Streamtube Approach to Upscaling Physical and Biogeochemical Heterogeneity in Subsurface Contaminant Transport

Reactive properties of aquifer materials play an important role in contaminant fate and transport on time scales ranging from days to decades.  Like physical properties of the subsurface, reactivity properties such as mineralogy, surface area, and/or microbial populations vary on one or more spatial scales.  However, the effect of such variability has received little attention relative to the decades of research on effects of physical heterogeneity on transport, that has been misplaced on macrodispersion of idealized (e.g., non-reactive or linearly reactive) solutes.  This is due in part to the complexity of combined flow, transport and reactions processes involving multiple species interacting at multiple rates, and in part to the difficulty of characterizing subsurface reactive properties in situ, that increases with depth.  Consequently, upscaling tools for dealing with the impact of geochemical/microbiological heterogeneity on realistic solute contaminants are severely limited.  Here we describe a simple upscaling method to simulate arrival distributions of a realistic solute contaminant, undergoing reactive transport in porous media with arbitrary physical and biogeochemical heterogeneity on potentially multiple scales. The approach relies on the streamtube-ensemble conceptual model of source-to-sink transport, and involves tracer testing using multiple reacting tracers to characterize biogeochemical heterogeneity along flow paths.  The tracer test supports inverse modeling to identify the distribution of solute flux over both travel time and cumulative reactivity, that is the cumulative contact time with reactive surfaces that solute experiences along a given flow path.  Because the flow paths are allowed to be nonuniform, this approach approximately upscales both physical and chemical (aquifer reactivity) heterogeneities.  We describe this scaling approach and examine dual-distributed solute flux within simulated aquifer materials, by using high-resolution numerical simulation of tracer tests to provide “data” that is then used in the inverse and upscaling methods, as it would be in an actual field test.