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The water curtain (WC) system has been applied to the greenhouse agricultural practice as an additional heat source during the cold season in South Korea. Thus, heavy groundwater extraction induces a drawdown of the groundwater level and an influx of adjacent surface water into the aquifers. Along with many reports on physicochemical transitions caused by groundwater-surface water mixing, not much knowledge exists about the resulting possible shifts and/or transitions of the subsurface and/or groundwater microbial community structures. Here, we studied a WC system's active site during a winter season to evaluate potential shifts of microbial community structures across the groundwater and surface water by next generation sequencing in combination with conventional physicochemical monitoring of groundwater. We found that there were shifts of groundwater microbial communities at groundwater (WJ-1 and WJ-2) near the adjacent stream, but there was a relatively delayed response of the community in the groundwater (WJ-3) located at some distance from surface water despite proximity to the wells of heavy groundwater extraction. [ABSTRACT FROM AUTHOR]

To understand how redox processes influence carbon, nitrogen, and iron cycling within the intrameander hyporheic zone, we developed a biotic and abiotic reaction network and incorporated it into the reactive transport simulator PFLOTRAN. Two‐dimensional reactive flow and transport simulations were performed (1) to evaluate how transient hydrological conditions control the lateral redox zonation within an intrameander region of the East River in Colorado and (2) to quantify the impact of a single meander on subsurface exports of carbon and other geochemical species to the river. The meander's overall contribution to the river was quantified by integrating geochemical outfluxes along the outside of the meander bend. The model was able to capture the field‐observed trends of dissolved oxygen, nitrate, iron, pH, and total inorganic carbon along a 2‐D transect. Consistent with field observations, simulated dissolved oxygen and nitrate decreased along the intrameander flow paths while iron (Fe2+) concentration increased. The simulation results further demonstrated that the reductive potential of the lateral redox zonation was controlled by groundwater velocities resulting from river stage fluctuations, with low‐water conditions promoting reducing conditions. The sensitivity analysis results showed that permeability had a more significant impact on biogeochemical zonation compared to the reaction pathways under transient hydrologic conditions. The simulation results further indicated that the meander acted as a sink for organic and inorganic carbon as well as iron during the extended baseflow and high‐water conditions; however, these geochemical species were released into the river during the falling limb of the hydrograph. Plain Language Summary: Hyporheic zones perform important ecological functions by linking terrestrial and aquatic systems within watersheds. Hyporheic zones can act as a source or sink for various metals and nutrients. Transient hydrologic conditions alter redox conditions within an intrameander hyporheic zone thus affecting the behavior of redox‐sensitive species. Here we investigate how transient hydrological conditions control the lateral redox zonation within an intrameander region of the East River and examine the contribution of a single meander on subsurface exports of carbon, iron, and other geochemical species to the river. The simulation results show that exports of carbon and iron are primarily hydrologically driven, yet depend upon intermittent oxic and reductive conditions resulting from river stage fluctuations. In addition, the net exports of different geochemical species increase as the river stage decreases. This study demonstrates the importance of including hydrologic transients, using a modern reactive transport approach, to quantify exports within the intrameander hyporheic zone at the riverine scale. Key Points: Intrameander hyporheic zones display distinct anoxic and suboxic regions, suboxic regions being localized along sides of the meander bendMicrotopographic features such as gullies show outsized impacts on redox processes in the hyporheic zonePermeability has a greater impact on biogeochemical zonation compared to the reaction pathways for transient hydrologic conditions [ABSTRACT FROM AUTHOR]

Travel time‐based representations of transport, a highly successful strategy for modeling conservative tracer transport in stream corridors, are extended to accommodate multicomponent reactive transport. Specifically, convolution representations used to model exchange of solute with the hyporheic zone are shown to be equivalent to solving one‐dimensional subgrid models in Lagrangian form coupled to the advection dispersion equation for the stream channel. Unlike the convolution‐based representations of previous travel time‐based stream transport models, the subgrid model generalizes to include multicomponent reactive transport with general nonlinear reactions. An example involving biomass growth, the establishment of redox zonation, and the resulting impact on denitrification rates demonstrate reach‐scale application of the new approach. Although simplified, those example simulations show some of the key phenomena associated with hyporheic zone denitrification that are not represented with conventional first‐order estimates. Key Points: Travel time formulations for hyporheic transport in stream corridors may be expressed as subgrid modelsThe alternative subgrid form accommodates multicomponent transport and nonlinear reactionsThe new multiscale framework provides a tractable path to reach‐to‐watershed scale reactive transport simulations of stream networks [ABSTRACT FROM AUTHOR]

