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The Salton Sea has experienced significant recession over the past two decades due to changes in the diversion of Colorado River water to the Salton Trough for agricultural irrigation. As a result, wetlands have emerged in some exposed playa areas along the Salton Sea, primarily in regions with extensive agricultural return flows and agricultural drainage. One notable wetland system, known as the Bombay Beach Wetlands, has formed on the north shore of the Salton Sea, in an area devoid of agriculture. In many other areas with limited or no agriculture, wetlands have failed to develop, leaving exposed playa surfaces as the Salton Sea recedes. These dry playa surfaces pose a significant threat to the health of local residents due to the presence of toxins contained in windblown dust associated with playa deposits. In this study, stable water isotope data, combined with other hydrological information, led to identification of two potential water sources for the Bombay Beach Wetlands. The first possibility proposes that thermal artesian waters alone contribute to the wetlands' water source, while the second hypothesis involves a combination of drainage from Salton Sea bank storage water mixing with the thermal artesian water. The thermal artesian water discharges into drainage channels that flow towards the Bombay Beach Wetlands, initially devoid of possible groundwater baseflow until reaching the wetlands. Studies were subsequently done along the full reach of the drainage channels receiving thermal artesian water. Dissolved solids content, P and N nutrients, arsenic, and stable water isotopes were tested synoptically along the drainage channels. Channel investigations led to the development of a novel model of salinization, which is linked to channel discharge, channel morphometrics, and channel incision. [ABSTRACT FROM AUTHOR]

It is unknown how antibiotics would behave after entering the hyporheic zone (HZ), which is an area where groundwater and surface water alternate continuously. In this study, the hydrolysis process in the HZ was investigated based on the intermediates identified by HPLC-Q-TOF-MS and FTIR, and the active sites of sulfamethoxazole (SMX) were predicted by density functional theory (DFT). The results showed that the hydrolysis rate of SMX during surface water recharged groundwater reached 38.94%, and the contribution rate of hydroxyl radicals reached 48.35%. In neutral and alkaline environments, SMX hydrolysed more quickly. This is due to the fact that ·OH, as the main precursor of OH-, is much higher in quantity under alkaline conditions. Inorganic anions such as NO3-, HCO3- and ${\rm CO}_3^{2-}$ CO 3 2 − may inhibit the hydrolysis of SMX by eliminating the reactive oxygen species generated in the HZ. In the process of groundwater recharging to surface water, the concentration of dissolved oxygen (DO) and the rate of SMX hydrolysis gradually reduced. Nitrification, hydroxylation and polymerisation are the main hydrolysis pathways of SMX. The hydrolysis products of SMX in the HZ are more plentiful and have a higher hydrolysis rate compared to the single oxygen environment. The study on the hydrolysis mechanism of SMX in this paper will provide a theoretical basis for the treatment of antibiotic pollution. The degradation pathways and influencing factors of sulfamethoxazole in the hyporheic zone. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to identify the behaviour of neutrally buoyant microplastics (specific density, 0.94) in different hydrodynamic conditions while focusing on combined wave–current conditions and the mixing across the hyporheic zone. For in-water-column microplastics, it was observed that the streamwise dispersion of neutrally buoyant microplastics is comparable to solute dye in both slow open-channel flow conditions and combined wave–current conditions. However, for in-bed microplastics, when compared to soluble tracers, the longer timespans associated with the hyporheic exchange process allowed the density effects to enhance the vertical exchange when compared to solutes. [ABSTRACT FROM AUTHOR]

Elevated 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. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to identify the behaviour of neutrally buoyant microplastics (specific density, 0.94) in different hydrodynamic conditions while focusing on combined wave–current conditions and the mixing across the hyporheic zone. For in-water-column microplastics, it was observed that the streamwise dispersion of neutrally buoyant microplastics is comparable to solute dye in both slow open-channel flow conditions and combined wave–current conditions. However, for in-bed microplastics, when compared to soluble tracers, the longer timespans associated with the hyporheic exchange process allowed the density effects to enhance the vertical exchange when compared to solutes.

Groundwater numerical modeling is a crucial scientific tool for understanding groundwater circulation and supporting regional water resource planning and management. The effectiveness of these models depends largely on the accuracy of hydrogeological parameters within aquifers, which are often spatially heterogeneous and randomly distributed due to complex geological and tectonic factors. Traditional modeling approaches frequently overlook this randomness, compromising the precision and resolution of groundwater simulations. This study focuses on a section of the Qingshui River in the Huaihe River Basin. Using field and laboratory data, probability distribution functions for key parameters like hydraulic conductivity, specific yield, and specific storage were developed. These functions were integrated into the groundwater model to reflect the inherent stochastic nature of aquifer properties. This integration significantly enhanced model accuracy, reducing the root mean square error of simulated water levels from 0.47-1.43 m to 0.13-0.16 m and improving the Nash-Sutcliffe efficiency coefficients (NSE) from -2.96-0.73 to 0.94-0.98. Additionally, the model facilitated analysis of the interactions between river and groundwater, particularly in the hyporheic zone, under various scenarios. It identified spatial and temporal variations in groundwater recharge dynamics and delay effects at different distances from the river channel. For instance, recharge rates at 50 m and 150 m from the river were 0.295 m/day and 0.015 m/day, respectively, indicating stronger recharge closer to the river. The study also assessed the impact of varying river flows, riverbed permeability, and irrigation practices on water exchanges between the river and groundwater. These factors were found to significantly influence the intensity of water exchange, seepage, and groundwater reserves. This research provides valuable insights for managing river-groundwater interactions and analyzing the ecological environment of surrounding groundwater systems, underscoring the importance of incorporating stochastic characteristics into groundwater modeling. [ABSTRACT FROM AUTHOR]

Hyporheic zones are commonly regarded as resilient and enduring interfaces between groundwater and surface water in river corridors. In particular, bedform‐induced advective pumping hyporheic exchange (bedform‐induced exchange) is often perceived as a relatively persistent mechanism in natural river systems driving water, solutes, and energy exchanges between the channel and its surrounding streambed sediments. Numerous studies have been based on this presumption. To evaluate the persistence of hyporheic zones under varying hydrologic conditions, we use a multi‐physics framework to model advective pumping bedform‐induced hyporheic exchange in response to a series of seasonal‐ and event‐scale groundwater table fluctuation scenarios, which lead to episodic river‐aquifer disconnections and reconnections. Our results suggest that hyporheic exchange is not as ubiquitous as generally assumed. Instead, the bedform‐induced hyporheic exchange is restricted to a narrow range of conditions characterized by minor river‐groundwater head differences, is intermittent, and can be easily obliterated by minor losing groundwater conditions. These findings shed light on the fragility of bedform‐induced hyporheic exchange and have important implications for biogeochemical transformations along river corridors. Plain Language Summary: The hyporheic zone is a small veneer connecting surface water and groundwater systems, which supports vital ecosystem services along river corridors. The current paradigm assumes this exchange zone is ubiquitous and relatively stable over space and time. Yet, the dynamic nature of the surface and subsurface process driving and modulating the exchange can lead to complex spatiotemporal dynamics where bedform‐induced hyporheic zones are only present for short periods or are absent. In this study, we investigated how persistent hyporheic zones are under typical groundwater dynamics. We found that bedform‐induced hyporheic zones are not as stable as usually assumed, and their presence is restricted to a narrow range of hydrological conditions. The findings reveal the fragility of hyporheic zones and offer new perspectives to conceptualize river connectivity processes. Key Points: We explore bedform‐induced hyporheic exchange under various seasonal‐ and event‐scale groundwater table fluctuation scenariosBedform‐induced exchange is intermittent and limited to a narrow range of conditions where river‐groundwater head differences are smallBedform‐induced exchange fragility highlights the need for better representations of turbulence and turnover drivers in water quality models [ABSTRACT FROM AUTHOR]

