Groundwater Exchange Research Articles

Transient Groundwater Flow Through a Coastal Confined Aquifer and Its Impact on Nearshore Submarine Slope Instability

AbstractRare in situ pore pressure and temperature data sets collected over more than one decade in a submarine landslide‐prone area off Nice (Southeast France) have been used to characterize a transient groundwater exchange between a coastal confined aquifer and the nearshore shallow‐water submarine shelf. The near‐coast study zone is highly exposed to geohazards and remains sadly famous for the 1979 catastrophic tsunamigenic submarine landslide with several casualties and substantial material damage. Measured pore pressure buildup does not appear to be synchronized with either the discharge of the nearby Var River or with precipitation in the Nice area. Unexpectedly, pore pressure fluctuations synchronizes well with seabed temperature trends indicating that the nearshore submarine shelf is mainly impacted by long smooth seasonal variations. We used the pore pressure and temperature data to describe the general transport mechanisms of pore waters and show that pore pressure diffusion is predominantly responsible for the near‐seabed pore pressure buildup. Based on our results and previously published geotechnical data, slope stability calculations reveal that groundwater exchange has a major impact on slope instability and shear zone formation in a prone‐to‐failure area. Minor earthquake‐induced ground accelerations seem to contribute to the nearshore slope instability by decreasing its factor of safety.

Contrasting Radium-Derived Groundwater Exchange and Nutrient Lateral Fluxes in a Natural Mangrove Versus an Artificial Canal

Artificial canals may function differently than the natural coastal wetlands, floodplains, and estuaries they often replace. Here, we assess the impact of canal estate development on saline groundwater exchange (tidal pumping) and associated nutrient fluxes. Time series observations of short-lived radium isotopes and dissolved nutrients were performed in a canal estate and a nearby mangrove creek in subtropical Australia. A mass balance model based on 223Ra (1.3 ± 0.4 and 3.4 ± 0.9 cm day−1 in the mangrove and canal, respectively) and 224Ra (2.8 ± 3.0 and 5.4 ± 4.6 cm day−1) revealed tidally driven groundwater exchange rates were ~ 2-fold greater in the canal. Lateral fluxes of total dissolved nitrogen (TDN) from the nearby estuary into the canal estate were comparable with the mangrove creek (8.4 and 9.1 mmol m−2 day−1 in the mangrove and canal, respectively). Groundwater flows into the canal released ~ 5-fold more TDN than the mangrove. As expected, mangroves appear to be more efficient at retaining groundwater-derived nitrogen than vegetation-stripped, sandy canals. Overall, this study demonstrates that land reclamation for canal estate development not only drives losses of ecosystem services, but also modifies groundwater and related nutrient exchange with coastal surface waters.

Modeling the surface water and groundwater budgets of the US using MODFLOW-OWHM

Assessments of groundwater and surface water budgets at a large scale, such as the contiguous United States, often separately analyze the complex dynamics linking the surface and subsurface categories of water resources. These dynamics include recharge and groundwater contributions to streamflow. The time-varying simulation of these complex hydrologic dynamics, across large spatial and temporal scales, remains a scientific challenge due to the complexity of the processes and data availability. In this study, groundwater fluxes and surface hydrologic processes are simulated across the contiguous US for 1950-2010. The simulation estimates the monthly water budget components, such as groundwater recharge, surface runoff, and evapotranspiration; streamflow in major rivers is routed while accounting for groundwater exchange. Human impacts are included through groundwater pumping, and climate variability is included, including variability in precipitation, temperature and potential evapotranspiration. The simulated groundwater level and river discharge have strong correlation with USGS observation wells and streamflow gages, with R2 values of 0.992 and 0.946, respectively. The simulated evapotranspiration is compared with three other published estimation methods, showing that it is able to capture the magnitude and seasonality of evapotranspiration over the Mississippi River basin. As such, the model is able to reasonably simulate the surface and groundwater budgets over the US, allowing for questions of the relative importance of climate and human impacts to be explored in the future.