Nitrate has been recognized as a global threat to environmental health. In this regard, the hyporheic zone (saturated media beneath and adjacent to the stream bed) plays a crucial role in attenuating groundwater nitrate, prior to discharge into surface water. While different nitrate removal pathways have been investigated over recent decades, the adsorption capacity of hyporheic sediments under natural conditions has not yet been identified. In this study, the natural attenuation capacity of the hyporheic-sediments of the Ghezel-Ozan River, located in the north-west of Iran, was determined. The sampled sediments (from 1 m below the stream bed) were characterized via XRD, FT-IR, BET, SEM, BJH, and Zeta potential. Nitrate adsorption was evaluated using a batch experiment with hyporheic pore-water from each study site. The study was performed in the hyporheic sediments of two morphologically different zones, including Z 1 located in the parafluvial zone having the clay sediment texture (57.8% clay) with smectite/Illite mixed layer clay type and Z 2 located in the river confluence area containing silty clay sediment texture (47.6% clay) with smectite/kaolinite mixed layer clay type. Data obtained from the batch experiment were subjected to pseudo-first order, pseudo-second order, intra-particle diffusion, and Elovich mass transfer kinetic models to characterize the nitrate adsorption mechanism. Furthermore, to replicate nitrate removal efficiencies of the hyporheic sediments under natural conditions, the sampled hyporheic pore-waters were applied as initial solutions to run the batch experiment. The results of the artificial nitrate solution correlated well with pseudo-second order (R 2 >95%; in both Z 1 and Z 2 ) and maximum removal efficiencies of 85.3% and 71.2% (adsorbent dosage 90 g/L, pH = 5.5, initial adsorbate concentration of 90 mg/L) were achieved in Z 1 and Z 2 , respectively. The results of the nitrate adsorption analysis revealed that the nitrate removal efficiencies varied from 17.24 ± 1.86% in Z 1 during the wet season to 28.13 ± 0.89% in Z 2 during the dry season. The results obtained by this study yielded strong evidence of the potential of hyporheic sediments to remove nitrate from an aqueous environment with great efficiency. [ABSTRACT FROM AUTHOR]

Stream temperature often varies diurnally and seasonally, and these variations propagate into hyporheic zones (HZs), forming dynamic and heterogeneous thermal patterns. The complex thermal distribution creates potential biogeochemical hotspots and hot moments. Yet, how diel temperature variations affect HZ nitrogen cycling is unknown. We thus conducted a series of multiphysics numerical simulations of nonisothermal fluid flow and multicomponent reactive solute transport to investigate this problem. We imposed a sinusoidally varying stream temperature representing diel warming and cooling and studied the effects of different temperature means and amplitudes on HZ nitrate removal efficiency inside a streambed with a dune. The results showed that the time‐variable nitrification, denitrification, and nitrate removal efficiency responded differently to the diel stream temperature signal. The temporal variation of spatially averaged nitrification rate tracks the stream temperature signal, whereas the spatially averaged denitrification variation pattern has a more complex connection to temperature. We observed a persistent hotspot where significant denitrification rates are present over the 24‐hr period. We further evaluated and estimated the bulk nitrate removal efficiency calculated by time integration of spatially averaged reaction rates over a day. For denitrification‐dominant systems, the bulk nitrate removal efficiency for cases with dynamic stream temperature was effectively the same as those with an equivalent constant mean temperature. Therefore, the bulk removal efficiency of a thermally dynamic diel system may be represented by an equivalent isothermal system given stable flow conditions. However, since large instantaneous variations in various rates were observed, the results imply that randomly timed field measurements are unlikely to be representative. This has to be considered for both past and future synoptic observational studies. Plain Language Summary: Stream temperature naturally varies over daily and seasonal periods. This could cause temperature‐dependent chemical and biological processes to vary simultaneously. Within the permeable streambed are areas called hyporheic zones where stream water circulates through in and out of the bed. The heat carried by this hyporheic flow that originates from the stream forms dynamic and complex temperature distributions in the hyporheic zone. The thermal patterns within the streambed could determine the presence of denitrification hotspots and hot moments since the hyporheic zone is an important site for many chemical reactions including nitrogen cycling. Through modeling the flow and reaction transport processes within the hyporheic zone, we found that denitrification hotspots persisted despite dynamic and complex hyporheic zone thermal patterns. Our simulation results can help explain previous observations of warmer stream temperature corresponding to lower in‐stream nitrate concentrations. The results also imply that instantaneous snapshots through synoptic chemical sampling or reaction rate quantification may not be representative since the results and interpretations from such sampling strategies will depend on the time the samples were taken. Despite exploring a large range of modeled instantaneous variations of stream temperature, the results show that the steady, mean temperature can be used to estimate the bulk nitrate removal efficiency of the hyporheic zone. Key Points: A multiphysics flow and transport model is used to examine the diel dynamics of temperature‐dependent C and N cycling in hyporheic zonesBiogeochemical hotspots persisted despite dynamic and complex hyporheic zone thermal patternsBulk daily nitrate removal efficiency can be estimated assuming steady temperatures despite large instantaneous variations under dynamic temperature conditions [ABSTRACT FROM AUTHOR]

Biogeochemical processes in the hyporheic zone (HZ) may retard the contaminants migration from groundwater to the river and vice versa. Anthropogenic activities may further complicate such processes. This study investigated the effects of dam‐induced hydrodynamics on biogeochemical transformation of contaminants using Cr as an example in the HZ of the Columbia River at the U.S. Department of Energy's Hanford Site. The flow velocities in the HZ were first simulated using the measured or averaged groundwater level and river stage at hourly, daily, weekly, and monthly time scales. The flow velocities were then incorporated into a reactive transport model with independently characterized biogeochemical reactions and kinetics. Simulation results indicated that hydrodynamics can significantly influence the rate and extent of Cr (VI) biogeochemical transformation, which was mainly controlled by the kinetic reduction of Cr (VI) to sparely soluble Cr (III) by sediment‐associated Fe (II) that can be regenerated by microorganisms with organic carbon as electron donor. The frequent flow direction reversals in the HZ induced by dam operations enhanced the rate of microbial consumption of bioavailable OC, which, if there was no organic carbon supply, can eventually terminate the Fe (II) regeneration mechanism and exhaust the HZ's redox capacity in reducing Cr (VI) to Cr (III). This study demonstrated the importance of hydrodynamics on biogeochemical transformation of contaminants in the HZ and of the time scales used in assessing the reactive transport of chemicals and contaminants in the HZ as the net supply of redox sensitive chemicals such as dissolved oxygen into the HZ is a function of the frequency of flow direction reversal. Key Points: Dam operations increase the frequency of flow direction reversal (FFDR) in the hyporheic zone (HZ)The FFDR depends on the time scales used in monitoring and simulations of hydrodynamics in the HZThe rate and extent of the biogeochemical transformation of contaminants in the HZ is a function of FFDR [ABSTRACT FROM AUTHOR]

