The potential for human activities to adversely affect the environment has become of increasing concern during the past three decades. As a result, the transport and fate of contaminants in subsurface systems has become one of the major research areas in the environmental/hydrological/earth sciences. An understanding of how contaminants move in the subsurface is required to address problems of characterizing and remediating soil and groundwater contaminated by chemicals associated with industrial and commercial operations, waste-disposal facilities, and agricultural production. Furthermore, such knowledge is needed for accurate risk assessments; for example, to evaluate the probability that contaminants associated with a chemical spill will reach an aquifer. Just as importantly, knowledge of contaminant transport and fate is necessary to design “pollution-prevention” strategies. A tremendous amount of research on the transport of solutes in porous media has been generated by several disciplines, including analytical chemistry (chromatography), chemical engineering, civil/environmental engineering, geology, hydrology, petroleum engineering, and soil science. This research includes the results of theoretical studies designed to pose and evaluate hypotheses, the results of experiments designed to test hypotheses and investigate processes, and the development and application of mathematical models useful for integrating theoretical and experimental results and for evaluating complex systems. While much of the previous research has focused on transport of nonreactive solutes, it is the transport of “reactive” solutes that is currently receiving increased attention. Reactive solutes are those subject to phase-transfer processes (e.g., sorption, precipitation/dissolution) and transformation reactions (e.g., biodegradalion). Of special interest in the field of contaminant transport is so-called nonideal transport. In the most general sense, nonideal transport can be described as transport behavior that deviates from the behavior that is predicted using a given set of assumptions. A homogeneous porous medium and linear, instantaneous phase transfers and transformation reactions are the most basic set of assumptions for ideal solute transport in porous media. As discussed in a recent review, transport of reactive contaminants is often nonideal (Brusseau, 1994). The potential causes of nonideal transport include rate-limited and nonlinear mass transfer and transformation reactions, as well as spatial (and temporal) variability of material properties.
Nonideal transport of reactive solutes in heterogeneous porous media: 5. Simulating regional-scale behavior of a trichloroethene plume during pump-and-treat remediation
