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of the site to 0 0025 ft/ft in the southern part of the site The steeper gradient near VM-4 is probably due to the low-permeability clay that is present in the northern part of the site This clay may confine groundwater to a slightly deeper depth by restricting flow from the west 4.3 Analytical Results Except for Total Petroleum Hydrocarbons in VM-1, concentrations decreased in all of the wells (Tables 2 and 3) Especially noteworthy is the sharp decline in TPH-g in VM-2 The change in hydrocarbon concentrations is much larger than the change in the depth to groundwater, and the two appear to be unrelated The laboratory noted that the gasoline chromatograms from V M-1 and VM-2 differ from the gasoline standard, and that two non-gasoline peaks were strong contributors to the concentrations reported These peaks may represent solvent compounds, several of which have been detected in these wells in the past The TPH-g concentrations are plotted and contoured in Figure 4 Because only two points have detectable concentrations, the contours are poorly controlled, however, this map can serve as a starting point for comparison to maps that will be prepared after interim remediation of contaminated soil 5.0 CONCLUSIONS AND RECOMMENDATIONS In view of the trend toward increasing groundwater contamination at the site, it will be necessary to continue groundwater monitoring on a quarterly basis through 1999 The next monitoring . event should take place in May Without approval from PHS/EHD, interim remediation of the site should not be cont--mplated Instead, the investigation should continue along the lines prescribed in the Tri-Regional Board Recommendations for Preliminary Evaluation and Investigation of Underground lank Sites (1990) The next step should be to prepare a Problem Assessment Report (PAR) for i eview by PHS/EHD and the Central Valley Regional Water Quality Control Board Once the PAR has been approved, the final step will be to prepare a Corrective Action Plan (CAP) (referred to as a Final Remedial Plan in the Tri-Regional Recommendations) Once the CAP has been approved, it can be sent out to competitive bidders and the cleanup can be awarded to the successful bidder The Tri-Regional Recommendations specify a number of items that must be addresEed in the PAR The work that has been performed to date provides the data necessary to complete several of these items, but before the PAR can be finished a few additional items are needed 1) An aquifer test to determine the groundwater flow properties of the water-bearing zone This would involve a one-day field test using monitoring well VM-1 2) An assessment of the feasibility of at least three alternative remediation methods This would involve conducting short-term pilot tests to model the effectiveness of each method at full scale We recommend testing three methods i 4