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be decreased to bi-weekly Data from the fourth quarter, including the laboratory analysis of the sample collected in December, indicate that hydrocarbon concentrations have continued to decrease gradually However, the San Joaquin Valley Air Quality Pollution Control Distnct'has denied Apache's request to redu&the frequency of monitoring Hence, Upgradient will continue to monitor on a weekly basis during 1996 Upgradient also recommended that vapor samples be collected from both extraction wells in the fourth quarter in order to quantitatively measure the effect of the system at both deep and shallow depths ' However, because of the presence of water in the deep well and recent efforts by Apache personnel to inject fresh air into this well, the deep and shallow wells were not sampled separately in December Laboratory analysis of the vapor sample collected in December did not confirm the suggestion by VR ' Systems that poor engine performance in the third and early fourth quarters of 1995 was due to an elevated CO2 concentration in the subsurface Rather, the combustion unit may have been affected by an abnormally high N2 concentration Based on both field and laboratory results, Upgradient believes that the vapor extraction unit may be nearing its effective limit, and supplementary fuel costs may begin to make this methodology less cost- effective than other methods during 1996 Furthermore, because hydrocarbon concentrations in vapor samples commonly exceed those measured in soil samples by a few hundred parts per million, it is likely that remediation is nearing completion Therefore, Upgradient recommends that soil samples be ' collected before the end of the first quarter of 199610 determine the actual concentrations in soil Contaminated groundwater was sampled only in MW-5, and concentrations continued to decrease since their high in June No contaminants were detected in wells MW-2, MW-3, MW4, MW-6, or MW-7 during any of the four monitoring events in 1995 Therefore, Upgradient recommends that ' sampling of these wells be reduced to annually, with the next sampling in_June 1996 The deep extraction well and MW-5 should be sampled quarterly, unless water is not present 1 i 1 5