Abstract
Efforts to include more detailed representations of biogeochemical processes in basin-scale water quality simulation tools face the challenge of how to tractably represent mass exchange between the flowing channels of streams and rivers and biogeochemical hotspots in the hyporheic zones. Multiscale models that use relatively coarse representations of the channel network with subgrid models for mass exchange and reactions in the hyporheic zone have started to emerge to address that challenge. Two such multiscale models are considered here, one based on a stochastic Lagrangian travel time representation of advective pumping and one on multirate diffusive exchange. The two models are formally equivalent to well-established integrodifferential representations for transport of non-reacting tracers in steady stream flow, which have been very successful in reproducing stream tracer tests. Despite that equivalence, the two models are based on very different model structures and produce significantly different results in reactive transport. In a simple denitrification example, denitrification is two to three times greater for the advection-based model because the multirate diffusive model has direct connections between the stream channel and transient storage zones and an assumption of mixing in the transient storage zones that prevent oxygen levels from dropping to the point where denitrification can progress uninhibited. By contrast, the advection-based model produces distinct redox zonation, allowing for denitrification to proceed uninhibited on part of the hyporheic flowpaths. These results demonstrate that conservative tracer tests alone are inadequate for constraining representation of mass transfer in models for reactive transport in streams and rivers.
Highlights
Efforts to understand and model the fate and transport of solutes through streams and rivers have focused much attention on regions of stagnant or relatively slow-moving water known as transient storage zones
Hyporheic exchange flow is important in the biogeochemical processing of nutrients (Duff and Triska, 2000; Böhlke et al, 2004; Mulholland et al, 2008), organic carbon (Grimm and Fisher, 1984; Battin et al, 2008), metals (Bourg and Bertin, 1993; Fuller and Harvey, 2000; Palumbo-Roe et al, 2012), and organic contaminants (Kim et al, 1995; Conant et al, 2004; Schaper et al, 2018)
Solid curves are the concentration in the channel at 3,000 m assuming the ADELS model and the dashed curve are from the equivalent Multirate Transient Storage Models (mTSM) model
Summary
Efforts to understand and model the fate and transport of solutes through streams and rivers have focused much attention on regions of stagnant or relatively slow-moving water known as transient storage zones. Helton et al (2011) evaluated common modeling approaches for modeling biogeochemistry in stream networks using scaling of in situ denitrification from headwater streams to river networks as a concrete example. They identify the need to include hydrologic exchanges between the stream channel and subsurface waters coupled to more mechanistic representations of coupled biogeochemical cycles and conclude that limitations in our current modeling approaches “restrict our ability to simulate biogeochemical dynamics among diverse river networks.”
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