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direction and rate from well to well, three wells have been chosen as examples Two of the wells are from the core of the plume and one is near the lateral margin of the plume Monitor well MW-2 is located near the mapped center of the groundwater plume,and TPH- g concentrations have been consistently lugher in this well than in the others (Table 3) Although both increases and decreases have been recorded, no consistent trend is obvious (Figure 23) For the first year (1994), the concentration was generally within the 30,000- 40,000 ppb range, and the water depth was about 29 feet In 1995, groundwater began to rise and the gasolme concentration followed suit, reaching a concentration of more than 50,000 ppb late in the year The concentration returned to the 30,000-40,000 ppb range in 1996, while groundwater continued to rise In 1997, the concentration began to fluctuate, dropping to its lowest recorded level in the first quarter (23,000 ppb) and then spiking to its highest recorded concentration of 66,000 ppb in the second quarter Since then, the concentration has again returned to the 30,000-40,000 ppb range and shown less guctuation The changes do not appear to be cyclic, nor do they show any relation to changes in the depth of groundwater I Sparging well SW-1 is also located near the point of release of the gasoline, and is withun the core of the groundwater plume The monitoring record for this well is shorter than that for MW-2,but a graph of the TPH-g concentration in the four samples collected from SW-1 shows a clearer trend (Figure 24) Gasoline concentrations have increased steadily, reaclung 124,000 ppb in the fourth quarter of 1998 The increase probably implies that gasoline has begun to dissolve out of the soil in the depth interval of 25-30 feet, where concentrations up to 1,400 ppm were detected during drilling of SW-1 However, the increase is somewhat surprising in view of two facts 1) BTEX compounds have shown the opposite trend, declining by more than 50% during the same period (Table 3), and 2) SW-1 is located within a few feet of two ORC injection borings and should have been affected by the ORC Monitor well MW-1 shows the opposite trend (Figure 25) Initially, the TPH-g concentration was 1,300 ppb, but it has declined steadily, reaching ND four times in the 1996-1998 period The decline occurred during the long-term rise of the static water level, but in view of the fact that neither MW-2 nor SW-1 have shown the same decline in TPH-g concentrations, it is unlikely that the trend in Figure 25 is due to the rise of groundwater above the screened interval Dilution due to increased groundwater flow is also an unlikely cause, because the groundwater gradient map (Figure 20) implies that groundwater flows more slowly in the vicinity of MW-1 than it does in the vicinity of MW-2 and SW-1, which are located near the permeability axis of hydrogeologic unit 2 Therefore, we attribute the decline to attenuation of the contaminants along the margin of the plume This attenuation has occurred despite local increases in TPH-g concentrations in the core of the plume (SW- 1),which is a further indication that contaminant migration is not occurring rapidly The one notable exception to the declining trend in MW-1 is the first quarter of 1998, when a concentration of 2,890 ppb was reported As previously discussed, however, this anomalous result was likely due to cross-contamination from the groundwater pump, and the data from this monitoring event are considered spurious 8