River Stage Fluctuations Research Articles

Employing generative adversarial neural networks as surrogate model for reactive transport modeling in the hyporheic zone

This study explores the application of advanced deep learning techniques, specifically conditional deep convolutional generative adversarial networks (cDC-GANs), to model and predict the complex dynamics of the hyporheic zone (HZ) driven by river stage fluctuations. The HZ is a critical transitional mixing zone between surface water and groundwater, where significant biogeochemical reactions occur. Traditional process-based reactive transport modeling methods are computationally intensive and require fine-scale parameterization. To overcome these challenges, we employ the cDC-GAN as a deep-learning-based surrogate model to predict zones of enhanced biogeochemical activity (i.e., hotspots) and reaction product concentrations in the HZ under varying bimodal sedimentary heterogeneity scenarios. The cDC-GAN model can efficiently capture nonlinear relationships between input data and output parameters without extensive parameterization, making it computationally less demanding. The model’s generative capabilities allow for the creation of new data instances, enabling the exploration of diverse scenarios and interpolation between observed data points. The model uses maps of liquid saturation and solute concentrations at different time steps to generate corresponding maps of reaction rates and reaction product concentrations. Results demonstrate the model’s qualitative and quantitative ability to capture complex relationships without relying on rigid assumptions about linearity or specific probability distribution functions. Notably, even amid variations in aquifer heterogeneity, the model consistently exhibits robust performance, validating its ability to adapt to dynamic geological settings. Our use of a simplified reaction scheme serves as a proof of concept for the cDC-GAN model’s ability to simulate solute transport in the HZ, opening avenues for future applications including more complex reaction networks within HZ system such as nutrient cycling, organic matter degradation, redox reactions, and beyond.

Tracking River's Pulse From Space: A Global Analysis of River Stage Fluctuations

AbstractRiver stage fluctuation (RSF) is one of the most important factors influencing the physical, chemical, and ecological aspects of rivers. Despite widespread interest in river stage variations, there is currently no global benchmark of RSF and their spatial patterns. Our understanding of these characteristics remains limited. We used Sentinel‐3 altimetry data to establish a benchmark data set for RSF in wide rivers (width >1 km). We conducted an initial investigation of the spatial patterns and inter‐annual variability associated with RSF. The results show a wide range of fluctuation amplitudes spanning from a mere 1 to 18 m. Notably, rivers in semi‐arid regions exhibit more pronounced fluctuations. Further analyses indicate that human activities play a significant role in RSF. The results are of substantial interest to the scientific community, as they are closely linked to critical hydrological processes, including floods, river‐floodplain dynamics, river‐groundwater interaction, greenhouse gas emissions, and river restoration.

Spatiotemporal Evolution of Riparian Redox Zonation in Response to River Stage Fluctuation and Dynamic Biofilm Growth

AbstractRiparian zones are one of the most complex redox‐dynamic systems, where sequential redox zones of dissolved oxygen, nitrate, manganese dioxide, ferric hydroxide and sulfate reduction commonly form. River stage dynamics and microbes are two key controls on nutrient transformation in the riparian zone. Microbial growth, on one hand, increases the biogeochemical reaction rate by altering the microbial population. On the other hand, decreases the permeability of the media due to clogging effects, thereby increasing the residence time of solutes. However, previous studies have often concentrated on steady‐state flow systems and overlooked the impacts of river stage fluctuations and dynamic microbial growth on redox zonation. In this study, we investigated the interactions among river fluctuations, nutrient supplies, microbial metabolisms, and sediment physical properties in a dynamic riparian system. A column experiment was conducted, and a one‐dimensional modeling framework that coupled flow, reactive solute transport, dynamic microbial growth and bioclogging processes was developed to study the spatiotemporal evolution of redox zonation in response to river stage fluctuations and microbial growth. Our results showed that two redox zonation patterns, including a typical sequence and an interlaced sequence, could form in the dynamic riparian zone. Increasing exogenous dissolved organic carbon concentration facilitates the occurrence of anomalous sequence redox zonation. Modeling results also indicated that ignoring microbial growth leads to a significant difference in the spatiotemporal characterization of the redox zonation, especially for a more permeable sediment. Our results implicate the occurrence of more complicated redox zonation in a dynamic riparian system than previously reported.

