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samples from these borings have been analyzed for hydrocarbon contaminants. All analyzed samples from MW-1 and RW-1 were contaminated with gasoline and BTE&X. No samples from any of the other borings were contaminated. These results indicate that soil contamination is limited to the soil beneath the former UST, and does not extend far beyond MW-1 (Figure 5). The zero line of vadose-zone n contamination hes between MW-1 and MW-3 on the east and between MWA and MW-4 on the north. Contamination probably extends southward into the alley behind the building, but underground utilities there prevent drilling to delineate the southern limit. Based on data from the other wells, it appears that gasoline migration was primarily vertical through the permeable upper fluvial sequence of silty to medium- grained sand, and reached the groundwater without significant lateral spreading within the vadose zone. ' Figure 7 shows the inferred zero line of soil contamination at the site. The plume is interpreted to have a cone or columnar shape, with a maximum radius of approximately 25 ' feet, based on the assumption that gasoline migration was essentially vertical through the permeable upper fluvial sequence. 4.3 Volume of Contaminated Soil Assuming that gasoline migration was primarily vertical through the sandy upper fluvial sequence, the vadose zone contaminant plume is roughly cylindrical in shape, with an estimated radius of 25 feet, a height of 70 feet, and a volume of 5,000 cubic yards. ' 5,0 GROUNDWATER CONTAMINATION ' 5.1 Time-Variance of Contamination Table 2 shows the concentrations in groundwater of all of the contaminants detected at this ' site since 1987. Figure 8 plots these concentrations versus time. It is evident that gasoline and all of the volatile hydrocarbons have been consistently detected in the two wells nearest the former tank, indicating that the hydrocarbons have not degraded naturally over time. ' However, concentrations have fluctuated widely, particularly in MW-1, rising during dry periods (10/92 and 10/93) and falling in wet periods (9/87 and 4/93). This pattern is surprising, because concentrations usually rise during the rainy season, when hydrocarbons ' desorbe from soil particles in the vadose zone and dissolve in groundwater as it rises. ' 5.2 Lateral Extent of Groundwater Contamination Judging from the fact that MW-1, MW-3, and MW-4 have been contaminated in each monitoring event, and MW-6, MW-7, and MW-8 have not been contaminated in any event, it appears that the lateral extent of groundwater contamination has remained relatively A&A PAdR.-N 9411'7