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basin. This basin has been a persistent feature for a number of years, as evidenced by the fact that at . other sites under investigation by Upgradient Environmental in the eastern portion of Stockton, the depth to groundwater has generally been greater than 50 feet below surface grade (30 to 50 feet below sea level). 2.3 Depth to Groundwater Another aspect of the site hydrogeology to consider in the conceptual model'is the variation in the depth to groundwater, both seasonally and long-term. Whereas large (more than 10 feet) seasonal variations are common at some sites, the seasonal variation at the Kwikee Foods site is on the order of 3 to 5 feet. This is illustrated by the curve in Figure 7, which shows the variations in the water level in Chevron's well MW-1 since 1986. In most years, the water depth reaches a maximum in the fourth quarter (November) and a minimum in the second quarter (May), but from 1997 to 2000 the minimum was reached during the first quarter and the depth had begun to decline by May. During those years, the depth rose rapidly early in the year and then gradually declined. Somewhat more dramatic than the seasonal changes in the depth to groundwater is the long-term trend. The water level was dropping in 1986 and apparently continued to do so until reaching a minimum of more than 20 feet below grade at the end of 1992. After that, the depth rose rather steadily and reached a minimum of less than 5 feet in February 1998. Since then, it has gradually declined more gradually, and has ranged between 8 and 12 feet below grade since December of 2000. 2.4 Fuel Release The final aspect of the site conceptual model to be considered is the release of fuel from the underground storage tanks, both at the Kwikee Foods site and at the Chevron site. Laboratory data from samples collected during the tank removals, as well as from borings drilled before and after the tank removals,,are useful in this regard. 2.4.1 UST Soil Samples Laboratory data from soil samples that were collected at the time of the Chevron tank removals (1987) have not been.reviewed-by Upgradient Environmental, but it is our understanding from SECOR, Inc. that Total Petroleum Hydrocarbons in the gasoline ranged as high as 11,000 mg/kg (parts per million—ppm). Samples collected during the Kwikee Foods tank removal (1998) contained as much as 27,000 mg/kg (Table 2). Hence, evidence of a release was detected in both tank pits. One-centimeter holes were also observed in the Kwikee tanks when they were removed; no information about the condition of the Chevron tanks was reviewed for.this report. However, judging from monitoring well data discussed in later sections, it appears that releases occurred at two or more locations at the Chevron site. 8