The hydrological and biogeochemical properties of the hyporheic zone in stream and riverine ecosystems have been extensively studied over the past two decades. Although it is widely acknowledged that sediment heterogeneity can influence biogeochemical reactions, little effort has been made to understand the role of heterogeneity on the spatiotemporal variability of riverbed redox conditions under changing flow dynamics at different timescales. Here we integrate a mechanistic model and field data to demonstrate that heterogeneity in permeability plays a vital role in modulating sediment redox conditions at both seasonal (annual) and event (daily‐to‐weekly) timescales, whereas heterogeneity in particulate organic carbon (POC) content only has a comparable influence on redox conditions at the seasonal timescale. These findings underscore the importance of accurately characterizing sediment heterogeneity, in terms of permeability and POC content, in quantifying biogeochemical dynamics in the riverbed and hyporheic zones of riverine ecosystems. Plain Language Summary: The redox condition of riverbed sediments is subject to the combined influence of hydrologic exchange flow and biogeochemical processes and is important for regulating the functioning of riverine ecosystems. Current understanding of the spatiotemporal pattern of sediment redox conditions especially with heterogeneity in consideration is limited, partially due to the lack of measurements and quantitative models. In this study, we integrate a mechanistic model and field data to reveal the role of sediment heterogeneity in controlling the redox condition under dynamic flow conditions. We demonstrate that heterogeneity in permeability modulates sediment redox condition at both seasonal and event timescales, and heterogeneity in particulate organic carbon is most prominent over multi‐month time intervals that reflect the balance between particulate organic carbon (POC) metabolism and time‐integrated oxygen influx. These findings highlight the importance of accurate characterization of sediment heterogeneity in both permeability and POC for predicting the dynamic redox shifts in riverbed sediments. Key Points: A reactive transport model was developed to quantify the impact of heterogeneity in permeability and particulate organic carbon (POC) concentration on sediment redox conditionsHeterogeneity in permeability controls sediment redox conditions at both seasonal (annual) and event (daily‐to‐weekly) timescalesThe effects of heterogeneity in POC occur over the monthly timescale, reflecting a balance between POC metabolism and the influx of oxygen [ABSTRACT FROM AUTHOR]

Characterizing the spatiotemporal variability of water fluxes at the stream‐groundwater interface is extremely challenging due to the lack of methods for estimating hyporheic flows at different scales. To address this, we demonstrate the potential of Active‐Distributed Temperature Sensing (DTS) methods for measuring and mapping hyporheic flow in a lowland stream. Experiments were conducted by burying a few hundred meters of heatable Fiber‐Optic cables within streambed sediments in a large meander, where permanent stream‐losing conditions are observed along the stream reach. We propose a new methodology to filter ambient temperature variations along the heated section of the DTS cable and to extend the application of Active‐DTS to losing streams. After data processing, the results show that, along lateral and longitudinal stream profiles, both thermal conductivity and water flux values follow normal distributions with relatively small standard deviations. Hyporheic fluxes vary by one order of magnitude. The absence of correlation between water fluxes within the hyporheic zone and streambed topography variations suggests that the variability is mainly controlled by local streambed heterogeneities. This means that the spatiotemporal variability of fluxes may be used as a marker of the variability of streambed hydraulic conductivities. The relatively low spatial variability (one order of magnitude) in hyporheic flow suggests a small variability of streambed properties. This is an important result for calibrating models assessing hyporheic processes, in which the hydraulic conductivity distribution is generally assumed. Additionally, measurements made over three years yield similar estimates showing the remarkable stability of hyporheic flows through time. Plain Language Summary: Characterizing the interactions between groundwater and surface water is extremely challenging although such interactions control water quality and ecosystems resilience to climate changes. Here, we used an innovative approach based on heated fiber optic cables, called Active‐Distributed Temperature Sensing, to image the spatial variability of hyporheic fluxes in a lowland stream. Our results show that the instrumental developments as well as the data processing methodology are very robust to accurately measure in‐situ the thermal conductivity of stream sediments and hyporheic fluxes within the streambed. Interestingly, groundwater flux variability was found relatively limited and not correlated to the morphology of the riverbed. In addition, measurements made over three years yield similar estimates showing the excellent reproducibility of the measurements and the remarkable stability of hyporheic flows through time. These results shed new light about the spatial and temporal variability of hyporheic fluxes in a lowland river. Key Points: Active‐Distributed Temperature Sensing was used in a lowland stream to assess and map the spatiotemporal variability of stream infiltrationAn innovative field setup and a new methodology was developed to remove ambient temperature variations from the raw temperature signalResults suggest relatively homogeneous streambed properties and show remarkable stability of hyporheic flow during few years [ABSTRACT FROM AUTHOR]

ABSTRACT This study characterized the hyporheic zone of a stretch of the Ipojuca River through analysis of the sediments and meiofauna of this environment. Samples were collected at two points (Point 1: 8°17'28.2” S; 35°59'38.6” W and Point 2: 8°17'48.1” S; 35°56'12.2” W) and in three periods: summer (November/2018), transition period (March/2018), and winter (June/2018). The most representative material was classified as silt, varying between coarse, medium, and fine. 678 meiofauna individuals were counted, distributed in four more representative taxa, Rotifera (41%), Nematoda (40%), Annelida (11%), and Copepoda (5%). The PERMANOVA analysis indicated significant differences for the period (p = 0.0009) and space (p = 0.0261) factors. There was no significant relationship between the meiofauna community structure and hyporheic environment variables. The observed differences may be linked to water quality, which also varied between the points and periods studied.

Groundwater (GW)–surface water (SW) interactions profoundly impact aquatic systems. In karst regions characterized by the coexistence of various media, the hyporheic zone (HZ) remains poorly understood. This study explores the HZ by examining karst development and various hydrological, hydrochemical, and biological processes. The research reveals high development of karst and difference in water environment may suggest a diversity of water sources beyond GW, leading to the formation of a karst hyporheic zone (KHZ). Four common KHZ types are proposed based on karst development, with a focus on spring outlet and karst window types. Interactions in these zones are driven by river backflow and GW abstraction in high- and low-water periods, respectively. KHZs may have water quality purification capabilities, but further research on biogeochemical processes is needed. This study provides preliminary insight into KHZs in karst water systems and suggests key scientific questions for future research in South China. [ABSTRACT FROM AUTHOR]

Hyporheic water exchange driven by groundwater-surface water interactions constitutes habitat conditions for aquatic biota. In our study, we conducted a field-research-based analysis of hyporheic water exchange to reveal whether the hyporheic water exchange differentiates particular Ranunculus sp. habitats. We measured the density of the stream of upwelling and hydraulic gradients of water residing in the hyporheic zone in 19 Polish rivers. We revealed that R. peltatus and R. penicillatus persist in habitats of considerably higher hyporheic water exchange upwelling flux (respectively 0.0852 m3∙d−1∙m−2 and 0.0952 m3∙d−1∙m−2) than R. circinatus, R. fluitans, and a hybrid of R. circinatus × R. fluitans (respectively m3∙d−1∙m−2; 0.0222 m3∙d−1∙m−2 and 0.0717 m3∙d−1∙m−2). The presented results can be used to indicate aquatic habitat suitability in the case of protection and management of ecosystems settled by Ranunculus sp. [ABSTRACT FROM AUTHOR]

The Supraśl River is a stable water ecosystem in North-Eastern Poland. This is indicated by the flow rate over many years, as well as the water quality monitored for years. Despite this, there are periods of oxygen deficiency in the water, causing massive fish kills. Our research shows that climate change resulting in an increase in the frequency of hydro-meteorological extremes may contribute to negative processes taking place in aquatic ecosystems. On the example of Supraśl Water System (SWS), we present an analysis of the phenomenon of suspension caused by the underground inflow, supplying the surface waters with a significant load of organic matter. The accelerated decomposition of peat formations in the Supraśl River catchment after heavy rains makes it easier for organic matter to penetrate into the aquatic ecosystem. Intensive flushing of valley and river bed sediments during summer floods has a particularly negative impact on the ecosystem of inland waters. If there are organic sediments on the bottom, their re-suspension is very dangerous. Then there is the immediate demand for oxygen, which can lead to deoxygenation of the water. Although this is a natural process, alarm and monitoring procedures should be established and countermeasures sought to respond in time and prevent mass fish kills. [ABSTRACT FROM AUTHOR]

