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feet BGS (-56 feet msl; the total depth of boring MW -9), as shown on cross-section A -A'. f i Groundwater was encountered in the borings at depths ranging from approximately 42 to 52 feet BGS (-23 to -33 feet msl). With the exception of boring H13, groundwater was subsequently measured at approximately 37 to 42 feet BGS (48 to -23 feet ms]). During the October 2006 monitoring event, the depth to groundwater in Site wells ranged from approximately 40 to 42 feet BGS (-21 to -23 feet msl). Historical quarterly monitoring data for r the Site indicate that the overall groundwater flow direction is generally to the east, with a hydraulic gradient of approximately 0.003 foot -per -linear foot. The depth to groundwater has ranged from approximately 40 to 60 feet BGS. The lowest depth to groundwater was reportedly measured at 60 feet BGS, by PSC in December 1992. hi April 2007, the depth to groundwater was approximately 35 to 39 feet BGS (ETIC, 2007b). Recent data indicate that the overall groundwater flow direction is generally to the east beneath the Site at a gradient of approximately 0.002 foot per foot. In 1994, PSC submitted a soil sample collected from boring MW -11 from 55 feet BGS for hydraulic conductivity analysis using ASTM Method D 5084 (Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter). The hydraulic conductivity of this soil sample was approximately 3.2 x 10"5 centimeters per second (cm/s), which is typical for a silty stratum. Pore pressure dissipation tests were performed on three CPT borings (CPT -1, CPT -2, and CPT -3) drilled in May 1999 to provide additional information regarding the aquifer's hydraulic conductivity. Hydraulic conductivities calculated from these CPT data ranged from 4.15 x 10-6 cm/s to 1.04 x 10-5 cm/s (PSC, 2003). In June 1991, Anderson Consulting Group performed a 24-hour pumping test. Groundwater was extracted from well MW -1 at an average rate of 6.7 gpm (ACG, 1991). 2.4 TOPOGRAPHY AND SURFACE WATER The land surface slopes toward the west in the vicinity of the Site at approximately 0.12 foot per 100 feet (USGS, 1987). The elevation of the Site is approximately 19 feet above mean sea level (msl) relative to the North American Vertical Datum (NAVD) of 1988, as surveyed by Morrow Surveying of West Sacramento, California, in July 2006. The Eastern San Joaquin Subbasin is drained by the San Joaquin River and several of its major tributaries namely, the Stanislaus, Calaveras, and Mokelumne rivers. The San Joaquin River flows northward into the Sacramento and San Joaquin Delta and discharges into San Francisco Bay (DWR, 2003). A storm water drainage ditch (approximately 1,000 feet long and less than five feet deep) extends westerly from the southwestern side of the facility towards a south -flowing drainage canal. This canal drains into the ephemeral Duck Creek, which is located about 900 feet south SL_AIR SCM and RI WP 4