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 ac
tion (RBCA) system. Tier 1 uses conservative assumptions and little site-sp
ecific data, while tier 2 requires more detailed site-specific data. This p
aper presents an engineering approach that can be used to predict maximum g
roundwater NAPL concentrations resulting from NAPL spills. This screening t
ool was developed by deriving an analytical solution for a partial differen
tial equation describing NAPL transport through the unsaturated zone. Model
processes include dissolution, advection, dispersion, sorption, volatiliza
tion, and first-order degradation. Results are obtained using two nondimens
ional parameters and engineering plots for a wide range of conditions. A br
ief 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.