A three-dimensional stratigraphic model of the Mississippi River Delta, USA: implications for river deltaic hydrogeology

A three-dimensional stratigraphic model was constructed of the upper 50 m of the Mississippi River Delta, southeastern USA. The model is used to understand hydrogeological connections among the Mississippi River, adjacent interdistributary bays and groundwater systems, and to understand how stratigraphic settings affect potential anthropically induced subsidence and erosion in the region. This study uses 619 geotechnical borings throughout the area along with the multiple-indicator natural neighbor (MINN) interpolation method to construct the model. Based on available data, the study focused on the Mississippi River reach from Head of Passes (river mouth) to Jesuit Bend (108 km upstream), covering an area of approximately 1,800 km2 and ranging in elevation from 3 to −46 m. The model shows typical basal coarse-grained sand bodies overlain by 10-m-thick blanket clay, which is interbedded frequently with silty and sandy sediments and occasionally with peat and organic clay. Sands are most abundant between elevations −10 and −35 m. The Mississippi River main channel incises the underlying sands, thereby providing pathways for river–groundwater exchange. Increased hydrologic loads may propagate along the flow paths potentially giving rise to high pore-water pressure and a resultant increase in land subsidence and thus local erosion of natural and local flood-control levees. This method of analysis may apply to other deltaic regions similarly subject to anthropically accelerated subsidence and erosion.

Tracing the Spatial Distribution of Whole-Lake Exchange of Groundwater and Lake Water in Low-Hydraulic Gradient Systems Using δ18O and Electrical Conductivity and Uncertain End-Member Mixing Analysis

δ18O and electrical conductivity (EC) were used successfully to trace the spatial distribution of whole-lake groundwater-lake exchange for a small (four ha) groundwater-fed lake situated in a low relief and low hydraulic gradient area. The method relies on quick sampling of shallow groundwater, direct analysis of EC in the field, and relatively in-expensive analysis of δ18O in the laboratory. Ternary uncertain end-member mixing analysis (precipitation, groundwater, and lake water) quantified the composition of water discharging to and recharging from the lake. The tracer distribution and mixing analysis were in agreement with the interpreted groundwater flow near the lake. The use of only one tracer (either δ18O or EC) gave the same results for the recharge segments, but the discharge segments changed the origin of the water from being groundwater to precipitation controlled. The two tracers complemented each other, especially with different signals in precipitation and groundwater. The uncertain end-members were assessed based on local (groundwater and lake water) and off-site (precipitation) data. The off-site data were found to be useful if it contained representative information on local-site seasonality (uncertainty, variance). Final end-member concentrations could explain the transience of the hydrology at the site (i.e., flooding of the area adjacent to the lake during periods with high precipitation, and variability of the δ18O signal in precipitation). This methodology potentially represents a new option to study groundwater-lake systems. The tracer information collected over only two days is useful by itself for developing the next steps like the quantification of fluxes based on other standard methods (Darcy approach, seepage meters, or temperature). The tracer information can provide quantitative estimation of inputs and outputs by using the mixing analysis.

Transient streambed hydraulic conductivity in channel and bar environments, Loup River, Nebraska

AbstractStreambed hydraulic conductivity (K) and vertical K (K v) are key controls on groundwater and surface water exchange and biogeochemical fluxes through the hyporheic zone, but drivers of transient hydraulic properties in different fluvial environments are poorly understood. This study combines hydrogeology, geophysics, and sedimentology to reveal mechanisms of K and K v transience in the upper 0.5 m of a sandy streambed during low discharge. Hydraulic tests (44 slug tests, 130 falling‐head permeameter tests) and 130 grain‐size analyses were repeated three times over 8 weeks on a 1,200 m2 grid spanning: (a) a channel with continuously flowing water and mobile bed load; (b) an adjacent mid‐channel bar that was stationary and infrequently submerged. Aerial photographs and ground‐penetrating radar show scour and complete reworking of fluvial sediments in the channel. Bar sediments below the water table remained immobile, but infrequent flows of moderate discharge reworked the uppermost few centimetres of the bar top. Despite differences in sediment mobility and stream flow characteristics across environments, K and K v exhibited order‐of‐magnitude differences in spatial heterogeneity and temporal variability in both the channel and bar. Mean K and K v values in the channel were comparatively stable over time. In the immobile bar, mean K declined 20% and K v declined 26% after increased discharge temporarily inundated the bar. Grain‐size distributions were steady across both environments over time, but repeat geophysical surveys of the bar show a decrease in electrical conductivity, likely from porosity reduction. These findings suggest that sediment dynamics and stream flow characteristics in different streambed environments are important drivers of K transience during low discharge conditions. Specifically, pore clogging can be an important mechanism of transience over short durations (weeks to months) in immobile sediments subject to infrequent flows and minor reworking.

