ARCHIVED REPORTS_XR0006242 - Laserfiche WebLink

Home Browse Search

April sampling and previous analytical data Copies of the laboratory analytical reports for April and August are included as Appendix A. Field observation forms for both sampling events, which include data on monitoring well purging and sampling, are included as Appendix B. Figures 2 and 3 represent the distribution of TPH-g and benzene, respectively, in the groundwater as indicated by the 13 August 1992 groundwater analytical results ' The analytical results for groundwater samples collected on 13 August 1992 show that TPH-g concentrations have increased in well Iocated near and downgradient of the former tank field ' Concentrations of TPH-g in samples collected from the wells along Pershing Avenue have not changed significantly TPH-g was found in samples collected from RW4 at a concentration of 120,000 µg/L and in V2 at a concentration of 220,000 ,ug/L; groundwater was not sampled from RW3 or V4 because of the presence of LPH The levels of TPH-g measured in samples from wells P1, 1W1, and P2 were 2,900, 650, and 2,100 µg/L, respectively. The distribution of benzene dissolved in the groundwater closely resembles that ' of TPH-g ' 6. WATER LEVEL RECORDS AND GRADIENT DETERMINATION ' Depths to groundwater were measured on 30 April and 13 August 1992. Table 1 shows groundwater elevations and thicknesses of LPH since January 1991. Figure 4 shows groundwater elevation isocontours for 13 August 1992. Groundwater surface elevations decreased by an average of 1.8 feet in the period between the gaugings of 30 April and 13 August 1992, consistent with groundwater surface elevations ' measured on 15 October 1991. The direction of groundwater flow remains roughly toward the east-northeast, which is consistent with previous measurements, at a hydraulic gradient of 0.009. 1 7. HISTORY ' 7.1 SITE HISTORY ' Until the transfer of ownership, Exxon had operated RS 7-3942 since April 1966. 7.2 HISTORY OF HYDROGEOLOGY IN VICINITY OF THE SITE The area lies within the Great Central Valley geomorphic province. This is a regional geosyncline filled with Quaternary and Tertiary sediments, up to 10,000 feet thick in the ' valley. The Great Central Valley is bounded to the east by the Sierra Nevada Range and to the west by Coastal Ranges geologic provinces tThese deposits contain fresh water up to a depth of about 1,000 feet in the Stockton area Below this depth water becomes brackish or saline (connate water). 73942(1)/p49=4 2