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• MEMORANDUM . CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD - CENTRAL VALLEY REGION Phone: (916) 361-5600 3443 Routier Road, Suite A ATSS Phone: 8-495-5600 Sacramento, CA 95827-3098 TO: i fi Ahtonia K. J. Vorster FROM: Marie T. McCrink Senior WRC Engineer Assoc. Engineering Geologist A,4444 DATE: 19 November 1990 SIGNATURE: / / ,4444 � L2 . SUBJECT: ARCO TERMINAL WORKPLAN FOR FURTHER SITE ASSESSMENT AND AQUIFER TESTING, PORT OF STOCKTON, SAN JOAQUIN COUNTY On 12 October 1990, Harding Lawson Associates (HLA) submitted a workplan on behalf of ARCO Products Company for additional site assessment and aquifer testing of ARCO's bulk terminal facility in the Port of Stockton, San Joaquin County. Two soil and ground water investigations have previously been conducted at this site. In 1984, Woodward-Clyde Consultants installed 6 shallow monitoring wells and found floating product in 3 of them. In 1990, HLA completed the following activities: drilled 6 soil borings to about 25 feet below ground surface (bgs) , and completed 5 as monitoring wells; conducted 6 cone penetrometer tests at about 60 feet bgs; installed 6 piezometers with 2 inch screens, 3 to an intermediate depth zone at 40 feet bgs, and 3 to deep zone at 60 feet bgs; conducted 6 laboratory soil permeability tests on samples collected at about 45 feet bgs; measured water levels in all wells and piezometers; and developed and samples all existing and new wells. Soil and water samples were analyzed for total petroleum hydrocarbons as diesel (TPH-D) and gasoline (TPH-G) , and for benzene, toluene, ethylbenzene, and xylene (BTEX) . In 1984, the depth to ground water ranged from 3 to 4 feet bgs, and the direction of ground water flow was to the southwest. During 1990, depth to ground water measured in the shallow and intermediate depth wells ranged from 5 to 8.25 feet bgs, while measurements from the deep wells ranged from 6.75 to 10 feet bgs. Ground water from the three zones is now flowing to the southeast. At the present time, ground water flow is away from the Stockton Deep Water Channel and the San Joaquin River. The average horizontal gradient is 0.003 feet per foot, the vertical gradient between the shallow and intermediate zone os 0.01 to 0.02 feet/foot, and the vertical gradient between the intermediate and deep zone is 0.5 to 0.6 feet/foot. The 1990 sampling and analysis program found that, in most cases, soil concentrations of TPH-D, TPH-G, and BTEX were highest between 8 and 13.5 feet bgs. Within this zone, TPH-D ranged from 250-660 mg/kg, TPH-G ranged from 350 to 2600 mg/kg, benzene ranged from 4.3 to 29 mg/kg, and total BTEX ranged from 48 to 448 mg/kg. During 1990, 11 monitoring wells and 6 piezometers were each sampled 2 times. Two wells had floating free product. Detectable concentrations of TPH-G and BTEX were found in all wells and piezometers. In shallow zone wells, TPH-G ranged from 2.5 to 280 mg/l , and benzene ranged from 1.3 to 34 mg/1 . In the intermediate and deep zone wells, TPH-G ranged from 0.24 to 80 mg/l , and benzene ranged from 0.0093 to 6.8 mg/1 . The current workplan proposes to continue the site investigation with the following: 1 . Install approximately 10 soil borings to about 10 feet bgs for further definition of vadose zone soils contaminated with petroleum hydrocarbons;