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is present in the northern part of the site This clay may confine groundwater to a slight], deeper . depth by restricting flow from the west 4.3 Analytical Results Concentrations of most hydrocarbons decreased in all of the wells (Tables 2 and 3) The only exception was tert-butanol (TBA), which was detected at a concentration of 43 parts per billion for the first time This result is somewhat suspicious, and may be due to laboratory error The detection in VM-3 of gasoline and BTEX compounds in August appears to have been a ane-time anomaly, and concentrations have now reverted to levels below detection limits 5.0 CONCLUSIONS AND RECOMMENDATIONS As anticipated, the depth to groundwater stabilized during the last part of the third qi arter and first part of the fourth quarter Despite the fact that the static water level is more thr n 10 feet above the screen in most wells, hydrocarbons continue to be detected at moderate conn,ntrations in two of the wells These results imply that the plume is recharged from the vadose zo pie during high-water months and begins to spread, and then probably slows down or staNlizes as groundwater recedes The source of the contaminants is probably the vadose-zone soil that exists in the depth interval 10-35 feet in the northwest corner of the property, in the area aroL nd VM-2 and HP-4 (Figure 2) We recommend excavation of this contaminated soil in order to re move the residual source We estimate that appoximately 1,000 cubic yards of soil should be rerr oved and transported to a suitable disposal facility, since there is no room on the site for on-site sreatment Now is the optimum time for removal, when the static water level is near its annual L Dw point However, it would not be possible to obtain regulatory approval for this before the wi iter rainy season begins Therefore, we recommend postponing the interim remedial work until the second quarter of 1999, after the rains have ended This work could take place in conjunctior with the removal of the waste-oil tank that was discovered in March 1998 in the sidewalk along Main Street A work plan for Interim Remediation (excavation) and a tank removal permi package should be prepared in January or February and submitted to San Joaquin County Envii onmental Health so that the work can be approved and performed in April or May Groundwater monitoring should continue on a quarterly basis through 1999 The next nLonitonng event should take place in February 1999 4