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-rH U 9 : 1 8 STATE OF CAUFORNIA-Erdeonmental F Ageney • PETE WILSON,G04-mor CALIFORNIA REGIONAL WATER QUALITY CONTROL HOARD CENTRAL VALLEY REGION 3643%utier Finae. Sulte A Sacramento,CA 95627-9098 PHONE: T16)255-3010 FAY (916)2a5-301 S 3 March 1993 Mr. Nick Nickerson Unocal Assessment and Remediation 2000 Crow Canyon Place, Suite 400 San Ramon, CA 94583 REYIfW OF WORKPLAN FOR THE PIJREGRO/8REA STOCKTON FACILITY Thank you for submitting the "Soil and Groundwater Investigation" workplan prepared by Condor. After reviewing the workplan, I have the following comments: Section 3.1 . We feel very strongly that a fourth monitoring well should be installed near the location of the former 2,2-0 aboveground tank. This fourth well is needed for two reasons: to assess any impact from the former tank, and to provide a better measurement of groundwater gradient. The gradient is so flat in this area that a slight measurement error could significantly impact the determination of gradient. A fourth well would provide four triangulations to measure the gradient, allowing for a greater degree of reliability. 5ection 3.3. Condor suggests a 20' screen length for the wells, but proposes to determine the final length while in the fieldAlthough the groundwater i has dropped dramatically in the last few years, t is staff's experience that a screen longer than 20' has the potential to intercept another aquifer or to dilute the sample beyond what is reasonable. Section 4.0. The flat groundwater gradient necessitates an extremely accurate well collar survey. Section 5.0. As discussed with Condor at an earlier date, samples must be analyzed for nitrate+nitrite, not Jost nitrate. When acidified with sulfuric acid, the holding time for nitrate+nitrite in water is 28 days. The holding time for TKN in water is also 28 days, not 48 hours as shown in Table 1. Because of the gasoline, diesel , and kerosene found in the soil during the 1991 excavation work, the Board will also require that the initial groundwater samples be analyzed for TAH-d and TPH-G (EPA methods 3510 and 5030) . 5ection 6 0 The groundwater samples collected on a quarterly basis must be analyzed for 1 ,2-D, nitrate+nitrite, and TKN. THP-4 and/or TPH-G analyses may also be required if the initial samples are positive for these constituents.