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MEMORANDUM CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD - CENTRAL VALLEY REGION 3443 Routier Road Phone: (916) 361-5600 Sacramento, CA 95827-3098 ATSS Phone: 8-495-5600 TO: Antonia K. J. Vorster FROM: John J. Tomko r, DATE: 11 December 1989 SIGNATURE: �7� A-- SUBJECT: PROPOSED DRINKING WATER WELL CONSTRUCTION, DEFENSE DEPOT TRACY, SAN JOAQUIN COUNTY On 4 December 1989 we received a report from the Tracy Defense Depot containing the design, construction, and installation specifications for two new drinking water wells. The new wells will be approximately 950 feet deep and will be screened within the Lower Tulare Formation (below the Corcoran Clay Member). The Upper Tulare Formation, to a depth of approximately 300 feet, will be sealed -off with a cement and sand slurry mixture. The wells are located along the northeast bearing property boundary (see attached location map). Based upon the ongoing RI/FS ground water investigation, the proposed well locations are not located within, nor downgradient, of any known ground water contamination areas. The sealing -off of the Upper Tulare Formation will minimize the hydraulic effect these wells may have on existing on-site contaminant plumes. The minimal lateral separation is approximately 1000 feet between the northern most well and Contamination Area 4. The extraction of water from the Lower Tulare Formation will likely have negligible effects on the migration of contaminants within the Upper Tulare Formation, due to the aquitard nature of the Corcoran Clay Member. If in the unlikely event that the production wells alter existing contaminant migration patterns, existing monitoring wells within the Upper Tulare Formation should detect these influences. If these changes are deemed significant then modifications to the ground water remediation program can be made to alleviate the problem. Therefore, based upon existing information I believe the proposed design and location of the two new production wells will not significantly impact existing contaminant migration patterns. However, the confidence associated with this conclusion can be increased if the production wells are located at the maximum distances possible from known contamination plumes. Based upon the lateral spacing (1500 feet) between the two new wells, it is possible to locate the new wells in the south corner of the Depot along the property boundaries and 1500 feet from the existing well #7 (see attached location map). These locations would place the wells at distances of 2400 and 2700 feet from Contamination Area 5 of the RI/FS and over 3000 feet from the more heavily contaminated areas (Areas 1 and 4). Obviously other restrictions may be limiting the Depots flexibility in locating the wells at the maximal lateral upgradient distance from known contamination areas. If there are no such restrictions, it is suggested that the new wells be placea(at the maximum lateral distance from known contamination areas. If this is not possible, the proposed locations (and the proposed design of the production wells) give us reasonable assurance that the wells will not influence contaminant plume migration patterns in the Upper Tulare Formation.