Groundwater Interactions Research Articles

Seasonal metal fluxes derived by the interaction of surface water and groundwater in an aquaculture estuary

Seasonal metal fluxes derived by the interaction of surface water and groundwater in an aquaculture estuary

Effect of depositional evolution on phosphorus enrichment in aquifer sediments of alluvial-lacustrine plain

Groundwater with high geogenic phosphorus (P) is increasingly concerned as a potential risk to surface water eutrophication. Although hydrogeochemical processes responsible for P mobilization in groundwater systems have been studied, the burial characteristics of P and the effect of depositional evolution on P enrichment in aquifer sediments remain unclear. In this study, aquifer sediments were collected from the Dongting Lake Plain (DTP) within the central Yangtze River Basin, a high P groundwater area, and the effect of depositional evolution on P enrichment was elucidated by comprehensively analyzing the lithology, grain size, geochronology, and geochemistry of the sediments, coupled with groundwater chemistry and sediment incubation experiments. The results showed that the contents of total organic carbon (TOC), iron (Fe), and P (the relative content of bioavailable phosphorus (BAP)) were higher in lacustrine sediments deposited under a warm-wet climate, but lower in fluvial sediments deposited under a cold-dry climate. During depositional evolution, the sedimentary facies mainly controlled the content of organic phosphorus (OP), while the paleo-climate controlled the content of both OP and Fe-bound inorganic P (FeP), which jointly affected total P content in aquifer sediments. Under the interaction of groundwater and sediment, the reductive dissolution of P-rich Fe (oxyhydr)oxides and the mineralization of OP in sediment continuously release P into groundwater. Notably, the rapid accumulation of alluvial sediments after the Last Glacial Maximum in the DTP and rapid evolution of Dongting Lake during the Holocene led to a large amount of organic matter (OM) and P buried in sediments, providing materials for P release in aquifers, which seriously threatens groundwater quality. This exploration can provide a new understanding of the enrichment of geogenic P in groundwater from the perspective of depositional evolution.

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.

Rainfall intensity affects the recharge mechanisms of groundwater in a headwater basin of the North China plain

Understanding the origins of groundwater and its movement from mountain to plain during different high intensity rainfall events is critical for conserving water supplies, determining water-use policies and controlling pollution. These factors are also the keys for understanding the dominant processes in hydrological models. In this study, groundwater resources and recharge processes during heavy precipitation were explored by using stable isotope tracers in the hilly area of Taihang Mountain. It was found that the δ2H and δ18O values of precipitation exhibited obvious precipitation amount effect during different precipitation intensity events. The stable isotopic values in groundwater and river water showed significantly varied during the single extreme heavy precipitation and the continuous heavy precipitation events. In the rainy season, precipitation amounts greater than 40 mm/d could effectively recharge the shallow groundwater in the study area. By comparing the spatial isotopic distribution of groundwater, soil water, and river water with precipitation, we showed distinct groundwater recharge patterns in terms of their water resources, timing, and the degree of river water and groundwater interaction during the single extreme heavy precipitation and the continuous heavy precipitation. After the single extreme heavy precipitation, the δ2H and δ18O values of groundwater, soil water, and river water showed stable over time and had the same similar variations, suggested that the groundwater recharge was mainly dominated by precipitation with preferential flow or bypass flow. While after the continuous heavy precipitation, the variation of δ2H and δ18O in all water is consistent with the previous precipitation, which shown a mixing effect of previous enrich precipitation and depleted heavy precipitation, suggested that groundwater source was dominated by a continuous recharge of previous heavier precipitation with translatory flow. The groundwater main recharge mechanism is not constant, but changes with rainfall intensity. The rainfall intensity play an important role in groundwater recharge change affecting runoff process. Overall, this paper presents a new insight to understand the effect of rainfall intensity on hydrological process, which could be used to provide vital information in the semi-humid and semi-arid regions where water resources are critical in climate change adaptation strategies.

