Groundwater-surface Water Interactions Research Articles

Impact of rainfed agriculture on spatio-temporal patterns of water balance and the interaction between groundwater and surface water in sub-humid plains

The expansion of rainfed agriculture, especially soybean cultivation in sub-humid plains, alters water balance and the exchange between groundwater-surface water (GW-SW). However, to date, there are no studies that analyze how these anthropic disturbances affect hydrological connectivity in these systems, especially the GW-SW interactions. The objective of this study is to analyze how the increase in rainfed agriculture affects the spatio-temporal patterns of the water balance and the GW-SW interaction. For this analysis, a coupled GW-SW flow model was implemented under land use and land cover (LULC) scenarios, to quantify the spatio-temporal dynamics for different components of water balance and GW-SW interactions for the upper creek basin of Del Azul. A simulation was carried out for a period of 13 years (2003–2015) on a daily scale and it was contrasted through three multitemporal LULC maps. The results point that substitution of natural pastures, the reduction of winter crops and the decrease of crop rotation, due to the increase of soybean monoculture in the basin under study, modifies the water balance, especially the annual rates of surface runoff and soil moisture which may increase between 3.5 and 9.4 % and between 1.4 and 4.4 % respectively, thus increasing the annual streamflows between 2.6 and 6.8 % and the groundwater heads between 0.2 and 0.6 m. This leads to changes in the hetereogeneity of the GW-SW interaction, a reduction between 0.3 and 3 % is observed in the discharge from the Pampeano aquifer to the Del Azul stream, while the recharge rates from the Del Azul stream to the Pampeano aquifer increase between 2 and 17.8 %. The application of the SWAT-MODFLOW model under LULC scenarios, improves the prediction of the regional hydrologic connectivity on sub-humid plains, because the hydrological processes occurring in the surface and non-saturated zone are governed by shallow groundwater dynamics.

Wetlands as integral parts of surface water–groundwater interactions in the Athabasca Oil Sands Area: Review and synthesis

Wetlands comprise unique water storage and conveyance mechanisms that maintain landscape integrity under the sub-humid climate in the Athabasca Oil Sands Area. In addition to their internal function, wetlands support a two-way hydrological connection to adjacent uplands and provide water for downstream water courses. Understanding the role of wetlands as integral parts of surface water (SW)–groundwater (GW) exchange can provide insights into the functioning of the hydrological system as a whole and contribute to thoughtful water management strategies and better coordination of monitoring efforts in the areas affected by oil sands (OS) activities. As such, this study summarizes the current state of hydrological knowledge on the role of wetlands in SW–GW interactions based on studies conducted within the Western Boreal Plains. In particular, the role of wetland soils and their properties in SW–GW interactions, the effects of wetlands on landscape hydrological connectivity and watershed runoff, and features of “wetland–aquifer” and “wetland–open waterbody” interactions were reviewed. Given that alterations of SW–GW interactions in wetlands can occur as a result of anthropogenic disturbances, the coordination of GW, SW, and wetland monitoring efforts and targeting areas where increased SW–GW exchange occurs would be beneficial for the economic and logistical efficiency of the OS monitoring network.

Tracer concentration mapping in a stream with hyperspectral images from unoccupied aerial systems