Alterações nas trocas interaquíferas resultantes de bombeamento a longo prazo: implicações para a recarga de águas subterrâneas em leito de rocha

Extensive pumping of aquifer systems alters the water budget components for individual aquifer units. This study evaluated long-term changes in alluvial-bedrock groundwater exchange within a highly pumped region of the Denver Basin Aquifer System, Colorado (USA). For a data set spanning five decades, bedrock-aquifer water levels were compared to the elevation of mapped stream alluvium in three-dimensional space, providing a description of spatiotemporal changes in the relationship between the alluvial and bedrock aquifers. Results clearly show increased downward hydraulic gradients (from the alluvial aquifer to underlying sedimentary bedrock); thus, recharge to the bedrock aquifer has increased over time. Variably saturated flow modeling was performed to investigate the dependence of recharge on the bedrock aquifer’s water-table position. Model simulations with realistic geologic heterogeneity demonstrate the potential for unsaturated conditions within the bedrock aquifer, which may eventually produce a hydraulic disconnection between the alluvial and bedrock aquifers, limiting further pumping-induced changes in recharge. These results have important implications for understanding and forecasting long-term water-budget changes in aquifer systems subjected to high pumping stress, especially those in semi-arid regions.

Nitrate bioreduction dynamics in hyporheic zone sediments under cyclic changes of chemical compositions

Hyporheic zone (HZ) is the hotspot of contaminant biotransformation in ecosystems that affects water quality in both groundwater and river water. HZ is characterized by its dynamic changes in water flow direction that lead to the cyclic changes in aqueous chemical compositions such as dissolved oxygen (DO), nitrate, dissolved organic carbon, and their concentrations. Here we report a study on the effects of cyclic changes in the chemical conditions on the rates of nitrate bioreduction in sediments from a hyporheic zone, and the corresponding dynamics of the synthesis and decay of nitrate reductases. The results indicated that the rate of nitrate bioreduction in the second and third cycles was faster than that in the first one because the nitrate reductases produced in the first cycle still partially maintained their reactivity. The presence of DO significantly decreased the rates of nitrate reduction and nitrate reductase synthesis, but such an inhibition effect diminished in the later cycle. The coupled changes in nitrate concentrations and functional enzymes and genes abundances were generally consistent with an enzyme-based biogeochemical kinetic model. Overall the results indicated that enzyme growth dynamics and functions were complexly affected by the cyclic changes of environmental conditions, and implied that HZ sediments have a memory-type system behavior in reducing nitrate that would be affected by the frequency and magnitude of groundwater and river water exchanges, and chemical compositions in the groundwater and river water.

The spread of nitrogen compounds in an active groundwater exchange zone within a valuable natural ecosystem

The aim of the paper is to present a methodical approach for developing a hydrodynamic model of groundwater flow and transport of nitrogen compounds in the area covered by the precision farming system. The study area covers the part of the Vistula River floodplain terrace near Warsaw (Poland), in the vicinity of nature reserves: Wyspy Zawadowskie, Wyspy Świderskie and Łęgi Oborskie. The components of the water balance in the analysed area were determined using the HELP (Hydrologic Evaluation of Landfill Performance) model. In reference to the doses of mineral nitrogen used during precise fertilisation, the scenarios of the spread of nitrogen ions in the soil-water environment were detailed. The results were presented graphically in the form of hydroisohypses and isolines of nitrate and ammonium concentrations. The activity of overbank flows and the effect of the geological settings on the course of aquifer layers in the active exchange zone were taken into consideration when describing groundwater flow. This allowed us to indicate potential paths of intense nitrate migration resulting from the evolution of the fluvial environment. Moreover, “hydrogeological windows” have been identified, where contaminants can freely migrate to usable aquifer in the major groundwater basin.

