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completion).At least one and preferably two boreholes shall be continuous cores. These boreholes shall be <br /> employed to further define any plume which may have originated from the former or current use practices <br /> at this site. The workplan shall consider historic low and high groundwater elevations in the defwtion of <br /> the vertical aspect of contaminant distribution at this site. All wells instafled and surveyed shall be <br /> referenced to an appropriate and designated bench mark. Well screen intervals shall not exceed 20' in total <br /> length. The Tracy,California regional hydrogeologic setting is known to have highly variable preferential <br /> flow paths with depth. A sand and gravel zone has been documented at this site as confirmed by bore hole <br /> logs in the saturated zone. Discrete (short) well screen intervals shall be placed in these zones in the <br /> predominant down gradient position to further characterize potential contaminant transport. <br /> Discuss your proposal for performing this work in the workplan requested below. Report the results of <br /> your investigation 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 <br /> that could spread the contamination particularly in the vertical direction. The conduit study is to <br /> include a detailed survey of all wells and other shallow transport pathways in the vicinity. Please contact <br /> the Department of Water Resources(DWR)and the local municipal utilities for procedures to request well <br /> and conduit information. Please include prints of historic aerial photos used as part of your study, and use <br /> data tables and maps to report the data collected as part of your survey. Report your results in the Soil and <br /> 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 geologic conditions and is reflected on geologic cross sections and fence diagrams. <br /> Additionally, you are required to prepare a rose diagram for groundwater gradient at this site, plot it on <br /> the groundwater contour maps. Plot and cross section drawings shall show the location of former and <br /> present tank systems. Report your results in the SWI Report requested below. <br /> Please determine(1)the approximate time frame of the MTBUAdditives/Solvents release first occurring at <br /> your site; (2) the use history of MTBF,/Addidves at your site; and (3) the history of all unauthorized <br /> releases and spills at your site. <br /> Additionally, if you have any MTBE, other fuel additives/blending agents, or any other contaminant data <br /> previously collected at your site that has not been submitted to PHS/EHD, you are requested to submit this <br /> data immediately and update cumulative data tables accordingly. <br /> We request that you employ approaches to estimating MTBE/Additive contaminant mass flux across plume <br /> transects or fences located perpendicular to the MTBE/Additive plume. We request the use of expedited <br /> site assessment tools and/or appropriately screened monitoring wells to provide data for these estimates. In <br /> deciding the location of transects and developing mass flux estimates, please consider the variable <br /> dissolution of MTBE and other contaminants from the source. We request that you install permanent <br /> monitoring wells and/or monitoring well clusters(screened at appropriate discrete depths with appropriate <br /> length of screen)to monitor the plume. Include your proposal for the installation of these wells in the work <br /> plan requested below. Please report your results in the SWI Report requested below. <br /> 2 <br />