A multidisciplinary geochemical approach to geothermal resource exploration: The Spezzano Albanese thermal system, southern Italy

This work aims to define the geothermal conceptual model of the Spezzano Albanese thermal system located in the northern sector of the Calabria Region, along the western edge of the Sibari Plain. The study confirms that the deep thermal reservoir of Spezzano Albanese is mainly hosted within the permeable levels of the evaporite deposits of the Messinian succession including siltstones with manganese mineralisation, halite, and gypsum-arenites. The thermal waters show discharging temperature between 20.2 and 26.6 °C and a high compositional variability, from Na–Cl to Na–Ca–HCO3. The compositional evolution (from Na–Cl to Na–HCO3) is accompanied by a decrease in both salinity and the concentrations of most dissolved constituents, including Cl, Br, B, Li, Na, K, Mg, Ca, and Ba. These variations are due to mixing between the thermal endmember, rich in Cl and related components, and low-salinity, cold shallow waters (temperature between 17.5 and 22.7 °C). The study points out that the thermal endmember derives by halite dissolution and more complex water-rock interaction processes involving (1) the dissolution of other solid phases of the Messinian deposits, as also confirmed by δ34S values of dissolved sulphate and sulphide, and (2) the precipitation of secondary solid phases (e.g., barite). The geothermometric modelling suggests that the thermal groundwaters in the deep reservoir are probably in equilibrium with either (i) quartz, calcite, disordered dolomite, low-albite, and K-feldspar, as well as with pyrophyllite and poorly crystalline kaolinite (as proxies of clay minerals) at temperatures of 65.5 ± 4.5 °C or (ii) quartz, calcite, disordered dolomite, low-albite, disordered adularia, laumontite and saponites at temperatures of 56.1 ± 4.3 °C, based on the first and second geothermometric model, respectively.The δ18O and δ2H values of water confirm a meteoric origin for the thermal waters with average recharge altitudes between 745 and 857 m a.s.l. These elevations are compatible with the recharge from the western side of the Esaro valley where evaporite successions are found close to the surface.The isotopic value of the dissolved CO2 associated to the Spezzano Terme water highlights its likely microbial origin, as recognised for other thermal circuits hosted in sedimentary rocks of the southern Apennines. Furthermore, the thermal endmember shows a noteworthy enrichment in CH4 with respect to air due to the interaction of groundwater with sediments rich in organic matter. Although methane could have a biogenic origin, the presence of a minor component of thermogenic methane in the gas phase dissolved in the Spezzano Terme waters cannot be completely excluded.The data obtained in this study allow to assume that the recharge meteoric waters descend to a maximum depth of about 1.1–1.4 km below the main emergence area and then the regional NE-SW fault systems probably act as a preferential pathway for the ascent of the thermal waters towards the surface. These waters discharge at Spezzano Albanese, where the crystalline-metamorphic units cropping out immediately upstream of the emergence area act as cap-rock favouring the final ascent towards the surface of the thermal waters.

Per- and polyfluoroalkyl substances (PFAS) contamination in a microtidal urban estuary: Sources and sinks

This study evaluates PFAS contamination and determines the major drainage sources to a temperate microtidal estuary, the Swan Canning Estuary, in Perth Western Australia. We describe how variability in these sources influences PFAS concentrations within this urban estuary. Surface water samples were collected from 20 estuary sites and 32 catchment sites in June and December from 2016 to 2018. Modelled catchment discharge was used to estimate PFAS load over the study period. Three major catchment sources of elevated PFAS were identified with contamination likely resulting from historical AFFF use on a commercial airport and defence base. Estuary PFAS concentration and composition varied significantly with season and spatially with the two different estuary arms responding differently to winter and summer conditions. This study has found that the influence of multiple PFAS sources on an estuary depend on the historical usage timeframe, groundwater interactions and surface water discharge.

