Analysis of Contaminant Transport through the Vadose and Saturated Zones for Source Screening

At complex sites there may be many potential source areas. Screening level analyses are useful to identify which of the source areas should be the focus of detailed investigation and analysis. A screening tool has been developed to evaluate the threat posed by waste sites on groundwater quality. This tool implements analytical solutions to simulate contaminant transport through the vadose and saturated zones and predict time-varying concentrations at potential groundwater receptors. The screening tool is developed within a user friendly, Microsoft ExcelTM based interface; however, care has been taken to implement rigorous solutions. The screening tool considers the following mechanisms: (a) Partitioning of soil contamination in to an equivalent dissolved concentration. For a time-invariant source, the solution is generalized from [3] for sorption and decay. For a time-varying source, the solution represents a special, degenerate, case of a solution implemented in ATRANS [2]; (b) One-dimensional (1D) transport of the dissolved contamination through the vadose zone considering 1D dispersion, equilibrium sorption, and first order transformation reactions. Steady state infiltration and moisture content are assumed; (c) Blending (mixing) of ambient water quality in the saturated zone with the contaminated water leaching from the vadose zone; and (d) Three-dimensional (3D) transport through the saturated zone using the formulation provided in [2], considering advection, dispersion, sorption, and first-order transformation reactions. The solution is derived using integral transform methods, following approaches adopted in [1] and [4]. Independent verification showed that the analytical techniques implemented in this study generate solutions that closely approximate those obtained using sophisticated numerical approaches, with a systematic over-estimate of the likely impact to groundwater that (predictably) stems from the use of a 1D approximation in the vadose zone. As a result, the screening tool enables rapid, conservative (i.e., protective) screening-level calculations and is also ideal for providing instruction on the underlying principles. References [1] Cleary, R.W., and M.J. Ungs. 1978. Analytical models for Groundwater Pollution and Hydrology. Report 78-WR-15, Department of Civil Engineering, Princeton University, Princeton, New Jersey. [2] Neville, C.J., 2005: ATRANS: Analytical Solutions for Three Dimensional Solute Transport from a Patch Source, Version 2, S.S. Papadopulos & Associates, Inc., Waterloo, Ontario. [http://www.sspa.com/Software/atrans.shtml] [3] Ogata, A., and R.B. Banks, 1961: A Solution of the Differential Equation of Longitudinal Dispersion in Porous Media, USGS Professional Paper 411 A. [4] Wexler, E., 1992. Analytical Solutions for One-, Two, and Three-Dimensional Solute Transport in Groundwater Systems with Uniform Flow, Techniques of Water Resources Investigations of the United States Geological Survey, Chapter B-7, Book 3, Applications of Hydraulics. USGS, Washington, DC.