Observing and modelling dynamic changes in temperature and volumetric fluxes within a riverbank aquifer under the influence of seasonal river stage fluctuations

AbstractSeasonal variation causes regular fluctuations in river water levels and water temperatures, which affects the distribution of temperature fields and seepage fields in riparian zones. In this paper, the monitoring device was used to continuously monitor Dongting Lake for a period of 1 year, and the temperature distribution characteristics of the riparian hyporheic zone under seasonal variation were analysed. Ten riparian hydrothermal coupling models based on different soil thermal conductivity models were verified, and the optimal model was used to simulate the influence of the flood process on the temperature field and seepage field of the riparian hyporheic zone. The results show that the changes in groundwater level and temperature in the riparian hyporheic zone had obvious lag and attenuation phenomena compared with the changes in river water level and ambient temperature. The temperature field of the riparian hyporheic zone changed obviously in the year. The vertical temperature gradient was large in summer and winter, the stratification phenomenon was obvious, and the temperature field fluctuation was small. The effect of the flood process on the temperature field and seepage velocity in the riparian hyporheic zone was obviously hysteretic. The effect of the flood‐withdrawing process on the temperature change of the shallow layer was greater than that of the flood‐rising process, while the response of the deep layer temperature change to flood‐rising and flood‐withdrawing processes was the opposite. The effect of floods on the variation in groundwater velocity was that the flood‐rising process was greater than the flood‐withdrawing process.

The competition between heterotrophic denitrification and DNRA pathways in hyporheic zone and its impact on the fate of nitrate

Dissimilatory nitrate reduction to ammonium (DNRA) and heterotrophic denitrification (DEN) are recognized as two key nitrate reduction pathways competing for both nitrate and dissolved organic carbon (DOC). However, the mechanisms and influencing factors governing this competition in the dynamic hyporheic system remain unclear. In this study, a numerical model that coupled flow, reactive solute transport and microbial activities was developed to investigate the competition between DEN and DNRA in the hyporheic zone. The modeling results indicate that DNRA hotspots are distributed along the hyporheic flow path. The greater the flow velocity, the farther the DNRA hotspots are from the water-sediment interface. In the simulation scenario involving river stage fluctuations, the relative contribution of DNRA resulting from these fluctuations under a small initial hydraulic gradient (0.001) of the hyporheic zone increased by 60% compared to the range observed under a large initial hydraulic gradient (0.01). The C/N ratio in river water does not dictate the prevailing microbial group at specific locations within the hyporheic zone. Instead, the river C/N ratio governs the competition outcomes across the entire hyporheic system, demonstrating a three-phase pattern. The nitrate removal efficiency in the hyporheic system approaches 100% at C/N = 3. As the stream C/N ratio increases, the contribution of DNRA gradually rises and can reach 80% (C/N = 10). At this point, the contribution of the heterotrophic denitrifying bacteria and their corresponding nitrate removal efficiency decreases to only about 20%. Disregarding the DNRA reaction might lead to an underestimation of nitrate removal efficiency of a hyporheic zone. Our investigation improves the understanding of nitrate fate in hyporheic zones and provides a scientific foundation for watershed management and pollution control.

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

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

Nitrate sink function of riparian zones induced by river stage fluctuations

River stage fluctuation (RSF) induced by tides, dam releases, or storms may lead to enhanced nitrogen cycling (N cycling) in riparian zones (RZ). We conducted a laboratory water table manipulation experiment and applied a multiphase flow and transport model (TOUGHREACT) to investigate the role of RSF in N cycling in the RZ. Coupled nitrification and denitrification occur in the water table fluctuation zone under alternating aerobic and anaerobic conditions. Nitrate removal was enhanced in the saturated and unsaturated zones of the RZ. The net nitrate reduction rate in groundwater increased with the increasing number of RSF, as a result, the cumulative water influx. The RZ nitrate sink function became stronger with increasing RSF amplitude and soil organic matter (SOM) content, and weaker with increasing [NH4+]/[NO3−] ratio and mineralization rate. RSF generally creates a hot moment for net nitrate removal in the RZ. However, a hot moment for the nitrate source function might occur if the [NH4+]/[NO3−] ratio of groundwater is much greater than 1 and/or if a large pool of nitrate accumulates in the topsoil over a prolonged dry period. The absence of oxygen diffusion in the model would overestimate the nitrate removal capacity of the RZ.

