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Ceofogrcuf?erfv+usTru Page 8 Sol)Groundwater Investigation Report George's Service Project No 425 2 August 5 1997 1 Although soil samples obtained from MW-4 and MW-5 were non-detect, field observations of soil condition (ie , screening by OVM) and groundwater sample test results suggest that the soil and groundwater have been impacted by petroleum hydrocarbons an these areas We feel that the most likely source for the contamination found in MW-4 and MW-5 is the pump island and associated piping 2 At this time the plume is not vertically defined The groundwater at 45 feet below ground surface is highly impacted with TPH-Gas, 9,000 ppb, and BTEX, 160 to 959 ppb The contamination has already migrated through the plastic clay layer at 40 feet below ground ' surface so it is possible that it has also migrated through the clay layer beneath the gravel layer and therefore poses a possible threat to the deeper groundwater resource used for domestic purposes The gravel layer at 45 feet may also provide a preferential path for down gradient contaminant migration 3 At this time at does not appear that the groundwater plume associated with the former UST's has migrated to the north to the point that it has impacted MW-7, 75 feet from the former UST's, or farther to MW-6 or the domestic wells However, if the same down ' gradient movement of the plume from pump island is hypothesized for the former UST's, the groundwater plume may have crossed the Durham Ferry Road and could be impacting neighboring properties It is also possible that the groundwater plume has migrated as far as MW-6 and MW-7 an the gravel layer but that it is at a greater depth then the wells are screened 4 Groundwater gradient is N20°W at 0 0024 ft/ft 4.2 Recommendations Geological Technics Inc makes the following recommendations based on the conclusions outlined above o Groundwater monitoring of the existing monitoring wells and domestic wells should be continued on a quarterly basis to evaluate groundwater quality at the site and determine if the initial groundwater monitoring results are reproducible Q Water levels should be collected on a monthly basis for a 12 month period to track the ' effects of pumping and recharge of the adjacent wells o GTI recommends that a passive soil gas survey be conducted at the site and on adjoamng properties to determine if off-sate migration of the groundwater plume has occurred Areas that should be addressed are to the north on the properties occupied by the residences and domestic wells and to the west and southwest under and across Highway 33 0 Due to MW-102 be so highly impacted at 45 feet below ground surface, GTI recommends that an additional deep discretely screened monitoring well be placed near the former UST's and target a depth of approximately 75 feet below ground surface r