Application of SWAT and MODFLOW models for characterization of surface–groundwater interaction in the Modjo River catchment, central Ethiopia

Application of SWAT and MODFLOW models for characterization of surface–groundwater interaction in the Modjo River catchment, central Ethiopia

Characterizing groundwater and surface water interaction using geological, environmental tracers (222Rn, EC, δ18O, and δ2H) and baseflow index methods for part of the Upper Awash and the adjacent Blue Nile Basin, Ethiopia

Characterization of groundwater-surface water interaction using a combined approach of geological, environmental tracers (222Rn, EC, δ18O, and δ2H) and baseflow index in a complex geological environment was carried out for part of the Upper Awash and adjacent Blue Nile Basins. In this work, total measurements of 59 water samples were used (52 from different reaches of main Rivers and tributaries, 4 from springs, and 3 from boreholes) for 222Rn analysis in dry and wet seasons, along with recent 8–20 years of daily stream flow records from 11-gauge stations. For sub-surface inference, regional geological structures and lineament density zones were determined using Kriging interpolation techniques. 222Rn values for the river and streams show 2.7–468 Bq/m3 during the rainy season and 17.1–1467 Bq/m3 for the non-rainy season. The credible justification related to moderate to high seasonal 222Rn values (100–400 Bq/m3) in the upstream parts of the two Basins, including areas falling along the surface water divide between the two adjacent Basins, shows a link between groundwater and surface water systems. In the same area, a very high mean annual baseflow index (BFI >0.5) is dominantly associated with high lineament density (0.9 < LD < 3.0), regional faults, and geologic units of highly permeable scoriaceous basalt, rift basalt, and fractured ignimbrite units. Relatively, low 222Rn (<100 Bq/m3) in both seasons and low baseflow index (BFI <0.35) values mainly in the central part of the Upper Awash basin are attributed to existing impervious superficial deposits (alluvial, tuff and ash). This, in turn, shows the poor connection between the groundwater and surface water systems in the central plain part of the Upper Awash Basin and, as also evidenced by the storage of floodwater for an extended time after the end of the rainy season, prove poor connectivity between groundwater and surface water systems. The very high 222Rn (400–1467 Bq/m3) with varied mean annual baseflow index (0.35 < BFI <0.5) at different stations in the extreme southern and southeastern parts of the Upper Awash Basin show Awash River is gaining from the aquifer system through highly fractured basalt and fractured ignimbrite geologic units. Evidence from analysis of measured electrical conductivity and stable isotopes (δ18O, δ2H) generally agree with the observed 222Rn value and baseflow analysis.

Assessing the influence of a dam reservoir on groundwater quality using geochemical and stable isotopes techniques

Stable Isotope (δ2H and δ18O) technique is employed to investigate the Dam water-groundwater interaction on the Bui hydropower project in Ghana, to ascertain; if surface water from the dam recharges the groundwater or if there is balance flow relationship between the two systems. The hydrochemical results showed that the pH of the groundwater (5.51–7.18) was slightly acidic to basic, predominantly due to lithology. The pH of the surface water (6.13–6.98) suggests decomposition of submerged organic matter at the impoundment of the dam. The hydrochemical facies showed signatures that result from groundwater interaction with soluble aquifer materials (feldspars, micas, and quartz). Stable isotope in the study area showed surface water is more enriched (δ18O = −2.46‰; δ2H −21.48‰) than rainwater (δ18O = −3.36‰; δ2H = −17.83‰), which is slightly enriched than groundwater (δ18O = −3.58‰; δ2H = −17.48‰). The stable isotope (δ2H-δ18O) biplot indicates that no interaction occurred between the groundwater and the Black Volta River; the groundwater in the catchment is of meteoric origin. D-excess confirms mineralization due to water-rock interaction while the biplot of Cl− versus δ18O delineated three (3) groups of water i.e., water being bridged by rocks; groundwater recharged by rainfall with little or no evaporation and evaporated groundwater. Residence time influences groundwater's ionic configuration which imparts aesthetic characteristics over time. The study confirms the feasibility of complimenting geochemistry with the isotope technique to investigate flow relationships between dam water and groundwater systems.