The release of anthropogenic chemicals to streams, stemming from contaminated sites, agriculture, urban sources, or accidental input, represents a significant threat to water resources and thus the health of humans and aquatic ecosystems. Predicting the transport and fate of chemicals is critical to quantifying contaminant concentrations and developing environmental quality standards (EQS). Tracer tests are a well-established tool for such hydrological investigations in various aquatic systems. In stream settings, the experiments have predominantly investigated longitudinal mixing and flow velocities by measuring the tracer concentration in a few discrete locations; few studies have focused on the transverse mixing properties. Recent progress in hyperspectral remote sensing from Unoccupied Aerial Systems (UAS) allows advancing the two-dimensional monitoring of tracer tests, by mapping the tracer concentration with a high spatial resolution in narrow streams with difficult accessibility. So far, such methods have mainly been demonstrated in controlled settings or in ocean waters, but rarely in optically complex streams. In this study, we evaluated the performance of a miniaturized hyperspectral imaging system and a consumer grade camera onboard a UAS, to map the concentration of the fluorescent tracer Rhodamine WT in a stream impacted by a contaminated site. In order to estimate tracer concentrations from the remotely sensed data, a ratio of the red and blue band was used for the RGB camera, while a vector-based method, estimating the spectral angle in regards to a reference spectrum, was applied for the continuum-removed hyperspectral data. The RGB camera performed well only in sections of the stream exposed to direct sunlight (R2: 0.83; nRMSE: 10.2%), failing to map the concentration in all locations, which included areas where direct sunlight was blocked by riparian trees (R2: 0.17; nRMSE: 26.19%). In contrast, the advanced spectral information allowed the hyperspectral- system to map the concentration well in all sections of the stream (R2: 0.78; nRMSE: 13.35%), regardless of illumination changes. This demonstrated the advantage of hyperspectral imaging systems for measuring water-leaving irradiance in hundreds of contiguous narrow spectral bands that also allow detecting finer absorption and emission features. The approach described here could help to improve the knowledge of contaminants mixing in streams, i.e. to predict the location of fully transverse mixing for contaminated sites discharging to streams via groundwater-surface water interactions, as well as general assumptions behind mixing and dilution models.

Microplastics in groundwater: An overview of source, distribution, mobility constraints and potential health impacts during the anthropocene

Microplastics (MPs) have already been detected in various environmental matrices like soil, sediment, and surface water, and recently in groundwater also. The occurrence of MPs in groundwater depends up on the transportation through recharge and may controlled by source and local hydrogeology, and partly on the process of surface water-groundwater interaction (SW-GW). Based on the available studies, we intended to establish a hypothetical overview on the source and process-dependent occurrence of MPs in groundwater across terrestrial and coastal aquifers. Groundwater recharge from agricultural stagnant water, losing streams near dumping sites and agricultural fields, effluents from wastewater treatment plants, septic system failure etc. are the potential sources of MPs in groundwater. The factors like sea level rise and tidal pumping are among the major factors which may control the migration of MPs in coastal aquifer along with the physical and chemical properties of the aquifer media. These MPs have another ecological concern as they can adsorb persistent organic pollutants as well as heavy metals and transfer them to animal tissues through food chain. Studies are being conducted mainly focusing the MP contamination in surface water, marine environment and soil, and very limited studies are available to address the source of MPs in groundwater. However, no such study has been done on the existence, profusion, or environmental factors that contribute to MP pollution in the groundwater in relation to the present climate change scenario. Understanding the extent of MP contamination in groundwater systems is necessary for developing effective management strategies and minimizing their impact on the environment and human health. This study focusses on the source along with the controlling factors of the migration of MPs towards groundwater including the effect of climate change.

Hydrologic changes in the Brazos River Basin and implications for Great Plains fishes

Alteration of natural flow regimes from surface water and groundwater resource development has adversely impacted aquatic habitats of resident fish throughout the Great Plains. Two endangered cyprinids, Sharpnose Shiner Notropis oxyrhynchus and Smalleye Shiner N.buccula, were historically found throughout the Brazos River Basin in Texas. However, their range has been greatly reduced by construction of impoundments and stream depletion from groundwater pumping. Successful recruitment requires flowing, unobstructed river reaches of sufficient velocity to suspend semi-buoyant fertilized fish ova. The wide, shallow stream habitats the shiners utilize are now limited to the Upper Brazos River Basin upstream of Possum Kingdom Reservoir. This study assesses the relative importance of long-term climate variability, reservoir construction, and groundwater development on Upper Brazos streamflow regimes by (1) calculating flow metrics (2) evaluating groundwater-surface water interactions and long-term groundwater inflows, and (3) using mixed-effects regression models and Poisson regression. Results show that fish habitat has been threatened through several impacted hydrologic processes: (1)groundwater development has resulted in reduced mean daily flows, peak flows, and zero-flow days, (2)upstream impoundments have increased zero-flow days and reduced mean daily flows, and (3)lower mean daily flows and peak flows now occur during droughts, suggesting water resource development—particularly after 1970—has exacerbated drought effects. We illustrate this approach using the Upper Brazos River Basin; however, the methodology applied can inform management actions to maintain spawning flows for streams in similar altered basins.