The hyporheic zone is a hotspot for biogeochemical cycling where interactions with mineral metals preserve the release and biodegradation of organic matter (OM). A small fraction of OM can still be exchanged between localized sediments and the overlying water column, and recent evidence suggests there exists a longitudinal structuring in sediment dissolved OM (DOM) chemistry across the continental United States (CONUS). In this study, we tested a hypothesis that water extractable sediment DOM chemistry could be explained by sediment metal contents and integrative watershed scale features at the CONUS scale. Crowdsourced samples were characterized for high resolution mass spectrometry and coupled with sediment metals determined via x‐ray fluorescence as well as with land cover and soil elemental information obtained from national databases. Our results highlight weak relationships between DOM chemistry and elemental composition at the CONUS scale indicating limited transferability of organo‐metal linkages into multi‐scale hydrobiogeochemical models. Plain Language Summary: The hyporheic zone is a vital area for chemical cycling that influences the breakdown of organic matter due to interactions with minerals and metals. Some organic matter moves between sediment and water. Recent findings show that sediment organic matter chemistry displays unique east‐west patterns across the continental United States. We tested whether crowdsourced sample sediment organic matter composition could be explained by its mineral metal composition and surrounding environmental features (land cover and soil metal composition). Results revealed weak connections between dissolved organic matter chemistry and elements, indicating limits in applying these connections to continental scale models. Key Points: Sediment dissolved organic matter chemistry is longitudinally structured across the continental United StatesSediment and soil mineral elemental compositions were weakly correlated with sediment dissolved organic matter compositionLocalized metal‐organic matter linkages likely have limited capacity to inform continental‐scale hydrobiogeochemical models [ABSTRACT FROM AUTHOR]

The hyporheic zone is a critical zone for the interaction between groundwater and surface water. The heterogeneous system composed of iron oxides and ferrous in the low-permeability zone of the hyporheic zone is vital for the abiotic natural attenuation of pollutants. This review summarized the universality, reduction ability, and influencing factors of the iron oxides and ferrous in groundwater. Chlorinated hydrocarbons are taken as typical pollutants to indicate the role of iron oxide and ferrous system in their abiotic natural attenuation process. The review indicates that the reducing ability of the iron oxide and ferrous system can be expressed by Eh which is influenced by pH, ferrous concentration, and natural organic matter. Whereas the rate constant of abiotic natural attenuation of pollutants can be quantitatively described by Eh. So far, the rapid and accurate determination of Eh and the establishment of quantitative relationships between various pollutants and Eh under complex hydrochemical conditions will be the key to evaluating the abiotic natural attenuation of pollutants in aquifers. [ABSTRACT FROM AUTHOR]

Streams impacted by historic mining activity are characterized by acidic pH, unique microbial communities, and abundant metal-oxide precipitation, all of which can influence groundwater-surface water exchange. We investigate how metal-oxide precipitates and hyporheic mixing mediate the composition of microbial communities in two streams receiving acid-rock and mine drainage near Silverton, Colorado, USA. A large, neutral pH hyporheic zone facilitated the precipitation of metal particles/colloids in hyporheic porewaters. A small, low pH hyporheic zone, limited by the presence of a low-permeability, iron-oxyhydroxide layer known as ferricrete, led to the formation of steep geochemical gradients and high dissolved-metal concentrations. To determine how these two hyporheic systems influence microbiome composition, we installed well clusters and deployed in situ microcosms in each stream to sample porewaters and sediments for 16S rRNA gene sequencing. Results indicated that distinct hydrogeochemical conditions were present above and below the ferricrete in the low pH system. A positive feedback loop may be present in the low pH stream where microbially mediated precipitation of iron-oxides contributes to additional clogging of hyporheic pore spaces, separating abundant, iron-oxidizing bacteria (Gallionella spp.) above the ferricrete from rare, low-abundance bacteria below the ferricrete. Metal precipitates and colloids that formed in the neutral pH hyporheic zone were associated with a more diverse phylogenetic community of nonmotile, nutrient-cycling bacteria that may be transported through hyporheic pore spaces. In summary, biogeochemical conditions influence, and are influenced by, hyporheic mixing, which mediates the distribution of micro-organisms and, thus, the cycling of metals in streams receiving acid-rock and mine drainage. IMPORTANCE In streams receiving acid-rock and mine drainage, the abundant precipitation of iron minerals can alter how groundwater and surface water mix along streams (in what is known as the “hyporheic zone”) and may shape the distribution of microbial communities. The findings presented here suggest that neutral pH streams with large, well-mixed hyporheic zones may harbor and transport diverse microorganisms attached to particles/colloids through hyporheic pore spaces. In acidic streams where metal oxides clog pore spaces and limit hyporheic exchange, iron-oxidizing bacteria may dominate and phylogenetic diversity becomes low. The abundance of iron-oxidizing bacteria in acid mine drainage streams has the potential to contribute to additional clogging of hyporheic pore spaces and the accumulation of toxic metals in the hyporheic zone. This research highlights the dynamic interplay between hydrology, geochemistry, and microbiology at the groundwater-surface water interface of acid mine drainage streams. [ABSTRACT FROM AUTHOR]

Abstract The hydrological and biogeochemical properties of the hyporheic zone in stream and riverine ecosystems have been extensively studied over the past two decades. Although it is widely acknowledged that sediment heterogeneity can influence biogeochemical reactions, little effort has been made to understand the role of heterogeneity on the spatiotemporal variability of riverbed redox conditions under changing flow dynamics at different timescales. Here we integrate a mechanistic model and field data to demonstrate that heterogeneity in permeability plays a vital role in modulating sediment redox conditions at both seasonal (annual) and event (daily‐to‐weekly) timescales, whereas heterogeneity in particulate organic carbon (POC) content only has a comparable influence on redox conditions at the seasonal timescale. These findings underscore the importance of accurately characterizing sediment heterogeneity, in terms of permeability and POC content, in quantifying biogeochemical dynamics in the riverbed and hyporheic zones of riverine ecosystems.

The fate of nutrients and contaminants in fluvial ecosystems is strongly affected by the mixing dynamics between surface water and groundwater within the hyporheic zone, depending on the combination of the sediment's hydraulic heterogeneity and dune morphology. This study examines the effects of hydraulic conductivity stratification on steady‐state, two‐dimensional, hyporheic flows and solute residence time distribution. First, we derive an integral transform‐based semi‐analytical solution for the flow field, capable of accounting for the effects of any functional shape of the vertically varying hydraulic conductivity. The solution considers the uneven distribution of pressure at the water‐sediment interface (i.e., the pumping process) dictated by the presence of dune morphology. We then simulate solute transport using particle tracking. Our modeling framework is validated against numerical and tracer data from flume experiments and used to explore the implication of hydraulic conductivity stratification on the statistics and pdf of the residence time. Finally, reduced‐order models are used to enlighten the dependence of key residence time statistics on the parameters characterizing the hydraulic conductivity stratification. Key Points: A new integral transform‐based semi‐analytical solution for hyporheic flows in stratified sediments is provided and tested against dataThe impact of hydraulic conductivity stratification on residence time distribution and its statistics is quantitatively analyzedROMs are used to approximate key residence time statistics in the space of variability of parameters characterizing the conductivity profile [ABSTRACT FROM AUTHOR]

Nitrogen (N) fixation is a fundamental mechanism by which N enters streams. Yet, because of modern N saturation, it is difficult to study the importance of N-fixation to stream nutrient budgets. Here, we utilized relatively simple and pristine McMurdo Dry Valley streams to investigate the role of N-fixing Nostoc abundance, streamwater dissolved inorganic N (DIN) concentration, and distance from the source glacier in regulating the elemental and isotopic composition of three microbial mat types (black, orange, and green) at the landscape scale. We found Nostoc-based black mats were the most enriched in δ15N, and δ15N signatures of mats increased where Nostoc was abundant, but did not surpass the atmospheric standard (δ15N ≈ 0‰). Furthermore, green and orange mat δ15N signatures became more depleted with increasing DIN, indicating that mats utilize glacial meltwater-sourced N when available. The distance from the source glacier explained limited variability in mat δ15N across sites, indicating the influence of individual stream characteristics on N spiraling. To further explore longitudinal N spiraling processes generating observed δ15Ν patterns, we developed a simple steady-state mathematical model. Analysis of plausible scenarios with this model confirmed that streams both have the capacity to remove allochthonous DIN over the plausible range of inputs, and that internal N sources are required to account for δ15N signatures and observed DIN concentrations at stream outlets. Collectively, these data and modeling results demonstrate that N-fixation exerts substantial influence within and across these streams, and is presumably dependent upon interconnected organic matter reserves, mineralization rates, and geomorphology. [ABSTRACT FROM AUTHOR]