Groundwater dynamics within a watershed in the discontinuous permafrost zone near Umiujaq (Nunavik, Canada)

Groundwater distribution and flow dynamics were studied in a small watershed located in the discontinuous permafrost zone near Umiujaq in Nunavik (Quebec), Canada, to assess the seasonal variations and perform a quantitative analysis of the water cycle in a subarctic watershed. Due to the complexity of the subsurface geology within the watershed, an integrated investigation was instrumental to provide a detailed understanding of the hydrogeological context as a basis for the water balance. Based on this water balance, for the two studied hydrological years of 2015 and 2016, the average values are 828 mm for precipitation, 337 mm for evapotranspiration, 46 mm for snow sublimation, 263 mm for runoff, 183 mm for groundwater exchange (losses with other aquifers outside the watershed), and 0 mm for change in water storage. Although these values likely have significant uncertainty and spatial variability, this water balance is shown to be plausible. It was also found that permafrost influences surface water and groundwater interaction, even if located in low-permeability sediments. It is expected that permafrost degradation will likely increase stream baseflow, especially in winter.

Controls over hydrogen and oxygen isotopes of surface water and groundwater in the Mun River catchment, northeast Thailand: implications for the water cycle

Stable isotopic composition (δ2H, δ18O) of river water, groundwater, and paddy water in the Mun River catchment, northeast Thailand, were determined to investigate the hydrological processes and the impacts of natural and anthropogenic activities on the water cycle. Quantities of δ2H (−93.9 to −25.4‰) and δ18O (−12.24 to −2.22‰) in river water in the wet season follow the trend: upper reaches > middle reaches ≈ lower reaches. Trends for δ2H (−52.3 to −22.0‰) and δ18O (−6.37 to −1.36‰) in the dry season are: upper reaches ≈ middle reaches > lower reaches. In the dry season, groundwater (δ2H: −57.5 to −34.6‰, δ18O: −8.24 to −4.40‰) shows a lighter isotopic composition, and paddy water (δ2H: −18.2‰, δ18O: −0.72‰) shows the highest isotopic composition. Spatial variation of δ18O and deuterium excess suggests that groundwater exchanges with surface water frequently. Rainfall and river water recharge groundwater in the wet season, and groundwater flows back to the river in the dry season, especially in the middle reaches. This process is most likely related to impoundment of the rivers by large dams. On the other hand, the lowest values of stable isotopes of river water are coincident with the extreme flooding that was produced by Tropical Storm Sonca in July 2017. This study contributes to a better understanding of hydrological processes in the Mun River catchment and provides a perspective on the application of stable isotopes to other large tropical monsoon catchments around the world.

Evaluation of surface water and groundwater interactions in the upstream of Kui river and Yunlong lake, Xuzhou, China

Understanding the interaction between groundwater and surface water is of great significance to the rational development and utilization of water resources and the protection of ecological environment. Here we present a combination of several methods (radon balance method, EC conservation method and flow balance method) for evaluating the hydraulic exchange of groundwater and surface water in a small watershed of Kui river and Yunlong lake. The exchange fluxes of Kui river, Yunlong lake and groundwater were estimated by one-dimensional steady state model and box model. The results indicate that the lake water is mainly recharged by groundwater from the surrounding mountains and the lake renewal time is approximately 26 days. Meanwhile, multiple transformations occur between groundwater and river water, and river water leakage is dominant. The existence of several tributaries complicates the hydraulic exchange, and groundwater discharge is also evident. Furthermore, the advantages and disadvantages of these three methods were analyzed. This study successfully demonstrated that radon and EC as tracers can be used to estimate the exchange flux between groundwater and surface water, particularly in areas lacking conventional hydrological data. Combining these three methods, the hydraulic exchange between groundwater and surface water can be evaluated more effectively. The analysis of hydraulic exchange is also crucial for Xuzhou area where water resources are extraordinary scarce and polluted.

