A. F. Aubeneau

Substrate size and heterogeneity control anomalous transport in small streams

In alluvial systems, substrate characteristics play a critical role in slowing downstream transport of both water and solutes. We present results from solute injection experiments testing the influence of sediment size (pea gravel versus coarse gravel) and heterogeneity (alternating sections versus well-mixed reaches) on solute transport dynamics in four experimental streams at the Notre Dame Linked Experimental Ecosystem Facility. The stream with pea gravel resulted in more long-term retention than the stream with coarse gravel, whereas both streams with heterogeneous substrate (alternated and mixed) fell between with similar late-time scaling. Inverse modeling of solute breakthrough curves suggested that residence times were distributed according to a truncated power law. While conservative solute transport in all four streams was anomalous, truncation times were influenced by sediment size, with the smaller pea gravel exhibiting a later truncation time than the coarse gravel, and the two streams with heterogeneous substrate having an intermediate cutoff. These results uniquely associate transport scaling with substrate characteristics in fluvial systems, revealing truncation time scales that had been previously predicted but not observed and quantified in field conditions. Because both benthic (i.e., substrate-water interface) and subsurface hyporheic regions are known biogeochemical hot spots, relating physical characteristics to the macroscopic transport behavior could be crucial to improve our estimates of solute export from fluvial systems.

Stochastic modeling of fine particulate organic carbon dynamics in rivers

The majority of particulate organic matter standing stock in streams is < 1 mm in diameter, and the mobile phase is primarily very fine particles. Such fine particles transport downstream in a series of deposition and resuspension events mediated by interactions with coarser bed sediment, yielding fine particle retention over a wide range of time scales. This retention controls the opportunity for biogeochemical processing of particulate organic carbon in streams. We present a conceptual model of particulate organic carbon transport in rivers categorized in three cyclic processes: (i) migration of fine particles from the water column to the underlying and surrounding sediments, (ii) fine particle transport and retention within the bed sediments, and (iii) resuspension of fine particles back to the water column. We developed a stochastic model to describe the transport and retention of fine suspended particles in rivers, including advective delivery of particles to the streambed, transport through pore waters, and reversible filtration within the streambed. We then apply this model to observations of fine particle transport in two small streams, and show that the stochastic mobile-immobile model supports improved interpretation of particulate organic carbon dynamics under base flow conditions. Analysis of in-stream solute and particle data shows that particles engage in multiple deposition and resuspension events during downstream transport, and that long-term retention in the streambed produces extended slow releases to the stream even during base flow conditions. We also show how multiscale stochastic modeling can be used to incorporate local observations of particle retention in predictions of whole-stream particle dynamics.

Effects of benthic and hyporheic reactive transport on breakthrough curves

In streams and rivers, the benthic and hyporheic regions harbor the microbes that process many stream-borne constituents, including O2, nutrients, C, and contaminants. The full distribution of transport time scales in these highly reactive regions must be understood because solute delivery and extended storage in these metabolically active zones control the opportunity for biogeochemical processing. The most commonly used transport models cannot capture these effects. We present a stochastic model for conservative and reactive solute transport in rivers based on continuous-time random-walk theory, which is capable of distinguishing and capturing processes not described by classical approaches. The model includes surface and subsurface storage zones with arbitrary residence-time distributions. We used this model to evaluate the effects of sorption and biological uptake on downstream solute transport. Linear or mildly nonlinear sorption in storage delays downstream transport without changing the fundamental shape of the breakthrough curves (BTCs). Highly nonlinear sorption isotherms can induce power-law tailing in stream BTCs. Model simulations show that sorption of commonly used solute tracers is not sufficient to explain the power-law tailing that has been observed in field tracer-injection studies, and instead, such tailing most probably reflects broad distributions of hyporheic exchange time scales. First-order biological uptake causes an exponential decline in in-stream tracer concentrations at the time scale of the uptake kinetics, thereby tempering power-law BTCs. The model can be used to calculate reach-scale reaction-rate coefficients in surface and subsurface storage from observed BTCs of co-injected conservative and reactive solutes, providing new capability to determine reaction-rate coefficients in storage zones with broad residence-time distributions.