CH4 emissions from inland water are highly uncertain in the current global CH4 budget, especially for rivers and streams due to sparse measurements and the uncertainty of measurements caused by turbulent water flow. A previous study has revealed that vertical hydrologic exchange flow (VHEF) is the main regulator of CH4 emissions from riverbed sediments. However, to what extent the understanding obtained from the plot‐scale can be extended to the reach scale and basin scale remains unknown. To address this challenge, we developed a process‐based model to estimate CH4 flux at the air‐water interface using the attributes available in the national hydrography data set. It calculates the annual mean flux of VHEF, CH4 production in sediments, and CH4 transport in the river channel in a sequential manner. Model performance is evaluated by CH4 efflux observed at the Hanford reach of the Columbia River. We show that reach‐wise sediment hydrologic and biogeochemical conditions estimated from the national hydrography data set could serve as a good indicator of CH4 emissions from rivers. Aerobic methane oxidation and export to the downstream are the dominant ways of total CH4 loss for the large lowland river. The hotspots of CH4 emissions are likely to be at the reaches with fine sediments and slow channel velocity. This study demonstrates the possibility of quantifying CH4 emissions at the reach scale and the modeling framework has the potential to be extended to the basin scale to improve estimates of CH4 emissions from lotic inland water. Key Points: A process‐based model is developed to estimate CH4 production and emission from rivers and streamsThe model demonstrates the possibility of quantifying CH4 emissions from rivers using the national hydrography data setThe modeling framework could be further extended to the basin scale to improve estimates of CH4 emissions from lotic rivers [ABSTRACT FROM AUTHOR]

Abstract The hyporheic zone is a hotspot for biogeochemical cycling where interactions with mineral metals preserve the release and biodegradation of organic matter (OM). A small fraction of OM can still be exchanged between localized sediments and the overlying water column, and recent evidence suggests there exists a longitudinal structuring in sediment dissolved OM (DOM) chemistry across the continental United States (CONUS). In this study, we tested a hypothesis that water extractable sediment DOM chemistry could be explained by sediment metal contents and integrative watershed scale features at the CONUS scale. Crowdsourced samples were characterized for high resolution mass spectrometry and coupled with sediment metals determined via x‐ray fluorescence as well as with land cover and soil elemental information obtained from national databases. Our results highlight weak relationships between DOM chemistry and elemental composition at the CONUS scale indicating limited transferability of organo‐metal linkages into multi‐scale hydrobiogeochemical models.

Microbial communities in the hyporheic zone are important for self-purification as the riverbed is metabolically active and responsible for the retention, storage, and mineralisation of organic matter transported in the surface water. Studies exploring the response of the hyporheic microbial community to flow regime alterations remain scarce. Here, we characterised the microbial community structure over downwelling, upwelling and intermediate hyporheic zone areas within and among gravel bars at high and low discharge levels to determine flow-induced changes in a dam-regulated river using 16S rRNA gene sequencing. At low discharge, microbial composition varied across the different areas of gravel bars, reflecting heterogeneity in local abiotic conditions. However, high discharge lead to microbial community homogenisation across the three areas of the gravel bars. Microbial communities across adjacent gravel bars remained dissimilar regardless of discharge levels, likely reflecting differences in bar morphology. Our results can help better understand how anthropogenic changes in hydrologic conditions can affect microbial diversity and function in the hyporheic zone. This knowledge will help us to understand better how changes in hydrological conditions affect ecosystem functions and services in rivers affected by human impacts.

Salmon spawning activities alter streambed morphology, forming a dune‐shaped egg nest called a redd. The spawning process increases redd sediment hydraulic conductivity, KD, by removing fines, creating an egg pocket with larger sediment grains such that egg pocket hydraulic conductivity, KEP, may be higher than KD. Salmon females may create one or more egg pockets within a single redd. Although the impact of redd shape and KD on redd‐induced hyporheic fluxes has been studied, the effects of streambed roughness, R, egg pocket permeability, and egg pocket location on egg pocket hyporheic fluxes, q‾ep ${\overline{q}}_{ep}$, (downwelling flows from the stoss side of the redd which may enter egg pockets) have not yet been quantified. This study investigates this knowledge gap with a set of numerical simulations supported by flume experiments. We simulated hyporheic flows for five egg pocket locations, five KEP values from 0.0025 to 0.02 m/s, and 12 rough streambed surfaces. Surface roughness was scaled from a measured streambed surface in two ways—only vertically (R1) and both vertically and horizontally (R2)—with scaling coefficients ranging from 0.5 to 3. The measured streambed surface had a median diameter, D50, of 1 cm and a standard deviation (σD) of 0.77 cm. The results indicate that the dimensionless flux into the egg pocket, q‾ep∗=q‾epKD ${\overline{q}}_{ep}^{\ast }=\frac{{\overline{q}}_{ep}}{{K}_{D}}$ increases noticeably with the downstream distance of egg pockets from the redd pit, and less strongly with KEP∗=KEPKD ${{K}_{EP}}^{\ast }=\frac{{K}_{EP}}{{K}_{D}}$. The near‐surface downwelling fluxes significantly increase with R1, but only negligibly with R2, and for deeper egg pockets, q‾ep∗ ${\overline{q}}_{ep}^{\ast }$ is minimally impacted by surface roughness. Our results suggest that the typical simplification of a smooth redd surface with a single redd hydraulic conductivity provides a good representation of the interstitial flow within the redd, and the effects of surface roughness and egg pocket hydraulic conductivity on q‾ep∗ ${\overline{q}}_{ep}^{\ast }$ fall within the uncertainty of the egg pocket location. Plain Language Summary: Salmonids lay their eggs in the streambed gravel, forming dune‐shaped egg‐nests, called "redds." Here we study water movement through the salmon redd, investigating egg pocket permeability, streambed roughness, and egg pocket locations within the redd. This flow is crucial as it delivers oxygenated water and nutrients to eggs while removing waste, influencing embryo survival. One key finding is that the egg pocket near the downstream end of the redd receives significantly more water, over five times more than the upstream‐most egg pocket. Interestingly, the presence of additional egg pockets within a redd doesn't substantially alter water flow into any specific egg pocket. While higher permeability in the egg pocket can increase the flow, its location has a more substantial impact; an 800% increase in permeability results in only about a 71% increase in flow into the egg pocket. Streambed roughness is less crucial for egg pockets deeper within the redd compared to the effect of redd's shape and egg pocket location. Our results indicate that treating redd as a single, smooth feature helps to understand the key factors affecting water flow into egg pockets. Information on egg pocket location, permeability, and riverbed roughness helps to better estimate the flow variations. Key Points: The interstitial flow toward the egg pocket increases with the distance from the pit, and toward the tailspill crestEgg pocket's higher hydraulic conductivity leads to a modest increase in interstitial flow toward itRedd shape primarily controls flows near egg pockets, with streambed roughness having minimal impact deeper within the redd [ABSTRACT FROM AUTHOR]

In many temperate streams, biological uptake of N acts to attenuate the transport of excess N from allochthonous anthropogenic imports. Relatively few studies have determined how this N uptake relates to the magnitude of physical vs biological N storage in the stream corridor, especially for intermittent systems where allochthonous N imports are often low and N transport may only occur during brief periods of streamflow. Glacial meltwater streams in the McMurdo Dry Valleys of Antarctica provide an excellent setting to quantify autochthonous N-cycling and storage processes supported by abundant algal mats and well-connected hyporheic zones. We combined historic point-scale sediment and periphyton sample datasets with remote sensing-based modeling of periphyton coverage to estimate how much N was stored in periphyton biomass and the hyporheic zone of a 5-km long McMurdo Dry Valley stream corridor (>100,000 m2). We contextualized these N storage calculations by estimating the magnitude of annual N imports to and exports from the stream corridor based on >2 decades of streamflow and surface water data, source glacier ice cores and meltwater data, and past studies of local aeolian deposition and biological N fixation rates. We found that in this highly oligotrophic system, stream corridor-scale N storage was ∼1000× that of total annual N import or export fluxes. More than 90% of this temporarily stored N was autochthonous organic matter in the shallow (<10 cm) hyporheic zone, which acts as a reservoir that sustains N availability in the water column. Despite its location in a polar desert devoid of higher-order vegetation, area-normalized N storage (∼40 g N/m2) was greater than that reported for streams at lower latitudes (∼1–22 g N/m2). We also demonstrated that NH4+ sorption to stream sediment may be an important physicochemical N storage mechanism that responds to short-term fluctuations in streamflow and governs the mobility of inorganic N. Altogether, this research illustrates the importance of quantifying N storage within stream corridors when evaluating the significance of internal cycling and physical retention processes that modulate N availability. [ABSTRACT FROM AUTHOR]

