Hyporheic exchange in recirculating flumes under heterogeneous bacterial and morphological conditions

Andrea Betterle,Malte Posselt,Mario Schirmer,Claudia Coll,Anna Jaeger,Jonathan P Benskin

doi:10.1007/s12665-021-09472-2

Abstract

Hyporheic exchange (HE) contributes to the biogeochemical turnover of macro- and micro-pollutants in rivers. However, the spatiotemporal complexity and variability of HE hinder understanding of its role in the overall functioning of riverine ecosystems. The present study focuses on investigating the role of bacterial diversity and sediment morphology on HE using a multi-flume experiment. A fully coupled surface–subsurface numerical model was used to highlight complex exchange patterns between surface water and the underlying flow field in the sediments. Under the experimental conditions, the surface water flow induced by bedforms has a prominent effect on both local trajectories and residence time distributions of hyporheic flow paths, whereas mean hyporheic retention times are mainly modulated by average surface flowrates. In case of complex bedform morphologies, the numerical model successfully reproduces the HE estimated by means of salt dilution tests. However, the 2D numerical representation of the system falls short in predicting HE in absence of bedforms, highlighting the intrinsic complexity of water circulation patterns in real scenarios. Finally, results show that higher bacterial diversities in the stream sediments can significantly reduce hyporheic fluxes. This work provides a framework to interpret micropollutants turnover in light of the underlying physical transport processes in the hyporheic zone. The study emphasizes the importance of better understanding the tradeoff between physically driven transport processes and bacterial dynamics in the hyporheic zone to quantify the fate of pollutants in streams and rivers.

Highlights

In streams and rivers, hyporheic exchange fluxes between surface water and sediment porewater are driven by a variety of physical processes playing on different temporal and spatial scales (Vogt et al 2010; Stonedahl et al 2010; Stanford and Ward 1988; Boano et al 2014; Lewandowski et al 2011; Krause et al 2012)
The discrepancies between modelling and experimental results highlight: (i) the limits of numerical models to reproduce the full range of complex processes featuring surfaceporewater interaction; (ii) the challenges in describing a slowly evolving biological system over a long period of time by means of a stationary description of the underlying hyporheic exchange mechanisms
The geometrical setup challenged the quantification of the boundary conditions, and the establishment of complex 3D flow fields in the bends may not have been adequately described by the 2D flow model

Summary

Introduction

Hyporheic exchange fluxes between surface water and sediment porewater are driven by a variety of physical processes playing on different temporal and spatial scales (Vogt et al 2010; Stonedahl et al 2010; Stanford and Ward 1988; Boano et al 2014; Lewandowski et al 2011; Krause et al 2012). Hyporheic flow processes control flow path distributions, exchange rates, exchange volumes, travel and residence times of water (and waterborne compounds) across the riverbed (Bencala and Walters 1983; Wörman et al 2002; Marion et al 2008; Mojarrad et al 2019a). These variables bear a substantial signature on the biogeochemical cycling of nutrients and pollutants in the sediments and on the development of. The ecosystem services associated with hyporheic exchanges have encouraged significant research efforts that, over the last few decades, have provided new mechanistic and phenomenological insights on the interactions between surface and subsurface waters (Boano et al 2014)

Methods

Results

Discussion

Conclusion