Fractal patterns in riverbed morphology produce fractal scaling of water storage times

River topography is famously fractal, and the fractality of the sediment bed surface can produce scaling in solute residence time distributions. Empirical evidence showing the relationship between fractal bed topography and scaling of hyporheic travel times is still lacking. We performed experiments to make high-resolution observations of streambed topography and solute transport over naturally formed sand bedforms in a large laboratory flume. We analyzed the results using both numerical and theoretical models. We found that fractal properties of the bed topography do indeed affect solute residence time distributions. Overall, our experimental, numerical, and theoretical results provide evidence for a coupling between the sand-bed topography and the anomalous transport scaling in rivers. Larger bedforms induced greater hyporheic exchange and faster pore water turnover relative to smaller bedforms, suggesting that the structure of legacy morphology may be more important to solute and contaminant transport in streams and rivers than previously recognized.

Biofilm growth in gravel bed streams controls solute residence time distributions

Streambed substrates harbor a rich biome responsible for biogeochemical processing in riverine waters. Beyond their biological role, the presence of benthic and hyporheic biofilms can play an important role in influencing large-scale transport of solutes, even for conservative tracers. As biofilms grow and accumulate biomass, they actively interact with and influence surface and subsurface flow patterns. To explore this effect, we conducted experiments at the Notre Dame Linked Ecosystems Experimental Facility in four outdoor streams, each with different gravel beds. Over the course of 20 weeks we conducted transport experiments in each of these streams and observed different patterns in breakthrough curves as biofilms grew on the substrate. Biofilms played a major role in shaping the observed conservative transport patterns. Overall, while the presence of biofilms led to a decreased exchange rate between the fast (mobile) and slow (immobile) parts of the flow domain, water that was exchanged tended to be stored in the slow regions for longer times once biofilms had established. More specifically, we observed enhanced longitudinal dispersion in breakthrough curves as well as broader residence time distributions when biofilms were present. Biofilm colonization over time homogenized transport patterns across the four streams that were originally very distinct. These results indicate that stream biofilms exert a strong control on conservative solute transport in streams, a role that to date has not received enough attention.

Covariation in patterns of turbulence‐driven hyporheic flow and denitrification enhances reach‐scale nitrogen removal

Co-injections of conservative tracers and nutrients are commonly used to assess travel time distributions and nutrient removal in streams. However, in-stream tracer data often lack information on long-term hyporheic storage, and removal rate coefficients are often assumed to be uniform despite plentiful evidence that microbially-mediated transformations, such as denitrification, exhibit strong spatial variability in the hyporheic zone. We used process-based particle tracking simulations to explore the coupled effects of spatial patterns in hyporheic flow and denitrification on reach-scale nitrogen removal. We simulated whole-stream nitrogen dynamics with exponential, layered, and uniform profiles of hyporheic denitrification. We also simulated nitrogen dynamics in Little Rabbit Creek, an agricultural headwater stream in the Kalamazoo River Basin (Michigan, U.S.) where vertical profiles of hyporheic denitrification were measured in situ. Covariation between porewater velocity and mixing causes rapid exchange in the near-surface bioactive region and substantially prolonged exchange in the deeper hyporheic. Patterns of hyporheic denitrification covary with patterns of hyporheic flow. This covariation directly controls tailing of in-stream breakthrough curves and hence reach-scale nutrient removal. Enhanced denitrification near the sediment-water interface strongly tempers breakthrough curve tails at timescales associated with flushing of the near-surface region, while more spatially uniform denitrification causes weaker tempering over a wider range of hyporheic exchange timescales. At the reach scale, overall nitrogen removal increases with heterogeneity of hyporheic denitrification, indicating that covariation between flow and denitrification – particularly the rapid flushing of highly bioactive regions near the sediment-water interface – controls whole-stream transformation rates.

Noise-Driven Return Statistics: Scaling and Truncation in Stochastic Storage Processes

In countless systems, subjected to variable forcing, a key question arises: how much time will a state variable spend away from a given threshold? When forcing is treated as a stochastic process, this can be addressed with first return time distributions. While many studies suggest exponential, double exponential or power laws as empirical forms, we contend that truncated power laws are natural candidates. To this end, we consider a minimal stochastic mass balance model and identify a parsimonious mechanism for the emergence of truncated power law return times. We derive boundary-independent scaling and truncation properties, which are consistent with numerical simulations, and discuss the implications and applicability of our findings.