Groundwater exfiltration pattern determination in the sub-arctic catchment using thermal imaging, stable water isotopes and fully-integrated groundwater-surface water modelling

Groundwater-surface water (GW-SW) interactions play an important function in northern catchments, but their dynamics and role in water, carbon, and nutrient balances are still poorly understood. In this case study, we (1) explored groundwater-surface water dynamics in a sub-arctic headwater watershed in Pallaslompolo, Finnish Lapland. (2) We compared fully-integrated physically-based simulations of the groundwater and surface water and their interactions with unique spatially distributed field datasets of drone-derived thermal infrared imaging (TIR) and surface peat pore stable water isotopes. Simulations and field observations point to similar GW-exfiltration areas, with an agreement between slightly over 50% for stable water isotopes and modeling and more than two thirds for thermal infrared and modeling. The agreement provides evidence that a fully-integrated GW-SW model with a simple parameterization approach can successfully identify the main features of GW-SW interactions in the high-latitude catchment. All methods suggest the complex nature of GW-SW exchange that is focused in topographical lowlands. Our simulation results also highlight how specific locations serve as groundwater exfiltration zones, surface water downwelling zones, or ‘mixed’ zones, where both interaction types occur at different times according to the hydrological state of the system. Modeling results suggest abrupt GW-SW interaction reversals – a phenomenon known to occur in peatland areas. To conclude, the three methods should be treated as complementary when describing and understanding GW-SW interactions within peatlands, but only the fully-integrated physically-based modeling can provide comprehensive estimates for both the spatial and temporal patterns and magnitudes of GW-SW interaction fluxes.

Regional evaluation of groundwater-surface water interactions using a coupled geohydrological model (SWAT+gwflow)

Study regionThe research is conducted for the Scheldt river basin, where seven major watersheds located in Belgium and partly in France are included in the analysis. Study focusA proper representation of groundwater–surface water interactions with (geo)hydrological models is possible via a coupled model. However, such models have disadvantages, such as complex code modifications and new tunings, and are computationally expensive. Therefore, their application on large spatial scales is limited. A newly developed model, SWAT+gwflow integrates the Soil Water Assessment Tool (SWAT+) with the groundwater module gwflow and has the potential to overcome these limitations. However, this coupled model has not yet been evaluated at a regional scale; hence, we present the evaluation of this model for regional studies using a global aquifer data over the seven watersheds in the Scheldt basin. Furthermore, we have investigated and quantified water balance components within the basin, with a focus on groundwater-surface water exchange. New Hydrological Insights for the RegionFrom the results (Nash-Sutcliffe efficiency (NSE) of 0.8–0.9 for all catchments based on monthly average streamflow during calibration and validation periods), we consider the model to be a good simulator of hydrology in the basin. In addition, the simulated groundwater head shows good agreement with observed well data (with a mean absolute error of less than 0.42 m). Also, the rivers in five of the seven watersheds are found to be strongly dependent on groundwater discharge to the streams. We conclude that (1) the SWAT+gwflow model is capable of accurately modeling hydrological processes and state variables in the seven watersheds using global aquifer data and limited computational time, (2) the climate-gridded dataset can successfully be used for (geo)hydrological studies, and (3) the groundwater-surface water interaction increases over the years (from 1975 to 2021) with a strong increment found in the Grote Nete (3.7 fold) and upper Scheldt (2.3 fold) watersheds. These results are, moreover, promising for data-scarce regions where geohydrological modeling relies on the use of global datasets, but the mere success of this modeling application does not guarantee the accuracy of the dataset for other locations, hence, further verification is required. Furthermore, although in this study, the gwflow module is integrated into the SWAT+ model, it could also be integrated into other surface water models for other studies.

