Hyporheic exchange in recirculating flumes under heterogeneous bacterial and morphological conditions

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.

Transformation of organic micropollutants along hyporheic flow in bedforms of river-simulating flumes

“…The results of the hydrodynamic model (as previously described by Betterle et al 38 ) highlighted that the flowpaths leading to Samplers A and B tended to overlap (i.e. they laid on the same streamline; Fig.…”

“…The hyporheic flow field feeding the respective PW samplers was simulated by a particle backtracking model as described in Betterle et al 38 . Simulations included a fully coupled 2D description of the joint surface and hyporheic flow, combining the Navier–Stokes equations for the surface flow and the Brinkman equations for the hyporheic flow.…”

“…At each sampler position, 10,000 particles (conservative compounds) were seeded in the model according to a bivariate normal distribution of a horizontal variance = 5 mm 2 and a vertical variance of = 2.5 mm 2 around the sampling location and tracked back to their likely origin at the sediment-surface water interface. As described in Betterle et al 38 , simulations identified the trajectories of water particles and provided an estimate of the probability distribution of flowpath lengths and travel times expected to be sampled at the four sampling locations. The results of the model were used to illustrate and compare the trajectories of the different flowpaths in the bedforms.…”

“…The transport equation can be written as: where R is the retardation coefficient (–), c is the concentration of a compound (µg L −1 ) at time t (h), D h (m 2 h −1 ) denotes the effective hydrodynamic dispersion coefficient, v (m h −1 ) the PW velocity along the specific flowpath, and k (h −1 ) is the first-order removal rate constant. The model was run independently for each flowpath because the hydrodynamic model demonstrated that Samplers A, B and C were not positioned on the same streamline 38 . Thus, for all four flowpaths, SW concentrations were set as time-varying upper boundary conditions.…”

“…In previous studies we described the experiment, in which in total 20 river-simulating flumes were set up to investigate the influence of bacterial diversity and hyporheic exchange on the fate of organic micropollutants in the SW. The previous studies addressed the complete setup and functioning of the experimental design 35 , the fate of micropollutants and the individual influence of the treatments in the SW 36 , 37 , and a hydrodynamic model estimating hyporheic exchange in the flumes 38 . The present study focuses on the transformation of micropollutants in the hyporheic zone, i.e.…”