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Phase II Environmental Site Assessment Manteca Multimodal Station Project, Southeast of Moffat Boulevard and Main Street Manteca, California Total Threshold Limit Concentrations (TTLC). Soluble Threshold Limit Concentrations (STLCs) and Toxicity Characteristic Leaching Procedure (TCLP): TTLCs and STLCs are standards set by the State of California Code of Regulations (CCR), Title 22, Chapter 11. TTLCs represent the total concentration of a constituent that may be present before a waste is classified as a California hazardous waste. STLCs represent the soluble concentration of a constituent that may be present before a waste is classified as a California hazardous waste. TCLPs are hazardous waste thresholds set by the USEPA. TCLPs represent the soluble concentration of a constituent that may be present before a waste is classified as a RCRA(Federal) hazardous waste. Comparison to Background Concentrations As shown in Table 3,the detected concentrations of metals were compared to naturally occurring background concentrations found in California soils(Bradford et al., March 1996). The detected concentrations of metals are within normal background ranges, except for the following: Zinc: Background concentrations of zinc found in California soils (non-contaminated sites) range from 88 to 236 mg/kg(Bradford et al.,March 1996). . The concentration of zinc detected in RB 14 at 0.5 feet below grade (601 mg/kg) exceeds the background range. Further discussion regarding the concentration of zinc in RB 14 is below. Lead: In addition, elevated levels of lead were detected in two soil samples(86.5 mg/kg in RB4 and 68.1 mg/kg RB 14). Although these concentrations are within the range of naturally occurring background concentrations found in California soils (12.4 to 97.1 mg/kg), in comparison to the lead concentrations detected in the other 15 soil samples collected from the site(1.85 to 26.6 mg/kg) these elevated concentrations appear anomalous and do not appear to be consistent with the background ranges at the site. Further discussion regarding the concentrations of lead in R134 and RB 14 is below. Comparison to SLs and CHHSLs In addition to the naturally occurring background ranges,the metal concentrations were compared to SLs and CHHSLs. None of the detected levels of metals exceeded their respective SLs or CHHSLs in residential or commercial/industrial soils, except for the following: • Arsenic: The detected concentrations of arsenic in soil samples collected from the site (ranging from 1.15 to 5.68 mg/kg)exceed the SLs and CHHSLs for arsenic in residential and commercial/industrial soil. However, for arsenic,normal background concentrations found in California soils are typically above the SLs and CHHSLs for both residential and commercial/industrial settings. Background concentrations of arsenic found in California soils (non-contaminated sites) range from 0.6 to 11.0 mg/kg and the arithmetic mean for arsenic in California soils (non-contaminated sites) is 3.5 mg/kg(Bradford et al.,March 1996). The SLs for residential and industrial settings for arsenic are 0.39 and 1.6 mg/kg,respectively. The CHHSLs for residential and industrial settings for arsenic are 0.07 and 0.24 mg/kg, respectively. The USEPA states that generally they do not require cleanup below natural background levels. In light of this fact and in our experience,regulatory agencies typically consider the use of local or regional background concentrations as the threshold concentration. The detected concentrations of arsenic in soil samples collected from the site (1.15 to 5.68 mg/kg) fall within the range of normal background concentrations of arsenic found in California soils (0.6 to 11.0 mg/kg). Rincon Consultants 12