Hyporheic zones are considered hot spots for numerically vast and phylogenetically diverse microbial communities. However, biogeochemical effects of hyporheic zones have rarely been investigated in detail because of the difficulty in accurately measuring denitrification in these zones. To date, little is known about the hydroecology of hyporheic zones. The effect of changes in hydraulic conditions on the community variations of indigenous microorganisms and water quality was examined based on the depth of the hyporheic zone. In particular, we report on the use of the pyrosequencing technique to elucidate denitrifying bacteria (DNB) community profiles combined with the stable isotope composition of nitrate and hydrological patterns in the hyporheic zones to reveal whether denitrification occurs. δ 15 N–NO 3 and δ 18 O–NO 3 values of nitrate were analyzed to evaluate the transformation processes of nitrate in upwelling and downwelling areas and mixed zones. The isotope values indicated different origins of water in upwelling and downwelling zones and that denitrification occurred predominantly in the upwelling areas. Analyses of microbial communities in the hyporheic zone showed that the new genera, species, and isotope data were associated with the hydrological uniqueness of the hyporheic zones. The 16S rRNA sequences were determined and phylogenetic analysis revealed that the DNB communities distributed and gathered the genus Comamonas denitrificans within the mixing patterns of the hyporheic zones and that the relative scarcity of these microbes in these zones was caused by the lack of appropriate substrates. The delineation of the surface water–groundwater mixing zone was quantitatively determined by systematically combining the hydrological and heat transfer analyses and by comparing denitrifying bacteria communities and N isotope data. This study showed that pyrosequencing and isotopic approaches are useful for evaluating the transformation processes of nitrate at the upwelling and downwelling points of a hyporheic zone. [ABSTRACT FROM AUTHOR]

Water quality is strongly related to a river’s ecosystem composition at landscape scale. Key water/sediment interfaces, referred to here as functional compartments (FCs), are the epilithic biofilm, the fine sediment at the bottom of the river and the macroporous medium or hyporheic zone, which is primarily located in the active channel where subsurface flow occurs. The function of each FC (consumption or production of organic matter and nitrate) at river reach scale was examined through modelling and calibrated within field measurements to quantify the regulation service for water quality improvement. These functions were tested on a river reach of a dominant interface function (DIF) river, the Garonne River in south-west France. It was found that: a) functions differed between the FCs as a result of biotic (organisms involved) and abiotic (hydrological and morphological) conditions, b) functions varied within each FC over time (measured, but not discussed) and c) FCs acted as transient storage zones and contributed to the development of the river’s self-purification capacity. It was concluded that in relation to the concept of river continuum, the upper reaches in the catchment were dominated by epilithic biofilm, where the main function was the production of organic matter (OM) (mean of 0.3 g·m −2 ·d −1 , range from 0.08 to −1.7), the middle course was dominated by the hyporheic zone, where the most important function was to serve as a nitrate sink (−1.25 g·m −2 ·d −1 , range from −45.6 to −0.19), and the downstream parts of the river, whose main function was the degradation of OM within the fine sediment FC (mean of -1000 g·m −2 ·d −1 , range from −960 to −1440). Hence, the morphological heterogeneity of rivers that mimic the natural mosaic of functional compartments with exchanges of water, organic matter and nutrients between compartments could contribute to enhancing their self-purification function and thus to improving water quality and system health. This study is a first step in testing the influence of the different FCs on the water quality regulation service provided by a river reach. The final objective was to be able to upscale the modelling of the different interfaces involved in a watershed’s water quality regulation service. The present study demonstrates the need to take the biophysical diversity of reaches into account in order to accurately model this regulation service. Thus the BIODIF concept of BIO-physical (biological and physical) Diversity for rivers Dominated by river-bed Interface Functions (DIF rivers) was investigated for its ability to identify the theoretical relationships between the structure and the function of regulation of a river. [ABSTRACT FROM AUTHOR]

The hyporheic environment is composed of a rich meiofauna that depends on water flow, organic matter, and oxygen. The concentration of meiofauna varies spatially (horizontal and vertical) and temporally. Several processes occur at this interface (groundwater and surface water), including biogeochemical processes involving meiofauna. A study of the hyporheic meiofauna community in the sediments of the Beberibe River, in Brazil, was conducted. Meiofauna attained peak densities of 942.8 organisms/10 cm2, with organisms distributed over six taxa, with 98% of them represented by rotifers, nematodes, and annelids. Density was higher in the dry season with an average of 653.1 organisms/10 cm2, and in more superficial sediments with an average of 739.6 organisms/10 cm2. Greater densities were found in coarser sediment with a higher percentage of organic matter. A river bank filtration (RBF) pilot project has been installed, with good results in the attenuation of contaminants and pathogens. Water Environ. Res., 90, 685 (2018). [ABSTRACT FROM AUTHOR]

