SITE INFORMATION AND CORRESPONDENCE

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� i r i You shall also evaluate the probability of the contaminants plume encountering preferential conduits that could spread the contamination particularly in the vertical direction. The conduit study is to include a detailed survey of all wells in the vicinity. Please contact The Department of Water Resources (DWR) for procedures to request well information. Please include prints of historic aerial photos used as . Part of Your study, and use data tables and maps to re p port the data collected as part of your survey. Report 4 your results in the SWI Report requested below. You are requested to properly characterize the hydrogeology and groundwater flow conditions in the vicinity of your site. During SWI activities, we request that you gather detailed lithologic information using borings, continuous core sampling, dual 'tube expedited site assessment methods, or cone penetrometry together with other methods to understand the hydrogeology at your site. Collection of soil samples 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. Report your results in the SWI Report requested below. Additionally, you are required to prepare a rose diagram for groundwater gradient at this site, plot it on the groundwater"contour maps, and update this diagram in'all future reports submitted for your site. Please employ all known information regarding the approximate time frame of the UST release first occurring at your site (February 20, 1995, 970 gallons); (2) the use history if known of MTBE / Fuel 3. Additives at your site; and(3)the history of all unauthorized releases and spills at your site. The workplan amendment should consider employing the approximate groundwater velocity and impact data to estimate plume dimensions and contaminant distribution. Using chromatograms from previously analyzed samples, the laboratory should be able to determine the presence or absence, and estimate/quantify the level of fuel additives (ie MTBE) present during previous sampling events in many cases. PHS/EHD has reviewed the data reports from WEST Laboratories dated December 5,,7, & 20, 1995. No fuel additive quantification::estimates are provided in the WEST report. Please report these amended findings in the SWI Report requested below. Additionally, if you have any MTBE, other fuel additives; 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 employ approaches to estimating MTBE/Fuel Additive contaminant mass flux across plume transects or fences located perpendicular to the MTBE/Fuel Additive plume. We request the use of expedited site assessment tools and/or appropriately screened monitoring wells to provide data for these estimates. In deciding the location of transects and developing mass flux estimates, please consider the variable dissolution of MTBE / Fuel Additives from the source. We request that you install permanent monitoring wells and/or monitoring well clusters (screened of appropriate discrete depths with appropriate length of screen)to monitor the plume. Include your proposal.for the installation of these wells in the work plan requested below. Please report your results in the SWI Report requested below. We request that you monitor the groundwater contaminant plumes on a quarterly basis. Additional wells will be required to define the down gradient extent of the plume if the plume continues to migrate or the first set of well fails to define the plume(s). Please discuss the results of your plume monitoring in your Quarterly Reports. In the event continued plume migration is identified, or impact to the Domestic Water Supply Well .is predicted please include recommendations.:for the installation of additional wells or remediation alternatives in the Quarterly Reports requested below. Please submit technical work plans and reports to San, Joaquin County Public Health Services Environmental Health Division(PHS/EHD),Attention: Ron Rowe, according to the following schedule: E. E n �