Hyporheic Exchange Flow Research Articles

Hydrologic connectivity and dynamics of solute transport in a mountain stream: Insights from a long-term tracer test and multiscale transport modeling informed by machine learning

The movement of solutes in a watershed is a complex process with multiple interactions and feedbacks across spatial and temporal scales. Modeling the dynamics of solute transport along diverse hydrologic pathways within watersheds – from hillslopes to stream channels and in and out of the hyporheic zones – is challenging but critically important, as these processes integrate and contribute to the biogeochemical functioning of the river corridor up to the river network scale. Here we use results from a long-term network-scale tracer test at the H.J. Andrews experimental forest in western Cascade Mountains, Oregon, USA to inform a multiscale framework for transport in stream corridors. The framework uses a Lagrangian-based subgrid model to represent the effects of hyporheic exchange flow and advective transport at stream network scales. The spatially and temporally resolved stream discharge needed for the transport model is imputed across the river system by an entity-aware long short-term memory network. Modeled concentrations show good agreements with the observations and exhibit power scaling laws indicative of a very wide range of timescales over which hyporheic exchange flow occurs. Our results demonstrate a data-informed modeling framework that links dynamical processes occurring at small scales to a network context to help understand how changes at reach scale cascade into network-scale effects, providing a useful tool for sustainable river basin management.

Hyporheic exchange flows in a mountainous river catchment identified by distributed temperature sensing

AbstractElevated stream temperatures under low‐flows, exacerbated by global warming, are a stressor that affects aquatic species directly or in combination with other stressors. Stream temperatures are influenced by energy fluxes across the air–water interface as well as by hydrological exchange processes occurring at the water–riverbed interface. Small‐scale stream temperature dynamics influenced by exchange flows are still underrepresented in stream temperature research. To investigate high‐resolution temperature dynamics and hydrological exchange processes at the sediment–water interface we applied fiber‐optic distributed temperature sensing (FO‐DTS) at two sites in the mountainous Kinzig catchment combined with mapping and measurement of additional environmental conditions. Two types of temperature anomalies could be observed at one site under conditions of low flow and high air temperature. Dampening effects coincided with riverine features such as pools, vegetation roots, fine sediment, and signs of streambank seepage which indicated hyporheic exchange flows. Increased heating of the substrate during the day was identified in shallow sections where sediment was exposed to the air and shading from riparian vegetation was patchy. At another site, at which the cable could not be buried because of the sediment composition, temperature anomalies in the overlying water indicated diffuse groundwater exfiltration. The results show that small‐scale processes in the hyporheic zone, low water tables, and riparian shading influence stream temperature in mountainous streams and can be identified with FO‐DTS under suitable conditions. The results improve our understanding of stream temperatures (in the hyporheic zone) and provide important information on how to improve hydrological modeling.

Influence of bank slope on sinuosity-driven hyporheic exchange flow and residence time distribution during a dynamic flood event

Abstract. This study uses a reduced-order two-dimensional (2-D) horizontal model to investigate the influence of the riverbank slope on the sinuosity-driven hyporheic exchange process along sloping alluvial riverbanks during a transient flood event. The deformed geometry method (DGM) is applied to quantify the displacement of the sediment–water interface (SWI) along the sloping riverbank during river stage fluctuation. This new modeling approach serves as the initial step focusing on the impact of the bank slope on the hyporheic exchange flux (HEF) and the residence time distribution (RTD) of pore water in the fluvial aquifer for a sinuosity-driven river corridor. Several controlling factors, including sinuosity, alluvial valley slope, river flow advective forcing and duration of flow, are incorporated into the model to investigate the effects of bank slope on aquifers of variable hydraulic transmissivity. Compared to simulations of a vertical riverbank, sloping riverbanks were found to increase the HEF. For sloping riverbanks, the hyporheic zone (HZ) encompasses a larger area and penetrated deeper into the alluvial aquifer, especially in aquifers with smaller transmissivity (i.e., due to increased hydraulic conductivity or reduced specific yield). Furthermore, consideration of sloping banks as compared to a vertical riverbank can lead to both underestimation and overestimation of the pore water travel time. The impact of bank slope on residence time was more pronounced during a flood event for high-transmissivity aquifer conditions, while it had a long-lasting influence after the flood event in lower-transmissivity aquifers. Consequently, the impact of bank slope decreases the travel time of water discharging into the river relative to base flow conditions. These findings highlight the need for (re)consideration of the importance of complex riverbank morphology conceptualization in numerical models when accounting for the HEF and RTD. The results have potential implications for river management and restoration and the management of river and groundwater pollution.

