ARCHIVED REPORTS_XR0007892 - Laserfiche WebLink

Home Browse Search

1 twater gradient is towards the south-southwest. The most recent water level data are provided in Table 5-2. Historical potentiometrxc surface contours for Former Tank Site #2 are provided in Appendix Q Flow gradient may vary because of tidal water-level fluctuations an the nearby Stockton Deep Water Channel. ' Perched ground water is locally situated beneath the former tank location in the vicamty of MW-2A. It appears that the ground water at MW-2A is perched and is not in communication with the ground water that was encountered in the other monitoring wells. This inference is based upon observations during drilling and supported by the fact that the ground water elevation is higher at this point than in the surrounding wells. Chemical OMM== Laboratory analyses of sail and ground water samples from Site #2 are summarized an Tables 5-3 and 5-4, respectively Complete ' laboratory analysis results and chain-of-custody forms are provided in Appendix R The site cross sections in Figures 5-4 and 5-5 display the estimated vertical extent of hydrocarbons in the soil. Figures 5-6 ' and 5-7 display the estimated horizontal extent of hydrocarbons in the soil at depths of 5 and 7 feet bgs, respectively Figure 5-8 shows the estimated horizontal extent of hydrocarbons in the ground water. The chemical occurrence in soils and ground water at Site #2 is discussed further below Hydrocarbons in Soils, In soils, TPH as gasoline or diesel and BTEX compounds have only been detected in bonngs MW-1A, MW-2A (only very low concentrations of benzene, ethylbenzene, and xylenes were detected in boring MW-2A at a depth of 10 feet), MW-3A, and B3-A. No hydrocarbons and associated compounds were detected in soils from any other borings (B1-A, B2-A, B-4A, MW-4A, and MW-5A) The lateral occurrence of chemicals in soils is generally defined by ' borings MW-1A, MW-2A, MW-3A, and B3-A, and the vertical occurrence is primarily hunted to the 5-to-7 5-foot bgs depth interval. The highest concentrations of chemicals in soils is centered around i boring MW-3A which is located a significant distance for east of the former underground tank. This data suggests another source area an the vicinity of MW-3A. However, this hypothesis is not supported by the results of the geophysical survey which found no evidence of any buried objects or debris in that ' vicinity. TPH as gasoline was detected in only 4 of 21 soil samples. The highest concentrations of TPH as gasoline were measured in boring ' MW-3A at a depth of 7 5 feet (1,200 mg/kg), boring B3-A at a depth of 6 feet (360 mg/kg), and en at on of TPH as1A at a depth 5 feet (240 gasoline gasoline decmreased In all three borings, the concentration 5-13 1