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1.4 REGIONAL GROUNDWATER USE Wells in the area are used primarily to supply water for agricultural and domestic purposes. Of the facilities in the area, the California Youth Authority and the Northern California Women's Facility both have wells that are thought to be used to supply drinking water. Review of Department of Water Resources (DWR) files (Beta Associates, 1988) indicates that 20 irrigation and domestic wells exist within 1 mile of the FSL(Table 1; Figure 2). The record review indicates that 9 of these wells are screened within 65 feet of the ground surface, and that 7 wells are screened below 130 feet. Only 6 of the 19 wells are down- or cross-gradient of the FSL (Table 1, Figure 2). Of these downgradient offsite wells, the nearest well (+ 200 feet) was recently purchased by Forward Inc., and bottled water is delivered to the tenants of the property. The next closest wells are approximately '/2-mile north, 1-mile northeast, and 1-mile east (cross- gradient) of the FSL. 1.5 MONITORING NETWORK The monitoring network at the FSL includes 19 groundwater monitoring wells, 12 vadose zone lysimeters, and 10 perimeter gas monitoring probes (Figure 1). Ten of the monitoring wells are positioned in well pairs (e.g., MW-1A, MW-1B) to accommodate fluctuating groundwater elevations in the area. The groundwater monitoring wells were typically constructed to depths between 70 and 110 feet. Except for the background lysimeter, which was constructed to a depth of about 10 feet, the lysimeters at the site were generally constructed to depths of 1 to 2 feet below the base of adjacent landfill cells. The perimeter gas probes were installed to depths of only approximately 5 feet. The two nearest offsite wells at 9621 and 9690 S. Austin Road are also monitored regularly. 2.0 VC IMPACTS 2.1 GROUNDWATER iVOCs have been detected sporadically, and at low concentrations, in monitoring well samples at the site since groundwater monitoring began in 1987. Before their abandonment in 1993, VOCs were most commonly detected in monitoring wells MW-5A and MW-513, near the southern border of U-B (Figure 1). Since then, VOCs have been most commonly detected at the northeast corner of the property, near the downgradient limits of the facility. As listed below, other than acetone and methylene chloride (common laboratory contaminants), VOCs were detected in samples from 8 monitoring wells during 1999 and 2000. VOCs were most commonly detected at trace levels (between the laboratory method detection limit [MDL] and practical quantitation limit [PQL]) and in samples 3 Geologic Associates