High-Resolution Sensor System to Investigate Solute Transport in Saturated Sediments

Highlights Innovative sensor system was developed to detect solute transport at very high time resolutions. The sensors measure electrical conductivity as voltage drops at a frequency of up to 10 Hz, allowing for detailed solute breakthrough curves (BTCs). One-dimensional solute transport equations were modeled numerically using the finite differences technique, and the results show good agreement between the modeled BTCs and those obtained in the laboratory for all materials. Abstract. Hyporheic exchange flow, the movement of river water into streambed sediments and its subsequent return to the surface water after suffering biogeochemical transformations, impacts a stream’s water quality and ecology. These processes, which are the basis for the self-purification capacity of river systems, depend on the quantity and travel time of the flow within the sediments. Different tracer-based approaches have been deployed for studying flow in saturated sediments, but few of these experimental methodologies are able to attain the high temporal resolutions that are needed to adequately capture the passage of a solute plume in a porous medium. To allow for a better description of such flows, we designed and fabricated an innovative, low-cost sensor system able to detect solute transport at multiple locations simultaneously at very high time resolutions. The sensors measure electrical conductivity as voltage drops at a frequency of up to 10 Hz, allowing for detailed solute breakthrough curves (BTCs). The fabricated sensors were tested in a laboratory experimental setup designed to examine solute transport under conditions that reproduce 1D flow in saturated sediments. The sensor system was able to successfully detect electrical conductivity (expressed as voltage drop) in real-time, at high temporal resolution, and at multiple locations. Solute BTCs collected with the system were consistent and highly repeatable under the same conditions. One-dimensional solute transport equations were modeled numerically using the finite differences technique, and the results showed good agreement between the measured and modeled BTCs for all materials. The proposed system is inexpensive compared to conventional EC probes, easy to operate, and sensitive to low solute concentrations. Keywords: Hyporheic exchange flow, Saturated sediments, Sensor, Solute transport.

Assessing Hydrology, Biogeochemistry, and Organic Micropollutants in an Urban Stream‐Aquifer System: An Interdisciplinary Data Set

AbstractUrban expansion leads to increasing water pollution, impacting both human health and ecosystems. This decline in water quality often stems from insufficient wastewater treatment, along with runoff from both urban and agricultural areas. Water quality degradation challenges our efforts for sustainable water management and hinders progress toward the UN's Sustainable Development Goals (SDGs), particularly SDG6. Within aquatic environments, the hyporheic zone—the subsurface area where surface water and groundwater mix—plays a crucial role in facilitating pollutant turnover and overall aquatic health. Mixing between surface water and groundwater generates diverse microhabitats in the streambed with varying levels of oxygen, temperature, and chemical composition, which in turn allows diverse microbial communities to strive. The complexity of exchange flows within the hyporheic zone and associated turnover processes of pollutants and nutrients can only be assessed with detailed, cross‐disciplinary data sets including data about hydrology, climatology, biogeochemistry, and the subsurface composition. However, integrated data sets of this kind are seldom available. As a result, the drivers behind pollutant dynamics in stream‐aquifer systems are still not fully understood. Addressing this knowledge deficit, we present a comprehensive unique data set from an urban stream‐aquifer system in Switzerland spanning over 6 months. Incorporating hydrometric, tracer, nutrient, microbial and organic micropollutant data, our data set can help to shed light on the intricate mechanisms governing hyporheic exchange flows, as well as nutrient and organic micropollutant cycling in urban environments.

