replacement screen at MW-13 is not considered necessary The proposed new well, MW-18, is a preferable location for monitoring eastward dissolNed phase transport from the former Unocal site The County has requested additional soil borings on the former Unocal facility This Work Plan does not propose additional soil characterization for the following reasons The Regional Board has previously agreed that additional soil sampling is not required, since " the soil contamination at the subject site has been adequately cleaned lip" (See attached letter) • In light of a completed soil cleanup, the concern and focus of investigations at this site appear to be solely groundwater Additional groundwater monitoring «ells as proposed in this Work Plan will address the fate and transport of dissolved constituents at this site The work will be performed in accordance with ERI's Field Procedures (Attachment E) and a site- specific Health and Safety Plan ERI will perform the following work as part of this investigation Task 1:Off-Site Access and Permitting • Pursue obtaining an encroachment permit to drill in City of Stockton right-of-way (i e Pacific _ • Avenue and Robinhood Drive) • Prepare and obtain a well permit from the County