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r <br /> SECTION 2 SITE SETTING AND BACKGROUND <br /> almost 180 degrees. This rise in groundwater levels in the Stockton area can be attributed to <br /> numerous City water supply wells shutting down. Further, three process-water supply <br /> wells located at the adjacent Del Monte Plant 33 facility were shut down in 1994. Between <br /> 1991 and July 1994,the groundwater flow direction was predominantly towards the <br /> northwest at an average groundwater flow direction and magnitude of 312°and 0.0006 ft/ft. <br /> From October 1995 through present, the groundwater flow direction has been consistently <br /> towards the southeast,at an average direction and magnitude of 143° and 0.001 ft/ft. The <br /> current groundwater flow direction and gradient magnitude is approximately 146°and <br /> 0.0004 ft/ft respectively. Figure 2-3 presents a summary of groundwater flow directions <br /> measured at the site since 1991.Figure 2-4 is a shallow groundwater contour map generated <br /> from groundwater elevations measured on March 28,2000. Vertical gradients have been <br /> calculated using monitoring well pairs MW-DIS-7S/7D,and MW DIS-8S/8D. The current <br /> vertical gradient is upward,at a magnitude of 0.0005 ft/ft in the area of MW-DIS-7S/7D, <br /> and downward at MW-DIS-8S/8D at a magnitude of approximately 0.0008 ft/ft. <br /> Prior to the switch in groundwater flow directions, the extent of the petroleum hydrocarbon <br /> plume was defined horizontally by monitoring wells MW-DIS-3,MW-DIS-8S,MW-DIS-9, <br /> and MW-DIS-10;and vertically by monitoring wells MW-DIS-7D and-8D. <br /> i . <br /> Since 1991,Del Monte has conducted quarterly groundwater monitoring and reported <br /> results to PHS/EHD.In February 1995,Del Monte initiated remediation at the Disco site by <br /> f installing an air sparging and soil vapor extraction(AS/SVE)system. The system consisted <br /> of two sets of AS/SVE wells and two sets of nested piezometers. The system was shut down <br /> in November 1995,when rising groundwater levels submerged the SVE wells.An estimated <br /> 8,787 pounds of TPH-gasoline were removed during the period of system operation, <br /> however,as indicated by the quarterly monitoring results at the site,elevated concentrations <br /> of gasoline constituents are still present in groundwater in the former tank and pump area. <br /> i <br /> f <br /> i <br /> l <br /> r <br /> � I <br /> t , <br /> f SFO/CPT REPORT-t.D000036700M 2-2 <br /> i <br />