Growth and contraction of headwater stream networks determine habitat extent, and open a window to the hyporheic zone. A fundamental challenge is observation of this process: wetted channel extent is dynamic in space and time, with wetted channel length varying by orders of magnitude over the course of a single storm event in headwater catchments. To date, observational data sets are produced from boots‐on‐the‐ground campaigns, drone imaging, or flow presence sensors, which are often laborious and limited in their spatial and temporal extents. Here, we evaluate satellite imagery as a means to detect wetted channel extent via machine learning methods trained on local surveys of wetted channel extent. Even where channel features are smaller than the imagery's spatial resolution, the presence of surface water may be imprinted upon the spectral signature of an individual pixel. For two catchments in northern California with minimal riparian canopy cover and highly dynamic wetted channel extent, we train a random forest model on RapidEye imagery captured contemporaneously with the existing surveys to predict wetted channel extent (accuracy >91%). The model is used to produce length‐discharge (L‐Q) relations and to calculate spatially distributed estimates of channel hyporheic flow capacity and exchange. A sharp break in hyporheic flow capacity occurs from main stem channels to lower order tributaries, resulting in a stepped L‐Q relationship that cannot be captured by traditionally used power law models. Remotely sensed imagery is a powerful tool for mapping wetted channels at high spatial resolution. Plain Language Summary: Streams contract, expand Flow exceeds capacity Detected, from space Key Points: Wetted channels can be mapped with high accuracy using machine learning and high spatio‐temporal resolution multispectral imageryCoincident observations of runoff and wetness state enable estimation of hydraulic properties of the hyporheic zoneThe scaling of hyporheic properties with contributing area exhibits punctuated break points in relation to stream network topology [ABSTRACT FROM AUTHOR]

• Summarized the pollution status of micro(nano)plastics in the hyporheic zone-groundwater system. • Introduced the sources and fate of micro(nano)plasticse in the hyporheic zone-groundwater system. • Analyzed the transport mechanisms of micro(nano)plastics within the hyporheic zone-groundwater system. • Proposed prospects for future studies based on the limitations of current studies. Micro(nano)plastics, as an emerging environmental pollutant, are gradually discovered in hyporheic zones and groundwater worldwide. Recent studies have focused on the origin and spatial/temporal distribution of micro(nano)plastics in regional groundwater, together with the influence of their properties and effects of environmental factors on their transport. However, the transport of micro(nano)plastics in the whole hyporheic zone-groundwater system and the behavior of co-existing substances still lack a complete theoretical interpretation. To provide systematic theoretical support for that, this review summarizes the current pollution status of micro(nano)plastics in the hyporheic zone-groundwater system, provides a comprehensive introduction of their sources and fate, and classifies the transport mechanisms into mechanical transport, physicochemical transport and biological processes assisted transport from the perspectives of mechanical stress, physicochemical reactions, and bioturbation, respectively. Ultimately, this review proposes to advance the understanding of the multi-dimensional hydrosphere transport of micro(nano)plastics centered on groundwater, the microorganisms-mediated synergistic transformation and co-transport involving the intertidal circulation. Overall, this review systematically dissects the presence and transport cycles of micro(nano)plastics within the hyporheic zone-groundwater system and proposes prospects for future studies based on the limitations of current studies. [Display omitted] [ABSTRACT FROM AUTHOR]

Biogeochemical processes capable of altering global carbon systems occur frequently in Earth’s Critical Zone–the area spanning from vegetation canopy to saturated bedrock–yet many of these phenomena are difficult to detect. Observation of these processes is limited by the seasonal inaccessibility of remote ecosystems, such as those in mountainous, snow- and ice-dominated areas. This isolation leads to a distinct gap in biogeochemical knowledge that ultimately affects the accuracy and confidence with which these ecosystems can be computationally modeled for the purpose of projecting change under different climate scenarios. To examine a high-altitude, headwater ecosystem’s role in methanogenesis, sulfate reduction, and groundwater-surface water exchange, water samples were continuously collected from the river and hyporheic zones (HZ) during winter isolation in the East River (ER), CO watershed. Measurements of continuously collected ER surface water revealed up to 50 μM levels of dissolved methane in July through September, while samples from 12 cm deep in the hyporheic zone at the same location showed a spring to early summer peak in methane with a strong biogenic signature (

Biogeochemical processes capable of altering global carbon systems occur frequently in Earth’s Critical Zone–the area spanning from vegetation canopy to saturated bedrock–yet many of these phenomena are difficult to detect. Observation of these processes is limited by the seasonal inaccessibility of remote ecosystems, such as those in mountainous, snow- and ice-dominated areas. This isolation leads to a distinct gap in biogeochemical knowledge that ultimately affects the accuracy and confidence with which these ecosystems can be computationally modeled for the purpose of projecting change under different climate scenarios. To examine a high-altitude, headwater ecosystem’s role in methanogenesis, sulfate reduction, and groundwater-surface water exchange, water samples were continuously collected from the river and hyporheic zones (HZ) during winter isolation in the East River (ER), CO watershed. Measurements of continuously collected ER surface water revealed up to 50 μM levels of dissolved methane in July through September, while samples from 12 cm deep in the hyporheic zone at the same location showed a spring to early summer peak in methane with a strong biogenic signature (

In the Willamette River, OR, main channel temperatures can be too warm for cold water fishes, causing fish to concentrate in secondary channel features that provide thermal refugia. However, temperature regimes vary among and within features. Improved understanding of physical processes controlling thermal regimes is needed. This study developed a dimensionless index for assessment of thermal refugia on the upper Willamette River. The novel hyporheic insolation (HIN) index uses minimal field measurements to predict thermal refugia resulting from buffering. Continuous water temperature measurements at one side channel, eight alcoves, and six beaver ponds provided data to ground truth calculated ℍin predictions. Water temperature records were first used to characterize stratification at sites. Calculation of the Richardson number, an index of stability, showed two well‐mixed sites and 13 stratified sites. At stratified sites, calculated ℍin values characterized the ratio of cooling flux from hyporheic discharge to heat flux from incoming solar radiation. As ℍin increased, measured temperatures at sites decreased. Despite overall scatter, a logarithmic fit to bin‐averaged ℍin values showed R2 = 0.91. Calculations suggest that secondary channel features characterized by stratification and cool hyporheic discharge can provide thermal refugia. Accordingly, the HIN index may serve as a practical tool grounded in physical processes governing temperature across a floodplain. [ABSTRACT FROM AUTHOR]

Water mites are a diverse but neglected meiofaunal group in interstitial habitats. In this study, water mites were sampled from the hyporheic zone of the upper Neretva river catchment in Bosnia and Herzegovina. In total, 10 obligate subterranean (hyporheobiontic) species were detected. Seven of them, i.e. Atractidespumilus (Szalay, 1946), Frontipodopsis reticulatifrons Szalay, 1945, Lethaxona pygmaea K. Viets, 1932, Paraxonopsis inferorum (Motaş and Tanasachi, 1947), P. vietsi (Motaş and Tanasachi, 1947), Erebaxonopsis brevipes Motaş and Tanasachi, 1947, and one halacarid species, Parasoldanellonyx typhlops K. Viets, 1933, were recorded for the first time for the fauna of Bosnia and Herzegovina. The first DNA barcoding data are provided for three families (Frontipodopsidae, Lethaxonidae, and Hungarohydracaridae), one subfamily (Stygomomoniinae), and 10 species, providing the first basis for a DNA barcode reference library of water mites from the interstitial habitats in South-Eastern Europe. In each of the four species, considered to have a wide Western Palaearctic distribution, i.e., A. pumilus, F. reticulatifrons, Hungarohydracarus subterraneaus, and Stygomomonia latipes, two separate lineages were distinguished. Fossil-calibrated modelling revealed that the diversification and speciation processes within the above-mentioned species started in the Late Miocene (7.3–11.8 Ma). [ABSTRACT FROM AUTHOR]

