Comparison of streamtube and three-dimensional models of reactive transport in heterogeneous media

Abstract

Field-scale transport of reactive solutes is influenced strongly by physical and geochemical heterogeneity over a wide range of spatial scales. Accurate numerical simulation requires fine spatial and temporal discretization to resolve the smallest scales of heterogeneity, in addition to coupling large numbers of chemical components participating in geochemical and microbiological reactions. Standard approaches based upon finite difference or finite element discretization of the governing set of advection-dispersion-reaction equations can lead to computational difficulties for large problems. To overcome these difficulties, several streamtube-based models have been presented in the literature. These models neglect transverse dispersion, and hence reactive transport occurs along independent one-dimensional streamtubes. Therefore, streamtube models are well-suited for applications where the objective is to calculate solute flux breakthrough across a control plane. We develop a reactive streamtube model by modification of the Finite Element Heat and Mass transfer code (FEHM), an existing flow and transport code. FEHM can simulate unsaturated zone conditions, heat flow, and multicomponent reactive transport, and has been used extensively by the Yucca Mountain Project. Because the same underlying numerical algorithms are used in both the FEHM and streamtube models, direct comparison of their relative performance is possible. We perform this comparison for a three-dimensional reactive transport example, to demonstrate the large potential computational advantages of the streamtube approach.