Analysis of nationwide groundwater monitoring networks using lumped-parameter models

Many countries maintain nationwide groundwater networks to monitor the status of their groundwater resources. For effective groundwater resource management, it is fundamental to understand the groundwater dynamics measured in the individual monitoring wells. Nationwide monitoring networks typically cover multiple aquifer systems with different degrees of environmental complexity. The analysis of the data of such networks thus requires flexible modeling approaches. In this study, we assessed the applicability and performance of lumped-parameter models using impulse response functions, as implemented in the Pastas software, to simulate hydraulic head data from the nationwide groundwater monitoring network in Switzerland. The selected 28 monitoring wells in the network are situated in unconsolidated, relatively shallow aquifers across Switzerland. Given the very diverse topography in the study area, snowmelt processes affect some aquifers, while groundwater–surface water interactions are important in the valleys. Different linear and nonlinear models were tested to take precipitation and potential evaporation into account, with a new model developed as part of this study to account for the effect of snow processes on recharge generation. After generally good fits in both the calibration and evaluation were achieved, the models were used to identify and quantify which stresses (e.g., precipitation, river stages) control the groundwater dynamics. The results show that precipitation and evaporation explain large parts of the measured dynamics, and about half of the monitoring wells in the network appear to be influenced by river stages. Explicitly accounting for snow processes in the recharge generating process is found to improve the simulation of the head dynamics across Switzerland, particularly for wells in high-altitude aquifers. This study demonstrates for the first time the applicability of lumped-parameter models using impulse response functions to model heads in Switzerland, and more generally, in climatic settings where snow processes are impacting the head dynamics.

Seasonal scour and siltation induced spatiotemporal variations in riverbed sediment leakage coefficients as measured via the thermal tracer method

Riverbed scour and siltation can change the hydraulic conductivity and thickness of sediments, leading to notable spatiotemporal variations of groundwater–surface water interaction. However, correlated quantitative research on the dynamic leakage coefficient (LC) remains lacking under riverbed scour and siltation. Based on dynamic monitoring and analysis of sediment medium properties, river-groundwater levels, and temperature profiles in the section of Liao River, this study compared the applicable conditions of different thermal analytical models and identified spatiotemporal variations in the LC values induced by riverbed scour and siltation. The results showed that: (1) The thermal tracer method combined with river-groundwater level monitoring can effectively quantify spatiotemporal variations in the LCs of riverbed sediment. Based on the sediment temperature presents diurnal fluctuations and gradient characteristics in different seasons, the LCs in summer, spring, and autumn compared to winter in seasonally frozen regions should be calculated using transient and steady-state analytical solution models, respectively. (2) The LCs ranged from 0.47 to 24.90 × 10-6 s−1, showed larger temporal variability than spatial, increased from the river bank to the center, and had larger values in autumn and winter. (3) Distinct from previous findings, the riverbed underwent a scouring process associated with reductions in the incoming sediment coefficient as the river stage shifted from high to low, eroding the surface 4–8 cm of sediment. During this process, the LC increased by 40 to 420%. Further, the influence of riverbed scour and siltation on the LC varied by river section. Near the riverbank, the sediment LC was primarily controlled by hydraulic conductivity related to the coarsening of sediment lithology; however, it was influenced by variations in both sediment thickness and hydraulic conductivity near the riverbed center. Collectively, these results further understand the mechanisms underlying changes in the sediment LC induced by riverbed scour and siltation, as well as provide a scientific basis for ensuring the sustainable development of riverside water sources under such changes.

