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Mr.M. Scott Mansholt—CEMC First Quarter 2007 Groundwater Monitoring and Sampling Report April 24,2007 US Can-Welty Road Page 2 of 6 Farm Service (WFS)is located directly northeast of the site across Highway 33 and has stored and distributed bulk liquid and dry fertilizers since 1969. In addition, a former gas station is located northeast of the site. REGIONAL/LOCAL HYDROGEOLOGY Regionally,the site is encompassed within the Tracy sub-basin(Groundwater Basin Number 5- 22.15) and is defined by unconsolidated to semi-consolidated sedimentary deposits that are 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 on the east and the Delta- Mendota sub-basin on the south.' These sub-basins are located within the larger San Joaquin Valley Groundwater Basin. The Tracy sub-basin is comprised of continental deposits of Late Tertiary to Quaternary age, including 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 below ground surface(bgs), and the lower aquifer is reported at 600 feet bgs.' Locally,the site is situated at an approximate elevation of 103 to 105 feet above mean sea level (MSL), and has a relatively flat topography that slopes gently downward to the northeast. Groundwater in the upper water-bearing zone flows in a northeasterly direction at an approximate gradient of 0.001 feet per foot(ft/ft)and is hydraulically controlled by low-permeable, unconsolidated sediments. Based on ongoing groundwater monitoring at the site, depth to groundwater varies from 20 to 40 feet bgs. Soils encountered appear to be fine-grained unconsolidated sediments, consisting mainly of lean clays and silts (CL,ML)and clayey and silty sands, (SC, SM)with smaller quantities of poorly graded sand and gravel(SP, GP). PETROLEUM DISTRIBUTION IN GROUNDWATER The RWQCB water quality objectives (WQOs) include total petroleum hydrocarbon(TPH) measurements for diesel products,but not for crude oil/Bunker C fuel oil. According to the Technical Approach to Site Evaluation and Decision-Making for the Historical Pipeline Portfolio (referred to as the Consistent Technical Approach [CTA]), diesel products are not associated with the Historical Pipeline Portfolio(HPP) sites and typically are not present at these sites.2 However,WQOs are applied to"dieselrange hydrocarbons"(extractable TPH, C107-C25) found in HPP site groundwater samples because no WQOs are established for crude oil/Bunker C fuel oil, and historical data indicates that diesel-range hydrocarbons are often detected at sites affected by crude oil/Bunker C fuel oil. Per the CTA, groundwater samples are quantified against a diesel standard so that the comparison to the WQOs is as direct as possible. 1. California Department of Water Resources,2003. California's Groundwater,Bulletin 118,2003 Update. 2. Geomatrix Consultants,Inc.,2005. Technical Approach to Site Evaluation and Decision-Making,Historical Pipeline Portfolio, Central Valley Region.February.