Effects of near-bed turbulence on microplastics fate and transport in streams

Quantifying the impact of hyporheic exchange is crucial for understanding the transport and fate of microplastics in streams. In this study, we conducted several Computational Fluid Dynamics (CFD) simulations to investigate near-bed turbulence and analyze vertical hyporheic exchange. Different arranged spheres were used to represent rough and permeable sediment beds in natural rivers. The velocities associated with vertical hyporheic flux and the gravitational force were compared to quantify the susceptibility of microplastics to hyporheic exchange. Four scenario cases representing different channel characteristics were studied and their effects on microplastics movements through hyporheic exchange were quantitatively studied. Results show that hyporheic exchange flow can significantly influence the fate and transport of microplastics of small and light-weighted microplastics. Under certain conditions, hyporheic exchange flow can dominate the behavior of microplastics with sizes up to around 800 μm. This dominance is particularly evident near the sediment-water interface, especially at the top layer of sediments. Higher bed porosity enhances the exchange of microplastics between water and sediment, while increased flow conditions extend the vertical exchange zone into deeper layers of the bed. Changes in the bedform lead to the most pronounced vertical hyporheic exchange, emphasizing the control of morphological features on microplastics transport. Furthermore, it is found that sweep-ejection events are prevailing near the bed surface, serving as a mechanism for microplastics transport in rivers. As moving from the water column to deeper layers in the sediment bed, there's a shift from sweeps dominance to ejections dominance, indicating changes of direction in microplastics movement at different locations.

Effects of a large-scale dam structure on upstream and downstream lateral hyporheic exchange and residence time distributions – The Xinglong Water Conservancy Dam, China

Anthropogenic in-stream structures such as large dams can alter stream hydrodynamics and cause river stage fluctuations that have the potential to affect hyporheic exchange flow (HEF) patterns between rivers and groundwater. However, the development of HEF patterns and residence time distribution (RTD) in dam-regulated riverbank aquifers has mostly been studied focusing on areas downstream of large dam structures while upstream areas have often been neglected. In order to investigate the HEF and RTD patterns under the influence of a large dam in upstream and downstream areas simultaneously, groundwater level data was collected from thirteen monitoring wells distributed over three monitoring profiles both upstream and downstream of XingLong water conservancy dam (XLD), in the Han river, China. Based on field observations, a two-dimensional, horizontal numerical model was built to assess the spatiotemporal evolution of lateral hyporheic exchange flow, the spatial extent of the hyporheic zone (HZ) and groundwater RTD under the influence of the XLD. Our results demonstrate that the impact of the XLD is different in upstream and downstream aquifer regions. The storage effect induced by the XLD caused significant groundwater recharge upstream of the dam over long period of model, while significant groundwater discharge occurs downstream of the dam. As the stage of the regulated Han River varies, the HZ along the upstream river boundary significantly enhanced by the dam whereas substantial HEF downstream was observed in response to rising or high river stage. Furthermore, the operation of the XLD resulted in some substantial flow around the dam characterized by short flow path lengths and high flow velocity, which consequently resulted in the development of a significant HZ in the perimeter surrounding the XLD. Residence time distributions showed that the water in the HZ downstream of the XLD is dominated by rejuvenated (younger) groundwater, whereas the HZ upstream of the XLD comprises a mix of older and younger waters. HEF in the immediate vicinity of the dam comprises both aged and rejuvenated discharged groundwater. Our findings emphasize the need to more holistically consider river stage dynamics in river corridors where large dams are operated to better assess and understand the consequences on riverbank storage as well as flow, transport and attenuation patterns in connected floodplain aquifers and their impact on the aquatic environment.

Geoelectric Characterization of Hyporheic Exchange Flow in the Bedrock‐Lined Streambed of East Fork Poplar Creek, Oak Ridge, Tennessee

AbstractA multimethod geoelectric survey was implemented between January and March 2022 along a 220‐m long reach of the bedrock‐lined streambed of East Fork Poplar Creek in Oak Ridge, Tennessee to identify locations of surface‐water and groundwater exchange and characterize the subsurface flow paths that convey water between the stream and flood plain. A waterborne self‐potential (WaSP) survey was completed in January 2022 to measure the electric streaming‐potential field in the stream. Electric resistivity tomography (ERT) was performed in March 2022 on the flood plain adjacent to the WaSP survey reach to map the electric resistivity distribution and characterize the hydrogeology and subsurface flow paths that facilitate surface‐water and groundwater exchange in the bedrock‐lined stream. The combination of WaSP and ERT data support the qualitative interpretation that surface‐water and groundwater exchange likely occurs along fractures in outcropping bedrock and along two fault lines that intersect the limestone creek bed.

