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. 4 3.1 Direct Contact with Diesel Fuel Under current site conditions, direct contact with diesel fuel in surface soil is limited by the fact that (1) the top 5 feet of soil is not impacted by diesel fuel, and (2) future plans for the site consist of development of a connection railroad track between two perpendicular tracks, eliminating public access Accordingly, exposure via direct dermal contact does not represent a complete pathway and will not be addressed in the assessment 4.3 2 Inhalation of Diesel Fuel Vapors Exposure to diesel fuel vapors is limited by the fact that 1) volatile compounds have not been detected at the site, 2) the soil partitioning studies demonstrated the absence of volatile organic compounds, 3) the site is outside with sufficient dispersion to dilute any vapors; 4) diesel, presumed to be the source, has very low volatile organic compound levels, and is generally not associated with vapor emissions, and 5) the age of the petroleum hydrocarbons associated with loss and degradation of any volatile organic compounds support the data findings 4 3.3 Migration to Groundwater The potential for migration of diesel fuel through soil to groundwater will be determined by environmental fate and transport modeling (Subsection 4 4) If the results show that one of the most mobile components of the TPH at the site (i e , tetradecane) will not reach groundwater, this pathway will be considered incomplete However, if groundwater impacts are predicted, exposures to diesel fuel in groundwater will be quantified 4.4 Evaluation of Exposure Point Concentrations Chemical concentrations in environmental media at the point of exposure must be estimated to calculate exposure In many cases, concentrations cannot be measured because of physical limitations (i e , under a building), time limitations (i.e , chemicals do not migrate to a location for 50 years), or limits of detection (i.e., exposure levels are below analytical limits) In the case of the Santa Fe Railway Company site, there is currently no groundwater impact, but to be protective, future impacts are evaluated Therefore, environmental fate and transport modeling was used to estimate the rate of petroleum W=»AW"E FNLU=W 26 0 4-4 ewe, 19"