River–aquifer interactions in a semi‐arid environment stressed by groundwater abstraction

Rivers and aquifers are, in many cases, a connected resource and as such the interactions between them need to be understood and quantified for the resource to be managed appropriately. The objective of this paper is to advance the understanding of river–aquifer interactions processes in semi‐arid environments stressed by groundwater abstraction. This is performed using data from a specific catchment where records of precipitation, evapotranspiration, river flow, groundwater levels and groundwater abstraction are analysed using basic statistics, hydrograph analysis and a simple mathematical model to determine the processes causing the spatial and temporal changes in river–aquifer interactions. This combined approach provides a novel but simple methodology to analyse river–aquifer interactions, which can be applied to catchments worldwide. The analysis revealed that the groundwater levels have declined (~ 3 m) since the onset of groundwater abstraction. The decline is predominantly due to the abstraction rather than climatic changes (r = 0.84 for the relationship between groundwater abstraction and groundwater levels; r = 0.92 for the relationship between decline in groundwater levels and magnitude of seasonal drawdown). It is then demonstrated that, since the onset of abstraction, the river has changed from being gaining to losing during low‐flow periods, defined as periods with flow less than 0.5, 1.0 or 1.5 GL/day (1 GL/day = 1 × 10⁶ m³/day). If defined as < 1.0 GL/day, low‐flow periods constitute approximately 65% of the river flows; the periods where the river is losing at low‐flow conditions are thus significant. Importantly, there was a significant delay (> 10 years) between the onset of groundwater abstraction and the changeover from gaining to losing conditions. Finally, a relationship between the groundwater gradient towards the river and the river flow at low‐flow is demonstrated. The results have important implications for water management as well as water ecology and quality. Copyright © 2012 John Wiley & Sons, Ltd.     

Groundwater–surface water interactions in a large semi‐arid floodplain: implications for salinity management

Flow regulation and water diversion for irrigation have considerably impacted the exchange of surface water between the Murray River and its floodplains. However, the way in which river regulation has impacted groundwater–surface water interactions is not completely understood, especially in regards to the salinization and accompanying vegetation dieback currently occurring in many of the floodplains. Groundwater–surface water interactions were studied over a 2 year period in the riparian area of a large floodplain (Hattah–Kulkyne, Victoria) using a combination of piezometric surface monitoring and environmental tracers (Cl⁻, δ²H, and δ¹⁸O). Despite being located in a local and regional groundwater discharge zone, the Murray River is a losing stream under low flow conditions at Hattah–Kulkyne. The discharge zone for local groundwater, regional groundwater and bank recharge is in the floodplain within ∼1 km of the river and is probably driven by high rates of transpiration by the riparian Eucalyptus camaldulensis woodland. Environmental tracers data suggest that the origin of groundwater is principally bank recharge in the riparian zone and a combination of diffuse rainfall recharge and localized floodwater recharge elsewhere in the floodplain. Although the Murray River was losing under low flows, bank discharge occurred during some flood recession periods. The way in which the water table responded to changes in river level was a function of the type of stream bank present, with point bars providing a better connection to the alluvial aquifer than the more common clay‐lined banks. Understanding the spatial variability in the hydraulic connection with the river channel and in vertical recharge following inundations will be critical to design effective salinity remediation strategies for large semi‐arid floodplains. Copyright © 2005 John Wiley & Sons, Ltd.     