Geographic and hydromorphologic controls on interactions between hyporheic flow and discharging deep groundwater

Hyporheic exchange flow (HEF) at the streambed–water interface (SWI) has been shown to impact the pattern and rate of discharging groundwater flow (GWF) and the consequential transport of heat, solutes and contaminants from the subsurface into streams. However, the control of geographic and hydromorphological catchment characteristics on GWF–HEF interactions is still not fully understood. Here, the spatial variability in flow characteristics in discharge zones was investigated and averaged over three spatial scales in five geographically different catchments in Sweden. Specifically, the deep GWF discharge velocity at the SWI was estimated using steady-state numerical models, accounting for the real multiscale topography and heterogeneous geology, while an analytical model, based on power spectral analysis of the streambed topography and statistical assessments of the stream hydraulics, was used to estimate the HEF. The modeling resulted in large variability in deep GWF and HEF velocities, both within and between catchments, and a regression analysis was performed to explain this observed variability by using a set of independent variables representing catchment topography and geology as well as local stream hydromorphology. Moreover, the HEF velocity was approximately two orders of magnitude larger than the deep GWF velocity in most of the investigated stream reaches, indicating significant potential to accelerate the deep GWF velocity and reduce the discharge areas. The greatest impact occurred in catchments with low average slope and in reaches close to the catchment outlet, where the deep GWF discharge velocity was generally low.

Hyporheic Exchange in a Meandering Pool‐Riffle Stream

AbstractInteractions among stream characteristics, hyporheic exchange (HE), and groundwater flow are a crucial control for physicochemical and biological reactions in hyporheic zone (HZ). Many previous studies solely investigated the HE induced by pool‐riffle bedforms or meanders but few research studied the interactions between them despite this pair of morphology generally coexisting in natural streams. Adopting similar laboratory and numerical methods as Huang and Chui (2021), https://doi.org/10.1029/2020WR029182 whose pool‐riffle streams were straight, this study adequately studied the impacts of hydraulic and morphologic factors on the HE in meandering pool‐riffle streams and highlighted the mutual influence among various factors such as bedform amplitude and sinuosity. The results show that meanders significantly complicate the three‐dimensional hyporheic flow paths and increase the flowrate and scale of HE. The HE flowrate increases linearly with discharge, bed slope, and sinuosity. The total HZ volume and maximum lateral and vertical extents of HZ generally increase with discharge. They also increase with bed slope and sinuosity but only up to a particular threshold, and they remain nearly constant or slightly decrease after exceeding the threshold. All HE characteristics exponentially decrease with both losing and gaining groundwater flow, and losing conditions have less significant effects on the HE compared with gaining conditions. The HE in stream bank can be larger than that in streambed, and the former is generally more influenced by discharge and sinuosity than the latter. Effects of bedform amplitude on HE exhibit very complex patterns due to the mutual influence of key factors.

Modeling Contaminant Microbes in Rivers During Both Baseflow and Stormflow.

Rivers transport contaminant microorganisms (including fecal indicator bacteria and human pathogens) long distances downstream of diffuse and point sources, posing a human health risk. We present a mobile‐immobile model that incorporates transport as well as immobilization and remobilization of contaminant microbes and other fine particles during baseflow and stormflow. During baseflow conditions, hyporheic exchange flow causes particles to accumulate in streambed sediments. Remobilization of stored particles from streambed sediments occurs slowly during baseflow via hyporheic exchange flow, while remobilization is vastly increased during stormflow. Model predictions are compared to observations over a range of artificial and natural flood events in the dairy contaminated Topehaehae Stream, New Zealand. The model outputs closely matched timing and magnitude of E. coli and turbidity observations through multiple high‐flow events. By accounting for both state‐of‐flow and hyporheic exchange processes, the model provides a valuable framework for predicting particle and contaminant microbe behavior in streams.

