A three dimension hyporheic model of the River Bure: Understanding the nutrient dynamics and the role of streambed heterogeneity

The hyporheic zone is often defined as the zone where mixing of surface water and groundwater occurs in shallow sediments beneath and adjacent to rivers. This mixing contributes to create unique biogeochemical conditions that may attenuate contaminants from either upstream surface water or groundwater under gaining and losing conditions. Hyporheic exchange results from differences in the channel near-bed head as it varies in space in response to interactions between surface flow and bed topography, with the interaction with the water table playing an important role too. Reactions of contaminants in groundwater also dependent on mixing between surface and subsurface water, which occurs in this zone. Therefore, representation of the profile of upwelling and downwelling exchange between surface water and groundwater have important consequences for contaminant transport. The present work studies nitrogen fate within a restored reach of the River Bure, Norfolk, United Kingdom. To this end, we confront numerical simulations of the hyporheic flow and tracer transport with field measurements of surface flow properties, nearby groundwater table and nitrogen compound concentration. We numerically model mixing between hyporheic flow paths induced by sediment, bedform, meanders on riverbed, and flow paths of adjacent upwelling of deeper groundwater. Results of the analysis indicate that despite the coarse topographical data and with limited surface water hydraulic data it is possible to define the spatial extent of hyporheic exchange and potential mixing zones for contaminants as a function of residence time. The proposed work has the potential to depict high residence time zones and biogeochemical reactivity in homogeneous and heterogeneous sediments. Furthermore, fieldwork analysis shows that in this site the hyporheic zone have a little effect on nutrient concentration. In addition, hydraulic modeling results indicate that streambed discharge significantly influences hyporheic exchange. Especially, the residence times under average stream discharge conditions are higher than the ones under the high discharge conditions mostly on the part that has riffle - pool morphology. From hydraulic point of view, heterogeneous domain has higher connectivity than the homogeneous fine sand subsurface set. Such that, subsurface flow has tendency to flow through high hydraulic conductivity zones, which is defined as tunneling effect, therefore, low conductivity zones have minor effect on hyporheic flow. The predictions based on the DaO2 index proves that heterogeneous sediment formations have more aerobic potential, however, prevailing anaerobic conditions occur mostly vicinity of low hydraulic conductivity zones