Propagation of hydropeaking waves in heterogeneous aquifers: effects on flow topology and uncertainty quantification

Topological flow properties are proxies for mixing processes in aquifers and allow us to better understand the mechanisms controlling transport of solutes in the subsurface. However, topological descriptors, such as the Okubo–Weiss metric, are affected by the uncertainty in the solution of the flow problem. While the uncertainty related to the heterogeneous properties of the aquifer has been widely investigated in the past, less attention has been given to the one related to highly transient boundary conditions. We study the effect of different transient boundary conditions associated with hydropeaking events (i.e., artificial river stage fluctuations due to hydropower production) on groundwater flow and the Okubo–Weiss metric. We define deterministic and stochastic modeling scenarios applying four typical settings to describe river stage fluctuations during hydropeaking events: a triangular wave, a sine wave, a complex wave that results of the superposition of two sine waves, and a trapezoidal wave. We use polynomial chaos expansions to quantify the spatiotemporal uncertainty that propagates into the hydraulic head in the aquifer and the Okubo–Weiss. The wave-shaped highly transient boundary conditions influence not only the magnitude of the deformation and rotational forces of the flow field but also the temporal dynamics of dominance between local strain and rotation properties. Larger uncertainties are found in the scenario where the trapezoidal wave was imposed due to sharp fluctuation in the stage. The statistical moments that describe the propagation of the uncertainty highly vary depending on the applied boundary condition.HighlightsDeterministic and stochastic scenarios to describe the groundwater flow field under river stage fluctuations during hydropeaking.Propagation of uncertainty of highly transient boundary conditions in the Okubo–Weiss metric.Highly transient boundary conditions can significantly affect mixing potential.

Influences of overlapped riparian groundwater mounds on interaction between surface water and groundwater

AbstractA losing river usually produces riparian groundwater mounds. Knowledge of influences of the mound on riparian hydrological processes is still limited. To fill the gap, we measured the annual regime of riparian groundwater at a cross‐section of a river channel in a semi‐arid valley where the water transfer is running. Results show that the hydrologic processes were totally controlled by the water transfer. The river runoff was dominated by the transferred water. Regulation of the water transfer caused river stage fluctuations that were companied by hydrochemical variations of the river water. The water transfer enhanced infiltration into the riparian zone. Consequently, the river water and the riparian groundwater kept connection during the one‐year observation. The annual recharge and the input of total dissolved solids (TDS) from the river to the riparian zone reached 195.8 m3 and 125.0 kg, respectively. Furthermore, the Pearson's r between the river stage and riparian groundwater level was 0.98 at 2 m, 0.94 at 20 m, and 0.93 at 40 m (α = 0.01) from the riverside, respectively. The curved river produces the convergence of hydraulic gradients, which drives the overlap of groundwater mounds. The overlap is evidenced by anomalies in water level, hydrochemistry, and temperature of the riparian groundwater at the cross‐section. When the river stage and adjacent groundwater level went down, the overlap locally reversed the hydraulic gradient to point to the river hindering the recharge from the river. Hence, the average daily recharge from the river decreased from 1.10 to 0.59 m3. The riparian groundwater discharged into the river after quick declines of the river stage. In total, 39.2 m3 of riparian groundwater discharged into the river accounting for 20% of the annual recharge from the river. The impact of the hydrochemical variation of the river water on the riparian groundwater was also blocked due to the overlap. Also 20% of the TDS input was returned to the river. The overlap reduced the impact of the water transfer on the local environment.

Nitrate Sink Function of Riparian Zones Induced by River Stage Fluctuations

Nitrate Sink Function of Riparian Zones Induced by River Stage Fluctuations

Aquifer Monitoring Using Ambient Seismic Noise Recorded With Distributed Acoustic Sensing (DAS) Deployed on Dark Fiber