A sustainable irrigation water management framework coupling water-salt processes simulation and uncertain optimization in an arid area

Field irrigation water management depends on interactions among crop yield, soil water/salt and groundwater/salt in arid irrigation area with shallow-saline groundwater. This paper presents a novel uncertainty simulation-optimization framework for irrigation water allocation and sustainable agricultural environment, which integrates simulation of physical processes of soil-groundwater water and salt balance into an uncertainty-based optimization model. The impacts of crop evapotranspiration, soil water and salt and groundwater levels are interactively involved in the simulation model. Uncertainties (economic and crop parameters, available water amount) presented as fuzzy boundary intervals and probability distribution functions are considered in the optimization model. This field irrigation water allocation framework emphasizes the role of field soil water and salt movement processes to decision-making of irrigation water allocation. Then, the proposed simulation-optimization framework was applied to a case study in the Hetao Irrigation District, an arid area of northwest China where soil salinity is a serious environmental problem induced by irrigation and shallow groundwater. Therefore, optimal irrigation water allocation solutions can be generated for providing decision makers with reliable decision options where the maximum system benefits resulting from sustainable agricultural production are desired. Furthermore, the results can support analysis of interrelationships among system benefits, water allocation planning and groundwater depth, soil salt content constraints. Scenario analysis (groundwater table depth (GTD) = 1, 1.5, 2, 2.5, 3 m and no groundwater exchange consideration) showed that the maximum net benefit could be [27469, 44818] Yuan with the groundwater table depth of 1.5 m. Also, the irrigation water allocation changed when the salt constraint was considered, which indicates that the results obtained by the developed framework can alleviate soil salinization to a certain degree. Therefore, this framework can provide more effective information for the irrigation water management and soil salinization control, which is meaningful for the sustainable development of irrigation agriculture.

Altered groundwater discharge and associated carbon fluxes in a wetland-drained coastal canal

Residential canal systems are becoming increasingly popular with the rising demand for absolute coastal waterfront properties. We hypothesize that canals alter groundwater-surface water connectivity and related carbon fluxes into coastal surface waters. Here, we quantified submarine groundwater discharge (SGD) in a residential canal system on Bribie Island (Australia) and associated carbon fluxes. SGD rates estimated from a radon (222Rn) mass balance model were 3.1 ± 1.5 cm d−1. These fluxes delivered 68 ± 44 and 70 ± 48 mmol m−2 d−1 of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) into the canal, respectively. Carbon dioxide (CO2) emissions to the atmosphere ranged from 15 to 28 mmol m−2 d−1. Multiple lines of evidence, including flux estimates and groundwater observations, converge to the conclusion that SGD was a major source of DOC and free CO2, but not carbonate alkalinity nor DIC to canal surface waters. In comparison to mangrove tidal creeks that often precede canals, the canal had (1) lower tidally-driven saline groundwater exchange rates but higher fresh groundwater discharge, (2) lower CO2 emissions to the atmosphere; and (3) acted as a driver rather than a buffer of local ocean acidification. These differences seem to be driven by the replacement of intertidal wetland vegetation with urban areas that prevent soil carbon accumulation and related biogeochemical processes around the canals. We suggest that decisions on canal construction should consider potential changes to groundwater-derived soil carbon losses and carbon cycling in receiving coastal waters.

