FIELD DOCUMENTS AND WORK PLANS 1990

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MEMORANDUM • CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD - CENTRAL VALLEY REGION 3443 Routier Road, Suite A Phone: (916) 361-5600 Sacramento, CA 95827-3098 ATSS Phone: 8-495-5600 TO: Antonia K. J. Vorster FROM: Camilla Williams Senior WRC Engineer Engineering Geologist DATE: 28 November 1990 SIGNATURE: SUBJECT: MINUTES OF 14 NOVEMBER 1990 MEETING, MARLEY COOLING TOWER COMPANY, SAN JOAQUIN COUNTY On 14 November 1990, a meeting was held to discuss the ground water extraction modeling, scheduling, the modification of the National Pollution Discharge Elimination System (NPDES) permit and the content of the quarterly monitoring reports for Marley Cooling Tower Company (MCTC) . A list of attendees is attached. GROUND WATER EXTRACTION MODELING A finite difference flow model (MODFLOW) has been built by Hargis and Associates to model the historical pumping centers in the area of MCTC in order to simulate potential extraction scenarios for the final ground water treatment system. Significant historical ground water features include a pumping center to the southeast of the site and a ground water high in the area of the Stockton Diverting Canal . Ground water flow directions have been toward the southeast to southwest from the 1950's to 1988. Since 1988, the flow direction has changed toward the east due to recent drought conditions and the associated dewatering of the aquifer due to increased withdrawals by supply wells. Ground water levels at MCTC have dropped from about 40 feet below ground surface (bgs) in 1985 to about 80 feet bgs in 1990. Hargis estimated that the pumping rate needed to cause the historical overdraft of the aquifer occurred at about 7000 gallons per minute (gpm) , based on the supply well pumping records from the local water district and on calculations using an analytical model . Ground water levels from Fall 1988 were used to build the model . Three flow conditions were simulated: historical flow to the south, January 1990 flow with a gradient of 0.002 feet per feet (ft/ft) to the east, and July 1990 flow with a gradient of 0.0002 ft/ft to the east. The model was built with three layers which correspond with the major water bearing zones. Layer 1 extends from the water table to a depth of about 100 feet bgs, Layer 2 extends from about 100 to about 200 feet bgs, and Layer 3 extends from about 200 feet to about 500 feet bgs. Two pumping cycles were simulated for each layer based on an extraction rate of 500 gpm, the proposed final extraction rate. Cycle 1 used extraction wells in the area of the site and Cycle 2 used extraction wells off-site to the south of the site. Monitor well MW-424 was used as a pivot extraction well for the two cycles and therefore, would be continuously pumping. MCTC has conducted contaminant mass loading studies and has concluded that about 90 percent of the contamination is within the immediate area of the site. The model runs indicated that complete capture of the chromium plume would occur for both cycles in all three layers using the historical and January 1990 flow conditions. However, complete capture would not occur using July 1990 flow conditions. If drought conditions abate this upcoming year and ground water flow returns to the historical