Abstract
Abstract. Surface water quality along river corridors can be modulated by hyporheic zones (HZs) that are ubiquitous and biogeochemically active. Watershed management practices often ignore the potentially important role of HZs as a natural reactor. To investigate the effect of hydrological exchange and biogeochemical processes on the fate of nutrients in surface water and HZs, a novel model, SWAT-MRMT-R, was developed coupling the Soil and Water Assessment Tool (SWAT) watershed model and the reaction module from a flow and reactive transport code (PFLOTRAN). SWAT-MRMT-R simulates concurrent nonlinear multicomponent biogeochemical reactions in both the channel water and its surrounding HZs, connecting the channel water and HZs through hyporheic exchanges using multirate mass transfer (MRMT) representation. Within the model, HZs are conceptualized as transient storage zones with distinguished exchange rates and residence times. The biogeochemical processes within HZs are different from those in the channel water. Hyporheic exchanges are modeled as multiple first-order mass transfers between the channel water and HZs. As a numerical example, SWAT-MRMT-R is applied to the Hanford Reach of the Columbia River, a large river in the United States, focusing on nitrate dynamics in the channel water. Major nitrate contaminants entering the Hanford Reach include those from the legacy waste, irrigation return flows (irrigation water that is not consumed by crops and runs off as point sources to the stream), and groundwater seepage resulting from irrigated agriculture. A two-step reaction sequence for denitrification and an aerobic respiration reaction is assumed to represent the biogeochemical transformations taking place within the HZs. The spatially variable hyporheic exchange rates and residence times in this example are estimated with the basin-scale Networks with EXchange and Subsurface Storage (NEXSS) model. Our simulation results show that (1), given a residence time distribution, how the exchange fluxes to HZs are approximated when using MRMT can significantly change the amount of nitrate consumption in HZs through denitrification and (2) source locations of nitrate have a different impact on surface water quality due to the spatially variable hyporheic exchanges.
Highlights
Defined, the hyporheic zone (HZ) is the area of the stream bed and stream bank in which stream water mixes with shallow groundwater (Runkel et al, 2003) or through which subsurface pathways begin and end at the stream (Cardenas, 2015)
Our study focuses on the effect of bedform-induced and sinuosity-driven hyporheic exchange derived under steady-state conditions on stream water quality in large rivers using the Soil and Water Assessment Tool (SWAT)-multirate mass transfer (MRMT)-R model we developed
Coupling reactive transport in channel and HZs and MRMT in a large river using the mean residence times and exchange fluxes calculated from Networks with EXchange and Subsurface Storage (NEXSS), our simulations show that HZs can attenuate the peak nitrate concentrations in the stream with mass transfer and biogeochemical reactions with a 11.6 % concentration reduction on average compared to the base case without MRMT
Summary
The hyporheic zone (HZ) is the area of the stream bed and stream bank in which stream water mixes with shallow groundwater (Runkel et al, 2003) or through which subsurface pathways begin and end at the stream (Cardenas, 2015). The HZ has been recognized as a critical component of a stream’s ecosystem (Boano et al, 2014; Boulton et al, 1998; Ward, 2016; Wondzell, 2011; Liao and Cirpka, 2011). It is a location of interacting physical, chemical, and biological systems (Ward, 2016). Numerical experiments may provide some insights on the role of HZs
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