Modelling lake water and isotope mass balance variations of Lake Azigza in the Moroccan Middle Atlas under Mediterranean climate

As many Mediterranean headwater catchments, the Moroccan Middle Atlas plays an important role in the highly vulnerable regional water resources. Mountain lakes are numerous in this region, and could be regarded as possible sentinels of hydro-climatic changes, using appropriate modelling tools able to simulate the lake-climate relation. We present a detailed study of Lake Azigza, based on a 4-year (2012–2016) observation period, including lake level measurements, isotope analyses of precipitation, lake and spring waters, and local meteorological data. The approach is based on a calibration of a daily time-step lake water and isotope mass balance model, fed by precipitation and evaporation rates, to estimate the ungauged components of the water balance. Results show the dominance of groundwater exchanges in the lake water balance, with significant interannual variations related to annual precipitation. At the annual time-step, groundwater inflow varies between twice and up to six times the amount of direct precipitation, while the groundwater loss reached up to five times evaporation. However, a significant decrease of groundwater loss is observed in 2016, suggesting that a threshold effect probably limits the seepage when the lake level decreases. This study underlines the importance of groundwater fluxes in the lake level variations for Lake Azigza, probably representative of many similar lakes in the Middle Atlas. The model was able to simulate the continuous lake level decrease (4 m) observed over 2012–2016 and can be further used to explore lake-climate relations at different timescales.

Characterization of Diffuse Groundwater Inflows into Stream Water (Part II: Quantifying Groundwater Inflows by Coupling FO-DTS and Vertical Flow Velocities)

Temperature has been used to characterize groundwater and stream water exchanges for years. One of the many methods used analyzes propagation of the atmosphere-influenced diurnal signal in sediment to infer vertical velocities. However, despite having good accuracy, the method is usually limited by its small spatial coverage. The appearance of fiber optic distributed temperature sensing (FO-DTS) provided new possibilities due to its high spatial and temporal resolution. Methods based on the heat-balance equation, however, cannot quantify diffuse groundwater inflows that do not modify stream temperature. Our research approach consists of coupling groundwater inflow mapping from a previous article (Part I) and deconvolution of thermal profiles in the sediment to obtain vertical velocities along the entire reach. Vertical flows were calculated along a 400 m long reach, and a period of 9 months (October 2016 to June 2017), by coupling a fiber optic cable buried in thalweg sediment and a few thermal lances at the water–sediment interface. When compared to predictions of hyporheic discharge by traditional methods (differential discharge between upstream and downstream of the monitored reach and the mass-balance method), those of our method agreed only for the low-flow period and the end of the high-flow period. Our method underestimated hyporheic discharge during high flow. We hypothesized that the differential discharge and mass-balance methods included lateral inflows that were not detected by the fiber optic cable buried in thalweg sediment. Increasing spatial coverage of the cable as well as automatic and continuous calculation over the reach may improve predictions during the high-flow period. Coupling groundwater inflow mapping and vertical hyporheic flow allows flow to be quantified continuously, which is of great interest for characterizing and modeling fine hyporheic processes over long periods.

Commemorating the 50th anniversary of the Freeze and Harlan (1969) Blueprint for a physically-based, digitally-simulated hydrologic response model

The year 2019 marks the 50th anniversary of a pioneering publication in hydrology. Allan Freeze and Richard Harlan published their Blueprint for a physically-based, digitally-simulated hydrologic response model (Freeze and Harlan, 1969) in this journal. Their vision was for a futuristic model that would integrate key processes and compartments in the hydrologic cycle: precipitation, evapotranspiration, overland runoff, infiltration and groundwater exchange (into and out of) surface water bodies, such as rivers and lakes. Today, the original Blueprint is a reality. Our paper commemorates the 50 year anniversary of the original Blueprint paper. Through personal communications with Allan Freeze, we document the history and genesis of this paper for the first time. We reflect on the uptake of the Blueprint into modern hydrology, the development of numerical models that enabled this, and the range of challenges being tackled by these models. Finally, we consider challenges and opportunities for the future of this area of modelling and hydrologic science.