Studies of a regulated dryland river: surface–groundwater interactions

Unlike rivers in humid regions, dryland rivers typically exhibit reduced flow in the downstream direction as a result of transmission losses, which include seepage of streamflow into the aquifer, evaporation, and transpiration. However, much remains to be learned about the nature of the exchange between surface water and groundwater in these landscapes, especially in terms of spatial and temporal variability. Our study focused on streambank seepage and groundwater flow in the alluvial aquifer, specifically on answering questions such as: Is there seasonal variability in seepage losses? Is seepage permanently lost? Can losses be reduced by killing riparian vegetation? To better understand the magnitude, variability, and fate of streambank seepage, we assessed river stages, groundwater hydraulic gradients, and groundwater flow paths at two sites along a reach of the Pecos River, a dryland perennial river in West Texas. We found that along this reach the river was losing water to the aquifer even under low‐flow conditions; but seepage was controlled by a number of different mechanisms. Seepage increased not only during high‐flow events but also when the groundwater level was declining owing to long periods of no irrigation release. Tamarix (saltcedar) control did not affect hydraulic gradients nor reduce streambank seepage and given that this reach of the Pecos River is a losing one, streamflow will not be enhanced by controlling saltcedar. These findings can be used to improve basic conceptual models of dryland river systems and to predict hydrologic responses to changes in the timing and magnitude of streamflows and to riparian vegetation management. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of channel transmission losses in a dryland river reach in north‐eastern Brazil using streamflow series,groundwater level series and multi‐temporal satellite data

Scarcity of hydrological data, especially streamflow discharge and groundwater level series, restricts the understanding of channel transmission losses (TL) in drylands. Furthermore, the lack of information on spatial river dynamics encompasses high uncertainty on TL analysis in large rivers. The objective of this study was to combine the information from streamflow and groundwater level series with multi‐temporal satellite data to derive a hydrological concept of TL for a reach of the Middle Jaguaribe River (MJR) in semi‐arid north‐eastern Brazil. Based on this analysis, we proposed strategies for its modelling and simulation. TL take place in an alluvium, where river and groundwater can be considered to be hydraulically connected. Most losses certainly infiltrated only through streambed and levees and not through the flood plains, as could be shown by satellite image analysis. TL events whose input river flows were smaller than a threshold did not reach the outlet of the MJR. TL events whose input flows were higher than this threshold reached the outlet losing on average 30% of their input. During the dry seasons (DS) and at the beginning of rainy seasons (DS/BRS), no river flow is expected for pre‐events, and events have vertical infiltration into the alluvium. At the middle and the end of the rainy seasons (MRS/ERS), river flow sustained by base flow occurs before/after events, and lateral infiltration into the alluvium plays a major role. Thus, the MJR shifts from being a losing river at DS/BRS to become a losing/gaining (mostly losing) river at MRS/ERS. A model of this system has to include the coupling of river and groundwater flow processes linked by a leakage approach. Copyright © 2012 John Wiley & Sons, Ltd.     

Modeling surface water–groundwater interaction in New Zealand: Model development and application

Most rivers worldwide have a strong interaction with groundwater when they leave the mountains and flow over alluvial plains before flowing into the seas or disappearing in the deserts, and in New Zealand, typically, rivers lose water to the groundwater in the upper plains and generally gain water from the groundwater in the lower plains. Aiming at simulating surface water–groundwater interaction nationally in New Zealand, we developed a conceptual groundwater module for the national hydrologic model TopNet to simulate surface water–groundwater interaction, groundwater flow, and intercatchment groundwater flow. The developed model was applied to the Pareora catchment in South Island of New Zealand, where there are concurrent spot gauged flows. Results show that the model simulations not only fit quite well to flow measurement but also to concurrent spot gauged flows, and compared to the original TopNet, it has a significant improvement in the low flows. Sensitivity analysis shows river flow is sensitive to the river losing/gaining rate instead of groundwater characteristic, while groundwater storage is sensitive to both river losing/gaining rate and groundwater characteristic. This indicates our conceptual approach is promising for nationwide modeling without the large amount of geology and aquifer data typically required by physically‐based modeling approaches.     

