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Antonia Vorster -2 - 30 July 2010
<br /> Chevron Coral Hollow and Grant ` : Roads
<br /> Tracy, Contra Costa County
<br /> the City of Tracy. Chevron repaired the pipeline and recovered about 840 gallons of fuel. Figure 1
<br /> shows the Site plan.
<br /> Actions: Investigation commenced in 1987.
<br /> C In 1987, Chevron excavated about 2,100 cubic yards of contaminated soil during repair of the
<br /> pipeline and installed three monitoring wells. One soil sample collected from the floor of the
<br /> excavation (20 feet below ground surface [bgs]) contained total petroleum hydrocarbons as
<br /> gasoline (TPHg) and TPH as diesel (TPHd) at 2 milligrams per kilogram (mg/kg) and
<br /> 840 mg/kg, respectively. Groundwater samples collected from the new monitoring wells did not
<br /> contain 3etroleum hydrocarbons.
<br /> 0 Between 1991 and 1995, Chevron drilled 10 monitoring wells and 17 direct push soil borings
<br /> north and south of Grant Line Road at depths of between 16 and 28 feet bgs. The maximum
<br /> soil concentrations were detected in the soil boring that was converted to monitoring well
<br /> MW-6. Monitoring well MW-7 contained the maximum TPHg and benzene concentrations at
<br /> 120,OOC micrograms per liter (Ng/L) and 16,000 pg/L, respectively. Tables 1 through 4 present
<br /> the soil, groundwater, and grab groundwater concentrations.
<br /> O In 1994 in conjunction with a dewatering project by the City of Tracy, Chevron extracted
<br /> groundwater from wells MW-6, MW-7, MW-12, and MW-13 to reduce the potential for polluted
<br /> groundwater to migrate to the dewatering wells. Chevron extracted about 300,000 gallons of
<br /> water during the 22-day event and discharged it to the City sewer system under permit. No
<br /> petroleum hydrocarbons were detected in the dewatering wells or in soil excavated during the
<br /> construction project. Groundwater samples collected one month after completion of the
<br /> extraction event showed increased concentrations in MW-5, MW-6, and MW-13 and decreased
<br /> concentrations in MW-7, MW-8, and MW-12.
<br /> C In 1996, Chevron drilled a soil boring to 50 feet bgs and collected a grab groundwater sample
<br /> to evaluate the vertical extent of groundwater pollution. TPHg and xylenes were detected at
<br /> 80 pg/L and 1.4 pg/L, respectively. Chevron then backfilled the boring to 26 feet bgs, installed
<br /> monitoring well MW-14, and sampled it for petroleum hydrocarbons. No petroleum
<br /> hydrocarbons were detected above laboratory detection limits.
<br /> O In March 1997, Chevron installed a dual phase extraction (DPE) system and operated it until
<br /> Novemoer. The system treated about 3,288,000 gallons of groundwater and removed an
<br /> estimated 440 pounds of hydrocarbons. Central Valley Water Board staff approved the
<br /> shutdown of the system when operation was no longer considered cost effective.
<br /> C Between 1991 and 2001, Chevron properly abandoned monitoring wells MW-1, MW-2, MW-3,
<br /> MW-4, MW-9, MW-10, MW-11, MW-13, and MW-14 to accommodate the expansion of Grant
<br /> Line Road and development of the property to the north. MW-1 contained sporadic hits of oil
<br /> and grE!ase in 1989. Petroleum hydrocarbon concentrations in the other wells were below
<br /> laboratory detection limits for a year prior to destruction.
<br /> O BetweE-n 1987 and 2010, Chevron conducted 40 monitoring events. Monitoring wells
<br /> MW-6, MW-8, and MW-12 are currently monitored semi-annually according to Monitoring and
<br /> Reporting Program No. R5-2006-0804 (MRP). MW-5 and MW-7 are no longer sampled.
<br /> Depth to water is about eight feet bgs, and flow is to the east. During the May 2010 sampling
<br /> event, .he highest concentrations of TPHg, TPHd, and benzene were 1,600 pg/L, 2,500 pg/L,
<br /> and 2 tag/L, respectively. MW-8 has not contained TPHg or benzene above the WQOs since
<br /> 2006. Concentrations in MW-12 were below the WQOs from 2007 until the November 2009
<br /> sampling event. MW-6 has had stable concentrations above the WQOs since 2007. Table 3
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