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Soil samples from this over-excavation were improperly collected and deemed invalid On July <br /> 2, 1990, three groundwater morutonng wells (MW-1 through MW-3) were installed in the <br />' vicinity of the Tank A excavation In addition, one soil boring (SB-1) was placed in the former <br /> excavation to venfy the presence of clean backfill Toluene was detected at a concentration of <br /> 0 077 ppm ata depth of 15 feet bgs in MW-1 TPH-gasoline was detected in SB-l at a <br /> concentration of 12 ppm under the east end of the former Tank A at a depth of 30 feet bgs <br /> Groundwater for the three monrtonng wells was not sampled until IT Corporation s efforts on <br />' May 17, 1996 <br />' 3.2 Summary of Groundwater Sampling Activities <br /> On May 17, 1996, IT Corporation monitored and sampled groundwater from each of the three <br /> on-site wells Based on suspect survey data groundwater was calculated to flow in a northerly <br /> direction at a hydraulic gradient of 0 047 feet/feet TPH-gasoline was detected in groundwater at <br />' concentrations of up to 770 micrograms per liter(,ccg/L) in MW-3 Benzene was also detected in <br /> groundwater at a concentration of 2 2 µg/L in MW-3 <br /> 4.0 Recommendations <br /> Soil and groundwater in the vicinity of former Tank A has been impacted by concentrations of <br /> gasoline hydrocarbons above allowable limits Specifically, concentrations of benzene in MW-1 <br />' of 1 7 E.cg/L and in MW-3 of 2 2 Acg/L exceed the Cal-EPA, Department of Health Services <br /> Drinking Water Maximum Contaminant Levels (MCL) of 1 0 ug/L Toluene, ethylbenzene and <br /> xylene concentrations are below MCL No MCL has been established for TPH-as-gasoline l <br />' Due to the elevated concentrations of benzene in the groundwater beneath the site, regulatory <br /> closure cannot immediately proceed as hoped An additional concern is the fact that the highest <br />' concentrations of gasoline hydrocarbons in groundwater were found in MW-3 which is <br /> apparently located downgradient of the former Tank A As such, the downgradient extent of <br /> impacted groundwater is not defined <br /> Recent regulatory guidelines issued by the State Water Resources Control Board indicate that <br />' certain sites may be classified as "low risk" and may be closed with concentrations above MCL <br /> after source removal, an evaluation of potential human health nsk factors and a penod of <br /> _ 7 <br /> 1 <br />