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{ ;f SUPPOSITION FOR CLOSURE Basis for Minor Alternative Source MW-6 is approximately 50 scaled feet north-northeast of the edge of the backfilled UST excavation. Hydropunch", HP-3, drilled on the north-northwest side of the backfilled UST excavation (see Figure 2) on April 8, 1994, yielded a water sample containing 160 µgll of benzene. This concentration is 42.8% of the amount reported in the sample collected from MW-6 on January 22, 1997. MW-6 is not downgradient to the east of the backfilled excavation as is MW-8. From its initial development in July 1995 through October 1996, MW-8 has never yielded gasoline-impacted water samples. The local flow direction has never been calculated in a northerly direction; it has always been calculated easterly between S770E and N810E. If the flow direction had been northerly from the location of the UST, a smear zone in the soil would account for higher reported concentrations in samples collected from MW-6 during winter months. And, given a presumed northerly flow direction, the concentrations reported in samples collected from MW-6 would be expected to be far less, if any, than those reported in HP-3. Thus, it is reasonable to assume that the lateral migration of groundwater could not have moved northerly to transport gasoline hydrocarbons against the local easterly gradient emanating from the slough. Given the long-term data, a secondary source other than the removed UST likely accounts for the fluctuating, seasonal concentrations reported in samples collected from MW-6. The reported concentrations in samples collected from MW-6 may be due to a minor spill into the soil between the slough shoreline and MW-6 which occurred during construction of the levee. Recharge water originating in the slough which contacts the residual gasoline hydrocarbons in the soil is yielded by MW-6 whenever the well is sampled. Reported TPH-g and benzene concentrations have been higher in samples :.� collected from MW-6 during winter-quarter monitoring activities; the concentrations decline when the water table drops in the spring, summer, and autumn. Tidal influences appear to have little effect, if any. Figures 4 and 5 are maps depicting the "Estimated Downgradient Decline of Benzene in Groundwater. Each map was prepared in accordance with a request of the SJCPHS-EHD utilizing laboratory data obtained on the dates July 7, 1995 and January '= 22, 1997, respectively. The maps exhibit estimated benzene isoconcentrations which might be transported downgradient from MW-6 beneath and just beyond the condominium complex. On July 7, 1995, benzene was reported at 199 µg11 in MW-6 and 124 µg11 in HP-9, a decline of 38% across a distance of 30 feet. This decline rate .� DAYNE L. FRARY, GEOLOGIST 6 1296-012.197