Modeling Expected Solute Concentration in Randomly Heterogeneous Flow Systems with Multicomponent Reactions

Many environmental problems require assessment of extensive reaction systems within natural subsurface flow systems exhibiting large physical and biogeochemical heterogeneity. We present an approach to couple stochastic advective-reactive modeling of physical solute transport (LaSAR) with the geochemical model PHREEQC for modeling solute concentrations in systems with variable flow velocity and multicomponent reactions. PHREEQC allows for general and flexible quantification of a multitude of linear and nonlinear geochemical processes, while LaSAR efficiently handles field-scale solute spreading in stochastic heterogeneous flow fields. The combined LaSAR−PHREEQC approach requires very modest computational efforts, thereby allowing a large number of reactive transport problems to be readily assessed and facilitating handling of quantifiable uncertainty in environmental model applications. Computational efficiency and explicit handling of field-scale dispersion without introduction of excessive fluid mixing that may impair model results are general advantages of the LaSAR compared with alternative solute transport modeling approaches. The LaSAR−PHREEQC approach is restricted to steady or unidirectional flow fields, and our specific application examples are limited to homogeneous reaction systems without local or transverse dispersion-diffusion, although these are not general methodological limitations. As a comprehensive application example, we simulate the spreading of acid mine drainage in a groundwater focusing on Zn<sup>2+</sup> and including relevant, major-component geochemistry. Model results show that Zn<sup>2+</sup> may be substantially attenuated by both sorption and precipitation, with flow heterogeneity greatly affecting expected solute concentrations downstream of the mine waste deposit in both cases.