Organizational Principles of Hyporheic Exchange Flow and Biogeochemical Cycling in River Networks Across Scales

AbstractHyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualizations of hyporheic exchange, residence time distributions, and the associated biogeochemical cycling in streambed sediments do not always accurately explain the hydrological and biogeochemical complexity observed in streams and rivers. Specifically, existing conceptual models insufficiently represent the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in streambed biogeochemical functioning, and the feedbacks between surface‐subsurface ecological processes, both within and across spatial and temporal scales. While simplified approaches to these issues are justifiable and necessary for transferability, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions and inadequate understanding and management of interconnected surface water and groundwater environments. This is particularly true at the landscape scale, where the organizational principles of spatio‐temporal dynamics of hyporheic exchange flow (HEF) and biogeochemical processes remain largely uncharacterized. This article seeks to identify the most important drivers and controls of HEF and biogeochemical cycling based on a comprehensive synthesis of findings from a wide range of river systems. We use these observations to test current paradigms and conceptual models, discussing the interactions of local‐to‐regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning. This improved conceptualization of the landscape organizational principles of drivers of HEF and biogeochemical processes from reach to catchment scales will inform future river research directions and watershed management strategies.

Sediment Reworking in Streambeds With Fine Sediment Deposits and Its Influence on Hyporheic Flow Regime

AbstractThe mobilization and mixing of sediments by the activities of in‐stream fauna, referred to as sediment reworking, constantly modifies the hydro‐physical properties of streambeds. This sediment‐organism interaction has been increasingly recognized to influence the hyporheic exchange flows in stream ecosystems, particularly in low flow environments (e.g., during base flow). In this work, we advance the knowledge of sediment reworking process by studying its impact on hyporheic exchange flows in streambeds with fine sediment deposits. Laboratory experiments are conducted in re‐circulating flumes following a control (only fine sediments) and treatment‐ (fine sediments + organisms) based design. The experiments involve studying the interaction of model organisms (Lumbriculus variegatus) with fine sediment (clay) deposits, and its subsequent influence on hyporheic flow regime in homogenous streambeds with fine sand, coarse sand, and gravel as substrate sediments. We observe that model organisms burrowed extensively into the fine sediment layer, mixed the clay particles with underlying grains, and eventually exposed the substrate sediments in the treatment flumes. Consequently, the treatment flumes exhibited greater solute penetration depth, shorter residence times, and higher hyporheic exchange flux compared to their respective control flumes. The results also suggest that the modification of hyporheic exchange flows depends on the overall reworking of the beds that is, the interaction of organisms with both substrate material and deposited fine sediments. It is critical to comprehend the influence of streambed inhabitants on mass and energy exchange across the sediment‐water interface as it has implications on the overall quality of both stream and groundwater.

Cross‐Validating Hydromechanical Models and Tracer Test Assessments of Hyporheic Exchange Flow in Streams With Different Hydromorphological Characteristics

AbstractHyporheic exchange flow (HEF) can generally be quantified through two different approaches. The first approach, which is deductive, entails physically based models, supported with relevant observations. The second approach includes inductive assessments of stream tracer tests using solute transport models, which provide a useful mathematical framework that allows for upscaling of results, but included parameters often have a vague physical base, which limits the possibilities of generalizing results using independent hydromorphological observations. To better understand how the physical basis of HEF‐quantifying parameters relates to stream hydromorphology at different spatial scales, we cross‐validated the results from (a) tracer test assessments using a 1D solute transport model that accounts for HEF and (b) an independent hydromechanical model that represents HEF driven by multiscale pressure gradients along the streambed interface. To parameterize the models, topographical surveys, tracer tests, and streambed hydraulic conductivity measurements were performed in 10 stream reaches, differing in terms of geomorphology, slope, and discharge. The results show that the models were cross‐validated in terms of the average exchange velocity, providing a plausible physical explanation for this parameter in small alluvial streams with low discharges, shallow depth, and moderate slopes. However, the hydromechanical model generally resulted in wider residence time distributions and occasionally higher average residence times compared to the tracer test assessments. From the cross‐validated multiscale hydromechanical model, we learned that water surface profile variations were the main drivers of HEF in all investigated streams and that spatial scales between 20 cm and 5 m dominated the estimated HEF velocity.

