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� r i • _ ' Several changes are noteworthy a)for the first time, groundwater is not shallowest in MW-7 ' b)for the first time, groundwater is shallower in MW-5 than in MW-3 c)for the first time, groundwater is not at the same depth in MW-2 and MW4 ' As a result of these irregularities, the groundwater gradient appears to be strikingly different from the normal situation (Figure 6) Rather than the usual groundwater low("trough") situated around MW-5, there appears to be a high surrounding the deep extraction well, with flow off this high toward the southeast, south, and southwest If subsequent monitoring confirms this pattern, it could indicate that air sparging into VE-1 has mounded the groundwater and locally reversed the groundwater flow direction The primary flow direction, however, continues to be toward the southeast 5.4 Groundwater Contamination t As in the previous five quarters, contaminants were detected in MW-5 (Table 2) The benzene and TPH-g concentrations reached a peak m May, when the water table was high, but declined as the water table dropped in the third quarter 1 6.0 CONCLUSIONS AND RECOMMENDATIONS Approximately 3,852,600 cu ft of vapors were withdrawn and combusted between July 1 and September 30, 1996 VOC concentrations (as measured by PID) in the shallow well vapor stream declined approximately 50% from the previous quarter, whereas concentrations in the deep well remained very low and constant The mean VOC concentration measured in the shallow well during ' the 9 monitoring events was 680 ppm Using this value, we calculate that 690 pounds of hydrocarbons were destroyed ' Contaminant concentrations in the groundwater sample from MW-5 were lower in the third quarter than in the second quarter of the year, repeating the pattern seen in 1995 This is probably due largely to the seasonal fall in the static water level, separating groundwater from residual gasoline in the ' vadose zone However, the TPH-g concentration in the third quarter of 1996 was only 36% of that in the third quarater of 1995, hence, it is possible that there has been an absolute decline in concentration ' Depth-to-groundwater measurements in this quarter suggest an anomalous change in the groundwater gradient that could be due to air sparging activities Because the flow pattern appears to differ so radically from what has been seen in previous quarters, we suggest that the depth to groundwater be ' measured again in October to check the validity of the August data In the previous report we speculated that vapor concentrations would increase as the static water level ' drops during the second half of the year and the capillary fringe and "smear zone" become more exposed This did not happen--vapor concentrations declined 'somewhat from their second-quarter levels and remained fairly constant This is probably a good sign, indicating that remediation is nearing ' completion 5 1 