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' 16�v Workplan for Additional Assessment 49 South Wilson Way May 4, 1998 Page 4 groundwater sampling down gradient (North-East) is required. At least one borehole shall be a continuous core. These boreholes shall be employed to further define any plume which may have originated from the former use practices at this site. Consideration of operational periods for this facility shall be employed when evaluating historic groundwater elevations. Any wells installed and surveyed shall be referenced to an appropriate and designated bench mark. Well screen intervals shall not exceed 20' in total length. The responsible party at this site is directed to include analyses of selected soil and all groundwater samples for TPH as gasoline, BTEX; by EPA Method 8015 and 8020, oxygenates and blending agents employing EPA Method 8260 (TAME, DIPE, MTBE, ETBE and TBA) until further written notice from this office. These analyses are required as a result of the potential for soil and groundwater plumes to be commingled from sources at this site, 101 and 102 South Wilson Way in Stockton. Selected soil samples shall include at a minimum the historic low and high groundwater elevation equivalent and the current capillary fringe. Additional samples may be'required at the discretion of the PHS/EHD site inspector. Discuss your proposal for performing this work in the workplan requested above. "You are requested to properly characterize the hydrogeology and groundwater flow conditions in the vicinity of your site. During your investigation activities, we request that you gather detailed lithologic information using borings, and continuous core sampling, with other methods to understand the hydrogeology at your site. Discrete soil samples collected from each significantly different lithological unit shall be analyzed by a -qualified laboratory for permeability, porosity, bulk density, moisture, and organic carbon content. We also request that you evaluate local groundwater flow conditions and establish a site-specific localized flownet that is dependent on geologic conditions and is reflected on geologic cross sections and fence diagrams. Additionally, you are required to:_ prepare a.rose diagram for groundwater gradient at this site, plot it on the groundwater contour maps. Plot and cross section drawings shall show the- location of the former and present tank systems when known. Report your- results ourresults in the Soil and Investigation(SWI)Report requested below. "Additionally, if you have any MTBE, other fuel additives/blending agents, or any other contaminant data previously collected at your site that has not been submitted to PHS/EHD, you are requested to submit this data immediately and _ update cumulative data tables accordingly. "We request that you monitor the groundwater contaminant plumes on a quarterly basis. Additional wells be required to define the down gradient extent of the plume if the plume continues to migrate or the first set of wells fail to define the plume(s). In event continued plume migration is identified, or impact to water supply wells is predicted please include recommendations for the installation of additional wells or remediation alternatives in the site Quarterly Reports." 4 199075-WFB Stockton.Workplan