Numerical modelling of stream–aquifer interaction: Quantifying the impact of transient streambed permeability and aquifer heterogeneity

Stream–aquifer interaction plays a vital role in the water cycle, and a proper study of this interaction is needed for understanding groundwater recharge, contaminants migration, and for managing surface water and groundwater resources. A model‐based investigation of a field experiment in a riparian zone of the Schwarzbach river, a tributary of the Rhine River in Germany, was conducted to understand stream–aquifer interaction under alternative gaining and losing streamflow conditions. An equivalent streambed permeability, estimated by inverting aquifer responses to flood waves, shows that streambed permeability increased during infiltration of stream water to aquifer and decreased during exfiltration. Aquifer permeability realizations generated by multiple‐point geostatistics exhibit a high degree of heterogeneity and anisotropy. A coupled surface water groundwater flow model was developed incorporating the time‐varying streambed permeability and heterogeneous aquifer permeability realizations. The model was able to reproduce varying pressure heads at two observation wells near the stream over a period of 55 days. A Monte Carlo analysis was also carried out to simulate groundwater flow, its age distribution, and the release of a hypothetical wastewater plume into the aquifer from the stream. Results of this uncertainty analysis suggest (a) stream–aquifer exchange flux during the infiltration periods was constrained by aquifer permeability; (b) during exfiltration, this flux was constrained by the reduced streambed permeability; (c) the effect of temporally variable streambed permeability and aquifer heterogeneity were found important to improve the accurate capture of the uncertainty; and (d) probabilistic infiltration paths in the aquifer reveal that such pathways and the associated prediction of the extent of the contaminant plume are highly dependent on aquifer heterogeneity.     

Influence of a thin veneer of low‐hydraulic‐conductivity sediment on modelled exchange between river water and groundwater in response to induced infiltration

A thin layer of fine‐grained sediment commonly is deposited at the sediment–water interface of streams and rivers during low‐flow conditions, and may hinder exchange at the sediment–water interface similar to that observed at many riverbank‐filtration (RBF) sites. Results from a numerical groundwater‐flow model indicate that a low‐permeability veneer reduces the contribution of river water to a pumping well in a riparian aquifer to various degrees, depending on simulated hydraulic gradients, hydrogeological properties, and pumping conditions. Seepage of river water is reduced by 5–10% when a 2‐cm thick, low‐permeability veneer is present on the bed surface. Increasing thickness of the low‐permeability layer to 0·1 m has little effect on distribution of seepage or percentage contribution from the river to the pumping well. A three‐orders‐of‐magnitude reduction in hydraulic conductivity of the veneer is required to reduce seepage from the river to the extent typically associated with clogging at RBF sites. This degree of reduction is much larger than field‐measured values that were on the order of a factor of 20–25. Over 90% of seepage occurs within 12 m of the shoreline closest to the pumping well for most simulations. Virtually no seepage occurs through the thalweg near the shoreline opposite the pumping well, although no low‐permeability sediment was simulated for the thalweg. These results are relevant to natural settings that favour formation of a substantial, low‐permeability sediment veneer, as well as central‐pivot irrigation systems, and municipal water supplies where river seepage is induced via pumping wells. Published in 2011 by John Wiley & Sons, Ltd.     

Effects of river discharge on hyporheic exchange flows in salmon spawning areas of a large gravel‐bed river

The flow magnitude and timing from hydroelectric dams in the Snake River Basin of the Pacific north‐western US is managed in part for the benefit of salmon. The objective of this research was to evaluate the effects of Hells Canyon Dam discharge operations on hydrologic exchange flows between the river and riverbed in Snake River fall Chinook salmon spawning areas. Interactions between river water and pore water within the upper 1 m of the riverbed were quantified through the use of self‐contained temperature and water level data loggers suspended inside of piezometers. The data were recorded at 20 min intervals over a period of 200 days when the mean daily discharge was 218–605 m³ s^?1, with hourly stage changes as large as 1·9 m. Differences in head pressure between the river and riverbed were small, often within ± 2 cm. Measured temperature gradients in the riverbed indicated significant interactions between the surface and subsurface water. At the majority of sites, neither hydraulic nor temperature gradients were significantly affected by either short‐ or long‐term changes in discharge operations from Hells Canyon Dam. Only 2 of 14 study sites exhibited acute flux reversals between the river and riverbed resulting from short‐term, large magnitude changes in discharge. The findings suggest that local scale measurements may not be wholly explanatory of the hydrological exchange between the river and riverbed. The processes controlling surface water exchange at the study sites are likely to be bedform‐induced advective pumping, turbulence at the riverbed surface, and large‐scale hydraulic gradients along the longitudinal profile of the riverbed. By incorporating the knowledge of hydrological exchange processes into water management planning, regional agencies will be better prepared to manage the limited water resources among competing priorities that include salmon recovery, flood control, irrigation supply, hydropower production, and recreation. Copyright © 2007 John Wiley & Sons, Ltd.     

