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WEISS ASSOCIATES G.Boggs 29 June 1988/page'4 to ground water extraction and/or seasonal variations in recharge. Therefore, at the time of the hydrocarbon leak, the water table may have been lower than it i£ +presently, allowing the leaked hydrocarbons to accumulate below the present water _table. Of the twelve saturated soil samples analyzed during previous phases of the site investigation (reported on 10 June and 15 October 1987), the 20.7 ft boring 13-2 sample was the only one containing hydrocarbons. This suggests that the distribution of hydrocarbons in saturated soil at the site is quite limited since they have been identified only in the vicinity of this sample. 4. RATIONALE FOR MW-4 Proposed monitoring well MW-4 will be located at the southwestern corner of' the tank pit to monitor ground water directly beneath and adjacent to the edge of the . concrete slab. As indicated by analytic results for samples-collected under the slab, the majority of leaked hydrocarbons appear to have migrated laterally along the slab and then downward into unsaturated soil. Leaked hydrocarbons move downward through unsaturated soil, by gravity, until they reach the water table,.where they become a source of hydrocarbons in ground water. Therefore, analysis of ground water samples from a monitoring well, located immediately adjacent to the concrete slab should provide the earliest detection of hydrocarbons in ground water. A source-area well such as proposed well MW-4 should ideally be screened at and above the water table. However, the upper 6 to 7 ft. of saturated soil beneath the southwest corner of the tank pit may be low-permeability silty clay. A well monitoring the water-bearing zone at 25 to 34 ft, and also screening into the unsaturated zone, would create a conduit for the downward migration of hydrocarbons, if they are present, through the low-permeability sediments. Therefore, if no water-bearing sediments are encountered above 25 ft during the drilling of the boring for MW-4, well MW-4 will screen only the continuous water bearing zone between 25 and 34 ft and will not screen into the unsaturated zone. If shallower water-bearing sediments are encountered, the well will be screened in the shallower sediments. REPORT PREPARATION Following the excavation closure, a comprehensive reportwill be prepared that includes: o A summary of all previous work at the site including all figures, boring logs and tabulated analytic and water level data. o Descriptions of field procedures for the tank pit excavation, sampling and backfilling.