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STATE OF CALIFORNIA-Environmental ProteAgency - • PETE WILSON, Governor <br /> CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD <br /> CENTRAL VALLEY REGION <br /> 3443 Routier Road, Suite A g <br /> Sacramento, CA 95827-3098 <br /> PHONE: (916) 255-3000 <br /> FAX: (916) 255-3015 <br /> 3 March 1993 MAR b 1993 <br /> Mr. Nick Nickerson <br /> Unocal Assessment and Remediation <br /> 2000 Crow Canyon Place, Suite 400 <br /> San Ramon, CA 94583 <br /> REVIEW OF WORKPLAN FOR THE PUREGRO/BREA STOCKTON FACILITY <br /> Thank you for submitting the "Soil and Groundwater Investigation" workplan <br /> prepared by Condor. After reviewing the workplan, I have the following <br /> comments: <br /> Section 3.1 . We feel very strongly that a fourth monitoring well should be <br /> installed near the location of the former 1,2-D aboveground tank. This fourth <br /> well is needed for two reasons: to assess any impact from the former tank, and <br /> to provide a better measurement of groundwater gradient. The gradient is so <br /> flat in this area that a slight measurement error could significantly impact <br /> the determination of gradient. A fourth well would provide four <br /> triangulations to measure the gradient, allowing for a greater degree of <br /> reliability. <br /> Section 3.3. Condor suggests a 20' screen length for the wells, but proposes <br /> to determine the final length while in the field. Although the groundwater <br /> has dropped dramatically in the last few years, it is staff's experience that <br /> a screen longer than 20' has the potential to intercept another aquifer or to <br /> dilute the sample beyond what is reasonable. <br /> Section 4.0. The flat groundwater gradient necessitates an extremely accurate <br /> well collar survey. <br /> Section 5.0. As discussed with Condor at an earlier date, samples must be <br /> analyzed for nitrate+nitrite, not just nitrate. When acidified with sulfuric <br /> acid, the holding time for nitrate+nitrite in water is 28 days. The holding <br /> time for TKN in water is also 28 days, not 48 hours as shown in Table 1 . <br /> Because of the gasoline, diesel , and kerosene found in the soil during the <br /> 1991 excavation work, the Board will also require that the initial groundwater <br /> samples be analyzed for TPH-D and TPH-G (EPA methods 3510 and 5030) . <br /> Section 6.0. The groundwater samples collected on a quarterly basis must be <br /> analyzed for 1,2-D, nitrate+nitrite, and TKN. THP-D and/or TPH-G analyses may <br /> also be required if the initial samples are positive for these constituents. <br /> 1 <br /> a <br /> ti� <br />