AbstractGroundwater is a critical resource for human activities worldwide, and a vital component of many natural ecosystems. However, the state and dynamics of water‐bearing aquifers remain uncertain, mostly due to the paucity of subsurface data at high spatial and temporal resolution. Here, we show that analysis of infrastructure‐generated ambient seismic noise acquired on distributed acoustic sensing (DAS) arrays has potential as a tool to track variations in seismic velocities (dv/v) caused by groundwater level fluctuations. We analyze 5 months of ambient noise acquired along an unused, 23 km‐long telecommunication fiber‐optic cable in the Sacramento Valley, CA, a so‐called “dark fiber." Three array subsections, ∼6 km apart, are processed and the stretching technique is applied to retrieve daily dv/v beneath each location. Near the Sacramento river, dv/v variations in the order of 2%–3% correlate with precipitation events and fluctuations in river stage of ∼1.5 m. In contrast, regions away (2.5 km) from the river do not experience large dv/v variations. These observations reveal short‐scale spatial variability in aquifer dynamics captured by this approach. Dispersion analysis and surface wave inversion of noise gathers reveal that seismic velocity perturbations occur at depths of 10–30 m. Rock physics modeling confirms that observed dv/v are linked to pore pressure changes at these depths, caused by groundwater table fluctuations. Our results suggest that DAS combined with ambient noise interferometry provides a means of tracking aquifer dynamics at high spatial and temporal resolutions at local to regional scales, relevant for effective groundwater resource management.

Understanding Contaminant Migration Within a Dynamic River Corridor Through Field Experiments and Reactive Transport Modeling

The behavior of a persistent uranium plume within an extended river corridor at the DOE Hanford site is dominantly controlled by river stage fluctuations in the adjacent Columbia River. The plume behavior is further complicated by substantial heterogeneity in physical and geochemical properties of the host aquifer sediments. Multi-scale field and laboratory experiments and reactive transport modeling were integrated to understand the complex plume behavior influenced by highly variable hydrologic and geochemical conditions in time and space. In this paper, we (1) describe multiple data sets from field-scale uranium adsorption and desorption experiments performed at our experimental well-field, (2) develop a reactive transport model that incorporates hydrologic and geochemical heterogeneities characterized from multi-scale and multi-type datasets and a surface complexation reaction network based on laboratory studies, and (3) compare the modeling and observation results to provide insights on how to refine the conceptual model and reduce prediction uncertainties. The experimental results revealed significant spatial variability in uranium adsorption/desorption behavior, while modeling demonstrated that ambient hydrologic and geochemical conditions and heterogeneities in sediment physical and chemical properties both contributed to complex plume behavior and its persistence. This research underscores the great challenges in adequately characterizing this type of site to model the reactive transport processes over scales of 10 m or more. Our analysis provides important insights into the characterization, understanding, modeling, and remediation of groundwater contaminant plumes influenced by dynamic surface water and groundwater interactions.

River Dynamics Control Transit Time Distributions and Biogeochemical Reactions in a Dam‐Regulated River Corridor

AbstractTransit time distributions (TTDs) exert important controls on biogeochemical processes in watershed systems. TTDs are often assumed to follow time‐invariant exponential, lognormal, or heavy‐tailed power law distributions in headwater or low‐order streams. However, under dynamic hydrological forcing, transit time could exhibit more complex distribution patterns with strong spatial and temporal variability. In this study, we used a numerical particle tracking approach to characterize TTDs along the Hanford Reach of the Columbia River under the influences of river stage fluctuations and evaluate the associated effects on biogeochemical reaction potentials within the river corridor. Particle tracking was conducted using velocity fields simulated by high‐resolution three‐dimensional groundwater flow models that capture both the river stage fluctuations and physical heterogeneity. Our results revealed that multifrequency flow variations led to multimodal TTDs that varied in time and space. Such characteristics can only be captured by multiyear numerical simulations supported by multiyear field monitoring. Dam‐induced high‐frequency (subweekly) flow variations increased additional hydrologic exchange flows with short (subweekly) transit times, which accounted for up to 44% of reactant consumption in the river corridor along the Hanford Reach. The dam‐induced river stage fluctuations have more significant impacts on faster biogeochemical reactions because they cause a larger fraction of shorter transit times. Numerical particle tracking provides an efficient alternative for characterizing TTDs for large complex systems where in situ field experiments are not feasible. Such a numerical approach is thus essential for improving large‐scale biogeochemical modeling from watersheds to basins.

