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Mr.M. Scott Mansholt—CEMC
<br /> Second Quarter 2006 Groundwater Monitoring and Sampling Report July 11,2006
<br /> Vernalis Pump Station Page 2 of 5
<br /> of 0.25 inch-thick wrapped steel, and was under the control of a subsidiary of Southern Pacific
<br /> Pipeline Company'.
<br /> From 1902 to 1935, Standard Oil(predecessor of Chevron)operated the Vernalis Pump Station,
<br /> which was the part of the former Old Valley Pipeline(OVP) system that transported heated heavy
<br /> petroleum(crude oil) from San Joaquin Valley oil fields in Bakersfield to a refinery in Richmond,
<br /> California. In addition, Bunker C fuel oil was sometimes transported from Richmond to Tracy,
<br /> California. The OVP was located along the northeastern side of the site and parallel to the
<br /> existing UPRR tracks and Highway 33. Site features included a boiler house, a pump house, a
<br /> sump, an incinerator, a rubbish pit, cottages, and three bulk-storage tanks with secondary
<br /> containment structures. These surface structures were located on the Arcos property. The
<br /> property located immediately southeast of the Arcos property was a former Tidewater Associated
<br /> Oil Company(TAOC;predecessor to Texaco and now Chevron)pump station. According to the
<br /> 1995 Environmental Site Assessment,the former pump station was owned by Vernalis
<br /> Enviropur-West in 1995, 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
<br /> Number 5-22.15) and is defined by unconsolidated to semi-consolidated sedimentary deposits
<br /> that are bounded by the Diablo Range on the west,the Mokelumne and San Joaquin rivers on the
<br /> north,the San Joaquin River on the east, and the San Joaquin-Stanislaus County line on the south.
<br /> The Tracy sub-basin is located adjacent to the eastern San Joaquin sub-basin on the east and the
<br /> Delta-Mendota sub-basin on the south.2 These sub-basins are located within the larger San
<br /> Joaquin Valley Groundwater Basin.
<br /> The Tracy sub-basin is comprised of continental deposits of Late Tertiary age to Quaternary age,
<br /> which include the Tulare Formation, Older Alluvium,Flood Basin Deposits, and Younger
<br /> Alluvium. The thickness of these formations ranges from a few hundred feet in the western
<br /> foothills to about 3,000 feet near the eastern margin of the basin. The Tulare Formation is
<br /> comprised of semi-consolidated,poorly sorted,discontinuous deposits of clay, silt, and gravel.
<br /> The Corcoran Clay is situated near the top of the Tulare Formation, separating groundwater in the
<br /> basin in what has been reported as two primary aquifers. The upper aquifer is reported to range
<br /> in thickness from 15 to 250 feet bgs, and the lower aquifer is reported at 600 feet bgs.2
<br /> Locally,the site is within the Central Valley Physiographic Province of California,has a
<br /> relatively flat topography that slopes gently downward to the northeast, and is situated at an
<br /> elevation of approximately 107 feet above mean sea level. Groundwater in the upper
<br /> 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
<br /> sediments. Generally, depth to groundwater near the site varies from 20 to 40 feet bgs. Soils
<br /> encountered appear to be fine-grained unconsolidated sediments, consisting mainly of lean clays
<br /> and silts(CL,ML),clayey and silty sands(SC, SM), and smaller quantities of poorly graded sand
<br /> and gravel(SP, GP).
<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
<br /> 1. Geological Technics,Inc., 1995. Preliminary Environmental Site Assessment:Vernalis Property,35500 Welty
<br /> Road,Vernalis,California. February.
<br /> 2. California Department of Water Resources,2003. California's Groundwater,Bulletin 118,2003 Update.
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