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Mr.John Johnson Project Y]3-2k-02,December 16,1993 Page 6 Conclusions and Recommendations Groundwater at Cheaperl Store #37 has demonstrated significant amounts of petroleum hydrocarbons at MW-4 for the entire monitoring year. Detectable levels have also been found at MW-Y. The apparent source of the petroleum hydrocarbons is the former location of the underground gasoline tanks kiowevei:, an investigation described in an Apnl, 20, 1993, report by Parker Environmental Services indicated high TPI-i-g concentrations west (upgradient) of the former tank location. This could indicate an off-site source. It dries nota ear that the petroleum hydrocarbon levels in the groundwater are diminishing at significant ate. Hydrocarbons increase with higher groundwater levelb, and decrease with lower levels, indicating a continued presence of gasoline in the soils above the groundwater near MW-4. It is uncertain given the current well configuration if petroleum hydrocarbons are remaining on-site or moving off-site. As shown in the gasoline concentration figure, MW-4, downgradient from the former tank location, has the highest concentration of T?H-g The TPH-g concentration north of MW-4 can only be estimated with the exibting well placements. The Regional Water Quality ContrO Board and county regulatory will not allow closure for this site given the past year's sampling resultb. Closure is typically granted when one to two years of quarterly monitoring results are non-detect Two alternative .strategies for site closure are available for this site. First, you can remove the apparent source of the petroleum hydrocarbons, the soil around the former underground gasoline tank location. This would eliminate the apparent continued release of hydrocarbons to the groundwater. Regulatory agencieb would also require the existing hydrocarbons be removed from the groundwater to minimum levels, probably regwring additional wells and a pump-and-treat system Second, a groundwater treatment system with a percolation gallery to flush the contaminated sails can be installed to remove both soil and groundwater contamination. Parker Environmental Services recommends that two additional monitoring wells be installed, cine northeast of MW-4, and one west of the existing tanks near Holly ' Drive. Mis would help define any possible on-site or off-site petroleum hydrocarbon migration. 51 Parker Environmental Service&, 190 Fist 7th Street, PAUbarg, CA 94565-2317 ' rholie (510) 439-1024 Fox (510) 439-2566