A composite modeling approach for subsurface transport of degrading contaminants from land-disposal sites

A composite modeling approach is presented for simulating the three-dimensional (3-D) subsurface transport of dissolved contaminants with transformation products. The approach is based on vertical infiltration and contaminant transport in the unsaturated zone and 3-D groundwater flow and contaminant migration in the saturated zone. Moisture movement and groundwater flow are considered to be steady, but contaminant transport is treated as transient. The model allows for advection, dispersion, linear or nonlinear equilibrium sorption, and first-order biochemical transformation of either a single contaminant species, or a multi-species, straight or branched, decay chain. The model is designed for regulatory decision making using Monte Carlo analysis. For such applications considerable emphasis is placed on computational efficiency and robustness of the model. An efficient and robust semi-analytical method is used to perform the steady-state solution for infiltration through the unsaturated zone. A variety of transport solutions, corresponding to transient or steady state, and linear or nonlinear sorption conditions, are incorporated in the model. For transient linear transport, the Laplace transform technique is used. The transformed unsaturated-zone transport equation is solved analytically; the transformed saturated-zone transport is solved numerically using the Laplace transform-Galerkin (LTG) technique which permits a nonuniform groundwater flow field, reflecting the influence of locally higher infiltration from the waste source. The model contains fully 3-D solutions for flow and transport in the saturated zone, as well as two-dimensional solutions for vertical cross-sectional and areal scenarios. Model formulations and solution schemes are verified by comparison against a fully 3-D, variably saturated flow and transport code for a hypothetical problem which represents a typical landfill. The model is applied also to simulate a controlled release field experiment and the model predictions compare well with groundwater monitoring data for the site.