Recently, antibiotics have been frequently detected in the hyporheic zone (HZ) as a novel contaminant. Bioavailability assessment has gradually attracted more attention in order to provide a more realistic assessment of human health risks. In this study, two typical antibiotics, oxytetracycline (OTC) and sulfamethoxazole (SMZ), were used as target pollutants in the HZ of the Zaohe–Weihe River, and the polar organics integrated sampler was used to analyze the variation of antibiotics bioavailability. According to the characteristics of the HZ, the total concentration of pollutants, pH, and dissolved oxygen (DO) were selected as major predictive factors to analyze their correlation with the antibiotics bioavailability. Then the predictive antibiotic bioavailability models were constructed by stepwise multiple linear regression method. The results showed that there was a highly significant negative correlation between OTC bioavailability and DO (P < 0.001), while SMZ bioavailability showed a highly significant negative correlation with total concentration of pollutants (P < 0.001) and a significant negative correlation with DO (P < 0.01). The results of correlation analysis were further verified by Principal Component Analysis. Based on the experimental data, we constructed eight prediction models for the bioavailability of two antibiotics and verified them. The data points of the six prediction models were distributed in the 95% prediction band, indicating that the models were more reliable and accurate. The prediction models in this study provide reference for the accurate ecological risk assessment of the bioavailability of pollutants in the HZ, and also provide a new idea for predicting the bioavailability of pollutants in practical applications. [ABSTRACT FROM AUTHOR]

Functional and oxygenated stream beds provide crucial habitat for multiple endangered stream taxa, including endangered freshwater mussels, fishes, and insect larvae. Stream bed restoration measures such as substrate raking are often applied to mitigate excess fine sediment introductions and stream bed colmation, yet such measures are controversial. In this study, we conducted a systematic experiment in which sites with stream bed raking and removal of macrophytes were monitored over two years and compared with before-treatment conditions and untreated reference sites in the Swedish Brånsån stream, which still contains a population of the endangered freshwater pearl mussel Margaritifera margaritifera. The stream bed restoration resulted in improved habitat quality, as evident from decreased substrate compaction, increased redox potential, and oxygen supply into the stream bed. In contrast to previous studies in Central European catchments with more intensive agricultural catchment uses, the effects of the restoration measure were much longer, extending over two years. Consequently, stream bed raking and macrophyte removal can be considered a useful and more long-lasting restoration measure than currently assumed, especially in streams where excess input of fine sediment has already been mitigated, where catchment land use is rather extensive, and where near-natural flow regimes still prevail. [ABSTRACT FROM AUTHOR]

The hyporheic zone (HZ), where surface water and groundwater interact in sediments beneath streams, presents unique conditions for nutrient dynamics, such as carbon and nitrogen (N) cycling. Organic carbon (OC) in aquatic systems has two distinct forms: dissolved organic carbon and particulate organic carbon (POC). OC affects N reduction and controls the occurrence of denitrification, a primary process by which nitrate (NO3−) is removed as gas (N2 or N2O) to the atmosphere. When POC is the predominant form of OC in streams, how POC influences NO3− reduction within a HZ remains unclear. We established a new reactive transport model incorporating POC transport and filtration processes to assess how POC deposition affects NO3− removal. Sediment permeability decreases as POC is filtrated in the streambed. Nitrate influx is reduced due to POC‐induced sediment clogging. The deposited POC in the shallow streambed serves as an OC source and supports higher biogeochemical reaction kinetics because OC is an important substrate promoting microbial activities. The filtrated POC pool enhances denitrification, thus causing a higher NO3− removal efficiency. In contrast, the POC pool is typically assumed to be distributed homogeneously in sediments, potentially causing a significant underestimation of stream‐borne NO3− removal. A lower hydraulic conductivity, a smaller POC grain size, and a larger POC filtration efficiency and in‐stream POC concentration allow for extensive POC deposition within the shallow streambed that favors nitrate removal. This study provides a better understanding of N processing and an accurate estimation of NO3− removal potential in HZs. Key Points: We developed a new hydrobiogeochemical model combining saturated flow, particulate organic carbon (POC) deposition and reactive solute transportDeposited POC near the sediment‐water interface reduces nitrate influx and enhances denitrification, causing a higher N removal efficiencyA higher POC concentration and filtration efficiency and a smaller sediment permeability and POC size enhance POC filtration and N removal [ABSTRACT FROM AUTHOR]

Contradictory interpretations of transient storage modeling (TSM) results of past studies hamper the understanding of how hydrologic conditions control solute transport in streams. To address this issue, we conduct 30 instantaneous tracer experiments in the Weierbach stream, Luxembourg. Using an iterative modeling approach, we calibrate TSM parameters and assess their identifiability across various hydrologic conditions. Near‐stream groundwater monitoring wells and LIDAR scans of the streambed are used to evaluate the area of the hyporheic zone and of the submerged sediments for each experiment. Our findings show that increasing discharge enhances parameters interaction requiring more samples of TSM parameters to obtain identifiable results. Our results also indicate that transient storage at the study site is influenced by in‐stream and hyporheic exchange processes during low discharge, likely due to the hyporheic zone's large extent and the relatively low water level compared to the size of slate fragments on the streambed. However, as discharge increases, in‐stream storage zones become part of the advective channel and the lower localized stream water losses to the adjacent groundwater suggests a decrease of the hyporheic exchange on transient storage. The results obtained were utilized to generate a hydrograph for the study site illustrating the dynamic evolution of in‐stream and hyporheic storage with varying discharge, providing insights into the expected influence of different transient storage processes prior to tracer experiments. Overall, our study enhances the understanding of the role of the hyporheic area and in‐stream storage zones in transient storage and helps estimate TSM parameters more accurately. Plain Language Summary: The transport of substances in stream networks is not only controlled by the velocity of the water, but also by the shape of the stream's channels and the continuous exchange of water between the stream and the saturated soil adjacent to its banks, that is, groundwater. A common approach to studying water movement in streams is to release a tracer—a substance that moves like water—into the stream water and measure its transport downstream. Although this method has provided useful results in the past, understanding the processes that control water transport and their changes over time is still challenging. Here, we studied the transport of a tracer in a stream reach at different times of the year and under varying flow conditions. We found that the exchange of water between the stream and groundwater, together with the slow movement of water between the stones in the stream channels, controlled the slowing of water transport downstream. However, as the amount of water in the stream channel increased, water velocity and eddies generated by the friction with the streambed sediments became the predominant factor in controlling downstream water transport, and the exchange between the stream channel and adjacent groundwater became less important. Key Points: Integrating groundwater data and streambed micro‐topography can improve the interpretation of transient storage model resultsParameter interaction in transient storage models increases with dischargeHyporheic and in‐stream storage can both contribute to increase and decrease of stream water transient storage when discharges increase [ABSTRACT FROM AUTHOR]

Dissolved oxygen (DO) is critical for aquatic ecosystems, however, few studies have focused on the long‐term DO dynamics in hyporheic zones, which are a function of both transport (hydrologic exchange between river and hyporheic zone) and uptake by biogeochemical reactions or respiration. We explore the dynamics of temperature and DO at 10, 20, and 35 cm depth beneath the East River, Colorado, from July–October 2017 (relatively normal water year) and April to October 2018 (comparatively low flow year), enabled by distinctive, in‐situ, high frequency (Δt = 5 min) sensors that provided continuous time‐series from the undisturbed study site over 14 months. We expect that hyporheic DO, which has a regular daily fluctuation pattern, is supplied by the surface water (at all times we estimate downwelling) and that diurnal hyporheic DO temporal patterns should be aligned with diurnal hyporheic temperature patterns. However, this was not found to be the case. Hyporheic DO becomes depleted briefly at 20 and 35 cm depths in 2017, and at all three hyporheic depths for extended periods in 2018. Whereas diurnal temperature fluctuations have consistent timings of maxima and minima, hyporheic DO rarely has as regular a pattern, and daily ranges are inconsistent. Rainfall events caused some of these changes to diurnal hyporheic DO patterns without repeatable effects. Antecedent snowpack conditions influence streamflow dynamics and therefore hyporheic DO dynamics in this alpine river. These results also point to the strong and variable influence of hyporheic microbial communities regulating hyporheic DO. Plain Language Summary: Much like humans, river ecosystems rely on oxygen (in this case dissolved into river water) to remain healthy and function properly. River water is known to exchange through riverbed sediments (this is called the hyporheic zone) carrying dissolved materials and heat into the bed. In the East River, a subalpine river outside of Crested Butte, Colorado, we deployed sensors in the river and at 10, 20, and 35 cm into the riverbed to measure temperature and dissolved oxygen concentrations every 5 min from July 2017 to October 2018. These data indicate that the riverbed goes anoxic (no oxygen) for short periods (a few days) at 20 and 35 cm depths in 2017, a typical flow year. But in the low flow summer of 2018, anoxia persists through the entire riverbed from mid‐June to October. Temperature in the riverbed follows a repeated daily pattern of high and low values at all locations, but the daily fluctuation of dissolved oxygen in the riverbed does not. During rainstorms, riverbed dissolved oxygen is affected. These results indicate that hyporheic bacteria vary their consumption of oxygen day‐to‐day. Key Points: Subdaily to annual hyporheic dissolved oxygen dynamics revealed by continuous in‐situ measurements in an undisturbed riverbed for >14 monthsIn two contrasting flow seasons, hyporheic dissolved oxygen is strongly controlled by both river discharge and antecedent conditionsCompared to repeated daily and seasonal patterns of hyporheic temperature, dissolved oxygen patterns are heterogeneous, aperiodic [ABSTRACT FROM AUTHOR]