Improved Representation of Groundwater–Surface Water Interactions Using SWAT+gwflow and Modifications to the gwflow Module

Recent water availability and scarcity problems have highlighted the importance of surface–groundwater interactions. Thus, groundwater models are coupled with surface water models. However, this solution is complex, needing code modifications and long computation times. Recently, a new groundwater module (gwflow) was developed directly inside the SWAT code to tackle these issues. This research assesses gwflow’s capabilities in representing surface–groundwater system interactions in the Dijle catchment (892.54 km2), a groundwater-driven watershed in Belgium. Additional developments were made in SWAT+gwflow to represent the interaction between the groundwater and soil (gwsoil). The model was calibrated for monthly mean streamflow at the catchment outlet (1983 to 1996) and validated for two periods (validation 1: 1975 to 1982 and validation 2: 1997 to 2002). It was found that the SWAT+gwflow model is better at representing the total flow (NSE of 0.6) than the standalone SWAT+ (NSE of 0.4). This was confirmed during two validation periods where the standalone model scored unsatisfactory monthly NSE (0.6 and 0.1), while the new model’s NSE was 0.7 and 0.5. Additionally, the SWAT+gwflow model simulations better depict the groundwater via baseflow and attain proper water balance values. Thus, in a highly groundwater-driven catchment, the simplified representation of groundwater systems by the standalone SWAT+ model has pitfalls. In addition, the modification made to the gwflow module (gwsoil) improved the model’s performance, which, without such adjustment, overestimates the streamflow via saturation excess flow. When including the gwsoil mechanism, thereby providing a more accurate representation of water storage and movement, groundwater is transferred to the soil profile, increasing the overall soil water content and thereby increasing lateral flow. This novel modification can also have implications for other distributed hydrological models to consider such exchanges in their modeling scheme.

Investigating key drivers of N2O emissions in heterogeneous riparian sediments: Reactive transport modeling and statistical analysis

Nitrous oxide (N2O) is a potent greenhouse gas that also contributes to ozone depletion. Recent studies have identified river corridors as significant sources of N2O emissions. Surface water-groundwater (hyporheic) interactions along river corridors induce flow and reactive nitrogen transport through riparian sediments, thereby generating N2O. Despite the prevalence of these processes, the controlling influence of physical and geochemical parameters on N2O emissions from coupled aerobic and anaerobic reactive transport processes in heterogeneous riparian sediments is not yet fully understood. This study presents an integrated framework that combines a flow and multi-component reactive transport model (RTM) with an uncertainty quantification and sensitivity analysis tool to determine which physical and geochemical parameters have the greatest impact on N2O emissions from riparian sediments. The framework involves the development of thousands of RTMs, followed by global sensitivity and responsive surface analyses. Results indicate that characterizing the denitrification reaction rate constant and permeability of intermediate-permeability sediments (e.g., sandy gravel) are crucial in describing coupled nitrification-denitrification reactions and the magnitude of N2O emissions. This study provides valuable insights into the factors that influence N2O emissions from riparian sediments and can help in developing strategies to control N2O emissions from river corridors.

Simulating drip irrigation in large-scale and high-resolution ecohydrological models: From emitters to the basin

Drip irrigation is deemed as a solution to the water conflict between agricultural and ecological needs in arid regions, but assessing hydrological impacts of drip irrigation remains challenging. This study developed a new approach to simulating drip irrigation at a basin scale, with the configuration of irrigation system explicitly represented and the emitter-scale wetted soil volume directly modeled, and incorporated the approach into HEIFLOW, a fully distributed and physically based ecohydrological model. The improved model, named HEIFLOW-drip, was used to study the potential implementation of drip irrigation in the Zhangye basin in Northwest China. Due to the recycling effect of irrigation return flow, the basin-scale water-saving efficiency (WSE) of drip irrigation fully implemented in the irrigated farmlands is 16.8%, less than half of its field-scale WSE (36.5%). This discrepancy indicates that accounting the water saved in fields may lead to notable overestimation of the basin-wide water saving by drip irrigation in basins with strong surface water-groundwater interactions. If fully implemented, drip irrigation would significantly alter the regional water balance, increasing the stream outflow by approximately 30%, while causing a decline in groundwater level. This tradeoff can be alleviated by tuning the management parameters of drip irrigation, which can achieve a synergistic effect of increasing the streamflow while preventing dramatic groundwater depletion. Another effective strategy is to implement drip irrigation in part of the basin. Given the intricate impacts of drip irrigation on hydrological processes, the determination of the scale and location of drip irrigation should be thought through at the basin level. Further analyses indicate the importance of proper management of the water saved by drip irrigation, which is crucial to prevent the paradox of irrigation efficiency. The findings of this study have great implications for addressing the complex water-food-ecosystem nexus in arid endorheic river basins.

