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11 February 2003 AGE_NC Project No. 98-0520 ' Page 5 of 21 ' Since initiation of groundwater monitoring in December 1999,groundwater elevations have ranged from approximately 13 to 19 feet above mean sea level. The ground water table underlying the site is generally in the form of a trough-like depression,with the base of the trough plunging northwest ' a low gradient of less than 0.01 ft/ft(Figure 7). Releases of petroleum hydrocarbons occurred from the on-site USTs. Once released to the ' subsurface, the contaminants migrated through the sandy Layer I to a vertical depth of approximately 20 feet bsg, at which point the contaminants encountered the less porous clayey silt of Layer 2 and began to migrate laterally as well as vertically;the direction of lateral migration Was ' dominantly towards the northwest, in the preferred ground water flow direction. Significant petroleum-hydrocarbon impact to soil in the vadose zone is present in borings B-2-20.25, ' B-4-20, MW-2-25 and MW-4-20, in the area.of the former USTs, and off-site well MW-9-20 (Table 2).TPH-g, TPH-d and TPH-rno are-present at maximum concentrations of 670 mg/kg(B-2- 20.25), 150 mg/kg(MW-9-20)and 120 mg/kg(B-2-20.25),respectively.Most of the contamination appeared to reside at the top of the clay layer, at approkimately 20 feet bsg. • 5.0. POTENTIAL IMPACT ONNEARBY SURTACE AND GROUND WATER Typically,'ground water would represent the migration medium for contaminants over significant ' horizontal distances. However, sites located in up-gradient or cross-gradient directions from the subject property are not likely to be negatively impacted by the subject property. In addition, first encountered ground water is not used for drinking or irrigation,,although all ground water in the ' State of California is considered to be of potential beneficial'use. 1 Drinking water at and near the site is supplied by the City of Manteca. The nearest city wells are located approximately 2,000 feet northeast of the site; based on the distance and cross-gradient ground water flow direction at the site,.it is unlikely for these wells to be contaminated by impacted ground water from the site. The nearest surface water features to the site are the San Joaquin River and Little John Creek.- located reek.located approximately 5 miles southwest and north, respectively, of the site. It is unlikely that impacted ground water at the site would represent a significant pollution threat to water in the San Joaquin River and Little John Creek, based upon their distances from the site. 6.0. SOIL REMEDIATION ALTERNATIVES ' Based on the distribution and concentrations of petroleum hydrocarbons in soil and ground water . on the subject site, and on site-specific conditions, AGE believes excavation and in-situ soil • Advanced CeoEnvironmental,Inc.