In this work, the deposition of clay-sized fine particles (d 50 = 0.006 mm) and its subsequent influence on the dune-induced hyporheic exchange are investigated. Fine sand (D 50 = 0.28 mm), coarse sand (D 50 = 1.7 mm), and gravel (D 50 = 5.5 mm) grains were used to form homogenous model streambeds; one control-no clay input, and two treatments-increasing clay inputs for each grain type. The results indicate that the clogging profiles of clay-sized sediments may not be predicted accurately using the previously proposed metric based on the relative sizes of infiltrating and substrate sediments. Further, the depositional patterns vary with the initial concentration of clay particles in the surface water. The assessment of clogging profiles in coarse-grained model streambeds also reveals a preferential infiltration of the clay particles in the hyporheic downwelling regions. The results from the dye tracer test suggest that the accumulation of clay particles altered the exchange characteristics in the treatment flumes. For each grain size, the treatment flumes exhibit lower hyporheic flux and higher median residence times compared to their respective control flumes. The dye penetration depths were lower in treatment flumes with fine and coarse sand compared to their respective control flumes. Interestingly, higher penetration depths were observed in treatment flumes with gravel compared to their respective control flume potentially due to the generation of preferential flow paths in the partially clogged gravel beds. The clogging altered the hyporheic fluxes and residence times in the coarse-grained model beds to a greater degree in comparison to the fine sand beds. Overall, our findings indicate that the properties of both fine and substrate sediments influence the clogging patterns in streambeds, and the subsequent influence of fine sediment clogging on hyporheic exchange and associated processes may vary across stream ecosystems.
Bioturbation occurs in streambeds by the action of a range of faunal species, but little is known about how it could modify the hyporheic exchange in streams. Previous experimental work investigating the effects of sediment‐biota interaction on exchange across the sediment‐water interface has been largely conducted in small mesocosms or infiltration columns that do not represent the lotic environment adequately. Therefore, the experimental findings from these studies may not be transferable to flowing water environments (e.g., streams). In this work, we first present a conceptual model demonstrating the causal pathways through which the sediment reworking and burrow ventilation processes (together referred to as bioturbation) could potentially modify the hyporheic flow regime. Next, to study the role of activities of faunal organisms in lotic environments and test some of the arguments presented in the model, laboratory experiments are conducted in re‐circulating flumes. The experiments involved investigating the modification of dune‐induced hyporheic flow due to the activities of a model bioturbating organism, Lumbriculus variegatus, following a control (without organisms) and treatment (with organisms) based experimental design. The sediment reworking activities such as surficial deposition of fecal pellets and burrowing by L. variegatus caused significantly higher hyporheic flux, longer mean residence times, and deeper solute penetration in the treatment flumes relative to the control flumes. We advocate that more intensive laboratory experiments and field investigations must be conducted to test the propositions put forward in the conceptual model and advance our understanding of the role of bioturbation process in fluvial ecosystems.
The two-way exchange of mass and energy across the sediment-water interface (SWI) in streams, referred to as hyporheic exchange, underpins several ecosystem functions such as natural processing of nutrients/pollutants (Bardini et al., 2012;Gandy et al., 2007) and supporting sub-surface ecology (Brunke & Gonser, 1997). Among the major drivers of hyporheic exchange, particularly at small scales, are the pressure gradients at the bed surface (arise due to interaction between surface water and morphological features like bedforms) and the bed permeability (a hydraulic property of sediments) (Bardini et al., 2012;Packman & Salehin, 2003;Storey et al., 2003). Two critical processes that can modify the morphology and permeability of streambeds, and consequently the hyporheic exchange flows are-fine sediment clogging and sediment reworking by in-stream faunal organisms (Shrivastava et al., 2020a). While the influence of fine sediment clogging on hyporheic exchange has garnered
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