There is little understanding of the variability in the structure and function of metazoan hyporheic communities across streams draining geologies that weather to produce different pore sizes and, by extension, different hydrological conditions. In this study we selected two catchments in each of three geologies that had differing values of hydraulic conductivity and porosity, and sampled four riffles in each catchment at high and low water levels and at two depths. We found clear differences in the physical template of streams draining different geologies and in the composition and abundance of communities inhabiting the hyporheic zones of streams draining chalk/sandstone and limestone geologies. However, we did not detect any significant differences in body size, biomass or functional measures (diversity, richness, redundancy) between the geologies. Our findings imply that ecosystem functioning in streams draining geologies that produce fine grain sediments may be similar to those draining geologies that produce coarse grained sediments irrespective of differences in the physical template and community structure. [ABSTRACT FROM AUTHOR]

The hydro-geological properties of streambed together with the hydraulic gradients determine the fluxes of water, energy and solutes between the stream and underlying aquifer system. Dam induced sedimentation affects hyporheic processes and alters substrate pore space geometries in the course of progressive stabilization of the sediment layers. Uncertainty in stream-aquifer interactions arises from the inherent complex-nested flow paths and spatio-temporal variability of streambed hydraulic properties. A detailed field investigation of streambed hydraulic conductivity ( K s ) using Guelph Permeameter was carried out in an intermittent stream reach of the Pavanje river basin located in the mountainous, forested tract of western ghats of India. The present study reports the spatial and temporal variability of streambed hydraulic conductivity along the stream reach obstructed by two Vented Dams in sequence. Statistical tests such as Levene’s and Welch’s t-tests were employed to check for various variability measures. The strength of spatial dependence and the presence of spatial autocorrelation among the streambed K s samples were tested by using Moran’s I statistic. The measures of central tendency and dispersion pointed out reasonable spatial variability in K s distribution throughout the study reach during two consecutive years 2016 and 2017. The streambed was heterogeneous with regard to hydraulic conductivity distribution with high- K s zones near the backwater areas of the vented dam and low- K s zones particularly at the tail water section of vented dams. Dam operational strategies were responsible for seasonal fluctuations in sedimentation and modifications to streambed substrate characteristics (such as porosity, grain size, packing etc.), resulting in heterogeneous streambed K s profiles. The channel downstream of vented dams contained significantly more cohesive deposits of fine sediment due to the overflow of surplus suspended sediment-laden water at low velocity and pressure head. The statistical test results accept the hypothesis of significant spatial variability of streambed K s but refuse to accept the temporal variations. The deterministic and geo-statistical approaches of spatial interpolation provided virtuous surface maps of streambed K s distribution. [ABSTRACT FROM AUTHOR]

The hyporheic zone, which is an important interface between the surface of the water and groundwater, is an active ecotone in a river ecosystem. The study on hyporheic zones and hyporheic exchange mechanism impose significant effects in assessing the development and utilization of water resources, and in maintaining and restoring the health of a river ecosystem. The present study systematically introduces the analytical model of a one-dimensional vertical steady-state and transient heat transport equation in the temperature tracer method, and combines and summarizes the research progress of temperature time-series data collection methods, signal analysis methods, and numerical methods, based on temperature tracing. Finally, the advantages and main problems of the temperature tracer method were compared and discussed, indicating that the uncertainty in temperature measurements, multi-dimensional groundwater flow, and impact of non-ideal conditions on these analytical models should still be emphasized in future studies on temperature tracer methods. [ABSTRACT FROM AUTHOR]

Abstract: The lack of a complete understanding of the complex reciprocal interactions between hydrological processes and the structure and function of microbial communities limits our ability to improve the predictions of microbial metabolism in streams. We report here on how overlying water velocity and losing and gaining flux interact with bacterial community structure and its activity to control oxygen consumption in a sandy streambed under high nutrient levels. We used an experimental flume packed with natural sediment and measured the bacterial biomass distribution and oxygen profiles in the streambed and across bed forms. Local oxygen consumption rates were calculated with a 1‐D numerical model (GRADIENT). Bacterial abundance and production rates varied across the bed form within 1 order of magnitude, while their taxonomic classes were similar across bed forms despite variations in flow conditions and sediment disturbance events. However, bacterial production rates were not directly correlated with bacterial abundance. On the other hand, oxygen consumption rates ranged over 4 orders of magnitude across the bed forms and were highly correlated with the vertical exchange flux between the water and the streambed. The results strongly suggest that under high nutrient levels, the system is, in general, transport limited and that predicting oxygen consumption rates depends on local vertical exchange fluxes. [ABSTRACT FROM AUTHOR]

