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Mr. James L. Barton <br /> November 27, 2006 <br /> Page 2 <br /> • The extent of hydrocarbon impacted groundwater attributable to the Site is defined by the existing <br /> monitoring well network. In consideration of the rate of groundwater movement beneath the Site <br /> and the lateral footprint covered by the existing monitoring well network, less frequent monitojing <br /> will enable detection of any changes in the transport of hydrocarbon constituents. <br /> • The abandonment of the former Robinhood Irrigation Well (RIW) and subsequent replacement <br /> with a new irrigation well to service the needs of San Joaquin Delta College have resulted in the <br /> stabilization of the natural groundwater flow direction (easterly) and gradient associated with the <br /> hydrocarbon plume. <br /> • Historic trends in hydrocarbon constituent concentrations show that the plume's lateral extent is <br /> stable. The active remediation system operation has resulted in hydrocarbon mass reduction in <br /> groundwater at the Site. <br /> Table 1, below, lists the criteria used in evaluating the extents and the time trends of hydrocarbon <br /> impacted groundwater at the Site and the associated justifications and recommendations for changes to <br /> the groundwater monitoring program. <br /> Table 1 <br /> Water Quality Criteria Used to Evaluate Groundwater Impacts and Justify Monitoring Changes <br /> Constituent Water Quality Driver Concentration in <br /> Micrograms Per Liter <br /> (NgfL) <br /> Total Petroleum Hydrocarbons as San Francisco Bay Regional Water Quality 100 <br /> Gasoline (TPHg) Control Board, Environmental Screening <br /> Level (where groundwater is a potential <br /> drinking water source), Taste and Odor <br /> Threshold <br /> Benzene California Primary Maximum Contaminant 1.0 <br /> Level (MCL) <br /> _ Toluene California Primary MCL 150 <br /> Ethylbenzene California Primary MCL 300 <br /> Xylenes (total) California Primary MCL 1,750 <br /> Ethylene dibromide (EDB) California Primary MCL 0.05 <br /> 1,2-Dichloroethane (1,2 DCA) California Primary MCL 0.5 <br /> Methyl t-butyl ether (MTBE) California Primary MCL 13 <br /> 1.1 Stabilization of the Natural Groundwater Flow Direction and Gradient <br /> The lateral and vertical extent of hydrocarbon-impacted groundwater associated with the Site resulted <br /> from the operation of the former RIW located at the San Joaquin Delta College. The former RIW <br /> ENSR AECOM <br />