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1 GEOMATRix vary accordingly) The direction of the horizontal hydraulic gradient ranges from ' approximately southeast to southwest A 10,000-gallon UST used for the storage of petroleum fuel was removed from the site on January 22, 1988 under the supervision of PHS/EHD Soil and groundwater assessment activi- ties, including the drilling and logging of soil borings and the installation of groundwater ' monitoring wells, were performed beginning in October 1997 by ATC Associates, Inc and by Haley & Aldrich, Inc on behalf of Conti Seven (7) groundwater monitoring wells (MW-I through MW-7) have been installed at the site and quarterly groundwater monitoring activities have been performed since 1998 Monitoring wells MW-1 through MW-6 have 20-foot well ' screens and are screened between depths of approximately 5 to 28 feet bgs Monitoring well MW-7 is screened across an approximately 30- to 35-foot depth interval ' Groundwater samples collected from monitoring wells installed within the affected soil area (MW-2, MW-3, and MW-4) have consistently been reported by analytical laboratories to be ' impacted by petroleum-fuel related analytes including total petroleum hydrocarbons in the diesel range (TPHd) Less consistently, groundwater samples have been reported to show detections of total petroleum hydrocarbons in the gasoline range (TPHg) Infrequent detections of one or more of the aromatic hydrocarbon compounds (benzene, toluene, ethylbenzene, and xylenes, collectively referred to as BTEX also have been reported in these wells (Table 2) TPHd and TPGg have been detected in groundwater samples collected from wells MW-2, MW-3, and MW-4 at concentrations that exceed water quality objectives (WQOs) established ' by the Regional Water Quality Control Board, Central Valley Region (RWQCB) The WQOs for TPHd and TPHg are 100 micrograms per liter (gg/L) and 5 µg/L, respectively, and are ' based on taste and odor thresholds for drinking water (Table 2) Methyl tertiary butyl ether (MTBE) has not been reported as detected in any soil or groundwater samples collected from the site With the exception p of the June 2004 groundwater sampling event, where reported laboratory ' results are considered anomalous and not representative of historical conditions, and the initial sampling of well MW-1, groundwater samples collected from perimeter monitoring wells MW-I and MW-6 are not reported by the laboratory to have contained detectable con- centrations of petroleum hydrocarbon constituents Monitoring well MW-5 was reported as having low TPH concentrations in June and November/December 2004 Comparison of the ' historical groundwater events to those from June 2004 demonstrate the anomalous nature of the . June 2004 results (see Table 2) 1 DOC_SafC%9000519963 001\ St qtr 2005E TxtVxt doc 2