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� v �y A S S 0 C I A T E S I N C direction of regional groundwater flow is generally to the east The groundwater basin has been developed for intensive irrigation, domestic, industrial, municipal, and stock use ' There are two surface streams near the site, the Mormon Slough approximately 500 feet to the southwest and the eastward extension of the Stockton Deep Water Channel(including MacLeod Lake) ' approximately 2,000 feet to the north-northwest Both bodies of water empty into the San Joaquin River approximately 2 miles downstream from the site 2_3 Site Geology and Hydroeeologv Based on information obtained from previous borings at the site, sediments underlying the site consist primarily of silt from ground surface to approximately 12 to 20 feet bsg From 12 to 20 feet to approximately 30 feet bsg, the sediments consist predominantly of sand and salty sand, with silt lenses varying in thickness from approximately 2 to 5 feet Geologic cross sections depicting the interpreted subsurface conditions are included in Appendix A(Plates 2 and 3) ' During drilling, groundwater was encountered at approximately 37 feet bsg From December 1988 to July 1999, static water levels have fluctuated from approximately 19 to 45 feet bsg, variations an depth to water between the wells during each monitoring event are apparently due to seasonal fluctuations and possible hydraulic influence caused by the Stockton Deep Water Channel which is located approximately 2,000 feet to the north-northwest ' Based on groundwater elevations measured on July 6, 1999, the direction of groundwater flow as generally toward the east-northeast at an approximate gradient of 0 002 f1/ft The direction of groundwater flow has ranged from southeast to northeast since groundwater monitoring was initiated in December 1988 3.0 SUMMARY OF SOIL AND GROUNDWATER CONDITIONS ' 3.1 Extent n of H drocarbon-Im acted Soil The lateral extent of gasoline hydrocarbons in soil is delineated by borings B-6 and B-7 on the west, boring B-5 on the south, boring B-4 on the east, and boring B-15 on the north. The lughest ' concentrations of residual gasoline hydrocarbons appear to be north and west of tank 73, from approximately 15 feet bsg to the top of the groundwater surface at the time of the investigation (42 feet bsg). Total petroleum hydrocarbons as gasoline (TPHg) were detected in soil samples at concentrations ranging from nondetect to 5,100 parts per million The extent of TPHg in soil is shown on Figure 3. ' 3_2 Extent of H drocarbon-Im acted Groundwater Based on the analytical results from groundwater samples collected from wells MW-1 through MW-5, RW-1, VW-4, VW-5s and VW-5d on July 6, 1999, and from wells PZ-1, VW-1, VW-2d and VW-3 on November 26, 1997, the lateral extents of groundwater impacted with TPHg, total W 162574 Oalpdot-Fs report dm 3