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f. Work Plan Addendum February 8, 1990 Fire Engine Company No. 2, Stockton, California AGS 38009-2 gradient across the site. The direction of ground-water flow on December 22, 1988, was evaluated to be to thelnorth,which was not in agreement with the inferred'southeastward regional gradient,used in the March 1988 work plan to locate the wells. The well locations and ground-water flow direction, are shown on Plate P-2. Results of laboratory analyses (Table i; indicated soil and water samples collected at 25 and 35 feet below grade from boring B-1 and well MW-1, contained detectable amounts of total petroleum hydrocarbons as gasoline (TPHg), total petroleum hydrocarbons as diesel (TPHd), benzene, toluene, ethylbenzene, and total xylene isomers (BTEX). Soil samples collected from borings B-2 and B-3, and water samples collected from wells MW-2 and MW-3 were below detection limits for TPHg, TPHd, and BTEX except for 0.03 ppm of TPHg from well MW-2. Laboratory results also indicated organic lead levels were below detection limits in the soil or water samples collected from the three borings and ~yells. Copies of the laboratory reports and the Chain of Custody Records are included in the .7 attachments to this letter. Because the December 1988 subsurface environmental investigation did not fully delineate the extent soil and water contamination,we intend to conduct the tasks listed below. This additional work will be conducted in accordance Regional Board Staff Recommendations for Initial Evaluation and Investigation Underground Tanks (Tri-Regional Recommendations), Appendix A of the Tri-Regional Recommendations, and the Leakine Underuround Fuel_TankManual guidelines. The tasks below will be conducted following the procedures previously approved by the Sari Joaquin Public Health Service (SJPHS) in our Work Plan 38009-1 and Addendum Letter dated November 7, 1988. Applied GeoSystems will: o Update the site safety plan. A copy of the safety plan is included with this letter. a Obtain a drilling and well installation permit from the SJPHS which is enclosed with this letter. o Drill four 8-inch-diameter soil borings in the locations shown on Plate P-2 to evaluate lateral and vertical limits of soil contamination near the former leaking storage tank. The borings will be terminated at the groundwater surface,which we estimate to be at 37 feet below grade, based on our December 1988 field work. Borings will be backfilled with a slurry of cement, bentonite, and water. o Drill a fifth 8-inch-diameter soil boring approximately 80 feet to the north and downgradient of T-5 to approximately 15 feet below ground water. The location of the fire engine building, trees, and fences prevent the safe operation of a drilling rig closer to the source of contamination. lip 4 2 ,4pP/led GeoS.YSA_'mS