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S . Request For Closure,Former Unocal Station#4123,Tracy,CA Apnl 19,2001 1) Contaminants remaining in the vadose zone must not reverse or threaten to reverse the + mass reduction rate of groundwater pollutants,discussed in#4 below. Since March 1997, depth to groundwater at the site has remained consistent, fluctuating between approximately 9 to 10 feet Given that petroleum hydrocarbon concentrations in well U-4 have significantly diminished over that period, and soil samples collected in 1995 suggest that little if any residual petroleum hydrocarbons remain in the vadose zone, there appears to be no threat to reverse the mass reduction rate of groundwater pollutants 2) Separate phase product has been removed to the extent practicable. The USTs, associated product lines and dispenser islands have been removed Soil containing residual hydrocarbons in the vicinity of the northern most UST cluster, except for one UST that has been properly'abandoned in place, was excavated to the extent possible No free product has been detected in any of the wells at this site Given that groundwater has fluctuated between approximately 9 and 10 feet bgs since March 1997, and petroleum hydrocarbons are non-detect in downgradient wells U-6 through U-8, any remaining residual petroleum hydrocarbons in the soil do not appear to be an on-going source of hydrocarbon impact 3) No existing water supply wells, deeper aquifers, surface waters or other receptors are . threatened by pollutants remaining in the aquifer. The nearest downgradient water supply well is located approximately 0 26 mile north of the site, and no surface water is located within 2,000 feet of the site Given that petroleum hydrocarbons are not detected in downgradient wells, and petroleum hydrocarbons detected in well U-4 have diminished significantly since the source (USTs) in that area was removed in 1996, any remaining residual petroleum hydrocarbons at the site do not present a risk to human health or the environment ` 4) The total pollutant mass remaining in the groundwater is decreasing at predicted rates and neither creates, nor threatens to create, a risk to human health and safety or future beneficial use(s)of the aquifer. The graph in Figure 7 shows TPHg and benzene concentrations in well U-4 Both TPHg and `benzene concentrations have diminished significantly since March 1997, and based on their trendlines, TPHg and benzene will reach their detection limits by January 2002, and May 2003, respectively Based on the current lack of consistently detectable hydrocarbon concentrations in monitoring wells at the site except for well U-4, the consistent groundwater flow direction, and distances to the nearest receptors, it appears unlikely that any residual hydrocarbons from the Unocal site represent an existing or potential threat to nearby receptors, and this site does not present a risk to human health and should be closed as low risk • 24007306 7 r