Enhanced Hydrological Simulations in Paddy-Dominated Watersheds Using the Hourly SWAT-MODFLOW-PADDY Modeling Approach

Accurate hydrological simulations are crucial for managing water resources and promoting sustainable agriculture in submerged paddy agricultural watersheds. The SWAT-MODFLOW, which couples the Soil and Water Assessment Tool (SWAT) and the Modular Groundwater Flow (MODFLOW) model, is a widely used tool for hydrologic simulations that consider surface water and groundwater (SW-GW) interactions. However, it falls short of effectively simulating the hydrological processes of submerged rice paddy field areas. To address this, we developed the hourly SWAT-MODFLOW-PADDY model, which enables integrated surface and groundwater simulations and effectively represents the hydrological responses of submerged paddy fields to high-resolution rainfall data. Our findings demonstrated that the hourly SWAT-MODFLOW-PADDY model could dynamically simulate soil moisture and runoff patterns in submerged paddy fields. Notably, the developed model showed enhanced performance throughout the entire period for hourly flow in the watershed, with an average coefficient of determination (R2) of 0.75, Nash and Sutcliffe efficiency (NSE) of 0.76, and percent bias (PBIAS) of 13.22 compared to the original model (R2 = 0.62, NSE = 0.70, PBIAS = 48.21). The model’s performance in predicting water quality was improved, and it highlighted the significant impact of complex hydrological mechanisms within submerged paddy fields on the spatial distribution of groundwater recharge and stream water volumes exchanged through SW-GW interactions. Given these promising results, the SWAT-MODFLOW-PADDY model could be a valuable resource for managing submerged paddy-dominated agricultural watersheds across various climates and regions.

How best to incorporate conjunctive water management into international water law: legal amendment, instrument coupling, or new protocol adoption?

International water conventions—e.g., the 1997 United Nations Convention on the Non-Navigational Uses of International Watercourses—include positive but insufficient focus on groundwater and its interaction with surface water. As such, a growing body of literature has proposed modifications to existing frameworks to enable consideration to surface and groundwater and their interactions. While this literature places considerable focus on coupling and amending existing legal frameworks, elaboration and evaluation of a new protocol on conjunctive water management comprises a key gap. To fill this gap, this paper seeks to answer the following question: does formulation and adoption of a new “conjunctive” protocol provide more value than existing proposals centered around modifications to existing law? This paper seeks to compare benefits associated with current proposals to strengthen the international legal framework for management of surface–groundwater interaction, vis-a-vis adoption of a new protocol on conjunctive management of transboundary freshwaters. To do so, the authors use doctrinal legal methods to analyze the existing main instruments globally assessing the degree to which they consider key interlinkages between surface water and groundwater. Then, the paper examines the concept of conjunctive water management and deduces tenets that should be pursued in shared waters to achieve this objective. To identify the preferred option to support conjunctive water management in international water law, the paper explores the degree to which existing proposals vs a new protocol enable an embrace of these tenets of conjunctive water management. The paper finds that while a new protocol may add greater value in advancing conjunctive water management, multiple options can and should be concurrently pursued. In particular, the authors argue that new protocols to the existing treaties must be adopted in combination with the amendment of the Draft Articles on the Law of Transboundary Aquifers. Benefits of doing so include more effective management of transboundary freshwater resources that are interconnected hydrologically, a less fragmented and more consistent international water regime, and ultimately more benefits accruing to the populations and environmental goods dependent on shared water resources.

Strategic Monitoring of Groundwater Quality Around Olusosun Landfill in Lagos State for Pollution Reduction and Environmental Sustainability

As urbanization and population increase in the megacity, there is a need for engineering intervention and strategic monitoring of groundwater around landfills for environmental sustainability, pollution reduction and public health. This study evaluated water’s physical and chemical parameters in wells and boreholes near the Olusosun landfill in Lagos State to determine how they impact groundwater quality. An Atomic Absorption Spectrometer (AAS) was used to evaluate groundwater samples obtained from five locations within the dump site. Some water parameters, such as dissolved oxygen (DO), iron (Fe), lead (Pb), manganese (Mn), and magnesium (Mg), had concentrations that were higher than the WHO, NESREA, and Nigerian Standard for Drinking Water Quality (NSDWQ) standard limits in some sampling sites, with mean concentrations of 0.33 mg.L-1, 0.04 mg.L-1, 0.74 mg.L-1, and 0.74 mg.L-1, respectively. A small amount of lead was identified in the groundwater of the study area. A major source of air and groundwater pollution, the Olusosun landfill has a detrimental impact on the health of those who live there. Solid waste, groundwater interactions, and contaminated migration into the nearby neighbourhood were studied. It was observed that the degradation of waste products in dump sites releases harmful leachate into the groundwater. Even though some heavy metal concentrations in the study area are still within WHO, NESREA, and NSDWQ standard limits, investigations and further monitoring should be conducted regularly to assess the concentrations of heavy metals in groundwater.