Patterns and dynamics of river–aquifer exchange with variably-saturated flow using a fully-coupled model

The parallel physically-based surface–subsurface model PARFLOW was used to investigate the spatial patterns and temporal dynamics of river–aquifer exchange in a heterogeneous alluvial river–aquifer system with deep water table. Aquifer heterogeneity at two scales was incorporated into the model. The architecture of the alluvial hydrofacies was represented based on conditioned geostatistical indicator simulations. Subscale variability of hydraulic conductivities (K) within hydrofacies bodies was created with a parallel Gaussian simulation. The effects of subscale heterogeneity were investigated in a Monte Carlo framework. Dynamics and patterns of river–aquifer exchange were simulated for a 30-day flow event. Simulation results show the rapid formation of saturated connections between the river channel and the deep water table at preferential flow zones that are characterized by high conductivity hydrofacies. Where the river intersects low conductivity hydrofacies shallow perched saturated zones immediately below the river form, but seepage to the deep water table remains unsaturated and seepage rates are low. Preferential flow zones, although only taking up around 50% of the river channel, account for more than 98% of total seepage. Groundwater recharge is most efficiently realized through these zones. Subscale variability of K_sat slightly increased seepage volumes, but did not change the general seepage patterns (preferential flow zones versus perched zones). Overall it is concluded that typical alluvial heterogeneity (hydrofacies architecture) is an important control of river–aquifer exchange in rivers overlying deep water tables. Simulated patterns and dynamics are in line with field observations and results from previous modeling studies using simpler models. Alluvial heterogeneity results in distinct patterns and dynamics of river–aquifer exchange with implications for groundwater recharge and the management of riparian zones (e.g. river channel-floodplain connectivity via saturated zones).     

Reducing monitoring gaps at the aquifer–river interface by modelling groundwater–surface water exchange flow patterns

This study investigates spatial patterns and temporal dynamics of aquifer–river exchange flow at a reach of the River Leith, UK. Observations of sub‐channel vertical hydraulic gradients at the field site indicate the dominance of groundwater up‐welling into the river and the absence of groundwater recharge from surface water. However, observed hydraulic heads do not provide information on potential surface water infiltration into the top 0–15 cm of the streambed as these depths are not covered by the existing experimental infrastructure. In order to evaluate whether surface water infiltration is likely to occur outside the ‘window of detection’, i.e. the shallow streambed, a numerical groundwater model is used to simulate hydrological exchanges between the aquifer and the river. Transient simulations of the successfully validated model (Nash and Sutcliff efficiency of 0·91) suggest that surface water infiltration is marginal and that the possibility of significant volumes of surface water infiltrating into non‐monitored shallow streambed sediments can be excluded for the simulation period. Furthermore, the simulation results show that with increasing head differences between river and aquifer towards the end of the simulation period, the impact of streambed topography and hydraulic conductivity on spatial patterns of exchange flow rates decreases. A set of peak flow scenarios with altered groundwater‐surface water head gradients is simulated in order to quantify the potential for surface water infiltration during characteristic winter flow conditions following the observation period. The results indicate that, particularly at the beginning of peak flow conditions, head gradients are likely to cause substantial increase in surface water infiltration into the streambed. The study highlights the potential for the improvement of process understanding of hyporheic exchange flow patterns at the stream reach scale by simulating aquifer‐river exchange fluxes with a standard numerical groundwater model and a simple but robust model structure and parameterization. Copyright © 2011 John Wiley & Sons, Ltd.     