Identification of floodplain and riverbed sediment heterogeneity in a meandering UK lowland stream by ground penetrating radar

Complex spatial heterogeneity in riverbed and floodplain sediments control the spatio-temporal exchange of groundwater and surface water in the hyporheic zone, inducing hot spots of microbial activity and biogeochemical cycling. However, the characterization of hyporheic exchange dynamics has thus far failed to adequately account for the complex subsurface heterogeneity of river bed sediments in a spatial explicitly manner, in particular for highly complex lowland river bed sediments. Here we demonstrate the ability of ground penetrating radar (GPR) to efficiently map floodplain and river bed sediment structures within a lowland meandering river. The aim of this study was to delineate the type and spatial extent of complex, texturally heterogeneous facies of high and low conductive streambed materials. GPR surveys in this study involved not only state-of-the-art terrestrial applications but also an aquatic survey conducted from a floating rig. The surveys revealed substantial sub-surface heterogeneity of depositional materials in the streambed and riparian zone. Eight characteristic radar facies were identified through the floodplain and ground-truthed against core samples and exposures of bank deposits. The majority of GPR profiles were dominated by trough-shaped depositional elements with erosional, curved, concave upward bounding surfaces, indicative of abandoned and chute stream channel structures. The identified abandoned channel structure was found to extend into the riverbed and to be filled by suspension fall-out fine-grained deposits (mud with organic matter and interbedded clay as indicated by observed signal attenuations). GPR proved to be a successful method to identify the spatial patterns of low conductivity peat and clay structures in the streambed and riparian zone of the investigated meander bend, highlighting its potential for larger scale analysis of these structures that have shown to control the exchange flow patterns between groundwater and surface water in lowland rivers.

Field Experiments of Hyporheic Flow Affected by a Clay Lens

As a typical water exchange of surface water and groundwater, hyporheic flow widely exists in streambeds and is significantly affected by the characteristics of sediment and surface water. In this study, a low-permeability clay lens was chosen to investigate the influence of the streambed heterogeneity on the hyporheic flow at a river section of the Xin’an River in Anhui Province, China. A 2D sand tank was constructed to simulate the natural streambed including a clay lens under different velocity of surface water velocity. Heat tracing was used in this study. In particular, six analytical solutions based on the amplitude ratio and phase shift of temperatures were applied to calculate the vertical hyporheic flux. The results of the six methods ranged from −102.4 to 137.5 m/day and showed significant spatial differences. In view of the robustness of the calculations and the rationality of the results, the amplitude ratio method was much better than the phase shift method. The existence of the clay lens had a significant influence on the hyporheic flow. Results shows that the vertical hyporheic flux in the model containing a clay lens was lower than that for the blank control, and the discrepancy of the hyporheic flow field on both sides of the lens was obvious. Several abnormal flow velocity zones appeared around the clay lens where the local hyporheic flow was suppressed or generally enhanced. The hyporheic flow fields at three test points had mild changes when the lens was placed in a shallow layer of the model, indicating that the surface water velocity only affect the hyporheic flow slightly. With the increasing depth of the clay lens, the patterns of the hyporheic flow fields at all test points were very close to those of the hyporheic flow field without a clay lens, indicating that the influence of surface water velocity on hyporheic flow appeared gradually. A probable maximum depth of the clay lens was 30 to 40 cm, which approached the bottom of the model and a clay lens buried lower than this maximum would not affect the hyporheic flow any more. Influenced by the clay lens, hyporheic flow was hindered or enhanced in different regions of streambed, which was also depended on the depth of lens and surface water velocity. Introducing a two-dimensional sand tank model in a field test is an attempt to simulate a natural streambed and may positively influence research on hyporheic flow.

Depth and vertical hydrodynamics constrain the size structure of a lowland streambed community.

Abundance-body mass (N-M) relationships are prominent macroecological patterns and provide an integrated measurement of the structure and energy flow through natural communities. However, little is known about how N-M relationships are constrained by local environmental conditions. Here, we quantify how sediment depth and direction of surface-groundwater exchange (vertical hydrodynamics), two major drivers of the streambed ecology, determine N-M scaling in a sandy lowland European stream. Streambed assemblages included flagellates, ciliates, meiofauna and macroinvertebrates, and spanned five orders of magnitude in body mass. We detected a significant interaction of body mass with depth and vertical hydrodynamics with a sharp reduction in N-M slopes in the hyporheic zone and under upwelling conditions. These results revealed that streambed assemblages become more size-structured as environmental constraints increase with direct implications for the metabolic capacity and functioning of the system.