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' Mr.M. Scott Mansholt—CEMC
<br /> Fourth Quarter 2006 Groundwater Monitoring and Sampling Report and
<br /> Annual Summary Report January 12,2007
<br /> Vernalis Pump Station Page 2 of 7
<br /> 0.25 inch-thick wrapped steel, and was under the control of a subsidiary of Southern Pacific Pipeline
<br /> ' Company'.
<br /> From 1902 to 1935, Standard Oil(predecessor of Chevron)operated the Vernalis Pump Station,which
<br /> was the part of the former Old Valley Pipeline(OVP)system that transported heated heavy petroleum
<br /> (crude oil)from San Joaquin Valley oil fields in Bakersfield to a refinery in Richmond, California. The
<br /> OVP was located along the northeastern side of the site and parallel to the existing UPRR tracks and
<br /> Highway 33. Site features included a boiler house, a pump house,a sump,an incinerator, a rubbish pit,
<br /> ' cottages, and three bulk-storage tanks with secondary containment structures. These surface structures
<br /> were located on the Arcos property. The property located immediately southeast of the Arcos property
<br /> was a former Tidewater Associated Oil Company(TAOC; predecessor to Texaco and now Chevron)
<br /> ' pump station. According to the 1995 Environmental Site Assessment, the former pump station was
<br /> owned by Vernalis Enviropur-West in 1995,and was used for bulk storage of petroleum and oil
<br /> products'.
<br /> REGIONAL/LOCAL HYDROGEOLOGY
<br /> Regionally, the study area is encompassed within the Tracy sub-basin(Groundwater Basin Number
<br /> 5-22.15)and is defined by unconsolidated to semi-consolidated sedimentary deposits that are bounded by
<br /> the Diablo Range on the west,the Mokelumne and San Joaquin rivers on the north,the San Joaquin River
<br /> on the east,and the San Joaquin-Stanislaus County line on the south. The Tracy sub-basin is located
<br /> adjacent to the eastern San Joaquin sub-basin on the east and the Delta-Mendota sub-basin on the south.2
<br /> These sub-basins are located within the larger San Joaquin Valley Groundwater Basin.
<br /> The Tracy sub-basin is comprised of continental deposits of Late Tertiary age to Quaternary age,which
<br /> include the Tulare Formation, Older Alluvium,Flood Basin Deposits,and Younger Alluvium. The
<br /> thickness of these formations ranges from a few hundred feet in the western foothills to about 3,000 feet
<br /> near the eastern margin of the basin. The Tulare Formation is comprised of semi-consolidated,poorly
<br /> sorted,discontinuous deposits of clay, silt,and gravel. The Corcoran Clay is situated near the top of the
<br /> Tulare Formation, separating groundwater in the basin in what has been reported as two primary aquifers.
<br /> The upper aquifer is reported to range in thickness from 15 to 250 feet bgs,and the lower aquifer is
<br /> reported at 600 feet bgs.2
<br /> 1 Locally, the site has a relatively flat topography that slopes gently downward to the northeast, and is
<br /> situated at an elevation of approximately 107 feet above mean sea level. Soils encountered appear to be
<br /> fine-grained unconsolidated sediments, consisting mainly of lean clays and silts(CL,ML), clayey and
<br /> ' silty sands(SC, SM), and smaller quantities of poorly graded sand and gravel(SP,GP). Groundwater in
<br /> the upper water-bearing zone flows in a northeasterly direction at an average gradient of approximately
<br /> 0.001 feet per foot(ft/ft)and is hydraulically controlled by low-permeable,unconsolidated sediments.
<br /> Generally, depth to groundwater near the site varies from 20 to 40 feet bgs.
<br /> PREVIOUS ENVIRONMENTAL ACTIVITIES
<br /> ' Bellini Property
<br /> 1994 and 1995 Site Investigations.' Geological Technics, Inc.performed a Preliminary Environmental
<br /> Site Assessment. The site assessment included review of public records, aerial photographs, on-site
<br /> reconnaissance,personal interviews, and subsurface investigation. On January 20, 1995, Osterberg and
<br /> Stewart Inc.,under the supervision of Geological Technics,drilled two soil borings, SB-1 and SB-2.
<br /> 1. Geological Technics,Inc., 1995. Preliminary Environmental Site Assessment: Vernalis Property,35500 Welty Road,
<br /> Vernalis, California. February.
<br /> 2. California Department of Water Resources,2003. California's Groundwater,Bulletin 118,2003 Update.
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