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V V 1967). Water bearing zones within these deposits in the vicinity of Stockton tend to fan deposits of sand and gravel between finer-grained sands, silts, and clays. This formation has been measured at up to 150 feet thick near the middle of the Central Valley basin, and overlays the Laguna and Mehrten Formations. The Mehrten Formation is generally considered to be the deepest formation with significant supplies of fresh water, and extends to as much as 1700 feet bgs in the Stockton area. r' L„ The Site overlies a portion of the San Joaquin Valley Groundwater Basin. According to the California Department of Water Resources, the Site is within the Eastern San Joaquin Sub basin (5-22.01) as depicted on interagency hydrologic maps prepared by the California Department of Water Resources (DWR) in August 1986. i The Site is approximately three-quarters of a mile south of the Calaveras River, which flows into the San Joaquin Delta, a defined "Surface Water Body" according to the California State �.. Water Resources Control Board's Water Quality Control Plan, Fourth Edition, for the Sacramento and San Joaquin River Basins (Basin Pian) adopted in 1998. The Calaveras River and its major tributaries are defined between the San Joaquin Delta (west of the Site) and the New Hogan Reservoir (east of the Site) as having several beneficial uses including municipal and domestic supply, agricultural, limited recreation, and several types of wildlife habitat, as well as potential usage for industrial process and service supply. Soil from the borings collected beneath the Site generally consists of silt and sand mixtures from the surface to 15 to 30 feet bgs. Soil consisting primarily of sand and gravelly sand was encountered in borings ASB2, ASB3, and ASB4 between 16 and 34 feet bgs. The soil encountered in ASB1 and ASB6 between 16 and 30 to 34 feet bgs consists of broad beds of silty sand, while the soil encountered in A E 5 �1, he same interval consists of narro beds of sand, silty sand, and silt-clay mix urs oil encountered between approximately to_34'3� feet bgs and S2to 60 eet bgs in all six borings generally consists of mixed silts and clays `-' with very little sand. Nfdfi +rve, I n AU y )Vb-' 5.0 GROUNDWATER MONITORING The most recent groundwater elevations in MW1 through MW3 were recorded on October 1, 2008. Groundwater elevations ranged from 56.07 to 56.61 feet bgs corresponding to -38.77 and -38.33 feet above mean sea level (MSL) Since 2004, groundwater elevations have ranged between -26.21 to -45.06 feet above MSL.-The hydraulic gradient, determined using the October 1, 2008, monitoring data, was 0.0020 feet per foot (ft/ft) trending to the 1 aea 7 southeast. Historically, the hydraulic gradient has ranged between 0.0000-M (Septe c1Q�2. /9 +-- 19, 2005) to 0.013 ft/ft (October 13, 2004). A summary of groundwater monitoring data is presented in Table 1. 6.0 CONTAMINANT DISTRIBUTION 6.1 Soil ( Laboratory analytical data from previous soil inve gations indicate the presence of petroleum hydrocarbon constituents (BTEX and TPHg)and fuel oxygenates MTBE and TBA in soil samples collected on-site. Specifically, these constituents were observed in borings r ,... Workplan to Conduct Subsurface Soil Vapor Intrusion Study 0MATC ATC Project 54.26874.0001 Page 3