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• • Mr. Ron Rowe Page 5 of 6 • Contaminant transport will be modeled using the computer program Flawpath II for Windows (Waterloo Hydrogeologic Inc.). This is a complete two-dimensional, finite difference groundwater flow and contaminant transport modeling package. The program can simulate complex hydrogeological systems including leaky, confined, and unconfined flow in anisotropic and heterogeneous media. The program supports spatially varying hydraulic conductivity, porosity, aquifer thickness and irregular model domains. Numerical modeling will be used in conjunction with analytical modeling and empirical contaminant distribution data to better describe and understand the characteristics of the site. The risk evaluation will be conducted using the computer program RBCA Tool Kit for Chemical Releases (Groundwater Services, Inc., 1998). This program incorporates the default chemical, toxicological, exposure, hydrogeological and physical parameters used in ASTM guidance for risk-based corrective actions (ASTM PS-104, 1998). The program also uses the transport/concentration models and equations from the ASTM document. Both the parameters and the equations are taken from USEPA and CalEPA guidance documents. Input parameters can be modified as needed by the incorporation of site-specific values in place of the default values. The program calculates the excess cancer risk and hazard index associated with exposure to existing contaminant levels under a variety of scenarios and will back- calculate contaminant levels that will result in a desired risk limit. TASK 5 EVALUATION OF ADDITIONAL REMEDIAL ALTERNATIVES Utilizing the existing information for the site, Ground Zero has evaluated the potential effectiveness of air sparging. We have concluded that the interbedded low and high permeability soils would make control and capture of the injected air ineffective for remedial purposes and may present a risk of further dispersion of gasoline hydrocarbons migrating under the site from offsite sources. We will reevaluate the potential effectiveness of air sparging using the additional data obtained from the proposed investigation. If appropriate, field testing will be proposed. We will also evaluate the cost effectiveness of continued or enhanced soil vapor extraction based upon the analytical results of the confirmation borings and the operational characteristics of the system. Should a significant mass of hydrocarbons be identified that is accessible to vapor extraction, additional vapor wells will be installed and soil vapor extraction resumed. TASK 6 REPORTING • The information developed from the investigation will be compiled in a summary report and submitted to PHS/EHD and the Regional Water Quality Control Board. The report mckk1mure\addmdum