Groundwater-surface water interactions in a semi-arid irrigated agricultural valley: A hydrometric and tracer-aided approach

The purpose of this study was to assess hydrological controls (e.g., rainfall, irrigation practices, river discharge, dam operation, evaporation) on surface (SW)- ground water (GW) interactions in an irrigated valley within semi-arid Patagonia Argentina (−65.49 W, −43.29 S). We combined different sampling designs (watershed/sub-watershed scales, longitudinal and monthly samplings) from 2015 to 2019 to investigate the temporal and spatial variation of hydrometrics, electrical conductivity (EC) and stable isotope composition of surface and ground water. Results showed that plant transpiration in the upper basin, evaporation in the middle basin and the reservoir dynamics modified water salinity and left an imprint in stable isotopes. Water tables in the irrigated valley were high (0.5–2 m level from soil surface) and presented higher salinity than river water. Groundwater salinity, temporal variation of water table levels and stable isotopes suggested that groundwater is subjected to evaporation, is recharged from field seepage and, at a lesser extent, from local rainwater. River salinity increased downstream of the irrigated valley during the whole study period (3 years), showing the effects of agriculture and urbanization. EC also responded to the opening and closing of irrigation channels. EC and daily discharge statistical analysis revealed that groundwater recharge the stream below a threshold discharge of 26 m.s−1; with river salinity increasing linearly as daily discharge decrease. This study illustrates the deep modifications that agricultural systems, mainly surface irrigation, produce on semiarid watersheds. Given that SW and GW components are currently not isolated and flow regulation and irrigation practices are playing a critical role in soil quality and river chemistry at low flow conditions, a conjunctive water management strategy must be implemented in order to prevent further land and water quality degradation.

Characterizing the transitory groundwater-surface water interaction and its environmental consequence of a riverside karst pool

Exchange between groundwater (GW) and surface water (SW) is a common occurrence in karst water systems through sinking stream disappearance or groundwater emergence. However, the transitory GW-SW interaction caused by river backflowing into a spring is poorly observed and understood. In this study, we present an approach for characterizing the influence of GW-SW interaction in a karst spring by integrating high-resolution hydrology, carefully selected hydrochemistry monitoring and precise microbe measurements. The spring-fed pool water conditions can be distinguished as high, medium, and low-water level periods in a hydrological year. The high-water level accounts for <1 % in a year, while it is associated with the hydrological regimes of backflooding states. The inflow of river backflow was found to be 4.4 times that of the natural discharge of spring water during a rainfall event. The duration of river intrusion into the spring or karst conduit could be assessed by jointly interpreting hydrography and physicochemical signatures, while the lasting environmental consequences should be evaluated together with biotic factors such as Escherichia coli. The GW-SW interaction induced by river backflow has led to the retention of river water in a pool, spring, and karst conduit for 132, 94, and 56 h, respectively. Despite turbidity returning to normal levels after 56 h, E.coli continued to persist for an extended duration. Our study reveals that despite the transient nature of GW-SW induced by river backflow on the hydrograph, they present a lasting risk of contamination from heavy metals, organic matter, and microorganisms. This extended influence can persist within a karst aquifer lacking a hyporheic zone. This research contributes to the quantification of processes involved in transitory GW-SW interaction in a karst spring, and it highlights the underestimation of GW-SW interactions in karst water systems, which might negatively impact water resources management.