Hyporheic transverse mixing zones and dispersivity: Laboratory and numerical experiments of hydraulic controls

Mixing of surface water and groundwater in shallow sediments is important to biogeochemical cycling and contaminant migration, and is often used to define the hyporheic zone. Yet knowledge of mixing processes in hyporheic zones is supported by surprisingly few rigorous lab or field observations, and differ from those in deeper groundwater by presence of enhanced head gradients, sediment heterogeneity, and temporal fluctuations. In a laboratory sediment (sand) tank we photographed a conservative dye to analyze transverse mixing zones between upwelling groundwater and bidirectional hyporheic exchange flows. We then conducted numerical modeling to investigate processes behind observed phenomena and estimate dispersivities. We found that transverse mixing zones were thin (i.e. mixing thickness measured in direction of steepest concentration gradient, δ, less than 5 cm), consistent with a small calibrated transverse dispersivity (~0.1 mm) and prior lab studies conducted at similar scales. In steady-state experiments and simulations, δ and estimated dispersion coefficients increased with the surface water head drop driving exchange flows. Given relatively constant deeper groundwater heads, increased Δh led to increased mixing zone length for both steady-state and transient conditions, indicating larger bedforms or weaker gaining conditions enhance subsurface mixing. However, Peclet number and flux-related dilution index simultaneously increased and decreased, respectively, indicating that enhancement of subsurface advection outpaced that of dispersion. In transient experiments and simulations, δ was greater than for steady-state, probably from temporary addition of longitudinal dispersion. During transient experiments, δ exhibited temporal noise, perhaps due to the mixing zone moving past varying patterns of sediment packing. Our results provide basic knowledge of mixing zone behavior in hyporheic zones with implications for hyporheic zone definitions, solute transport, mixing-dependent reaction, and water quality.

An Integrative Conceptualization of Floodplain Storage

AbstractFloodplains perform diverse functions, including attenuation of fluxes of water, solutes, and particulate material. Critical details of floodplain storage including magnitude, duration, and spatial distribution are strongly influenced by floodplain biogeochemical processes and biotic communities. Floodplain storage of diverse materials can be conceptualized in the form of a budget that quantifies inputs, outputs, and storage within the floodplain control volume. The floodplain control volume is here defined as bounded on the inner edges by the banks of the active channel(s), on the outer edges by the limit of periodic flooding and the deposition of fluvially transported sediment, on the underside by the extent of hyporheic exchange flows and the floodplain aquifer, and on the upper side by the upper elevation of living vegetation. Fluxes within the floodplain control volume can also change the location, characteristics, and residence time of material in storage. Fluxes, residence time, and quantities of material stored in floodplains can be measured directly; inferred from diverse types of remotely sensed data; or quantitatively estimated using numerical models. Human activities can modify floodplain storage by: hydrologically and/or geomorphically disconnecting channels and floodplains; altering fluxes of water and sediment to the river corridor; and obliterating floodplains through alluvial mining or urbanization. Floodplain restoration can focus on enlarging the functional floodplain, reconnecting the channel and floodplain, restoring natural regimes of water, sediment, and/or large wood, or enhancing the spatial heterogeneity of the channel and floodplain. Each form of floodplain restoration can increase floodplain storage and resilience to disturbances.

Spatial and temporal dynamics of nitrogen exchange in an upwelling reach of a groundwater‐fed river and potential response to perturbations changing rainfall patterns under UK climate change scenarios

