Top: Depiction of hyporheic exchange in the stream/river corridor with a schematic of our new multiscale model for reactive transport. Bottom: Nitrate concentration from a watershed-scale simulation of microbially driven denitrification.
A new multiscale simulation approach enables more detailed representation of biogeochemical processing of carbon, nutrients, and metals in basin-scale models
Highly localized biogeochemical hotspots are widely recognized to play an outsized role in controlling the transformation of nutrients and contaminants in streams and rivers, but they are difficult to represent in field-scale models. In collaboration with the IDEAS–Watersheds project, we developed a new multiscale representation of transport and transformation in stream corridors. The new model, CHAnnel Network with Lagrangian Subgrid (CHANLS), associates a subgrid model for hyporheic zone reactive transport with each channel grid cell. The subgrid models are written in Lagrangian form with hyporheic age replacing travel distance. This novel application of stochastic subsurface hydrology theory makes it possible to efficiently represent an ensemble of flowpaths and account for flowpath diversity through the biogeochemically active hyporheic zone. We implemented the CHANLS conceptualization in the opensource community software ATS using the Alquimia application programming interface to access the biogeochemical reaction modeling capability in the PFLOTRAN simulator. Methods for estimating key model parameters related to hyporheic exchange rates and hyporheic residence times from stream tracer tests were developed and tested.
For the first time, detailed, fine-scale understanding of nutrient, carbon, and metal biogeochemical processes in the hyporheic zone can be incorporated at the societally relevant scales needed to understand effects on downstream water quality.
In contrast to traditional methods for modeling reactive transport in streams and rivers, the CHANLS conceptualization allows more detailed biogeochemical process understanding to be represented at the appropriate fine spatial scale of those processes while remaining tractable at field scales. The implementation in ATS provides a platform for next-generation water quality modeling tools.
Painter, S. L., A. Jan, and E. T. Coon. “Accounting for the biogeochemical effects of hyporheic exchange flows in network-scale models of reactive transport in streams and rivers.” Water Resources Research. In preparation.
Painter, S. L. 2018. “Multiscale framework for modeling multicomponent reactive transport in stream corridors.” Water Resources Research. 54(10): 7216–230. DOI: 10.1029/2018WR022831.
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The ORNL Mercury SFA is sponsored by the Subsurface Biogeochemical Research (SBR) program within the U.S. Department of Energy's Office of Biological and Environmental Research.