Considering heterogeneity in porous media pore size and connectivity is essential to predicting reactive solute transport across interfaces. However, exchange with less‐mobile porosity is rarely considered in surface water/groundwater recharge studies. Previous research indicates that a combination of pore‐fluid sampling and geoelectrical measurements can be used to quantify less‐mobile porosity exchange dynamics using the time‐varying relation between fluid and bulk electrical conductivity. For this study, we use macro‐scale (10 s of cm) advection–dispersion solute transport models linked with electrical conduction in COMSOL Multiphysics to explore less‐mobile porosity dynamics in two different types of observed sediment water interface porous media. Modeled sediment textures contrast from strongly layered streambed deposits to poorly sorted lakebed sands and cobbles. During simulated ionic tracer perturbations, a lag between fluid and bulk electrical conductivity, and the resultant hysteresis, is observed for all simulations indicating differential loading of pore spaces with tracer. Less‐mobile exchange parameters are determined graphically from these tracer time series data without the need for inverse numerical model simulation. In both sediment types, effective less‐mobile porosity exchange parameters are variable in response to changes in flow direction and fluid flux. These observed flow‐dependent effects directly impact local less‐mobile residence times and associated contact time for biogeochemical reaction. The simulations indicate that for the sediment textures explored here, less‐mobile porosity exchange is dominated by variable rates of advection through the domain, rather than diffusion of solute, for typical low‐to‐moderate rate (approximately 3–40 cm/day) hyporheic fluid fluxes. Overall, our model‐based results show that less‐mobile porosity may be expected in a range of natural hyporheic sediments and that changes in flowpath orientation and magnitude will impact less‐mobile exchange parameters. These temporal dynamics can be assessed with the geoelectrical experimental tracer method applied at laboratory and field scales. [ABSTRACT FROM AUTHOR]

Benthic macroinvertebrates inhabit the streambed sediments of temporary streams during drying events. Fine sediment (< 2 mm in diameter) deposition and clogging of interstitial pathways reduces the connectivity between benthic and subsurface habitats, potentially inhibiting macroinvertebrate vertical movements. Direct observations within subsurface sediments are, however, inherently difficult. As a result, confirmation of macroinvertebrate vertical movement, and the effect of fine sediment, is limited. We used laboratory mesocosms containing transparent gravel sized particles (10-15 mm) to facilitate the direct observation and tracking of vertical movements by Gammarus pulex in response to water level reduction and sedimentation. Seven sediment treatments comprised two fine sediment fractions (small: 0.125-0.5 mm, coarse sand: 0.5-1 mm) deposited onto the surface of the substrate, and a control treatment where no fine sediment was applied. We found that G. pulex moved into the subsurface gravel sediments in response to drying, but their ability to remain submerged during water level reduction was impeded by fine sediment deposition. In particular deposition of the coarser sand fraction clogged the sediment surface, limiting vertical movements. Our results highlight the potential effect of sedimentation on G. pulex resistance to drying events in streams. [ABSTRACT FROM AUTHOR]

The hyporheic zone (HZ) plays an important role in stream ecology. Previous studies have mainly focused on the factors influencing the HZ in the steady state. However, the exchange between surface water and groundwater in the HZ can become transient during a storm. This study investigates the impacts of different rainfall patterns (varying in intensity and duration) on the HZ under transient conditions. A two-dimensional numerical model of a 10-m long and 2-m deep domain is developed, in which the streambed consists of a series of dunes. Brinkman-Darcy and Navier-Stokes equations are respectively solved for groundwater and surface water, and velocity and pressure are coupled at the interface (i.e., the streambed surface). To compare the results under different transient conditions, this study proposes two indicators, i.e., the influential time (IT, the time required for the HZ to return to its initial state once it starts to change) and the influential depth (ID, the maximum increment in the HZ depth). To detect the extent to which the HZ undergoes significant spatial changes, moving split-window and inflection point tests are conducted. The results indicate that rainfall intensity (RI) and rainfall duration (RD) both display logarithmic relationships with the IT and ID with high coefficients of determination, but only between certain lower and upper thresholds of the RI and RD. Moreover, the distributions of the IT and ID as a function of the RI and RD are mapped using the surface spline and kriging interpolation methods to facilitate future prediction of the IT and ID. In addition, it is observed that the IT has a linear negative correlation with the groundwater response while the ID is not affected by different groundwater responses. All of the derived relationships can be used to predict the impacts of a future rainfall event on the HZ. [ABSTRACT FROM AUTHOR]

The LOMOS-mini is a new experimental system monitoring transient water and heat fluxes in the hyporheic zone at a sub-daily resolution. Given the coupling of high-frequency hydraulic head gradient and temperature measurements, its innovation when compared to currently available monitoring systems is to operate under transient conditions and to allow the quantification of coupled water and heat exchanges with an unprecented fine temporal resolution, such as the rainfall event time scale. The LOMOS-mini is low-cost, easy to construct from individual microelectronic components and has an autonomy of several months in the field. Its robust implementation protocol makes it suitable for a wide variety of geological environments, from soft loamy, sandy, clayey streambeds to compact colluvial environments. Hydraulic head gradients are measured using the pressure sensor technology presented in Greswell et al. (2009) . The sensibility of pressure estimates to ambient temperature is demonstrated; the measurement error is evaluated to decrease by up to one order of magnitude when accounting for temperature in the calibration relationship. Coupled with a physically-based hydrothermal model, the LOMOS-mini provides transient estimates of vertically distributed Darcy fluxes, conductive and advective heat fluxes. The methodology is illustrated on a colluvial streambed case study. [ABSTRACT FROM AUTHOR]

