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•b Ge (a ' afTecfucics Inc. � N4001 Page 3 MCGranahan(Gawne Road) Work Pian-Fate&Transport Model Project#251.2 March 21, 1997 4.0 CLIMATOLOGICAL CHARACTERISTICS The National Oceanic and Atmospheric Administration (NOAA) is the agency of the Commerce Department that maintains weather data collection stations at many localities throughout the United States. One of these stations is in Stockton. The Stockton data for precipitation, temperature, evapotranspiration, and storm events will be used. The long term averages along with monthly statistical parameters for this station are built into the climate data base of SESOIL. 5.0 CHEMICAL CONSTANTS The principal substance of concern at the site is gasoline which is a mixture of numerous individual chemicals, primarily with carbon chains ranging from C5 to C14. Aromatic hydrocarbon compounds contained within gasoline constitute the principal human health risk. Benzene (C6) is the primary chemical of human health risk and is a known carcinogen. Based on toxicity characteristics (reference dose) and the relatively high mobility of specific compounds, benzene is considered to be a compound of significant importance. It has a low reference dose and is relatively mobile. For these reasons, benzene has been selected as the chemical to be modeled in this study. The SESOIL program contains the chemical properties of benzene in its data base, and the data base will be used in this study. To provide the most conservative estimates of the benzene levels, it will be assumed that no biodegradation will occur. 6.0 HYDROGEOLOGY SESOIL is primarily a vadose zone model. No groundwater monitoring wells have been installed and so site specific groundwater hydrogeologic information is not available. The saturated zone,at this site will not be modeled. 7.0 APPLICATION The ranges of input characteristics will be used as input to the SESOIL program. A series of runs will be made using the data to generate a range of likely scenarios estimating the transport of the residual vadose zone benzene and its loading to the underlying groundwater, if any. It is proposed that the entire Layer I (including all its sub-layers) will be considered as the benzene loading zone. Based on available information, a range of initial concentration of contaminants within Layer 1 soil will be considered for the loading rate. The model will thea be executed to predict the rate of vertical movement of the initially loaded contaminant. The results will be presented in a modeling report at the conclusion of the modeling runs. The complete set of details with appropriate references will be documented in the modeling report.