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1999 peak in MW-6 is that it represents the passage of the May 1998 peak in MW-1 discussed previously, and that approximately 18 months elapsed before this peak reached MW-6. In this regard, the very large peak observed in MW-6 in May 1994 may thus be more significant than it might otherwise appear; like the 1999 peak, it occurred more than 12 months after a concentration peak was observed in MW-1 (in April 1993). Hence, the water data from these two wells support the argument that a wave of gasoline-impacted groundwater, migrating eastward (downgradient) from the Chevron UST cavity, first impacted MW-1 and then subsequently reached MW-6 a year later. This occurred not once, but twice between 1993 and 1999. As a further test of this hypothesis, Figure 10 compares the concentration trends in MW-1 with those in MW-3. The graphs strongly support the notion that the 1999 concentration peak in MW-6 records eastward migration of a pulse of contaminated groundwater from the Chevron UST facility. Unlike MW-6, MW-3 is located west (upgradient according to Figure 6) of the Chevron UST facility. Hence, if the 1994 and 1999 peaks in MW-6 are indeed due to eastward migration of the groundwater plume, no such peak should have occurred in MW-3. From September 1987 until August 1995, the curve for MW-3 is strikingly similar to that for MW-1. Concentrations peaked in both wells in April 1993, not May 1994. However, concentrations in MW-3 did not rise again after August 1995, as they did in MW-1, but began a long and continuous decline, with no peaks in 1998 or 1999. 2.4.4 Summary In summary, there is abundant evidence that gasoline was released from the Chevron UST's (and perhaps other appurtenances, such as piping or dispensers), and that this gasoline leached both laterally and downward. It entered the thin sand bed that is present at that site in the upper lithologic unit at a depth of 10 feet (Figure 3, cross sections A-A' and B-B') and downward into the middle lithologic unit, where it impacted groundwater sometime prior to 1986. On the contrary, there is very little evidence that gasoline released from the Kwikee UST's escaped the tank pit. This may explain why the hydrocarbon concentrations that were detected in the Kwikee UST samples were as much as 2.5 times greater than those in the Chevron UST samples. Whereas a significant proportion of the gasoline in the Chevron tank pit leached into the underlying soil and groundwater, thereby lowering the residual concentrations within the tank pit, all or nearly all of the gasoline that leaked from the Kwikee UST's remained encased in the tank pit, unable to penetrate-the low-permeability clay of the upper lithologic unit. 2.5 Schematic Model The foregoing discussions are summarized into a schematic block diagram of the two sites in Figure 11. The model predicts that under the prevailing eastward or northeastward groundwater flow, petroleum hydrocarbons dissolved in shallow groundwater in the middle lithologic unit will migrate to the east, and that over time gasoline compounds will be detected in wells that are located east (downgradient) of the leak points. In this scenario, any gasoline leaked from the Chevron UST's will first impact monitoring well MW-1, which is located immediately east of the tank cavity, and will subsequently migrate to MW-6 and KF-3. The data discussed in section 2,4.3 11 _