A Simple Analytical Model for Interpretation of Tracer Tests in Two-Domain Subsurface Flow Systems

Abstract.

Two-domain subsurface flow systems are common in nature and in engineered wetlands and similar passive treatment systems. To expedite analysis of flow and solute transport processes in such systems, a simple mathematical model for one-dimensional flow and transport in such systems has been derived by combining separate analytical solutions for the partial differential equations that describe (i) straight-forward transport (by advection and mechanical mixing only) in a rapid-flow domain of large pores, and (ii) transport in a similar system that is in intimate contact with an essentially stagnant micro-porous domain, with which it exchanges solutes solely by molecular diffusion. Two tracer tests were performed in passive mine water treatment systems that were anticipated to comprise coupled ‘fast’ and ‘stagnant’ porous domains. Application of the new analytical model readily yielded breakthrough curves very similar to those observed. Besides providing corroboration of the two-domain concept as an explanation for solute transport in these passive treatment systems, the new model yields a fractionation coefficient, which may be a useful objective index of the modal hydraulic behaviour of natural and man-made subsurface flow systems.