AbstractWe report the complex spatial and temporal dynamics of hyporheic exchange flows (HEFs) and nitrogen exchange in an upwelling reach of a 200 m groundwater‐fed river. We show how research combining hydrological measurement, geophysics and isotopes, together with nutrient speciation techniques provides insight on nitrogen pathways and transformations that could not have been captured otherwise, including a zone of vertical preferential discharge of nitrate from deeper groundwater, and a zone of rapid denitrification linking the floodplain with the riverbed. Nitrate attenuation in the reach is dominated by denitrification but is spatially highly variable. This variability is driven by groundwater flow pathways and landscape setting, which influences hyporheic flow, residence time and nitrate removal. We observed the spatial connectivity of the river to the riparian zone is important because zones of horizontal preferential discharge supply organic matter from the floodplain and create anoxic riverbed conditions with overlapping zones of nitrification potential and denitrification activity that peaked 10–20 cm below the riverbed. Our data also show that temporal variability in water pathways in the reach is driven by changes in stage of the order of tens of centimetres and by strength of water flux, which may influence the depth of delivery of dissolved organic carbon. The temporal variability is sensitive to changes to river flows under UK climate projections that anticipate a 14%–15% increase in regional median winter rainfall and a 14%–19% reduction in summer rainfall. Superimposed on seasonal projections is more intensive storm activity that will likely lead to a more dynamic and inherently complex (hydrologically and biogeochemically) hyporheic zone. We recorded direct evidence of suppression of upwelling groundwater (flow reversal) during rainfall events. Such flow reversal may fuel riverbed sediments whereby delivery of organic carbon to depth, and higher denitrification rates in HEFs might act in concert to make nitrate removal in the riverbed more efficient.

Macrophyte Controls on Urban Stream Microbial Metabolic Activity

Urban rivers worldwide are affected directly by macrophyte growth, causing reduced flow velocity and risks of flooding. Therefore, cutting macrophytes is a common management practice to ensure free drainage. The impacts of macrophyte removal on transient storage dynamics and microbial metabolic activity of wastewater-fed urban streams are unknown, preventing any assessment of the hydrodynamic and biogeochemical consequences of this management practice. Slug tracer injections were performed with the conservative tracer uranine and the reactive tracer resazurin to quantify the implications of macrophyte cutting on stream flow dynamics and metabolism. Macrophyte cutting reduced mean tracer arrival times in managed stream reaches but did not significantly decrease whole-stream microbial metabolic activity. In fact, transient storage indices were found to have increased after cutting, suggesting that macrophyte removal and the resulting increase in flow velocity may have enhanced hyporheic exchange flow through streambed sediments. Our results evidence that macrophyte cutting in nutrient-rich urban streams does not necessarily lead to lower in-stream storage and metabolism but that the gain in hyporheic exchange and streambed microbial metabolic activity can compensate for reduced in-stream storage. Increased stream flow resulting from macrophyte removal may therefore even enhance nutrient and pollutant attenuation capacity of streambed sediments.

High-Resolution Integrated Transport Model for Studying Surface Water-Groundwater Interaction.

Transport processes that lead to exchange of mass between surface water and groundwater play a significant role for the ecological functioning of aquatic systems, for hydrological processes and for biogeochemical transformations. In this study, we present a novel integral modeling approach for flow and transport at the sediment-water interface. The model allows us to simultaneously simulate turbulent surface and subsurface flow and transport with the same conceptual approach. For this purpose, a conservative transport equation was implemented to an existing approach that uses an extended version of the Navier-Stokes equations. Based on previous flume studies which investigated the spreading of a dye tracer under neutral, losing and gaining flow conditions the new solver is validated. Tracer distributions of the experiments are in close agreement with the simulations. The simulated flow paths are significantly affected by in- and outflowing groundwater flow. The highest velocities within the sediment are found for losing condition, which leads to shorter residence times compared to neutral and gaining conditions. The largest extent of the hyporheic exchange flow is observed under neutral condition. The new solver can be used for further examinations of cases that are not suitable for the conventional coupled models, for example, if Reynolds numbers are larger than 10. Moreover, results gained with the integral solver provide high-resolution information on pressure and velocity distributions at the rippled streambed, which can be used to improve flow predictions. This includes the extent of hyporheic exchange under varying ambient groundwater flow conditions.

A mathematical approach to improving the representation of surface water–groundwater exchange in the hyporheic zone

AbstractIt is well known that land surface topography governs surface–groundwater interactions under some circumstances and can be separated in a Fourier-series spectrum that provides an exact analytical solution of both the surface and the underlying three-dimensional groundwater flows. We evaluate the performance of the current Fourier fitting process by testing on different scenarios of synthetic surfaces. We identify a technical gap and propose a new version of the approach which incorporates the spectral analysis method to help identify the statistically significant frequencies of the surface to guide the refinement and mesh. Our results show that spectral analysis is the method that can help improve the accuracy of representing the surface, thus further improving the accuracy of predicting the bedform-driven hyporheic exchange flows.