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' Pombo Ranch Facility August 17,2004 <br /> Subsurface Investigation Report& <br /> Second Quarter Groundwater Monitoring <br /> 4.0 CONCLUSIONS AND RECOMMENDATIONS <br /> ' Contaminants of concern have been identified as gasoline range petroleum hydrocarbons; <br /> benzene, toluene, ethyl benzene, and xylenes sourced from the former underground <br /> storage tank which were at or below the maximum contaminant level for the compounds. <br /> ' MTBE was detected at low concentrations in a water sample collected from one well <br /> (MW4) in this investigation. MTBE was not detected in soil samples during the <br /> investigation. The isolated occurrence of MTBE should be confirmed by additional <br /> ' monitoring of weII MW4. Soil contamination was identified in samples collected at the <br /> water table in down gradient direction. The contamination is probably a relic smear zone <br /> from the initial release from the underground storage tanks. <br /> A survey of well reports by the <br /> Department of Water resources faded to identify reports <br /> of wells located within 200 feet of the site. Several reports were located for wells located <br /> at the northwest corner of section 30, 4000 to 5000 feet away. Well reports or <br /> construction details were not available for the nearby domestic water wells. <br /> Wf IL- <br /> The lateral and vertical extent of soil and groundwater contamination has been defined in <br /> this phase of the environmental investigation. Soil contamination appears to be restricted <br /> ' to the immediate vicinity of the former underground storage tank and down gradient in <br /> what appears to be a smear zone. Groundwater contamination is present in <br /> concentrations dust above or below the established clean up standards. The site appears <br /> to be undergoing in situ bioremediation as shown by the reduced dissolved oxygen <br /> concentrations and low contaminant values. <br /> ' Quarterly monitoring of the existing wells should continue to confirm the low <br /> contaminant concentrations and continuing biodegradation. The nearby domestic wells <br /> should be included in the quarterly monitoring. If the current low groundwater <br /> contaminant concentrations and lack of impact in the domestic wells is confirmed over <br /> twelve months of monitoring, the site should be proposed for closure as low risk. <br /> 1 <br /> 1 <br /> 1 <br /> 1 <br /> r <br /> rI 1 <br />