An Integrated Experimental and Modeling Approach to Predict Sediment Mixing from Benthic Burrowing Behavior

Bioturbation is the dominant mode of sediment transport in many aquatic environments and strongly influences both sediment biogeochemistry and contaminant fate. Available bioturbation models rely on highly simplified biodiffusion formulations that inadequately capture the behavior of many benthic organisms. We present a novel experimental and modeling approach that uses time-lapse imagery to directly relate burrow formation to resulting sediment mixing. We paired white-light imaging of burrow formation with fluorescence imaging of tracer particle redistribution by the oligochaete Lumbriculus variegatus. We used the observed burrow formation statistics and organism density to parametrize a parsimonious model for sediment mixing based on fundamental random walk theory. Worms burrowed over a range of times and depths, resulting in homogenization of sediments near the sediment-water interface, rapid nonlocal transport of tracer particles to deep sediments, and large areas of unperturbed sediments. Our fundamental, parsimonious random walk model captures the central features of this highly heterogeneous sediment bioturbation, including evolution of the sediment-water interface coupled with rapid near-surface mixing and anomalous late-time mixing resulting from infrequent, deep burrowing events. This approach provides a general, transferable framework for explicitly linking sediment transport to governing biophysical processes.

Turbulence Links Momentum and Solute Exchange in Coarse‐Grained Streambeds

The exchange of solutes between surface and pore waters is an important control over stream ecology and biogeochemistry. Free-stream turbulence is known to enhance transport across the sediment-water interface (SWI), but the link between turbulent momentum and solute transport within the hyporheic zone remains undetermined due to a lack of in situ observations. Here, we relate turbulent momentum and solute transport using measurements within a streambed with 0.04 m diameter sediment. Pore water velocities were measured using endoscopic particle image velocimetry and used to generate depth profiles of turbulence statistics. Solute transport was observed directly within the hyporheic zone using an array of microsensors. Solute injection experiments were used to assess turbulent fluxes across the SWI and patterns of hyporheic mixing. Depth profiles of fluctuations in solute concentration were compared with profiles of turbulence statistics, and profiles of mean solute concentration were compared to an effective dispersion model. Fluorescent visualization experiments at a Reynolds number of Re 27,000 revealed the presence of large-scale motions that ejected tracer from the pore waters, and that these events were not present at Re 5 13,000. Turbulent shear stresses and high-frequency concentration fluctuations decayed greatly within 1–2 grain diameters below the SWI. However, low-frequency concentration fluctuations penetrated to greater depths than high-frequency fluctuations. Comparison with a constant-coefficient dispersion model showed that hyporheic mixing was enhanced in regions where turbulent stresses were observed. Together, these results show that the penetration of turbulence into the bed directly controls both interfacial exchange and mixing within a transition layer below the SWI. Plain Language Summary Streams and rivers continuously exchange water with their underlying sediments in a region called the hyporheic zone. This zone is a hotspot of transformation for many societally relevant chemicals, including carbon, nutrients, and contaminants. Accurate predictions for how much transformation occurs in the hyporheic zone requires an improved understanding of how reactive chemicals are transported into, and within, this region of a riverbed. Although fluid turbulence can be the dominant process controlling surface-subsurface exchange in gravel-bed streams, its influence is poorly understood due to the difficulty of measuring turbulent fluid velocities and concentrations within the streambed. In this experimental study, we show that turbulence strongly couples surface waters with hyporheic waters in a thin layer where the water column and stream sediments meet. As a result, fluid transport and mixing are enhanced several centimeters into the hyporheic zone of gravel-bed streams. These findings support recent theoretical arguments that surface and subsurface waters are not independent and must instead be treated as a single unit to accurately model solute, particulate and pollutant transport in streams and rivers.

Substrate-specific biofilms control nutrient uptake in experimental streams

Substrate heterogeneity and biofilm colonization in streams vary across both time and space, but their relative contribution to reach-scale nutrient uptake is difficult to partition. We performed multiple short-term nutrient additions over a 4-mo colonization sequence in 4 small, groundwater-fed, experimental streams. We quantified the influence of substrate size (pea gravel vs cobble) and heterogeneity (alternating sections vs well mixed) on the uptake of NH4+, NO3−, and soluble reactive P (SRP) and transient storage properties. In general, the effect of benthic substrate on uptake velocity (vf) and areal nutrient uptake (U) were inversely related to substrate size, and both metrics were highest in the stream lined with pea gravel, lowest in cobble, and intermediate in streams with alternating and mixed substrates. Substrate trends were consistent among solute types, but the magnitude of uptake differed. Uptake generally was higher for NH4+ than for NO3− and SRP in these open-canopy systems. Algal biomass controlled temporal patterns of nutrient uptake but reduced exchange of water between the stream channel and transient storage zone (k1) such that k1 decreased as nutrient uptake increased. Our results uniquely demonstrate that substrate heterogeneity and substrate-specific biofilms interact to influence biogeochemical cycling in streams, with implications for the role of substrate in restoring ecosystem function in impaired systems.