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4.0 CONCLUSIONS AND RECOMMENDATIONS ' This phase of environmental corrective action included the installing, surveying and sampling three groundwater monitoring wells Soils encountered beneath the site consist of a fining- upward sequence of fine-grained siliciclastic sediments The depth to static groundwater beneath the site ranges from 7 74 to 8 16 feet bgs Groundwater flow is to the north at a relatively low gradient of 0 004 foot/foot _ Only the groundwater sample collected from MW-2 contained detectable concentrations of petroleum hydrocarbons The only compound detected at MW-2 was TPH-as-diesel at 140 µg/I As such, the plume of impacted groundwater appears to have migrated from the area of the former USTs at least 30 feet to the northeast Based on the presence of free product previously observed in close proximity to the former USTs and the relatively low concentration of diesel in groundwater at MW-2, contaminant migration appears to be slow and probably stable This is ' supported by the relatively flat groundwater gradient Groundwater testing for physiochemical and microbial parameters indicated that nitrate concentrations and both general and selective microbial populations are relatively low However, the relative percentage of selective degraders to the total number of heterotrophs is high These results suggest that the existing microbes are capable of degrading the contaminant The population of naturally occurring bio organisms would likely be enhanced by the addition of nutrients, namely nitrate and oxygen No water wells or other sensitive receptors appear to be threatened by the plume of impacted groundwater The two wells identified as closest to the site could not be physically located Based on the results of the well installation and sampling and water well survey, Ramage Environmental recommends the following course of environmental corrective action 1 Monitor and sample groundwater from the three monitoring wells on a quarterly basis for at least three more quarters The groundwater samples should be analyzed for TPH-as- gasoline, BTEX and MTBE by Modified EPA Methods 8015 and 8020 Since the other oxygenate compounds are absent, no further testing by EPA 8260 is necessary 2 Based on the results of four quarters of groundwater monitoring and sampling and on the results of previous soil and groundwater grab samples collected at the site, evaluate the risk posed by residual petroleum hydrocarbons on human health and the environment 3 Based on the findings of the risk assessment, request regulatory closure or proceed with further evaluation of possible remedial alternatives 12