Evaluating the Groundwater and Surface Water Interaction in Southwest Iraq Using Environmental Isotopes Technique

This study aims to use the environmental isotopes technique to evaluate the interaction between groundwater and surface water at a location between two cities in the southwest of Iraq; namely Al-Kifl and Al-Samawa. The salinity of the Euphrates River water increases sharply as it passes in this area, to a level that affects its usage for municipal purposes. A total of 111 samples were collected from the rivers, drainages, springs, shallow and deep wells, and from Sawa Lake, and all the samples were subjected to chemical (TDS, SO4, Cl, and B) and isotopic (deuterium and oxygen-18) analyses. The chemical and isotopic results showed no interference between the quality of the groundwater (from wells and springs) and Sawa Lake water with the Euphrates River water, in the study area, and hence, neither the groundwater nor Sawa Lake affects the river water quality. Statistically, the t-test analysis showed significant differences among those water samples (p-values less than 0.05 for almost all the chemical and isotopic parameters). On the other hand, drainage water showed a strong relationship to the river water, which indicates a high and strong influence of the drainage water on the river water quality when it mixes with the Euphrates River water.

Changes in the surface water – Groundwater interactions of the Murray-Darling basin (Australia) over the past half a century

The Murray-Darling Basin is Australia’s most stressed water resource and most contentious in the sharing of those water resources. Our hydrological models used for water management have no information on the surface water – groundwater interactions within a reach and so implicitly assume that their relationship is stationary. This study seeks to test this assumption by looking at the direction of surface water – groundwater exchanges and how they have changed over the 49-year period from 1970 to 2019 at the whole of basin scale (492 river reaches). This was achieved by comparing the groundwater level at 59,340 bores to the elevation of the surface water at the closest point on the stream network. Each point evaluated on the stream network was classified as either gaining (water from groundwater) or losing (water to groundwater) at an annual scale. These point scale observations were aggregated to the reach scale (as used in river systems models) for analysis of trends though time. The proportion of bores predicting losing conditions was falling through the 1970’s and 1980’s before reaching a minimum of 48% in 1995/96. From this period the proportion of bores predicting losing conditions increased to a maximum at the end of the analysis period of 78% in 2018/19. This finding at the point scale is replicated at the reach scale. The reaches with greater than 80% of bores predicting losing conditions had a minimum in 1993/94 (at 40%) and increased to 66% at the end of the analysis period. These observations are consistent with the management challenges of the time. In the 1980’s and 1990’s gaining conditions leading to increasing salt loads in the streams caused by groundwater discharge were a major problem for downstream water users. Since the turn of the century drought and water scarcity have been the major challenges with falling groundwater levels leading to increases in losing conditions. This study has shown that the surface water – groundwater interactions are not stationary and have been changing through time with many reaches having changed direction from gaining to losing throughout the analysis period. This is highlighting a deficiency in our hydrological models that cannot replicate this change in flux direction and therefore leading to errors in predicting low flows.