Analysis of groundwater and river stage fluctuations and their relationship with water use and climate variation effects on Alto Grande watershed, Northeastern Brazil

Sustainable use of water resources on the behavior of groundwater table and surface water level variations, especially in phreatic sedimentary aquifers, where a strong interaction between stream flow and groundwater normally takes place. In the present study, precipitation, water level fluctuation, river flow and pivot installation data from an important agricultural frontier with an increasing water use for irrigated agriculture and a downward precipitation trend were compared with surface and water level fluctuation monitoring data. The main purpose was to assess possible impacts on the aquifers and to analyze the variation in groundwater table and surface water levels due to long-term downward precipitation trend (climate variation effects) and increasing water demand (irrigation). A growing number of central pivots installed for irrigation are withdrawing water from the Urucuia Aquifer System (UAS) in western Bahia State, Northeastern Brazil, one of the leading agricultural frontiers of the world. Field surveys provided hydrogeological aspects that led to the construction of a hydrogeological conceptual model. Monitoring groundwater and surface water through Brazilian Geological Survey's Integrated Groundwater Monitoring Network (RIMAS-CPRM), satellite images and Hydrological Information System (HidroWeb-ANA) network formed the databases for the study. This data was compared with rainfall data and evolution of irrigated areas captured by satellite image analysis. Results indicate that reduction of rainfall volumes from 1990 to 2018 in the headwaters of Alto Grande basin, due to cyclic droughts, and an increase in surface and groundwater exploitation due to flourishing agribusiness activity has caused a considerable groundwater level drawdown. The drawdown of up to 6.63 m points to the need of strong management actions, which should provide a long-term sustainable use of water resources (superficial and underground) within Alto Grande watershed. This study is part of the actions undertaken to provide the scientific and technical basis to secure the availability of water and a sustainable agricultural activity, since an important part of the Cerrado, the Brazilian savanna ecosystem, and considerable proportion of São Francisco river flow, lies in the study area.

Fractal nature of groundwater level fluctuations affected by riparian zone vegetation water use and river stage variations

Groundwater systems affected by various factors can exhibit complex fractal behaviors, whose reliable characterization however is not straightforward. This study explores the fractal scaling behavior of the groundwater systems affected by plant water use and river stage fluctuations in the riparian zone, using multifractal detrended fluctuation analysis (MFDFA). The multifractal spectrum based on the local Hurst exponent is used to quantify the complexity of fractal nature. Results show that the water level variations at the riparian zone of the Colorado River, USA, exhibit multifractal characteristics mainly caused by the memory of time series of the water level fluctuations. The groundwater level at the monitoring well close to the river characterizes the season-dependent scaling behavior, including persistence from December to February and anti-persistence from March to November. For the site with high-density plants (Tamarisk ramosissima, which requires direct access to groundwater as its source of water), the groundwater level fluctuation becomes persistent in spring and summer, since the plants have the most significant and sustained influence on the groundwater in these seasons, which can result in stronger memory of the water level fluctuation. Results also show that the high-density plants weaken the complexity of the multifractal property of the groundwater system. In addition, the groundwater level variations at the site close to the river exhibit the most complex multifractality due to the influence of the river stage fluctuation.

Spectral Analysis of River Resistance and Aquifer Diffusivity in a River‐Confined Aquifer System

AbstractHydraulic connections between a river and an adjacent aquifer are controlled by the river resistance and aquifer diffusivity. In this paper, we derive a spectral solution linking the power spectrum of river stage fluctuations to that of the hydraulic head of a confined aquifer by means of a physical scaling factor. The physical scaling factor represents an algebraic expression of the river resistance and aquifer diffusivity and is included in an exact spectral solution derived herein. Statistical measures of the aquifer diffusivity and river resistance are provided by fitting the solution versus observed groundwater hydraulic head obtained at several distances and/or frequencies. At a study site in the middle reach of the Yangtze River and downstream of the Three Gorges Dam in China, we find systematic damping of the hydraulic head variations with distance from the river, which follows a fractal pattern driven by the river stage. In general, the estimated parameters are consistent with results reported in the literature, which supports the validity of the proposed spectral approach, although the paper discusses advantages and limitations due to application conditions.

