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3 4 March 1992 (GeoAudit CV 30-34E3-1.47) Page. 2 of 4 GROUNDWATER GRADIENT In 1987 , the groundwater gradient was to the north-northeast during.• both;,sampling events performed by Weiss Environmental. MW-1 was the only.; one of three monitoring wells that would have been down- gradient from the former tank cavity. The present situation is unclear, because the depth of 60. 12 ft in MW-1, as reported by Earth Systems Environmental, is at or just below the level of the bottom perforations in MW-2 and MW-3 (60 ft and 59 ft, respectively) . If water levels at the locations of MW-2 and MW-3 are just below the perforated intervals, at perhaps 60. 05 and 60. 10 ft, these wells would still be up-gradient from MW-1 but would be too shallow to encounter the water table. This possibility is not unlikely, since the water levels in MW-1 and MW-3 were within 0. 03 ft of each other during both sampling events in 1987 , and the level in the up-gradient well (MW-2) was 0. 4 to 0. 6 ft above those in the other wells. Due to the uncertainty of the groundwater gradient, San Joaquin County EHD has requested that at least two additional- monitoring wells be installed to measure the present gradient and to determine FT the ,lateral extent of soil and groundwater contamination. In addition, the County has asked that a plan to remove the floating product from MW-1 be included. PROPOSED WELL LOCATIONS We propose to -insta1-1 one 'monitoring -well °north of MW-1, and -a 'second-well near MW-2 to determine whether groundwater in this up- Lgradient position is still__contaminated. (Figure 2) Details of the drilling and sampling procedures are provided on the following pages. 1) Notify the San Joaquin County Environmental Health Division (SJEHD) 48 hours prior to work initiation. 2) Drill two monitoring wells using truck-mounted, continuous- flight, 8-inch diameter hollow-stem augers using the well design depicted in Figure 3 . 3) Obtain discrete soil samples at rtefoot intervals ' in each boring, n- ' using a split-tube sampler loaded with three-pre--, ", cleaned brass tubes. 4) Decontaminate sampling equipment in a tri-sodium phosphate solution (TSP) and rinse twice with water prior to each sampling run.