The influence of local hydrogeologic forcings on near‐stream event water recharge and retention (Upper San Pedro River,Arizona)

The rise in stream stage during high flow events (floods) can induce losing stream conditions, even along stream reaches that are gaining during baseflow conditions. The aquifer response to flood events can affect the geochemical composition of both near‐stream groundwater and post‐event streamflow, but the amount and persistence of recharged floodwater may differ as a function of local hydrogeologic forcings. As a result, this study focuses on how vertical flood recharge varies under different hydrogeologic forcings and the significance that recharge processes can have on groundwater and streamflow composition after floods. River and shallow groundwater samples were collected along three reaches of the Upper San Pedro River (Arizona, USA) before, during and after the 2009 and 2010 summer monsoon seasons. Tracer data from these samples indicate that subsurface floodwater propagation and residence times are strongly controlled by the direction and magnitude of the dominant stream–aquifer gradient. A reach that is typically strongly gaining shows minimal floodwater retention shortly after large events, whereas the moderately gaining and losing reaches can retain recharged floodwater from smaller events for longer periods. The moderately gaining reach likely returned flood recharge to the river as flow declined. These results indicate that reach‐scale differences in hydrogeologic forcing can control (i) the amount of local flood recharge during events and (ii) the duration of its subsurface retention and possible return to the stream during low‐flow periods. Our observations also suggest that the presence of floodwater in year‐round baseflow is not due to long‐term storage beneath the streambed along predominantly gaining reaches, so three alternative mechanisms are suggested: (i) repeated flooding that drives lateral redistribution of previously recharged floodwater, (ii) vertical recharge on the floodplain during overbank flow events and (iii) temporal variability in the stream–aquifer gradient due to seasonally varying water demands of riparian vegetation. Copyright © 2011 John Wiley & Sons, Ltd.     

Dynamic river–groundwater exchange in the presence of a saline,semi‐confined aquifer

Understanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field‐based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater (<500 μS/cm and <100 years old) and a semi‐confined artesian aquifer containing old and saline groundwater (electrical conductivity > 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re‐established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low‐salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m³/m/day compared with 26 and 3 m³/m/day for the same periods via mass balance using Radon (²²²Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.     

The effects of floods on the temperature of riparian groundwater

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.     

Hillslope run‐off thresholds with shrink–swell clay soils

Irrigation experiments on 12 instrumented field plots were used to assess the impact of dynamic soil crack networks on infiltration and run‐off. During applications of intensity similar to a heavy rainstorm, water was seen being preferentially delivered within the soil profile. However, run‐off was not observed until soil water content of the profile reached field capacity, and the apertures of surface‐connected cracks had closed >60%. Electrical resistivity measurements suggested that subsurface cracks persisted and enhanced lateral transport, even in wet conditions. Likewise, single‐ring infiltration measurements taken before and after irrigation indicated that infiltration remained an important component of the water budget at high soil water content values, despite apparent surface sealing. Overall, although the wetting and sealing of the soil profile showed considerable complexity, an emergent property at the hillslope scale was observed: all of the plots demonstrated a strikingly similar threshold run‐off response to the cumulative precipitation amount. Copyright © 2014 John Wiley & Sons, Ltd.     

Improvement of surface run‐off in the hydrological model ParFlow by a scale‐consistent river parameterization

We propose an improvement of the overland‐flow parameterization in a distributed hydrological model, which uses a constant horizontal grid resolution and employs the kinematic wave approximation for both hillslope and river channel flow. The standard parameterization lacks any channel flow characteristics for rivers, which results in reduced river flow velocities for streams narrower than the horizontal grid resolution. Moreover, the surface areas, through which these wider model rivers may exchange water with the subsurface, are larger than the real river channels potentially leading to unrealistic vertical flows. We propose an approximation of the subscale channel flow by scaling Manning's roughness in the kinematic wave formulation via a relationship between river width and grid cell size, following a simplified version of the Barré de Saint‐Venant equations (Manning–Strickler equations). The too large exchange areas between model rivers and the subsurface are compensated by a grid resolution‐dependent scaling of the infiltration/exfiltration rate across river beds. We test both scaling approaches in the integrated hydrological model ParFlow. An empirical relation is used for estimating the true river width from the mean annual discharge. Our simulations show that the scaling of the roughness coefficient and the hydraulic conductivity effectively corrects overland flow velocities calculated on the coarse grid leading to a better representation of flood waves in the river channels.     

