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6.0 CONCLUSIONS <br /> This risk assessment evaluated the potential site-related environmental impacts and human <br /> health risks associated with the presence of petroleum hydrocarbons in soil beneath the <br /> Santa Fe Railway Company site in Stockton, California. The potential health risks have <br /> been evaluated by estimating the migration of petroleum hydrocarbons in site soil <br /> This risk assessment is based upon extensive site-specific data including 1) soil <br /> investigations, 2) soil geophysical data, and 3) partitioning studies Previous soil <br /> investigations have characterized the horizontal and vertical extent of petroleum <br /> hydrocarbon-impacted soil Soil samples were obtained to determine site-specific <br /> geophysical parameters such as bulk density, intrinsic permeability, moisture content, and <br /> organic carbon contents Partitioning studies were conducted to simulate the migration of <br /> petroleum hydrocarbons through soil by determining the chemical constituents of the <br /> petroleum hydrocarbons which may partition into percolating water These studies were <br /> developed to provide an overestimate of the amount of chemicals partitioning into <br /> percolating water <br /> The straight-chained alkane, tetradecane, the smallest and most water soluble chemical <br /> found in the water from the partitioning studies was modeled as a conservative chemical <br /> surrogate for the entire petroleum hydrocarbon mixture Other components of petroleum <br /> hydrocarbons would be expected to move much slower than tetradecane The highest <br /> concentration of petroleum hydrocarbons detected in each layer was used as the total <br /> uniform concentration within that layer <br /> The results of the modeling demonstrate that the petroleum hydrocarbons have only limited <br /> migration potential in site soil Over a 99-year period, tetradecane, used as a conservative <br /> surrogate chemical, migrated less than one foot Therefore, it is concluded that the <br /> petroleum hydrocarbons will not impact the groundwater beneath the site. Therefore, there <br /> are no complete exposure pathways at the site. In addition, the most recent groundwater <br /> sampling conducted by ERM-West (January, 1993), reporting non-detect results for total <br /> extractable petroleum hydrocarbons for the three on-site wells, confirms these findings <br /> G ro6M IMAWAFE FNL%b&W 26 93 b-1 Chcaaak• 19" <br />