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

Abstract

Modeling and experimental studies demonstrate that ambient groundwater reduces hyporheic exchange, but the implications of this observation for stream N‐cycling is not yet clear. Here we utilize a simple process‐based model (the Pumping and Streamline Segregation or PASS model) to evaluate N‐cycling over two scales of hyporheic exchange (fluvial ripples and riffle‐pool sequences), ten ambient groundwater and stream flow scenarios (five gaining and losing conditions and two stream discharges), and three biogeochemical settings (identified based on a principal component analysis of previously published measurements in streams throughout the United States). Model‐data comparisons indicate that our model provides realistic estimates for direct denitrification of stream nitrate, but overpredicts nitrification and coupled nitrification‐denitrification. Riffle‐pool sequences are responsible for most of the N‐processing, despite the fact that fluvial ripples generate 3–11 times more hyporheic exchange flux. Across all scenarios, hyporheic exchange flux and the Damköhler Number emerge as primary controls on stream N‐cycling; the former regulates trafficking of nutrients and oxygen across the sediment‐water interface, while the latter quantifies the relative rates of organic carbon mineralization and advective transport in streambed sediments. Vertical groundwater flux modulates both of these master variables in ways that tend to diminish stream N‐cycling. Thus, anthropogenic perturbations of ambient groundwater flows (e.g., by urbanization, agricultural activities, groundwater mining, and/or climate change) may compromise some of the key ecosystem services provided by streams.

Modeling the Effects of Turbulence on Hyporheic Exchange and Local‐to‐Global Nutrient Processing in Streams

“…In the opposite limit ( α ≈ ψ ≪ 1) the model predicts that the uptake velocity is controlled exclusively by nutrient transport and processing within the streambed ( ). While other conceptualizations of nutrient fate and transport in the hyporheic zone are possible (e.g., Azizian et al, , ; Gomez‐Velez & Harvey, ; Grant et al, ; McCluskey et al, ; Trauth et al, ), to our knowledge equation is unique in its simplicity and explicit accounting of turbulence‐induced mixing above and below the bed (as well as bedform pumping, see next).…”

Aquatic Carbon‐Nutrient Dynamics as Emergent Properties of Hydrological, Biogeochemical, and Ecological Interactions: Scientific Advances

“…In this special issue, Li et al () highlight how fine‐scale (<25 cm) patterns of hyporheic flow and microbially mediated denitrification can affect nitrogen removal rates at the stream reach (hundreds of meters) and larger spatial scales. Azizian et al () demonstrate how nitrogen processing in stream hyporheic zones can also be influenced by the spatiotemporal structure of groundwater flow, whereby higher rates of vertical groundwater flux generally diminish stream nitrogen processing. Accordingly, turbulence‐driven hyporheic flow dynamics at the submeter scale, and groundwater flow at the riffle‐pool sequence scale, represent nested processes that simultaneously modulate stream nitrogen processing (Azizian et al, ; Li et al, ).…”

“…In this special issue, Li et al (2017) highlight how fine-scale (<25 cm) patterns of hyporheic flow and microbially mediated denitrification can affect nitrogen removal rates at the stream reach (hundreds of meters) and larger spatial scales. Azizian et al (2017) demonstrate how nitrogen processing in stream hyporheic zones can also be influenced by the spatiotemporal structure of groundwater flow, whereby higher rates of vertical However, observing fine-scale heterogeneity and upscaling that information remains a challenge. As such, there is a need for multiscale approaches to understanding eco-hydrobiogeochemical dynamics.…”

“…Water Resources Research groundwater flux generally diminish stream nitrogen processing. Accordingly, turbulence-driven hyporheic flow dynamics at the submeter scale, and groundwater flow at the riffle-pool sequence scale, represent nested processes that simultaneously modulate stream nitrogen processing (Azizian et al, 2017;Li et al, 2017).…”

Improved representation of groundwater at a regional scale – coupling of mesocale Hydrologic Model (mHM) with OpeneGeoSys (OGS)

“…Van Meter et al (2017) found that current nitrogen fluxes in rivers can be dominated by groundwater legacies. Moreover, stream-subsurface water interactions may be significant in modulating the human and environmental effects of nitrogen pollution (Azizian et al, 2017). To assess 15 the response of groundwater to climate change, a physically based groundwater representation including lateral subsurface flow is urgently needed (Scibek and Allen, 2006;Green et al, 2011;Ferguson et al, 2016) .…”