The Use of Isotopic Techniques to Determine the Sources of Groundwater Recharge and Interaction with Surface Water in Southern Baghdad –Iraq

Isotope Hydrology has become a widely used technique for managing the global water crisis. The study of the Isotopic content of Groundwater and surface water and the study of chemical analysis is an important source of water sustainability. The study area is located south of Baghdad- Iraq three regions (Al-Dora, Al- Rustumiya and Al-Tuwaitha). Many samples were taken in the study area (15) samples Groundwater (well) and surface water (Rivers Tigris and Diyala). The results of the isotopic study indicated that there is a clear interaction in the values of the two stable isotopes between groundwater and surface water (Tigris and Diyala Rivers). In the wells (-41.5,-42.95 ‰) for deuterium (2H) (-6.77, -7.15 ‰) Oxygen-18 (18O) and the Tigris and Diyala rivers (-40 , - 39.47 - -7.34,-7.2 ‰) respectively ((2H), ((18O)) in the study area. Depending on these variables, indicates isotopic signatures similar between groundwater and surface water in the study area, helped in this the geological structures of the city of Baghdad (the sedimentation of the sedimentary plain). Through the results of isotope Tritium (H-3), (indicator to know the age of water and Recharge of water) we showed a clear convergence between the concentrations of tritium in the surface water of the rivers (Tigris and Diyala) (4.8 ± 0.064) TU and the wells (4.5 ±0.045) TU in the region, which indicates that the supply of water from wells is modern and continuous, and confirms the overlap between surface water and groundwater in the study area. It also indicated the high values of electrical conductivity in wells samples because most of the wells in the study area are shallow, where salt concentrations are high due to increased evaporation and are not suitable for drinking.

Radium Isotopes and Hydrochemical Signatures of Surface Water-Groundwater Interaction in the Salt-Wedge Razdolnaya River Estuary (Sea of Japan) in the Ice-Covered Period

The interaction of surface water and groundwater is important in the ecology of coastal basins, affecting hydrological conditions, oxygen regime, carbon, and nutrient exchange. This study demonstrates a dynamic connection between the salt-wedge region and its underlying aquifer in the eutrophic estuary. In winter, this estuary is covered with ice, and the river flow is at its lowest; that is why the specific response to groundwater discharge is best marked in this season. Groundwater admixture was detected in the salt-wedge region by highly active radium isotopes: 223Ra—4.80 ± 0.42 dpm 100 L−1, 224Ra—55.37 ± 1.1 dpm 100 L−1, and 228Ra—189.71 ± 4.66 dpm 100 L−1. The temperature of groundwater and river water was about +4 °C and 0 °C, respectively; that of seawater was −1.6 °C, and temperature increased up to +2.3 °C in the surface water–groundwater interaction region. Groundwater admixture is accompanied by a lower level of oxygen concentration of 52 μmol/kg; at that time, the maximum oxygen concentration in the salt-wedge region was 567 μmol/kg. In waters with a high activity of radium isotopes, there was a maximum partial pressure of CO2—4454 μatm at the range 100–150 μatm in the salt-wedge region and also observed extremum of NH4+, NO2−, and dissolved phosphorus. The surface water–groundwater interaction through anoxic sediment can form localized anaerobic areas despite the general oxygen supersaturation of eutrophic estuary waters and also cause local recycling of nutrients from bottom sediments.

Exploring Climate-Driven agricultural water shortages in a Snow-Fed basin using a water allocation model and Machine learning

Water allocation models (WAM) can capture complex feedbacks between water infrastructure, water supply, water demand, and water rights structure. They are an important tool for water managers to reduce conflict and aid in future water resource planning. Future streamflow reductions and shifts in timing are anticipated in snow-dominated watersheds of the western United States due to projected reductions in snow water storage. These basins tend to be water-limited and rely on groundwater to support agricultural demand. Water managers in the snow-fed, agricultural Walker River Basin (WRB) in California and Nevada use a dynamically coupled WAM and physically based groundwater flow model as a decision support tool. Large computation time is required to simulate groundwater dynamics, limiting the number of model scenarios that can feasibly be run to test alternative management scenarios under climate change. Machine learning (ML) techniques have been used to capture complex nonlinear hydrologic dynamics in other contexts, but to our knowledge there have not been efforts to use ML to increase computational efficiency in modeling groundwater responses to agricultural water use. We use extreme gradient boosting to replace the groundwater sub-model for the WRB decision support tool, resulting in accurate estimates of reservoir storage (monthly error almost always < 5% of reservoir capacity), streamflow (total outflow errors from −3% to 20% depending on scenario) and agricultural water shortages (-12% to 18% error in total shortage depending on scenario). For comparison, we found that omitting groundwater modeling from the WAM results in a larger overprediction of outflows (2 to 58%) and underprediction in agricultural water shortages (0 to 41%) across a range of scenarios. We use this validated ML method to explore the sensitivity of agricultural water availability to a range of streamflow volumes and streamflow timings. Results indicate that agricultural water shortages are more sensitive to annual streamflow volume in comparison to shifts in streamflow timing. Our model shows that reservoirs can buffer the impact of altered flow timing on agricultural production, except under extreme conditions in which all precipitation falls as rain (no snow storage) and streamflow peaks in the winter. Future work will explore these thresholds in more detail. While the results here are site-specific, the methodology for coupling a WAM with a ML representation of surface–groundwater interactions can be applied to a wide variety of water resource modeling applications. These results also provide a stark example of the importance of snowpack storage and groundwater management to western US water infrastructure.

