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December 22, 1998 Harding Lawson Associates <br /> 4034405 <br /> Margaret Lagono, Supervising RENS <br /> San Joaquin County Public Health Services <br /> Environmental Health Division <br /> ' Page 8 <br /> Risk Based Screening Levels* for Commercial/Industrial Receptors and <br /> Maximum Detected Concentrations for <br /> Chemicals of Concern COCs <br /> COC Ingestion/dermal Inhalation from Maximum <br />' /inhalation from Subsurface Soils Detected <br /> Surface Soils (mg/kg) Concentrations <br /> (m (mg/kg) <br /> Benzene 2 9E+00 1 32E-01 <1 OOE-2 <br /> Toluene 1 15E+04 RES 1 OOE-1 <br /> Ethylbenzene 187E+04 RES 110E-2 <br />' Total xylenes 2 08E+05 RES 1 60E-1 <br /> Napthalene 1 90E+03 RES c5 OE-01 <br /> Benzo (a) Pyrene 4 74E-01 RES <2 5E-01 <br />' Notes <br /> * = Associated with a target risk level of one-in-a-million or hazard quotients of unity for non-carcinogens <br /> RES = Selected risk level is not exceeded for pure compound at any concentration <br /> = Analyte concentration is not detected at (is less than) indicated reporting limit _ <br /> ASTM RBSLs were adjusted to reflect Cal/FTA dope factors (Cal/EPA, 1994) <br /> As is shown in the table above, none of the maximum concentrations of COCl on Site exceed their <br /> respective RBSLs for the RPESs identified for the Site On the basis of this Tier 1 evaluation and the <br /> inherent conservative nature of default assumptions made, the impacted soils on Site are unlikely to pose <br /> a significant risk to human health or safety <br />' DISCUSSION <br />' As previously discussed above, the concrete USTs and sump were closed in place by RESNA under the <br /> supervision and guidance of PHS/EHD personnel by backfilling with a cement slurry in May 1994 <br /> (RESNA, 1994) Prior to closure activities, AEMC collected two soil samples from each of three soil <br /> borings angled-drilled beneath the USTs/sump in July 1990 (AEMC, 1990) TPHd was detected in two of <br /> the six samples submitted for analysis and no BTEX was reported in any sample A TPHd concentration <br /> of 370 mg/kg was detected beneath the eastern end of the sump in the 7-foot bgs sample (Plate 7) and 730 <br /> mg/kg TPHd was detected in a sample near the eastern side of the USTs at a depth of approximately 10 5 <br /> feet bgs (Plate 8) <br /> RESNA collected six additional samples in November 1993 consisting of two samples from beneath each <br /> of the USTs and the sump The maximum TPHd concentration of 600 mg/kg was detected in the sample <br /> collected beneath the eastern end of the sump at a depth of eight feet bgs (Plate 7) This same sample <br /> reportedly contained 1 3 milligrams per liter (mg/L) of soluble TPH quantified against a diesel standard <br /> The laboratory rioted that the TPH reported was a non-diesel mix and it is unknown if the extraction <br /> method used an acid of deionized water for this analyses As discussed below, the reported 1 3 mg/L of <br /> soluble TPHd, is in conflict with the soluble data collected during this investigation <br /> . One of the objectives of this investigation was to further assess the vertical extent of soil contamination <br />' near the source with boring SB-1 Impacted soils were encountered at depths of 5, 10, and 15 feet bgs in <br />