Movements of Juvenile Bull Sharks in Response to a Major Hurricane Within a Tropical Estuarine Nursery Area

Predicting the responses of animals to environmental changes is a fundamental goal of ecology and is necessary for conservation and management of species. While most studies focus on relatively gradual changes, extreme events may have lasting impacts on populations. Animals respond to major disturbances such as hurricanes by seeking shelter, migrating, or they may fail to respond appropriately. We assessed the effects of Hurricane Irma in 2017 on the behavior and survival of juvenile bull sharks (Carcharhinus leucas) within a nursery of the Florida coastal Everglades using long-term acoustic telemetry monitoring. Most of our tagged sharks (n = 14) attempted to leave the shallow waters of the Shark River Estuary before the hurricane strike, but individuals varied in the timing and success of their movements. Eight bull sharks left within hours or days before the hurricane, but three left more than a week in advance. Nine of 11 bull sharks (~ 82%) eventually returned to the array within weeks or months of the storm. Six of these returning individuals were detected in a different coastal array in nearshore waters ca. 80 km away from the mouth of the estuary during their absence. The remaining three bull sharks moved downstream relatively late (after the hurricane) and may have died. We used binomial generalized linear mixed models to estimate the probability of presence within the array as a function of several environmental variables. Departure from the array was predicted by declining barometric pressure, increasing rate of change in pressure, and potentially fluctuations in river stage. Juvenile bull sharks may weigh multiple environmental cues, perceived predation risk, their own physical size, and shifting prey resources when making decisions during and after hurricanes.

Dam Operations and Subsurface Hydrogeology Control Dynamics of Hydrologic Exchange Flows in a Regulated River Reach

AbstractHydrologic exchange flows (HEFs) across the river‐aquifer interface have important implications for biogeochemical processes and contaminant plume migration in the river corridor, yet little is known about the hydrogeomorphic factors that control HEFs dynamics under dynamic flow conditions. Here, we developed a 3‐D numerical model for a large regulated river corridor along the Columbia River to study how HEFs are controlled by the interplays between dam‐regulated flow conditions and hydrogeomorphic features of such river corridor system. Our results revealed highly variable intra‐annual spatiotemporal patterns in HEFs along the 75‐km river reach, as well as strong interannual variability with larger exchange volumes in wet years than dry years. In general, the river was losing during late spring to early summer when the river stage was high, and river was gaining in fall and winter when river stage was low. The magnitude and timing of river stage fluctuations controlled the timing of high exchange rates. Both river channel geomorphology and the thickness of a highly permeable river bank geologic layer controlled the locations of exchange hot spots, while the latter played a dominant role. Dam‐induced, subdaily to daily river stage fluctuations drove high‐frequency variations in HEFs across the river‐aquifer interfaces, resulting in greater overall exchange volumes as compared to the case without high‐frequency flows. Our results demonstrated that upstream dam operations enhanced the exchange between river water and groundwater with strong potential influence on the associated biogeochemical processes and on the fate and transport of groundwater contaminant plumes in such river corridors.

Spectral analysis of continuous redox data reveals geochemical dynamics near the stream–aquifer interface

AbstractChanges in streamflow and water table elevation influence oxidation–reduction (redox) conditions near river–aquifer interfaces, with potentially important consequences for solute fluxes and biogeochemical reaction rates. Although continuous measurements of groundwater chemistry can be arduous, in situ sensors reveal chemistry dynamics across a wide range of timescales. We monitored redox potential in an aquifer adjacent to a tidal river and used spectral and wavelet analyses to link redox responses to hydrologic perturbations within the bed and banks. Storms perturb redox potential within both the bed and banks over timescales of days to weeks. Tides drive semidiurnal oscillations in redox potential within the streambed that are absent in the banks. Wavelet analysis shows that tidal redox oscillations in the bed are greatest during late summer (wavelet magnitude of 5.62 mV) when river stage fluctuations are on the order of 70 cm and microbial activity is relatively high. Tidal redox oscillations diminish during the winter (wavelet magnitude of 2.73 mV) when river stage fluctuations are smaller (on the order of 50 cm) and microbial activity is presumably low. Although traditional geochemical observations are often limited to summer baseflow conditions, in situ redox sensing provides continuous, high‐resolution chemical characterization of the subsurface, revealing transport and reaction processes across spatial and temporal scales in aquifers.

Geochemical Exports to River From the Intrameander Hyporheic Zone Under Transient Hydrologic Conditions: East River Mountainous Watershed, Colorado

AbstractTo 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.