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Kennedy/Jenks Consultants <br /> In northern OU-5 elevated arsenic concentrations (>50 mg/kg) accompany elevated lead <br /> concentrations, typically are found in surface soil samples, and may be related to slag ballast <br /> fragments observed on the ground surface. However, over most of the Railyard, the arsenic <br /> concentrations in soil range between about 2 mg/kg and 10 mg/kg with relatively few samples <br /> containing arsenic concentrations between 10 mg/kg and 20 mg/kg, suggesting that the site <br /> background arsenic concentration may be in the range of 2 mg/kg to 10 mg/kg. <br /> tSome of the surface samples at the former Railyard have probably been impacted with arsenic. <br /> Deeper samples are considered more representative of naturally-occurring arsenic <br /> concentrations. The mean arsenic concentration of the deeper soil samples (the non-surface <br /> samples) is 5.2 mg/kg. The mean arsenic concentration of all of the surface samples (including <br /> those with elevated concentrations) is 16.8 mg/kg. <br /> The mean arsenic concentration of 5.2 mg/kg in deeper samples at the Railyard is consistent <br /> with the published mean background concentrations of arsenic in California and western U.S. <br /> soils. This further supports the conclusion that arsenic concentrations found in the deeper <br /> samples are naturally occurring. The 95% upper tolerance limits was estimated for the deeper <br /> samples using ChemstatTM v. 6.0.0 software. The 95% upper tolerance limit on the mean, based <br /> on the deeper samples, is 9.24 mg/kg. Samples with arsenic exceeding this concentration would <br /> be evaluated as statistically different from the background samples. <br /> • <br /> DRAFT Phase I Remedial Investigation Report Page 38 <br /> Former Tracy Railyard, Tracy, California <br /> g:lis-groupladmingob10510565777.18_uprr109-reportslrirpAtext.doc <br />