SITE INFORMATION AND CORRESPONDENCE

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Wendy L. Cohen -2- • 40 ctobei 1996 In response to the City of Tracy's notification of an unauthorized discharge, the Board inspected GP on 28 August 1984. Samples of water from the stormwater pond contained chloroform, other trihalomethane compounds, and ethylene glycol. Chloroform concentrations ranged between 19,000 and 88,100 micrograms per liter (µg/1). The specific conductance was as high as 22,000 micromhos per centimeter. The Board issued a Cleanup and Abatement Order on 25 September 1984. The Order required GP to abate the discharge and threat of discharge, determine the extent of contamination in subsurface soils and groundwater, and take necessary remedial actions at the site. Subsequent investigations by GP determined the lateral and vertical extent of soil contamination. Groundwater also was determined to be contaminated with chloroform. Remediation activities included removing all water from the stormwater pond and 1,700 cubic yard of sludge and soil containing chloroform, ethylene glycol and other trihalomethanes. Ground water recovery wells and a ground water treatment system with an air stripper recovered chloroform from the shallow ground water. Hydrogeology The site is in the northwestern portion of the San Joaquin Valley. Sediments in the San Joaquin Valley consist of an interbedded sequence of clay, silt, sand, and gravel. A regional aquitard, the Corcoran Clay, divides the ground water system into an upper unconfined aquifer and a lower confined aquifer. Surface soils in the vicinity of the site consist mainly of silt to approximately 15 feet below ground surface (bgs). This silt layer is underlain by sand and gravel to approximately 40 feet bgs. Alternating layers of the silt and medium-to-coarse sands extend to approximately 100 feet bgs. Clay predominates between approximately 100 feet bgs and the Corcoran Clay which is present at approximately 250 feet bgs. The upper, unconfined aquifer extends from approximately 45 to 55 feet bgs to the top of the Corcoran Clay at about 250 feet bgs. In the vicinity of the site, the Corcoran Clay is between 80 and 90 feet thick. The confined aquifer occurs between 325 and 330 feet bgs at the site and extends down to bedrock at approximately 1,500 feet bgs. The site is at the western edge of the San Joaquin Valley and near the western edge of the Corcoran Clay. For this reason, the confining clay layer may not be continuous within two miles of the site. In addition, there are several unconfined and composite irrigation wells in the area surrounding the site. These wells create interconnections between the two aquifers and the two bodies of ground water. The upper and lower aquifers are thus considered essentially interconnected. Water level measurements at the GP site have shown that ground water in the upper unconfined aquifer has consistently flowed to the northwest between 1985 and 1996. Most of the monitoring wells containing chloroform are approximately 50 feet deep and are screened from 19 to 39 feet. Two monitoring wells which contain chloroform are 70 feet deep and are screened from 55 to 70 feet. Sixty to 100 percent of the City of Tracy's water is supplied from the Delta-Mendota Canal. When the canal cannot supply 100 percent of the city's demand, water from the city's 10 ground water wells