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1 22 October 1996 AGE-NC Project No. 95-0100 ' Page 6 of 9 ' corrections to the original CAP and the results of the risk based computer modeling program. 3.0. COMPUTER AIDED FATE AND TRANSPORT MODELING 3.1. SESOIL MODEL 1 As an alternative to active soil remediation, a fate modeling program was ran for the site to better assess the need for site remediation. The American Petroleum Institute's (API) Risk/Exposure ' Assessment Decision Support System(DSS) is a modeling program developed to estimate human exposure and risk from contaminated sites. Several modeling options are contained in DSS, including SESOIL and AT123D. SESOIL uses information obtained from the site assessment of contaminants,hydrogeo logical,and meteorological information to estimate the mass of contaminants that will reach ground water over a given time frame.AT123D can be used to determine contaminant concentrations in a receptor well located down-gradient-from a contaminant source. AT123D can ' use the information generated by SESOIL as the input data. Chemical properties and health risk data for 26 chemicals of possible concern are supplied in the program including benzene,toluene,ethylbetizene and xylenes.Total petroleum hydrocarbons(TPH) is not an individual chemical with definite properties, and therefore is not included on the list of ' chemical. choices. Output can be displayed in several way, including annual reports, tables and graphs. ' 3.2. SITE SPECIFIC MODEL DATA . ' The impacted soil in the area of concern was divided into three lithologic layers. The top layer consists of hydrocarbon-impacted clay and sandy clay from surface grade to 6.1 meters (18 feet); the second layer.is the impacted clayey gravel from 5.8 meters to 11.8 meters (18 to 38 feet); and ' the third layer is the impacted sand from'l L8 meters to. 17 meters(38 to 54 feet):The lateral extent of impacted soil was estimated to be 8 meters. Figure 3 - Stratigraphic Cross Section depicts the distribution of the impacted soil. A sample calculation of the mass of contaminates.for the model is ' depicted in Appendix E. The highest concentrations of hydrocarbons were detected in samples collected from the clay located '—"- —'=fro"—the surface to a depth'-of 18 166t bsg. Anaverage of the concentrations of the impacted soil was assumed for the intervals' of differing soil composition The masses of benzene, toluene,. ' ethylbenzene and xylenes were calculated for each layer. Soil characteristics were estimated from . all boring soil description data collected at the site. Climatic parameters were estimated from historical Iocal weather conditions.