Engineering Approach for Tier 2 RBCA Evaluations at NAPL-Contaminated Sites

Abstract

Groundwater contaminated with non-aqueous-phase liquids (NAPLs) poses human health risks and evaluation of these risks requires estimated groundwater NAPL concentrations. Site NAPL concentrations are typically evaluated using the U.S. Environmental Protection Agency's tiered risk-based corrective action (RBCA) system. Tier 1 uses conservative assumptions and little site-specific data, while tier 2 requires more detailed site-specific data. This paper presents an engineering approach that can be used to predict maximum groundwater NAPL concentrations resulting from NAPL spills. This screening tool was developed by deriving an analytical solution for a partial differential equation describing NAPL transport through the unsaturated zone. Model processes include dissolution, advection, dispersion, sorption, volatilization, and first-order degradation. Results are obtained using two nondimensional parameters and engineering plots for a wide range of conditions. A brief study is provided to demonstrate the application of the model. Because the engineering plots are easy to use and facilitate sensitivity analyses, the model is recommended for screening and RBCA tier 2 analyses.