Abstract: Bed form‐driven hyporheic exchange is vital to biogeochemical processes occurring within aquatic sediment. The integrated effects of hyporheic processes in a bed form ultimately impact watershed‐scale water quality. However, much of what is understood regarding bed hyporheic exchange is based on idealized two‐dimensional bed forms despite the prevalence of potentially complex three‐dimensional (3‐D) bed forms in sandy riverbeds. We thus examined the impact of bed form three‐dimensionality on hyporheic exchange. Bed form three‐dimensionality was represented by two groups of geometric parameters: (1) crest planform curvature and (2) transverse and longitudinal wavelength and amplitude. A wide variety of synthetic bed forms was generated based on the geometric parameters. Then, surface flow over and hyporheic flow through each bed form was calculated using a 3‐D multiphysics computational fluid dynamics model implemented across a range of Reynolds Number. We found that hyporheic exchange is sensitive to both types of parameters that determine the bed form three‐dimensionality. Hyporheic exchange is dominated however by the three‐dimensionality caused by out‐of‐phase superimposed sinusoidal surfaces. The results of the complex flow models were synthesized into simple equations for predicting hyporheic flux and bulk residence time based on bed form longitudinal and transverse wavelengths, bed form height, and Reynolds Number. [ABSTRACT FROM AUTHOR]

Abstract: The transition area between rivers and their adjacent riparian aquifers, which may comprise the hyporheic zone, hosts important biochemical reactions, which control water quality. The rates of these reactions and metabolic processes are temperature dependent. Yet the thermal dynamics of riparian aquifers, especially during flooding and dynamic groundwater flow conditions, has seldom been studied. Thus, we investigated heat transport in riparian aquifers during 3 flood events of different magnitudes at 2 sites along the same river. River and riparian aquifer temperature and water‐level data along the Lower Colorado River in Central Texas, USA, were monitored across 2‐dimensional vertical sections perpendicular to the bank. At the downstream site, preflood temperature penetration distance into the bank suggested that advective heat transport from lateral hyporheic exchange of river water into the riparian aquifer was occurring during relatively steady low‐flow river conditions. Although a small (20‐cm stage increase) dam‐controlled flood pulse had no observable influence on groundwater temperature, larger floods (40‐cm and >3‐m stage increases) caused lateral movement of distinct heat plumes away from the river during flood stage, which then retreated back towards the river after flood recession. These plumes result from advective heat transport caused by flood waters being forced into the riparian aquifer. These flood‐induced temperature responses were controlled by the size of the flood, river water temperature during the flood, and local factors at the study sites, such as topography and local ambient water table configuration. For the intermediate and large floods, the thermal disturbance in the riparian aquifer lasted days after flood waters receded. Large floods therefore have impacts on the temperature regime of riparian aquifers lasting long beyond the flood's timescale. These persistent thermal disturbances may have a significant impact on biochemical reaction rates, nutrient cycling, and ecological niches in the river corridor. [ABSTRACT FROM AUTHOR]

Microbially mediated ammonium oxidation is a major process affecting nitrogen transformation and cycling in natural environments. This study investigated whether ion exchange process can affect microbially mediated aerobic oxidation of ammonium in a hyporheic zone (HZ) sediments from the Columbia River at US Department of Energy’s Hanford site, Washington State. Experiments were conducted using synthetic groundwater and river water to investigate their effect on ammonium oxidation. Results indicated that ammonium sorption through ion exchange reactions decreased the rate of ammonium oxidation, apparently resulting from the influence of the ion exchange on dissolved ammonium concentration, thus decreasing the bioavailability of ammonium for microbial oxidation. However, with the decrease in dissolved ammonium concentration, the sorbed ammonium released back to aqueous phase, and became bioavailable so that all the ammonium in the suspensions were oxidized. Our results implied a dynamic change in ammonium oxidation rates in an environment such as at HZ where river water and groundwater with different chemical compositions exchange frequently that can affect ammonium sorption and desorption through ion exchange reactions. [ABSTRACT FROM AUTHOR]

Hyporheic zone is an important region of nitrogen removal in river systems. Hyporheic exchange generally leads to heterogeneous redox environments, which are conducive to nitrogen transformation. This study seeks to determine the influence of hyporheic upwelling fluxes on inorganic nitrogen (NH 4 + , NO 3 − , and NO 2 − ) concentrations in the sediment pore water of the Weihe River, China. The patterns and magnitudes of hyporheic water exchange on 12 August 2016 were derived by a one-dimensional heat transport model, and inorganic nitrogen concentrations in the pore water, surface water, and groundwater were obtained. The results indicated that hyporheic water exchange was characterized by upwelling at each point during the test period. Moreover, NH 4 + dominated the hyporheic zone from 0 to 45 cm, likely due to organic nitrogen mineralization. Additionally, a non-linear relationship was observed between NH 4 + concentrations and upwelling fluxes. This relationship was derived by analyzing the effect of upwelling on biogeochemical activity and nitrogen transformation. Notably, increasing upwelling fluxes less than 400 mm/d resulted in high NH 4 + concentrations, whereas fluxes exceeding 400 mm/d led to low NH 4 + concentrations. Overall, the variations in inorganic nitrogen associated with hyporheic water exchange are of great importance for controlling nitrogen pollution and maintaining sustainable health in river systems. [ABSTRACT FROM AUTHOR]

