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3.2 Extent of Groundwater Contamination Although Clearwater Group concluded in 1995 that the extent of groundwater contamination had been delineated, Clearwater did not include a map of the gasoline plume in its report Subsequently, however, Clearwater prepared a time-averaged isocontour map of benzene in groundwater and included it in the Corrective Action Plan We have revised this map slightly for this report to correct minor errors in the location of some of the wells, include TPH-g contours, and make the map correspond to a specific date (November 14, 1996) (Figure 13) We have also prepared an updated version of this map, using data from the December 7, 1998 sampling event (Figure 14) The decline in the size of the plume is notable The historical groundwater monitoring data (Table 3) indicate that gasoline and BTEX concentrations have fluctuated over time, and that different wells exhibit different trends, which suggests changes in the size and shape of the plume over time Benzene concentrations declined overall in MW-2 until early 1996, but since that time the concentration has remained relatively flat at about 200 parts per billion (Figure 15) A steadier and more persistent decline has occurred in MW-5 (Figure 16) This accounts for the smaller size of the plume in Figure 14 in comparison to Figure 13 3.3 Hydrogeology 3.3.1 Hydrogeologic Units Boring data indicate that there are three hydrogeologic units underlying the site that are relevant to the distribution of hydrocarbons These units are best illustrated in cross sectional view (Figures 17 and 1 S) Water-bearing characteristics of these units are interpreted from their gram size and bedding features, no hydraulic conductivity tests or aquifer tests have been performed to quantitatively measure these properties Unit 1 is a silt bed, but it grades from clayey silt in the middle part of the unit to sandy silt near the base Near the ground surface it has weathered to various shades of brown, but below a depth of 10 feet it is gray to white in color An aquitard that contains relatively little water, unit 1 is approximately 23 feet thick but gradually thickens from south to north and west to east across the site 1t interfingers with and grades downward into unit 2,winch is an aquifer consisting of very-fine-to-medium-grained sand This unit also thickens slightly from south to north, but thins very abruptly in an east-west direction(Figure 17) and has an incised, erosional base These features, and its linear geometry in map view (Figure 19) imply that this unit is a stream-channel deposit Hydrogeologic unit 3 is another aquitard This unit was penetrated in four of the monitor wells (MW-1 through MW-4) and the CPT boning, and the description of the unit varies from boring to boring In CPTB-7, Clearwater described it as dark green silty clay, in MW- 4, Clearwater described it as dark olive silt In MW-1 through MW-3,GeoAudit described it as bluish-gray to greenish-gray clayey silt to sandy silt In CPTB-7, which penetrated the entire unit, it is 11 feet flick and is underlain by fine-grained sand 5