Floodplain connection buffers seasonal changes in urban stream water quality

Urban streams in the Northeastern United States have large road salt inputs during the winter, increased nonpoint sources of inorganic nitrogen and decreased short‐term and permanent storage of nutrients. Restoration activities that re‐establish connection between streams and riparian environments may be effective for improving urban stream water quality. Meadowbrook Creek, a first‐order stream in Syracuse, NY, provides a unique setting to explore impacts of stream–floodplain connection because it flows along a negative urbanization gradient, from channelized and armoured headwaters to a broad, vegetated floodplain with a riparian aquifer. In this study, we investigated how reconnection to groundwater and introduction of riparian vegetation impacted urban surface water chemistry by making biweekly longitudinal surveys of stream water chemistry in the creek from May 2012 until June 2013. We used multiple methods to measure groundwater discharge rates along the creek. Chloride concentrations in the upstream, disconnected reach were influenced by discharge of road salt during snow melt events and ranged from 161.2 to 1440 mg/l. Chloride concentrations in the downstream, connected reach had less temporal variation, ranging from 252.0 to 1049 mg/l, because of buffering by groundwater discharge, as groundwater chloride concentrations ranged from 84.0 to 655.4 mg/l. In the summer, there was little to no nitrate in the disconnected reach because of limited sources and high primary productivity, but concentrations reached over 1 mg N/l in the connected reach because of the presence of riparian vegetation. During the winter, when temperatures fell below freezing, nitrate concentrations in the disconnected reach increased to 0.58 mg N/l but were still lower than the connected reach, which averaged 0.88 mg N/l. Urban stream restoration projects that restore floodplain connection may impact water quality by storing high salinity road run‐off during winter overbank events and discharging that water year‐round, thereby attenuating seasonal fluctuations in chloride. Contrary to prior findings, we observed that floodplain connection and riparian vegetation may alter nitrate sources and sinks such that nitrate concentrations increase longitudinally in connected urban streams. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface water–groundwater exchange dynamics in a tidal freshwater zone

In coastal rivers, tides can propagate for tens to hundreds of kilometres inland beyond the saltwater line. Yet the influence of tides on river–aquifer connectivity and solute transport in tidal freshwater zones (TFZs) is largely unknown. We estimate that along the TFZ of White Clay Creek (Delaware, USA), 11% of river water exchanges through tidal bank storage zones. Additional hyporheic processes such as flow through bedforms likely contribute even more exchange. The turnover length associated with tidal bank storage is 150 km, on the order of turnover lengths for all hyporheic exchange processes in non‐tidal rivers of similar size. Based on measurements at a transect of piezometers located 17 km from the coast, tides exchange 0.36 m³ of water across the banks and 0.86 m³ across the bed per unit river length. Exchange fluxes range from ?1.66 to 2.26 m day^?1 across the bank and ?0.84 to 1.88 m day^?1 across the bed. During rising tide, river water infiltrates into the riparian aquifer, and the downstream transport rate in the channel is low. During falling tide, stored groundwater is released to the river, and the downstream transport rate in the channel increases. Tidal bank storage zones may remove nutrients or other contaminants from river water and attenuate nutrient loads to coasts. Alternating expansion and contraction of aerobic zones in the riparian aquifer likely influence contaminant removal along flow paths. A clear need exists to understand contaminant removal and other ecosystem services in TFZs and adopt best management practices to promote these ecosystem services. Copyright © 2015 John Wiley & Sons, Ltd.     

