Hydraulic controls of in‐stream gravel bar hyporheic exchange and reactions

Abstract

Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three-dimensional numerical model of a natural in-stream gravel bar (20 m 3 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater-borne nitrate when mixed with stream-sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log-space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures.

The effect of losing and gaining flow conditions on hyporheic exchange in heterogeneous streambeds

“…The penetration depth may be important for defining the reactive zone in the streambed; however, we postulate that the volume of the hyporheic flow cell may be a better proxy for a general evaluation of hyporheic reactivity. This is because the volume and the magnitude of q H will dictate the residence time, which has been shown to be a good predictor for biogeochemical reactions in the hyporheic zone [ Zarnetske et al ., ; Marzadri et al ., ; Trauth et al ., ]. One of the most striking results from the numerical modeling was the sharp decrease in the volume of the hyporheic flow cell, even under a relatively small q bot .…”

Influence of Hydrological Perturbations and Riverbed Sediment Characteristics on Hyporheic Zone Respiration of CO2 and N2

“…Dual‐Monod maximum growth rate coefficients for AR and DN were 1E−4 and 1E−5 and were chosen from published ranges ( M = mol/L of each species and s = seconds) (Rosenzweig et al, ; Thullner, Zeyer, & Kinzelbach, ; Trauth et al, ). NI was included with a maximum specific growth rate coefficient of 1E−4 .…”

Modeling the Impact of Stream Discharge Events on Riparian Solute Dynamics

“…The van Genuchten‐Mualem approach is utilized for the estimation of the unsaturated hydraulic conductivity (Mualem ; van Genuchten ) whereas tortuosity is calculated by Millington formula (Millington ). The MIN3P code has been used for simulating a variety of problems in contaminant transport and stream‐groundwater interaction studies (Mayer et al ; Trauth et al , ; Trauth and Fleckenstein ). Although MIN3P is fully capable of simulating reactive transport, in this study we are focusing on non‐reactive solute transport because the objective of this study is to investigate how conservative solute transport is affected by changing hydraulic conditions.…”