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configuration of the plume is unusual and difficult to explain It does not appear to be elongate parallel to the Modesto Aquifer, which is presently above the static water level It is particularly difficult to explain the presence of diesel in GT-9, which is upgradient from most of the other wells at this tune 4.0 RECOMMENDATIONS Groundwater monitoring began at this site in 1990, but 1999 was the first year in which sampling was performed on a regular quarterly basis Over the years, a total of ten samples have been collected from GT-3, fewer samples have been collected from the other wells Numerous fluctuations in contaminant concentrations have been noted in this and previous monitoring reports, but a clear trend toward decreasing concentrations has not yet been established, partly due to simultaneous changes in water depth that have caused the static water level to drop below the contaminated zone or rise above the well screens Therefore, it will probably be necessary to continue monitoring through the year 2000 in order to demonstrate that the groundwater plume is stable or shrinking in size In the event that the plume is not stable, the regulatory agency may require some form of remediation to either reduce groundwater concentrations or lower the risk of further groundwater contamination by reducing concentrations within the soil of the Modesto Formation above the static water level In order to assess the feaibility of groundwater remediation, we recommend conducting a short-term pumping test of GT-10 A work plan for this test was submitted in August 1999, and an addendum to the work plan was submitted in October If San Joaquin County PHSIEHD approves the addendum, we recommend performing the pumping test in the middle to latter part of the first quarter(late February or early March) of 2000, when the static water level should be within the contaminated portion of the Modesto Aquifer i I 4