Spatio‐Temporal Variations in Stream–Aquifer Interactions Following Construction of Weirs in Korea

The “Four Major Rivers Restoration Project” was conducted to secure sufficient water resources, introduce comprehensive flood control measures, improve water quality, and restore river ecosystems in Korea. As a part of the project, 16 sites were dredged and weirs were installed in the Han, Geum, Yeongsan, and Nakdong Rivers from late 2010 to early 2012. Groundwater data were obtained from 213 groundwater monitoring wells near the four major rivers to analyze the impacts of weir construction on the nearby groundwater flow system. The groundwater level and chemical characteristics were analyzed to investigate how the groundwater flow system and water quality changed following weir construction. Our results show that the groundwater level immediately increased with increased river levels following weir construction. In addition, the hydrologic condition of some rivers upstream of the weirs was changed from gaining to losing streams. Consequently, the direction of groundwater flow changed from perpendicular to parallel to the river, and groundwater downstream of the weir became recharged from the area upstream of the weir. This should affect groundwater quality, which should become similar to the river water; however, this change has not yet been observed. Therefore, both further monitoring of the groundwater quality and further hydrogeochemical analysis are required for quantitative evaluation of the effects of weir construction in the study area.     

Disconnected Surface Water and Groundwater: From Theory to Practice

When describing the hydraulic relationship between rivers and aquifers, the term disconnected is frequently misunderstood or used in an incorrect way. The problem is compounded by the fact that there is no definitive literature on the topic of disconnected surface water and groundwater. We aim at closing this gap and begin the discussion with a short introduction to the historical background of the terminology. Even though a conceptual illustration of a disconnected system was published by Meinzer (1923) , it is only within the last few years that the underlying physics of the disconnection process has been described. The importance of disconnected systems, however, is not widely appreciated. Although rarely explicitly stated, many approaches for predicting the impacts of groundwater development on surface water resources assume full connection. Furthermore, management policies often suggest that surface water and groundwater should only be managed jointly if they are connected. However, although lowering the water table beneath a disconnected section of a river will not change the infiltration rate at that point, it can increase the length of stream that is disconnected. Because knowing the state of connection is of fundamental importance for sustainable water management, robust field methods that allow the identification of the state of connection are required. Currently, disconnection is identified by showing that the infiltration rate from a stream to an underlying aquifer is independent of the water table position or by identifying an unsaturated zone under the stream. More field studies are required to develop better methods for the identification of disconnection and to quantify the implications of heterogeneity and clogging processes in the streambed on disconnection.     

Quantifying River‐Groundwater Interactions of New Zealand's Gravel‐Bed Rivers: The Wairau Plain

New Zealand's gravel‐bed rivers have deposited coarse, highly conductive gravel aquifers that are predominantly fed by river water. Managing their groundwater resources is challenging because the recharge mechanisms in these rivers are poorly understood and recharge rates are difficult to predict, particularly under a more variable future climate. To understand the river‐groundwater exchange processes in gravel‐bed rivers, we investigate the Wairau Plain Aquifer using a three‐dimensional groundwater flow model which was calibrated using targeted field observations, “soft” information from experts of the local water authority, parameter regularization techniques, and the model‐independent parameter estimation software PEST. The uncertainty of simulated river‐aquifer exchange flows, groundwater heads, spring flows, and mean transit times were evaluated using Null‐space Monte‐Carlo methods. Our analysis suggests that the river is hydraulically perched (losing) above the regional water table in its upper reaches and is gaining downstream where marine sediments overlay unconfined gravels. River recharge rates are on average 7.3 m³/s, but are highly dynamic in time and variable in space. Although the river discharge regularly hits 1000 m³/s, the net exchange flow rarely exceeds 12 m³/s and seems to be limited by the physical constraints of unit‐gradient flux under disconnected rivers. An important finding for the management of the aquifer is that changes in aquifer storage are mainly affected by the frequency and duration of low‐flow periods in the river. We hypothesize that the new insights into the river‐groundwater exchange mechanisms of the presented case study are transferable to other rivers with similar characteristics.