Ambient groundwater flow diminishes nitrate processing in the hyporheic zone of streams

What is it about?

Humans generate vast quantities of bioavailable nitrogen for agricul- tural production, and much of the excess ends up in rivers and streams. As the nitrogen flows downstream, some of it is naturally removed by streambed sediments through a process known as hyporheic exchange. In this paper, we set out to answer the question: how does the movement of groundwater into (or out of) a stream affect the removal of nitrogen by hyporheic exchange? Multiscale and multi-physics model simula- tions suggest that groundwater movement across the streambed can ‘‘shut down’’ the physical and biologi- cal processes that remove stream nitrogen. Hence, stream-groundwater interactions may play an important role in modulating the human and environmental impacts of nitrogen pollution.

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Why is it important?

There is an urgent need for modeling tools that can provide realistic estimates for the nitrate uptake veloc- ity in support of regulatory, ecological, and sustainability goals, including implementation of total maximum daily loads for nitrogen impaired streams [French et al., 2006], stream restoration efforts to improve the retention and removal of bioavailable nitrogen [Craig et al., 2008], and long-term forecasts of the effects of land-use and climate change on water resources at the watershed scale [Grathwohl et al., 2013]. To this end, several process-based models for vf have been proposed, but these: (1) do not consider the multiscale nature of hyporheic exchange flows and, in particular, the impact of ambient groundwater flow on hypo- rheic exchange; (2) rely on simplified conceptualizations of mixing within streambed sediments (e.g., a well- mixed box); (3) neglect important steps in the N-cycle (e.g., nitrification and ammonification); and/or (4) adopt pseudo-first-order kinetic descriptions of denitrification [Stream Solute Workshop, 1990; Runkel, 2007; Yang and Wang, 2010]. The first limitation is particularly concerning given that groundwater resources are increasingly under stress from urbanization, agricultural activities, groundwater mining, and climate change [Walsh et al., 2005; Green et al., 2011; Askarizadeh et al., 2015]. The effects of ambient groundwater flow on in-stream ecosystem services are largely unknown [Boulton et al., 2010; Grathwohl et al., 2013; Wondzell, 2015]. In this paper, we develop and test a simple and scalable process-based model for estimating the nitrate uptake velocity that addresses the limitations identified above. In particular, our model accounts for: (1) hyporheic exchange at multiple scales together with ambient groundwater flow in gaining or losing streams; (2) the broad residence time distributions characteristic of hyporheic exchange; (3) key biogeo- chemical reactions associated with N-cycling, including respiration, ammonification, nitrification, and deni- trification; and (4) the nonlinear nature of the pertinent biogeochemical reaction rates, including Monod kinetics for aerobic respiration and denitrification, and second-order kinetics for nitrification. Using this modeling framework, we systematically evaluate how changing ambient groundwater flow is likely to affect N-cycling in the hyporheic zone of streams, and compare our predictions to previously published reach- scale measurements of nitrate removal.