Hydrochemical and isotopic characteristics of the Lodwar Alluvial Aquifer System (LAAS) in Northwestern Kenya and implications for sustainable groundwater use in dryland urban areas

Groundwater is a crucial resource for dryland regions such as this, where surface water resources are limited and unreliable. This paper presents a study of the Lodwar Alluvial Aquifer System (LAAS) in northwestern Kenya and its hydrochemical and isotopic characteristics, with the goal of understanding how to sustainably manage the groundwater system. As a result, the paper focuses on elucidating the hydrogeochemical and river-groundwater interaction (using environmental isotopes and major ion chemistry) of an aquifer that is located in the north-western dryland of Kenya. The study utilised environmental isotopes of oxygen-18 (18O), deuterium (2H), and tritium (3H) as tracers for establishing recharge sources and origin of groundwater. A sampling campaign involving 112 water samples was conducted to establish isotopic compositions of rain, spring, surface water (rivers, scoop holes, dams and lake) and groundwater at the peak of the wet season in May 2018. The tritium values in the study area ranged from 1.1 to 2.4 TU. Considering the median values of δ18O and δ2H in surface water and groundwater samples, four clusters emerge based on the degree of enrichment; Cluster 1 comprises the lake water (δ18O = +6.01, δ2H = +41.9); Cluster 2 is the Turkwel and Kawalase river water with slightly positive relative to VSMOW and with different δ2H values (+7.6‰ versus −9.8‰). The third cluster is the groundwater of the Shallow Alluvial Aquifer (SAA) and the Deep Aquifer (DA) (δ18O = −0.96‰ and −0.70‰; and δ2H = +0.4‰ and +0.6‰, respectively). The last cluster comprises the most isotope-depleted waters of water pans, scoop holes, Intermediate Aquifer (IA) and Turkana Grit Shall Aquifer (TGSA) with median values ranging from −2.87‰ to −2.48‰ for δ18O and −8.6‰ and −16.4‰ for δ2H. While the SAA is mainly recharge by the Turkwel River, a relationship is observed between the values deuterium in the Kawalase (−9.6‰ VSMOW) and IA (−8.6‰ VSMOW). Understanding recharge sources and aquifer vulnerability of similar strategic aquifers helps scientists appropriately advise policymakers and the water community who develop sustainable water use, aquifer protection and conservation strategies. In addition, the study contributes scientific evidence of isotopic compositions of groundwater in the Horn of Africa. Furthermore, the evidence of surface water-groundwater interaction presents a case of a fragile dryland ecosystem. Future work will involve the installation of piezometers in strategic aquifer zones to monitor groundwater levels in relation to river gauging data to quantify the amount of recharge and establish the impacts of rainfall variability in the upstream catchment.

Stable isotopes reveal the surface water-groundwater interaction and variation in young water fraction in an urbanized river zone

Based on 30 sampling campaigns on precipitation, river water and groundwater in Qingbaijiang urban catchment, southwest China, this study reports the spatio-temporal evolution of water isotopes under the impacts of human activities. The seasonal variation of groundwater-surface water interaction is quantitatively evaluated by isotope-aid hydrograph separation, and the young water fraction (Fyw) in river water at different spatial locations is estimated based on the seasonal periodicity of isotopes. The results show that the stable isotopes in river water and groundwater in Qingbaijiang urban catchment grow enriched from upstream to downstream, reflecting the transformation of the dominant water source from plateau river water to local precipitation and the influence of evaporation. The operation of water conservancy projects and construction of impervious surface in urban areas increase the evaporation intensity of river water. The results of the end-member mixing analysis (EMMA) demonstrates that the average annual contributions of river water to near-bank groundwater are 60.17%, 23.55% and 24.90% in the upper, middle and lower parts, respectively. The periodic storage and release of water led by the water conservancy projects enhances the groundwater recharge in the upstream river, while the water exchanges between river water and groundwater in the middle and lower parts of the catchment are reduced. The Fyw at 8 river sections of Qingbaijiang River estimated by the isotopic amplitude ratio method ranges from 0.067 to 0.194, with an overall increasing trend from upstream to downstream. The young water fraction is negatively correlated with mean elevation, mean slope and vegetation coverage, while positively correlated with the proportion of anthropogenic land use and the proportion of low-permeable soil. The construction of impervious surfaces and drainage systems in urban and rural areas is an important driver for the elevated young water fraction.

