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5.0 RISK CHARACTERIZATION <br /> This section of the risk assessment integrates the exposure assessment and the toxicity <br /> assessment into a quantitative characterization of potential health risk In addition, the risk <br /> characterization presents a discussion of the conservatism and uncertainty in the various <br /> components of the risk assessment including the toxicity assessment, environmental fate and <br /> transport modeling, and the exposure assessment. The focus of this risk assessment was on <br /> estimating the migration potential of petroleum hydrocarbons in site soil. The <br /> environmental fate and transport modeling predicted that the petroleum hydrocarbons would <br /> not impact groundwater The focus of this section is on the results and the uncertainty of <br /> the modeling approach <br /> 5.1 Uncertainty Analysis <br /> This risk assessment is based on the use of conservative methods and assumptions in all <br /> phases of the assessment Because direct measurements were not available for some of the <br /> criteria used in the environmental fate and transport modeling, conservative assumptions <br /> and methodology were necessarily employed to eliminate the possibility of underestimating <br /> migration and potential exposures This practice, although commonly used in risk <br /> assessment, introduces a significant level of conservatism into the risk assessment results by <br /> relying on upperbound values rather than statistically-based best estimate values In order <br /> to completely characterize any potential environmental impact, this uncertainty must be <br /> taken into consideration by the risk manager The petroleum hydrocarbon migration <br /> potential presented in this assessment for the Santa Fe Railway Company site is considered <br /> conservative because of the following environmental fate and transport modeling <br /> assumptions. <br /> • Conservative assumptions were made in the model regarding the lateral and <br /> vertical extent of soil containing petroleum hydrocarbons Therefore, the total <br /> mass migrating was overestimated <br /> • The study area (i e., the area over which leaching occurs) for all layers was <br /> represented by the largest area of petroleum hydrocarbon-impacted soil in any <br /> layer. <br /> G'060MIMAWAFE FNL%b.W 26 93 5-1 Ch=R.%e 19" <br />