With the rapid development of tourism and agriculture in the alluvial fan and lacustrine sedimentary plain on the western side of Erhai Lake in Dali, Yunnan Province, large amounts of NH4 +-N, NO3 --N, and NO2 --N pollutants have been generated by agricultural cultivation at the margins of the alluvial fan and by human activities within the alluvial fan. The infiltration of the 18 streams in the Cangshan Mountains drives the nitrogen-contaminated groundwater to continuously discharge into Erhai Lake, which further increases the risk of eutrophication of the water bodies of the alpine lake. The hyporheic zone has a purifying effect on the infiltrated surface water. To investigate the migration and removal patterns of nitrogen in the hyporheic zone in the alluvial fan, a simulated infiltration system of the hyporheic zone was designed, including three hydrological processes affecting the migration of the aforementioned nitrogen pollutants, namely, the infiltration of urban domestic sewage in the alluvial fan, the vertical infiltration of fertilizer applied at the margins of the alluvial fan, and the alternate recharge of the hyporheic zone by surface water and groundwater at the apex of the alluvial fan. The experimental results revealed that the hyporheic zone has good NH4 +-N and NO3 --N removal efficiencies. Their removal in the alluvial fan mainly relies on denitrification during their vertical infiltration, which generally involves two processes with opposite effects: (1) a positive-effect process involving the alternate adsorption of anions and cations in the hyporheic zone, accompa nied by nitrification and denitrification, where denitrification is the primary nitrogen removal step and nitrification plays a supplementary role, leading to pollutant concentration reduction and water purification, and (2) a negative-effect process involving nitrification and anion/cation adsorption followed by the release of Ca2+ and Mg2+ into the aqueous phase, leading to increased total hardness of the water. During the alternate recharge of the hyporheic zone by surface water and groundwater at the apex of the alluvial fan, NO3 --N removal and water purification are mainly achieved through denitrification and partial ly through dissimilatory nitrate reduction to ammonium. [ABSTRACT FROM AUTHOR]

Salmonids create an egg nest, called a redd, in hyporheic streambed gravel to bury their eggs, characterized by a pit and a hump topography, resembling a dune. Embryos' survival depends on downwelling oxygen‐rich stream water fluxes, influenced by the redd shape, stream hydraulics, and the hydraulic conductivity of the redd sediment, K. We hypothesize that downwelling fluxes increase with stream discharge and redd aspect ratio (AR = A/L, with A, the redd amplitude and L, its length), and they can be predicted using a set of dimensionless numbers, which include the stream flow Reynolds (Re) and Froude (Fr) numbers, AR, and the redd relative submergence (A/Y0, with Y0, the water depth). We address our goal by simulating the surface and subsurface flows with numerical hydraulic models linked through the near‐bed pressure distribution quantified with a two‐phase (air‐water) two‐dimensional surface water computational fluid dynamics model, validated with experiments. We apply the modeling approach to five redd sizes, which span the field observed range (from ∼1 to ∼4 m long), and by increasing discharge from shallow (0.1 m) and slow (0.15 m/s) to deep (8 m) and fast (3.3 m/s). Results confirm that downwelling fluxes increase with discharge and redd aspect ratio due to the increased near‐bed head gradient over the redd. Averaged downwelling fluxes are a function of Re, Fr, AR, and hydraulic conductivity of the redd and not of the undisturbed streambed material. The derived regression equation may help evaluate regulated and unregulated flow impacts on hyporheic flows during embryo incubation. Key Points: Redd‐induced hyporheic exchange increases with discharge and redd aspect ratioHyporheic downwelling flows are a function of surface Reynolds and Froude numbers and redd aspect ratioDisturbed material hydraulic conductivity can be used to predict the downwelling flux instead of categorical heterogeneity for the sediments [ABSTRACT FROM AUTHOR]

The San Joaquin River (SJR) in California is purported to receive high nitrate loadings from surrounding agricultural lands through both surface and groundwater inputs. To investigate the potential removal of nitrate (NO3-) from surface and ground water sources, the spatial variations in dinitrogen (N2) gas concentrations and direct measurements of sediment denitrification potential (DNP), with amended NO3- and carbon (C) treatments, were investigated in the summer along a 95-km reach of the San Joaquin River. Excess N2 in hyporheic porewaters ranged from

Microplastics are ubiquitous in the river environment, although their abundance in sediment profiles has received little attention. The river hyporheic zone (HZ) sediment is the area influenced by surface and groundwater flow dynamics, and pollutants are more likely to be transported vertically in this area, thus entering the groundwater. Understanding the microplastic abundance and composition in the HZ sediment is crucial for microplastic pollution management. Hence, this study investigated the vertical distribution and characteristics of microplastic in the HZ sediment of the Yangtze River (Nanjing section). The results show that the abundance of microplastics in the HZ sediment ranged from 207 ± 95 to 1817 ± 467 items/kg dry wet in a vertical profile. With the increase of sediment depth, the abundance of microplastics decreased obviously in most sites, whereas the proportions of pellet shapes and smaller sizes of microplastics increased only at S1. No significant variation was found in the microplastic colors between different depth layers. Polypropylene and polyethylene were the dominant polymer types in all sediment samples. These results provided insights into the understanding of the microplastic fates in a river HZ region. [ABSTRACT FROM AUTHOR]

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Fine particle clogging and faunal bioturbation are two key processes co‐occurring in the hyporheic zone that potentially affect hyporheic exchange through modifications in the sediment structure of streambeds. Clogging results from excessive fine sediment infiltration and deposition in rivers, and it is known to decrease matrix porosity and potentially reduce permeability. Faunal bioturbation activity may compensate for the negative effect of clogging by reworking the sediment, increasing porosity, and preventing further infiltration of fines. Although both processes of clogging and bioturbation have received significant attention in the literature separately, their combined effects on streambed sediment structure are not well understood, mostly due to the lack of a standard methodology for their assessment. Here, we illustrate a novel methodology using X‐ray computed tomography (CT), as proof of concept, to investigate how, together, clogging and bioturbation affect streambed porosity in a controlled flow‐through flume. By visualising gallery formations of an upward conveyor macroinvertebrate; Lumbriculus variegatus as a model species, we quantified bioturbation activity in a clogged streambed, focusing on orientation, depth, and volume at downwelling and upwelling areas of the flume. Gallery creation increased the porosity of the streambed sediment, suggesting a potential improvement in permeability and a possible offset of clogging effects. We illustrate the promising use of X‐ray CT as a tool to assess bioturbation in clogged streambeds, and the potential role of bioturbation activity supporting hyporheic exchange processes in streambeds, warranting further studies to understand the extent of bioturbation impacts in natural systems. [ABSTRACT FROM AUTHOR]

Although river ecosystems constitute a small fraction of Earth’s total area, they are critical modulators of microbially and virally orchestrated global biogeochemical cycles. However, most studies either use data that is not spatially resolved or is collected at timepoints that do not reflect the short life cycles of microorganisms. To address this gap, we assessed how viral and microbial communities change over a 48-hour period by sampling surface water and pore water compartments of the wastewater-impacted River Erpe in Germany. We sampled every 3 hours resulting in 32 samples for which we obtained metagenomes along with geochemical and metabolite measurements. From our metagenomes, we identified 6,500 viral and 1,033 microbial metagenome assembled genomes (MAGs) and found distinct community membership and abundance associated with each river compartment (e.g., Competibacteraceae in surfacewater and Sulfurimonadaceae in pore water). We show that 17% of our viral MAGs clustered to viruses from other ecosystems like wastewater treatment plants and rivers. Our results also indicated that 70% of the viral community was persistent in surface waters, whereas only 13% were persistent in the pore waters taken from the hyporheic zone. Finally, we predicted linkages between 73 viral genomes and 38 microbial genomes. These putatively linked hosts included members of the Competibacteraceae, which we suggest are potential contributors to river carbon and nitrogen cycling via denitrification and nitrogen fixation. Together, these findings demonstrate that members of the surface water microbiome from this urban river are stable over multiple diurnal cycles. These temporal insights raise important considerations for ecosystem models attempting to constrain dynamics of river biogeochemical cycles.