Climate-warming-driven changes in the cryosphere and their impact on groundwater–surface-water interactions in the Heihe River basin

Abstract. The Heihe River basin in northwest China depends heavily on both anthropogenic and natural storage (e.g., surface reservoirs, rivers and groundwater) to support economic and environmental functions. The Qilian Mountain cryosphere in the upper basin is integral to recharging these storage supplies. It is well established that climate warming is driving major shifts in high-elevation water storage through loss of glaciers and permafrost. However, the impacts on groundwater–surface-water interactions and water supply in corresponding lower reaches are less clear. We built an integrated hydrologic model of the middle basin, where most water usage occurs, in order to explore the hydrologic response to the changing cryosphere. We simulate the watershed response to loss of glaciers (glacier scenario), advanced permafrost degradation (permafrost scenario), both of these changes simultaneously (combined scenario) and projected temperature increases in the middle basin (warming scenario) by altering streamflow inputs to the model to represent cryosphere-melting processes, as well as by increasing the temperature of the climate forcing data. Net losses to groundwater storage in the glacier scenario and net gains in the permafrost and combined scenarios show the potential of groundwater exchanges to mediate streamflow shifts. The result of the combined scenario also shows that permafrost degradation has more of an impact on the system than glacial loss. Seasonal differences in groundwater–surface-water partitioning are also evident. The glacier scenario has the highest fraction of groundwater in terms of streamflow in early spring. The permafrost and combined scenarios meanwhile have the highest fraction of streamflow infiltration in late spring and summer. The warming scenario raises the temperature of the combined scenario by 2 ∘C. This results in net groundwater storage loss, a reversal from the combined scenario. Large seasonal changes in evapotranspiration and stream network connectivity relative to the combined scenario show the potential for warming to overpower changes resulting from streamflow. Our results demonstrate the importance of understanding the entire system of groundwater–surface-water exchanges to assess water resources under changing climatic conditions. Ultimately, this analysis can be used to examine the cascading impact of climate change in the cryosphere on the resilience of water resources in arid basins downstream of mountain ranges globally.

Hydrogeochemical similarities and groundwater-surface water interactions for the karst hydrological system of northwest Rwanda

Hydrogeochemical similarities and groundwater-surface water interactions for the karst hydrological system of northwest Rwanda

Combining spectral analysis and geochemical tracers to investigate surface water–groundwater interactions: A case study in an intensive agricultural setting (southern Guatemala)

An increase in the frequency of severe hydrological events has highlighted the importance of sustainable water management in intensive agricultural regions. In a warming climate, improved understanding and stewardship of water resources are needed to guarantee water supply, ensure food security, and build resilience against extreme events. In this study, we evaluate a framework that combines spectral analysis and geochemical tracers as a potential tool for (1) gaining valuable insights into surface water (SW)–groundwater (GW) interactions, and (2) providing guidance for improved water management in an intensive agricultural basin in southern Guatemala. The framework proves to be useful in revealing important water dynamics, exposing key feedback mechanisms for water availability and quality. With the use of power density functions and hydrochemistry (T, pH, EC, and major ions), two specific interaction regimes (influent and effluent) were identified and delimited for the main watercourse. These segments are estimated to interact at high rates with the shallow aquifer in the river channel proximities and would lose influence towards the basin flanks. Furthermore, the δ2H and δ18O values indicate that regional groundwater flow systems play an essential role in the basin groundwater recharge. Lastly, we established three influence zones that depict the spatial extent of the SW-GW interactions within the basin. With these zones, we provide recommendations that will allow for further investigation and application into better water management strategies regulating groundwater development and land use activities within the agricultural context of the area.

Transformation of dissolved organic matter and related arsenic mobility at a surface water-groundwater interface in the Hetao Basin, China

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