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i <br /> \.001 <br /> Fuel additives may be present in the subsurface environment at this site. Data generated to date is <br /> inconclusive for the presence or absence of blending agents (preliminary analytical data indicates the <br /> presence of MTBE,subsequent data indicates MTBE is not present). You are required to analyze all water <br /> and selected soil samples for Tertiary Butanol, MTBE, DIPE, ETBE, and TAME employing EPA 8260 for <br /> all samples until further written notice from this office (see CVRWQCB and PHS/EHD attachments <br /> regarding these analytes). All wells shall be sampled the same day. Selected soil samples shall include at a <br /> minimum the historic low and high groundwater elevation equivalent and the current capillary fringe. <br /> Traditional TPH as gasoline and BTEX analyses are also required. Additional soil and ground water <br /> samples may be required to be analyzed as directed and at the discretion of the PHS/EHD site inspector. <br /> A workplan prepared by CH2MHILL dated April 1997 has been reviewed considering the most recent <br /> quarterly ground water monitoring events (August 12, 1997 sampling data received by PHS/EHD on <br /> October 6, 1997)and an addendum is required. Additional boreholes shall be distributed at this site which <br /> employ the findings of the comprehensive site hydrogeologic cross sections. At least four boreholes shall <br /> be continuous cores. All continuous cores shall be collected in approved sampling sleeves. These <br /> boreholes shall be appropriately logged and employed to further define any plume which may have <br /> originated from the former or current use practices at this site. The workplan shall be revised to consider <br /> historic low and high groundwater elevations in the definition of the vertical aspect of contaminant <br /> distribution at this site. San Joaquin County Flood Control records indicate that ground water in this area <br /> of Stockton may have been in excess of 110 feet below surface grade during the fall of 1978. Conventional <br /> monitoring well networks existing and proposed for this Underground Fuel Tank (UST) leak site may be <br /> insufficient to properly locate and define the extent or existence of Petroleum Hyrocarbon/Fuel Additive <br /> plumes as evidenced by the vertical and horizontal variations of contamination observed at this site. <br /> Boreholes completed as monitoring wells must be placed at the site with appropriate discrete screen <br /> intervals based on site specific findings(preferential pathways/contaminant loading)not arbitrary depths as <br /> proposed (each borehole may require more than one well completion). Well screen intervals shall not <br /> exceed 20' in total length. Well screen and filter pack construction shall not act as a vertical or horizontal <br /> contaminant transport mechanism. Preferably,monitor well screened intervals shall not exceed the height <br /> of the hydrogeologic interval under investigation (preferential pathway). Once vertical characterization is <br /> approximated for each source area, lateral contaminant definition may commence. All wells installed and <br /> surveyed shall be referenced to an appropriate and designated bench mark. Discuss your proposal for <br /> performing this work in the workplan addendum requested below. Report the results of your investigation <br /> to this agency within 30 days of completion of the approved scope of work. <br /> Please note, you are requested to immediately pursue any off-site access agreements that you may <br /> need to complete your investigation activities. <br /> You shall also evaluate the probability of the contaminants plume encountering preferential conduits that <br /> could spread the contamination particularly in the vertical direction. The conduit study is to include a <br /> detailed survey of all wells in the vicinity. Please contact the Department of Water Resources (DWR) for <br /> procedures to request well information. Please include prints of historic aerial photos used as part of your <br /> study, and use data tables and maps to report the data collected as part of your survey. Report your results <br /> in the Soil and Water Investigation(SWI)Report requested below. <br /> You are requested to properly characterize the hydrogeology and groundwater flow conditions in the <br /> vicinity of your site. During SWI activities, we request that you gather detailed lithologic information <br /> using borings, continuous core sampling, dual tube expedited site assessment methods, or cone <br /> penetrometry together with other methods to understand the hydrogeology at your site. Discrete soil <br /> samples collected from each significantly different lithological unit shall be analyzed by a qualified <br /> laboratory for permeability, porosity, bulk density, moisture, and organic carbon content. We also request <br /> that you evaluate local groundwater flow conditions and establish a site-specific localized flownet that is <br /> dependent on hydrology and geologic conditions and is reflected on hyrdrogeologic cross sections and <br /> fence diagrams. Additionally, you are required to prepare a rose diagram for groundwater gradient at this <br /> 2 <br />