ARCHIVED REPORTS_ANNUAL 2009 GROUNDWATER MONITORING AND SAMPLING REPORT

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Mr.M. Scott Mansholt CEMC ' Annual 2009 Groundwater Monitoring and Sampling Report April 15,2009 Alden Park Page 2 of 6 The reservoir was emptied, cleaned, and decommissioned after operations ceased in 1957. The area was sold and used for farming throughout the 1960s. Residential demand in Tracy increased, and the area was developed by Renown Enterprises into the Rancho Pacifica, Westchester Green (in the 1970s), and Tennis Villa(in the 1980s)housing developments. The present park (Alden Park) was constructed during the development of Rancho Pacifica. ' REGIONAL/LOCAL HYDROGEOLOGY The site is within the Tracy Sub-basin (Groundwater Basin Number 5-22.15) and is defined by the areal extent of unconsolidated to semi-consolidated sedimentary deposits bounded by the ' Diablo Range on the west, the Mokelumne and San Joaquin rivers on the north, the San Joaquin River on the east, and the San Joaquin-Stanislaus County line on the south. The Tracy Sub-basin is located adjacent to the eastern San Joaquin Sub-basin to the east and the Delta-Mendota Sub- basin to the south.' These sub-basins are located within the larger San Joaquin Valley Groundwater Basin. ' The Tracy Sub-basin consists of continental deposits of Late Tertiary to Quaternary age, which include the Tulare Formation, Older Alluvium, Flood Basin Deposits, and Younger Alluvium. The thickness of these formations ranges from a few hundred feet in the western foothills to ' about 3,000 feet near the eastern margin of the basin. The Tulare Formation is comprised of semi-consolidated, poorly sorted, discontinuous deposits of clay, silt, and gravel. The Corcoran Clay is situated near the top of the Tulare Formation, separating groundwater in the basin in what ' has been reported as two primary aquifers. The upper aquifer is reported to range in thickness from 15 to 250 feet bgs, and the lower aquifer is reported at 600 feet bgs. Generally, depth to groundwater in the Tracy sub-basin varies from 5 to 10 feet bgs'. Groundwater flow at the site remains relatively consistent to the north to north-northwest at a gradient of approximately 0.003 feet per foot (ft/ft). PETROLEUM DISTRIBUTION IN GROUNDWATER ' The petroleum present in the groundwater is highly degraded Bunker C oil in both the separate and dissolved phases. The presence of dissolved-phase Bunker C oil is indicated by total petroleum hydrocarbons quantified as diesel (TPHd) analyses. Both SPO and dissolved-phase oil are limited to the vicinity of Alden Park, and extend down gradient in the direction of MW-5, MW-6, and MW-7. Low levels of polynuclear aromatic hydrocarbons (PAHs) are present in the down-gradient to cross-gradient direction. The constituents of potential concern (COPCs) at the site are TPHd and PAHs. The distribution and extent of SPO and dissolved hydrocarbons has shown very little variation since monitoring began in 1995. The plume of affected groundwater I� is stable. PREVIOUS ENVIRONMENTAL ACTIVITIES I' Several environmental investigations have been conducted at the site since 1971, and are briefly discussed as follows: • Site assessments were conducted to determine the extent of hydrocarbon-affected soil and ' groundwater in May 1971, October 1986, September 1987, and April 1996. An area of hydrocarbon-affected soil and groundwater centered at the former reservoir location was 1. Department of Water Resources(DWR),2003. California's Groundwater,Bulletin 118,2003 Update.