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' Mr. M. Scott Mansholt—CEMC
<br /> First Quarter 2007 Groundwater Monitoring and Sampling Report April 24,2007
<br /> ' Vernalis Pump Station Page 2 of 5
<br /> ground surface (bgs)during 1985,was constructed of 0.25 inch-thick wrapped steel, and was under the
<br /> control of a subsidiary of Southern Pacific Pipeline 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, and was used for bulk storage of petroleum and oil 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.z
<br /> 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 towards the east to southeast 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 /> ' PETROLEUM DISTRIBUTION IN GROUNDWATER
<br /> The RWQCB water quality objectives(WQOs) include total petroleum hydrocarbons (TPH)
<br /> measurements for diesel products, but not for crude oil/Bunker C fuel oil. According to the Technical
<br /> Approach to Site Evaluation and Decision-Making for the Historical Pipeline Portfolio(referred to as the
<br /> Consistent Technical Approach[CTA]), diesel products are not associated with the Historical Pipeline
<br /> Portfolio(HPP) sites and typically are not present at these sites.' However, WQOs are applied to diesel-
<br /> '
<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.
<br /> 3. Geomatrix Consultants,Inc.,2005. Technical Approach to Site Evaluation and Decision-Making,Historical Pipeline
<br /> Portfolio, Central Valley Region. February.
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