The Moszczenica is a lowland river, which is a third-order river with a length of 55 km; it flows into the Bzura River in central Poland. The objective of this study was to evaluate two important factors in the exchange between surface water and groundwater in the hyporheic zone: a considerable change in water flow conditions and various origins of riverbed sections, natural and artificial. To identify the spatial variation of the hydraulic gradient in the hyporheic zone of the river, a gradientmeter was applied. The measurements show that at low water stages, upwelling was dominant, with an evidently inactive zone, whereas downwelling was inconsiderable. However, the morphology of the riverbed changed during the flood flow, and downwelling clearly dominated. Upwelling zones retained their activity despite a major change in hydrological conditions. Present studies on the artificially dug Moszczenica Canal have documented outflow of water from an artificial medieval canal to a naturally formed drainage base. This means that despite the passage of hundreds of years, the natural drainage base of the Moszczenica River is still active. Studies have demonstrated the applicability of the gradientmeter for evaluating the interaction between surface and groundwaters in the hyporheic zone.

The distribution of antibiotic resistance genes (ARGs) in water sources potentially threatens drinking water safety. However, the sources of antibiotic resistome in groundwater are still under-investigated. Here, we evaluated the profiles of antibiotic resistome in peri-urban groundwater and its associated water sources (river and mountain spring) to characterize the antibiotic resistome from natural water sources on groundwater resistome. A total of 261 antibiotic resistome were detected in groundwater, mountain spring, and river samples. The relative abundances of ARGs and mobile genetic elements (MGEs) were significantly higher in the river samples than in spring water and groundwater samples. The resistome profiles were similar between groundwater and spring water but differed from the river samples. According to source tracking results, the groundwater resistome was likely to be derived from springs (28.0%−50.0%) and rivers (28.6%−48.6%), which share the same trend for the source tracking of bacterial communities. Bacterial α-diversity, bacterial β-diversity, and MGEs directly or indirectly affected the ARGs in groundwater samples. Although the abundance of groundwater resistome was not elevated by river and spring water, groundwater resistomes were diverse and may be derived from both river and spring water. We highlight the importance of groundwater resistome and its association with potential water sources, providing a better understanding and basis for the effective control of the ARG proliferation and dissemination in groundwater from exogenous water bodies in the future.

The widespread spread of antibiotic resistance genes (ARGs) in hyporheic zone (HZ) has become an emerging environmental problem due to their potentially harmful nature. In this research, three different oxygen treatment systems were set up to study the effects of oxygen changes on the abundance of ARGs in the HZ. In addition, the effects of temperature and salinity on ARGs were investigated under aerobic and anaerobic systems, respectively. The bacterial community composition of sediment samples and the relationship with ARGs were analyzed. The explanation ratio and causality of the driving factors affecting ARGs were analyzed using variation partitioning analysis (VPA) and structural equation model (SEM). The relative abundance of ARGs and mobile genetic elements (MGEs) in the anaerobic system increased significantly, which was higher than that in the aerobic system and the aerobic-anaerobic interaction system. The experiment of salinity and temperature also further proved this result. There were many bacterial communities that affected tetracycline and sulfonamide ARGs in sediments, and these host bacteria are mainly concentrated in Proteobacteria, Firmicutes and Bacteroidetes. VPA and SEM further revealed that the abundance of ARGs was mainly influenced by changes in bacterial communities and oxygen conditions, and horizontal gene transfer (HGT) of MGEs also had a positive effect on the spread of ARGs. Those findings suggest that complex oxygen conditions in the HZ alter bacterial communities and promote MGEs-mediated horizontal transfer, which together lead to the spread of ARGs. This study has value as a reference for formulating effective strategies to minimize the propagation of ARGs in underground environment. Highlights: Multidrug resistance of ARGs is more likely to be generated under interactive conditions in hyporheic zone. The anaerobic system had the fastest proliferation rate of ARGs, which was more conducive to the spread of ARGs than the aerobic and aerobic-anaerobic interactive systems. During the dynamics of ARGs, mexF was more sensitive to oxygen conditions, and sul1 and tnpA-04 also had significant differences under different temperature and salinity conditions. Most of the bacteria in the sediments of the hyporheic zone were strongly correlated with ARGs, and most of them had effects on tetracyclines and sulfonamides ARGs. [ABSTRACT FROM AUTHOR]

Denitrification in the hyporheic zone (HZ) of river corridors is crucial to removing excess nitrogen in rivers from anthropogenic activities. However, previous modeling studies of the effectiveness of river corridors in removing excess nitrogen via denitrification were often limited to the reach‐scale and low‐order stream watersheds. We developed a basin‐scale river corridor model for the Columbia River Basin with random forest models to identify the dominant factors associated with the spatial variation of HZ denitrification. Our modeling results suggest that the combined effects of hydrologic variability in reaches and substrate availability influenced by land use are associated with the spatial variability of modeled HZ denitrification at the basin scale. Hyporheic exchange flux can explain most of spatial variation of denitrification amounts in reaches of different sizes, while among the reaches affected by different land uses, the combination of hyporheic exchange flux and stream dissolved organic carbon (DOC) concentration can explain the denitrification differences. Also, we can generalize that the most influential watershed and channel variables controlling denitrification variation are channel morphology parameters (median grain size (D50), stream slope), climate (annual precipitation and evapotranspiration), and stream DOC‐related parameters (percent of shrub area). The modeling framework in our study can serve as a valuable tool to identify the limiting factors in removing excess nitrogen pollution in large river basins where direct measurement is often infeasible. Key Points: Hyporheic exchange flux controls the spatial variation of denitrification across reaches with different sizes and land usesThe combination of hyporheic exchange flux and stream dissolved organic carbon (DOC) explains the differences in denitrification for different land use streamsD50, stream slope, precipitation, evapotranspiration, and shrub area can explain most of the spatial variability in denitrification [ABSTRACT FROM AUTHOR]

• >20,000 groundwater-surface water (GW-SW) interaction papers published (1970–2023). • GW-SW interactions research is expanding beyond the hydrological sciences. • Future research should look to upscale site-specific findings to inform management. Interactions between groundwater and surface water sustain groundwater-dependent ecosystems and regulate river temperature and biogeochemical cycles, amongst many other processes. These interactions occur in freshwater environments including rivers, springs, lakes, and wetlands, and in coastal environments via tidal pumping, submarine groundwater discharge, and seawater intrusion. Here, we explore groundwater-surface water interactions research using bibliometric analyses of titles, abstracts, and keywords from 20,275 journal papers published between 1970 and 2023 extracted from Scopus. Analyses show that research into groundwater-surface water interactions is highly multi-disciplinary, with growing contributions from the social and biological sciences. The number of groundwater-surface water interactions papers is rapidly increasing with over 1200 papers published per year since 2020. Drawing on our data-driven approach and expert knowledge, we synthesise current research trends and identify critical future research directions. Despite the thousands of papers on groundwater-surface water interactions, important processes are still difficult to quantify or predict at meaningful spatial scales to inform water-resources management. We see benefits in future groundwater-surface water interactions research focusing on: (1) using new technologies including internet-of-things-based sensors, uncrewed vehicles, and remote-sensing approaches for data collection to inform groundwater-surface water interactions at large scales, (2) seeking approaches to upscale site-specific findings to better inform management, and (3) continuing the movement towards multi-disciplinary investigations to better inform the understanding of groundwater-surface water interactions and processes that will enable better management outcomes. [ABSTRACT FROM AUTHOR]