Abstract: Shallow groundwater interacts strongly with surface water across a quarter of global land area, affecting significantly the terrestrial eco‐hydrology and biogeochemistry. We examined groundwater behavior subjected to unimodal impulse and irregular surface water fluctuations, combining physical experiments, numerical simulations, and functional data analysis. Both the experiments and numerical simulations demonstrated a damped and delayed response of groundwater table to surface water fluctuations. To quantify this hysteretic shallow groundwater behavior, we developed a regression model with the Gamma distribution functions adopted to account for the dependence of groundwater behavior on antecedent surface water conditions. The regression model fits and predicts well the groundwater table oscillations resulting from propagation of irregular surface water fluctuations in both laboratory and large‐scale aquifers. The coefficients of the Gamma distribution function vary spatially, reflecting the hysteresis effect associated with increased amplitude damping and delay as the fluctuation propagates. The regression model, in a relatively simple functional form, has demonstrated its capacity of reproducing high‐order nonlinear effects that underpin the surface water and groundwater interactions. The finding has important implications for understanding and predicting shallow groundwater behavior and associated biogeochemical processes, and will contribute broadly to studies of groundwater‐dependent ecology and biogeochemistry. [ABSTRACT FROM AUTHOR]

• EDC pollution more severe in surface water and sediments than groundwater in China. • Surface-groundwater interaction plays a decisive role in EDCs migration. • The removal process of EDCs needs improved removal technologies for water security. • The EDCs pollution and high risk in China require strict monitoring and management. Endocrine Disrupting Compounds (EDCs) are ubiquitous in soil and water system and have become a great issue of environmental and public health concern since the 1990s. However, the occurrence and mechanism(s) of EDCs' migration and transformation at the watershed scale are poorly understood. A review of EDCs pollution in China's major watersheds (and comparison to other countries) has been carried out to better assess these issues and associated ecological risks, compiling a large amount of data. Comparing the distribution characteristics of EDCs in water environments around the world and analyzing various measures and systems for managing EDCs internationally, the significant insights of the review are: 1) There are significant spatial differences and concentration variations of EDCs in surface water and groundwater in China, yet all regions present non-negligible ecological risks. 2) The hyporheic zone, as a transitional zone of surface water and groundwater interaction, can effectively adsorb and degrade EDCs and prevent the migration of high concentrations of EDCs from surface water to groundwater. This suggests that more attention needs to be paid to the role played by critical zones in water environments, when considering the removal of EDCs in water environments. 3) In China, there is a lack of comprehensive and effective regulations to limit and reduce EDCs generated during human activities and their discharge into the water environment. 4) To prevent the deterioration of surface water and groundwater quality, the monitoring and management of EDCs in water environments should be strengthened in China. This review provides a thorough survey of scientifically valid data and recommendations for the development of policies for the management of EDCs in China's water environment. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Mn(II)-curtain formed at the river-groundwater interface. • Release of CO 2 pressure facilitates Mn carbonate formation. • High HCO 3 – and Mn2+ from groundwater flow promote Mn carbonate formation. Manganese (Mn) is an abundant valent-variable element whose transformation plays an essential role in pollutant fates. This study investigated the distributions of total Mn (T-Mn) and its existing forms, such as exchangeable Mn (EX-Mn), those bound to carbonate (Carb-Mn), iron (manganese) oxides (Ox-Mn), organic matter or sulfide (Om-Mn), and residual lattice combined state Mn (Res-Mn) in three two-dimension sediment profiles of the riparian zone at the lower reaches of Hanjiang River, China, with different water flow directions caused by the dam. The mechanism for the distribution of the Mn forms was discussed through correlation analysis with other sediment indices and modelling studies using the hydrogeochemical simulation software PHREEQC. Results showed that Res-Mn dominated the whole profiles, and the Carb-Mn and Ex-Mn were accumulated and formed an "Mn(II) curtain" at the interface of groundwater and river water, most likely derived from reducing Ox-Mn and Om-Mn at low Eh of this area. The correlation analysis results demonstrated that T-Mn positively correlated with Fe, Ca, Mg, and P in the sediments, inferring the deposition of the primary geological environment or coprecipitation in the riparian zone. Both EX-Mn and Carb-Mn were positively correlated with the average depth of the sampling site and the moisture content of sediments, indicating that the reductive environment below the water level was more favourable for the accumulation of EX-Mn and Carb-Mn. Ox-Mn tended to accumulate above the groundwater level, and Om-Mn were mainly enriched in the subsurface of farmland. The modelling results suggested that the formation of "Mn(II) curtain" was related to the release of CO2 pressure, high HCO3– concentration from the regional groundwater or the aerobic decomposition of TOC from river water, and the low Eh caused by the depth or exhaustion of the DO from river water. This study helps us understand the hydrogeochemical process and environmental fate of Mn and related pollutants in the riparian zone. [ABSTRACT FROM AUTHOR]

