• Exposure Concentrations

For use in risk-based corrective action (RBCA) analyses, simple analytical fate-and-transport models can provide a cost-effective means of estimating exposure concentrations and developing risk-based soil and groundwater remediation standards. Under ASTM E-1739 "Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites, " such models are recommended as a conservative first step under Tiers 1 and 2 of the site evaluation process, prior to use of more complex numerical modeling methods under Tier 3. However, the reliability of an analytical model as a conservative predictor of chronic exposure levels depends upon proper characterization of key physical and chemical parameters. This paper reviews a system of analytical fate-and-transport models compiled expressly for use with the ASTM RBCA Standard and provides practical guidelines for measurement and/or estimation of key input parameters for each model. Contaminant transport pathways addressed in this paper include soil-to-air volatilization, soil-to-groundwater leaching, lateral air transport, and lateral groundwater transport. Parameter selection guidelines discussed in this paper relate specifically to the analytical expressions listed in Appendix X.2 of ASTM E-1739. However, these guidelines are generally applicable to a broad range of soil, air, and groundwater transport models. RBCA Spreadsheet System The RBCA Spreadsheet System, developed by Groundwater Services, Inc. (GSI), is designed to complete all calculations required for Tiers 1 and 2 of the ASTM RBCA planning process (Conno