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Ms.Lori Duncan
<br /> November 15,2011
<br /> RE:2911 E.Fremont St,Stockton,CA
<br /> Page 7 of 14
<br /> levels of tetrachloroethene (PCE)were detected in groundwater samples at levels below the Maximum
<br /> Contaminant Level (MCL) of 5.0 micrograms per liter.
<br /> Contaminant concentrations of residual contaminants left in place,based upon analytical results of soil
<br /> samples collected from soil borings at a depth greater than 18.5 feet(the depth of the excavation) and
<br /> confirmation soil samples collected from the excavation floor and sidewalls, were compared to the US
<br /> EPA Region 9 Regional Screening Levels(RSLs)for commercial/industrial property(November 2010).
<br /> The RSLs were prepared based upon human health risk"to provide default screening tables to assist in
<br /> decision-making concerning CERCLA hazardous waste sites and to determine whether levels of
<br /> contamination found at the site may warrant further investigation or site cleanup, or whether no further
<br /> investigation or action may be required."
<br /> In comparing site concentrations to RSLs, the following can be noted:
<br /> • The soil sample collected from boring B2 at a depth of 20 feet bgs(collected April 2010 prior to
<br /> site excavation activities)contained ethylbenzene,naphthalene,and 1,1,2,2-tetrachloroethane at
<br /> concentrations exceeding their respective RSLs.
<br /> • The soil samples collected from the excavation floor at a depth of approximately 18.5 feet bgs
<br /> contained 1,1,2-trichloroethane at concentrations exceeding its RSL.
<br /> • The soil sample collected from the south excavation wall at a depth of approximately 14 feet bgs
<br /> contained naphthalene, 1,1,2,2-tetrachloroethane, and 1,1,2-trichloroethane at concentrations
<br /> exceeding their respective RSLs.
<br /> The above comparison suggests that residual concentrations of ethylbenzene, naphthalene, 1,1,2,2-
<br /> tetrachloroethane,and 1,1,2-trichloroethane in soil at a depth of approximately 18.5-20 feet,as well as in
<br /> the south wall of the excavated area, may pose a human health risk.
<br /> For further evaluation,concentrations of residual co to ants were also compared to the Environmental
<br /> Screening Levels(ESLs)established by the Californ�ter Quality Control Board, San Francisco Bay
<br /> Region (RWQCB SF Bay, November 2007). The ESLs were developed to address environmental
<br /> protection goals presented in the Water Quality Control Plan for the San Francisco Bay Basin, but are
<br /> generally used as a screening tool throughout California. The ESLs are considered to be conservative.
<br /> The RWQCB SF Bay guidance document states that under most circumstances "the presence of a
<br /> chemical in soil,soil gas or groundwater at concentrations below the corresponding ESL can be assumed
<br /> to not pose a significant, long-term(chronic) threat to human health and the environment. Additional
<br /> evaluation will generally be necessary at sites where a chemical is present at concentrations above the
<br /> corresponding ESL.Active remediation may or may not be required depending on site-specific conditions
<br /> and considerations."
<br /> In comparing site concentrations to ESLs, several samples from depths between 18.5 and 20 feet
<br /> contained concentrations of toluene,ethylbenzene,xylenes,naphthalene, 1,1,2,2-tetrachloroethane,and
<br /> 1,1,2-trichloroethane above their respective ESLs. The final ESLs are generally based upon protection oV
<br /> groundwater, in other words the potential of the contaminant to leach into drinking water sources. In
<br /> 10759-EXC SUMMARY LTR
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