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� Me rnor2ndum• <br /> CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD • CENTRAL VALLEY REGION <br /> 1.3443 Routier Road Phone: (916) 361-5600 <br /> Sacramento, CA 95827-3098 ATSS: 8-495-5600 <br /> TO: Antonia K.J. Vorster FROM: Gary Reents <br /> Senior Engineer Area Engineer <br /> DATE: 15 June 1986 SIGNATURE: <br /> SUBJECT: PROPOSED WORKPLAN FOR THE TRACY SERVICE CENTER, PRELIMINA(FY UNDERGROUND <br /> TANK INVESTIGATION, SAN JOAQUIN COUNTY <br /> I have reviewed the subject workplan, dated 20 March 1987, prepared by Pacific <br /> Gas and Electric (PGandE) . Overall , the workplan is comprehensive and well <br /> thought out. However, I de have the following comments and/or questions: <br /> 1. Has the underground tank in questions been removed? If so, was the surround- <br /> ing soil contamination delineated and mitigated? If not, these tasks should <br /> be initiated immediately. The workplan does not discuss this aspect of the <br /> project. <br /> 2. A report and proposal for specific well locations should be prepared after <br /> Phase I soil gas work and submitted for regulatory concurrence. Phase II work <br /> should not be initiated until approval is received. <br /> 3. Although two-inch wells are acceptable, four-inch wells have the advantages of <br /> better development, easier purging and sampling, are much more useful for <br /> aquifer testing, can possibly be used for extraction/recovery wells, and can <br /> still be installed by the same drilling technique. I recommend that PGandE <br /> consider installing four-inch monitoring wells on-site. <br /> 4. Accurate water levels cannot be obtained from monitoring wells within the <br /> floating product plume. Immiscible, lighter than water liquids actually float <br /> on the capillary zone, not the free water table. Wells screened across the <br /> floating product zone will fill with product, displacing the water level <br /> inside the well downward to a degree dependent upon the thickness of the <br /> capillary zone. The end results are a thicker than reality product zone and a <br /> lower than reality water level in monitoring wells. To further confuse the <br /> situation, the water table is, in reality, depressed relative to the local <br /> water table, due to the floating product. <br /> These two factors prevent computation of "corrected" water levels, based on <br /> density differences. Accordingly, on-site gradients must be determined by a <br /> minimum of three monitoring wells outside the area of floating product. <br /> 7 1. ,li, <br /> SEP 1 <br /> ENPERHEALTH <br /> MIT/SERV CES <br />