Denitrification in the banks of fluctuating rivers: The effects of river stage amplitude, sediment hydraulic conductivity and dispersivity, and ambient groundwater flow

Abstract

Hyporheic exchange induced by periodic river fluctuations leads to important biogeochemical processes, particularly nitrogen cycling, in riparian zones (RZs) where chemically distinct surface water and groundwater mix. We developed a two‐dimensional coupled flow, reactive transport model to study the role of bank storage induced by river fluctuations on removing river‐borne nitrate. Sensitivity analyses were conducted to quantify the effects of river amplitude, sediment hydraulic conductivity and dispersivity, and ambient groundwater flow on nitrate removal rate. The simulations showed that nitrification occurred in the shallower zone adjacent to the bank where oxic river water and groundwater interacted while denitrification occurred deeper into the aquifer and in the riverbed sediments where oxygen was depleted. River fluctuations greatly increased the amount of nitrate being removed; the nitrate removal rate increased as river amplitude increased. Similarly, increasing hydraulic conductivity increased overall nitrate removal since it expanded the denitrifying zone but decreased efficiency. In contrast, increasing sediment dispersivity increased the removal efficiency of nitrate because it promoted mixing between electron acceptors and donors. The presence and direction of ambient groundwater flow had a significant impact on nitrate removal rate when compared to neutral conditions. A losing river showed a larger nitrate removal rate, whereas a gaining river showed a smaller nitrate removal rate. Our results demonstrated that daily river fluctuations created denitrification hot spots within the RZ that would not otherwise exist under naturally neutral or gaining conditions.

Model‐Based Analysis of the Effects of Dam‐Induced River Water and Groundwater Interactions on Hydro‐Biogeochemical Transformation of Redox Sensitive Contaminants in a Hyporheic Zone

“…The high frequency in FDRs enhanced DO supply, which increased the rate of OC consumption and shorten the duration of the HZ in reducing Cr (VI) (Figure g). The importance of DO has also been reported in previous studies (e.g., Gu et al, ; Mcmahon et al, ; Shuai et al, ). It should be noted that though FDRs could supply more DOC to the HZ, its effect was outcompeted by the effect of DO because of the low DOC level in Columbia River at Hanford Site.…”

Drought Conditions Maximize the Impact of High‐Frequency Flow Variations on Thermal Regimes and Biogeochemical Function in the Hyporheic Zone

“…Our system characteristics are representative of most large gravel river with large bank storage, high HEF rates, oxygenated, and low DOC conditions in both river water and groundwater. While the conceptual model and findings could vary in other river corridor systems, such as those with finer sediments, low HEF rates, different river/subsurface geochemical conditions, and reaction pathways (Knights et al, ; Shuai et al, ; Trauth & Fleckenstein, ), the numerical modeling approaches and analyses used to identify the effects of high‐frequency flow variations are general and readily transferable.…”

Shear Flows Accelerate Mixing Dynamics in Hyporheic Zones and Hillslopes

“…This includes in particular the transient dynamics of solute mixing and heat transfer in the hyporheic zone (Hester et al, 2017;Kaufman et al, 2017;Marzadri et al, 2013;Zheng & Cardenas, 2018). Furthermore, acceleration of mixing rates by shear flows in hillslopes may influence the statistics of concentrations in rivers (e.g., Kirchner & Neal, 2013;Shuai et al, 2017) and concentration-discharge relationships (e.g., Godsey et al, 2009;Neilson et al, 2018).…”