Porewater arsenic mobility above the groundwater table has been recognized as a potential cause of arsenic-rich groundwater, but the processing pathways of dissolved organic matter (DOM) in that hyporheic zone and their effect on porewater arsenic release remain poorly understood. To address these issues, two porewater profiles were sampled in a surface water-groundwater interaction zone from the Hetao Basin, China, to monitor the porewater geochemistry and DOM molecular characteristics. The results show that the porewater arsenic, Fe(II), and DOC concentrations were all significantly higher than those of the intruding pond water, and were located above the conservative mixing model lines. This indicates a net release of these solutes from the sediment. By comparing the porewater with pond water DOM, we found that the carboxyl-rich alicyclic molecules (CRAM) were selectively preserved, carbohydrates and aliphatics/proteins were preferentially consumed, and low O/C-ratio compounds with high bioproduction index (I_bioprod) and terrestrial index (I_terr) were produced. The transformation of CHO to CHOS compounds also represented a pathway of recalcitrant DOM production. The produced recalcitrant organic compounds mostly contributed to the elevated porewater DOC concentrations, but their contribution decreased along the filtration path. The consumption of labile DOM compounds would be responsible for Fe(III) hydroxide reduction and arsenic release. The generated recalcitrant DOM may also be a driver of porewater arsenic mobility by acting as electron shuttles. This study highlights the importance of the hyporheic zone in shaping shallow groundwater DOM composition and the potential contribution to arsenic enrichment. [Display omitted] • Porewater arsenic (As) and DOC concentrations were higher than those in pond water. • Higher DOC concentration in porewater was related to recalcitrant DOM production. • The production of recalcitrant DOM decreased with depth in the hyporheic zone. • Labile DOM compound degradation triggers the Fe(III) oxide reduction and As release. [ABSTRACT FROM AUTHOR]

• Numerically evaluated photosensitive tracer as a strategy to separate surface and hyporheic transient storage contributions • Jointly fitting day and night tracer data using two-storage zone model delineated hyporheic and surface transient storage • Single storage model or day-only tracer data led to biases in the parameter estimates, yet good model-data fit • Good model fit to tracer test data does not guarantee accurate estimates of stream function We numerically evaluated photosensitive tracers as a potential strategy for separating the effects of surface and hyporheic storage zones (SSZs and HSZs, respectively) on stream corridor transport. Correctly separating HSZ and SSZ effects is critical to estimating the hydro-biogeochemical function of a stream because HSZs and SSZs expose solutes to significantly different biogeochemical conditions, like sunlight exposure, microbial processes, and oxygen availability. Our numerical experiments used a multiscale river-corridor transport model implemented in the ATS code, which accommodates multiple storage zones with distinct travel time distributions and biogeochemical reactions. For parameter inferences, we used Bayesian inverse modeling. We found that breakthrough curves for photo-decaying tracers from day and night injection can delineate surface and hyporheic transient storage contributions, but only when interpreted jointly through a two-storage zone model. Numerical experiments that used only daytime injection or interpreted breakthrough curves with a single storage zone model yielded good fit to breakthrough curves, but parameter estimates were biased and controlling processes misattributed, examples of good model fits for the wrong reasons. Using those biased parameter estimates in reactive transport simulations resulted in significantly different projections of denitrification, which underscores the potential for stream function to be mischaracterized if tracer tests are interpreted through an inappropriately simplified model for transient storage. More generally, this study highlights the role of modeling in evaluating the experimental design and identifying the potential of system mischaracterization and its implications. [ABSTRACT FROM AUTHOR]

• Small microplastic particles infiltrate into the hyporheic zone similar to solutes. • Integral modeling of surface water and hyporheic zone enhances the model accuracy. • Transport of larger microplastic particles in the hyporheic zone is retarded. Despite the significance of rivers and streams as pathways for microplastics (MP) entering the marine environment, limited research has been conducted on the behavior of MP within fluvial systems. Specifically, there is a lack of understanding regarding the infiltration and transport dynamics of MP across the streambed interface and within the hyporheic sediments. In this study, transport and retention of MP are investigated using a new numerical modeling approach. The model is built as a digital twin of accompanying flume experiments, which are used to validate the simulation results. The model accurately represents particle transport in turbulent water flow and within the hyporheic zone (HZ). Simulations for transport and infiltration of 1 µm MP particles into a sandy streambed demonstrate that the advection-dispersion equation can be used to adequately represent particle transport for pore-scale sized MP within the HZ. To assess the applicability of the modeling framework for larger MP, the experiment was repeated using 10 µm particles. The larger particles exhibited delayed infiltration and transport behavior, and while the model successfully represented the spatial extent of particle transport through the HZ, it was unable to fully replicate hyporheic transit times. This study is the first to combine explicit validation against experimental data, encompassing qualitative observations of MP concentration patterns and quantification of fluxes. By that, it significantly contributes to our understanding of MP transport processes in fluvial systems. The study also highlights the advantages and limitations of employing a fully integrated modeling approach to investigate the transport and retention behavior of MP in rivers and streams. [Display omitted] [ABSTRACT FROM AUTHOR]

In a recent study of denitrification dynamics in hyporheic zone sediments, we observed a significant time lag (up to several days) in enzymatic response to the changes in substrate concentration. To explore an underlying mechanism and understand the interactive dynamics between enzymes and nutrients, we developed a trait-based model that associates a community's traits with functional enzymes, instead of typically used species guilds (or functional guilds). This enzyme-based formulation allows to collectively describe biogeochemical functions of microbial communities without directly parameterizing the dynamics of species guilds, therefore being scalable to complex communities. As a key component of modeling, we accounted for microbial regulation occurring through transcriptional and translational processes, the dynamics of which was parameterized based on the temporal profiles of enzyme concentrations measured using a new signature peptide-based method. The simulation results using the resulting model showed several days of a time lag in enzymatic responses as observed in experiments. Further, the model showed that the delayed enzymatic reactions could be primarily controlled by transcriptional responses and that the dynamics of transcripts and enzymes are closely correlated. The developed model can serve as a useful tool for predicting biogeochemical processes in natural environments, either independently or through integration with hydrologic flow simulators.