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A. K. J. Vorster -2- 19 November 1990 ARCO Terminal , Port of Stockton San Joaquin County 2. Install ten 4-inch diameter, intermediate zone monitoring wells to approximate depths of 40 feet bgs. These wells will further define ground water contamination in a 10-15 foot thick intermediate depth zone, composed mostly of very fine grained sand; 3. Install ten 4-inch diameter, double cased, deep zone monitoring wells to approximate depths of 70 feet bgs. These wells will further define deep zone contamination found in a coarse grained sand unit extending an unknown thickness below the intermediate zone. A maximum of 4 soil samples from intermediate and deep zones combined will be collected from each deep well . Soil samples will be analyzed for TPH-D, TPH-G, and BTEX; 4. Install 2 triple cased, very deep wells to approximately 150 feet bgs. These borings will test the bottom of the first aquitard beneath the deep sandy aquifer. All deep and very deep borings will be mud rotary drilled and electrically logged; 5. Two intermediate and 2 deep zone pumping wells will be installed for conducting pumping tests, and will be constructed so that they can be used as ground water extraction wells in the future; 6. All wells will be developed, water levels will be measured, and samples for TPH-D, TPH-G, and BTEX will be collected from all wells except the pumping test wells; 7. Two types of aquifer tests will be performed. Initially, 30 short duration slug tests will be conducted which includes all currently existing and proposed wells. Slug test information will used to design a long term aquifer test. Pumping tests will be conducted in the 4 pumping wells; and 8. Finally, HLA will prepare a report documenting all work conducted during this phase of the site investigation, and will make recommendations for further work, if necessary. COMMENTS: This workplan thoroughly addresses the next phase of this site investigation. I recommend that HLA proceed as soon as possible with the workplan as submitted. However, after all the slug test and pumping test data are collected, a letter report should be submitted reporting that data and all details of the planned aquifer test. Previous investigations have not completely defined the lateral and vertical extent of ground water contamination. The current workplan will hopefully complete the definition of vertical extent. However, the definition of lateral extent will require investigating beyond the ARCO property. The possibility of drilling beyond the ARCO property boundary should be pursued. If possible, zero contaminant concentration lines or points at which ARCO's plume intersects with a plume from another source should be determined. MTM