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Antonia K. J. Vorster 2 December 18, 1989 <br /> Based upon water level measurements taken during the Phase I investigation the <br /> direction of ground water flaw within the uppermost water bearing zone was <br /> determined to be towards the east-northeast at a gradient of about 0.002. The <br /> ground water flow direction of the deeper zones, penetrated by the municipal <br /> wells, was determined to be towards the cone of depression caused by the pumping <br /> of LVMD #1 and #2. It was also determined that there exists a static water level <br /> differential of approximately 12 to 14 feet between the upper and lower water <br /> bearing zones. The consultant concluded based upon existing hydrogeological data <br /> that these water bearing zones appear to be hydraulically separate. <br /> On 23 March 1989, Leedshill-Herkenoff, Inc. , the technical consultant on the <br /> Lincoln Village Center ground water investigation submitted to us a Phase II <br /> workplan. This workplan included a soil gas sur•:ey to identify PCE hotspots <br /> associated with the potentially leaking sewer lines and laterals from the three <br /> dry cleaners. The information gained from this survey was to be used in locating <br /> additional monitoring wells and/or soil borings. This workplan also included <br /> the installation of three shallow monitoring wells. The preliminary locations <br /> of these monitoring wells were included in the workplan. The three wells are <br /> approximately 500 feet from the existing contaminated monitoring well (MW#4) , <br /> and will be constructed to a depth of 70 feet below ground surface. See attached <br /> map. The wells are proposed to be constructed 50 lineal feet from any sewer <br /> line, and to be sealed-off to 50 feet below the ground surface. These <br /> specifications were apparently based upon the requirements of the San Joaquin <br /> County Environmental Health Department. Finally, the workplan called for the <br /> sampling and analyses of the four existing wells and the three new wells for EPA <br /> 601 compounds. On 19 July 1989 we conditionally approved this workplan. <br /> On 6 December 1989, the results of the recently completed soil gas survey, recent <br /> analytical results from the LVMD wells and onsite monitoring wells, and a revised <br /> workplan for the Phase II field investigation were submitted. A brief discussion <br /> on the contents of this submittal follows: <br /> SOIL GAS SURVEY <br /> The soil gas survey consisted of the sampling of 41 soil gas sampling <br /> points. Initially nineteen soil gas sampling points were spaced at 100- <br /> foot intervals along the sewer lines originating from each dry cleaning <br /> establishment. Another 22 sampling points were then taken to provide <br /> lateral definition, to confirm the initial sampling results, and to <br /> determine a vertical concentration profile. The vertical profiling was <br /> conducted to attempt to differentiate between soil and ground water <br /> contamination sources. <br /> Unfortunately, due to lithological differences, the vertical profiling of <br /> contaminant gasses was inconclusive. Apparently, low permeability soils <br /> (indicated by high vacuum readings) severely restrict the migration of <br /> contaminants in, not only the liquid phase, but also the gaseous phase. <br /> Therefore, vertical profiling of PCE concentrations were not able to <br /> distinguish between soil contamination and ground water contamination. <br /> However, there were indications that the sewer lines were potential sources <br /> of the PCE in the soil gas. Decreases in PCE soil vapor levels with <br />