Impact of limestone caves and seawater intrusion on coastal aquifer of middle Andaman

Seawater intrusion has become a common problem in coastal and island aquifers with the rise in climate change that greatly affects the majority of developing countries. The island hydrology is very complex and associated with a unique set of environmental characteristics with the dynamic interaction of groundwater, surface water, and seawater. Further, Sea level rise, erratic rainfall, and over-extraction of groundwater triggered salt-water intrusion. A study on seawater intrusion and the effect of limestone caves on groundwater was carried out in middle Andaman using a combination of ionic ratios of major ions. A total of 24 samples and a reference sample from the sea were collected and analysed using ICP, spectrophotometer, and flame photometer. A combination of 10 ionic ratios Cl/HCO3, Ca/(HCO3 + SO4), (Ca + Mg)/Cl, Ca/Mg, Ca/Na, Cl/(SO4 + HCO3), Ca/SO4, K/Cl, Mg/Cl, and SO4/Cl was used to assess the dissolution of limestone minerals and the level of saltwater intrusion into groundwater. The geospatial method was used to extract and combine all the hydrogeochemical parameters and ionic ratios in the GIS platform. Durov plot was used for the interpretation of groundwater chemistry and the identification of natural processes controlling the hydrogeochemistry of the area. The dominance of Ca-HCO3 and Na-HCO3 was confirmed in 48% and 24% of the sample respectively. The equiline graph of chloride with other major ions showed the enrichment of alkali and alkaline earth metal salt in groundwater. Schoeller's diagram depicted the dominance of Cl, Ca, and the sum of CO3 and HCO3 in seawater near Mayabunder. The lower concentration of Na with respect to Cl (64%) and Ca (100%) showed the presence of a reverse ion exchange process. Further, the correlation matrix showed a strong relationship between Cl, K, Ca, and Na. The analysis of X-ray diffraction of the rock samples confirmed the presence of limestones such as Aragonite, Calcite, Chlorite, Chromite, Dolomite, Magnetite, and Pyrite in the study area. The integration of ionic ratios showed moderately affected and slightly affected saline regions in 44% and 54% of the region respectively. Finally, the role of tectonic activities and active lineaments connected to the sea was found to play a major role in the intrusion of seawater where interconnected faults created an opening for surface water to recharge groundwater leading to the deep aquifer.

Assessing surface-groundwater interactions for sustaining spring wetlands of the Great Artesian Basin, Australia

The Great Artesian Basin (GAB) is one of the world’s largest actively recharging aquifers. Groundwater discharges from the GAB sustain numerous spring wetlands, which have great ecological, scientific, and socio-economic significance. However, groundwater extraction and variation over time have had an impact on the quantity and area of spring wetlands with a 38% decline in extent since 1900. A major barrier to understanding variability in surface–groundwater interactions in spring wetlands of the GAB is the lack of observational data across critical spatial and temporal scales. Satellite observations have the ability to overcome this barrier and allow the evaluation of spring wetland responses to groundwater storage (GWS) variation. We investigated how GWS, and its associated drivers such as evapotranspiration (ET), soil moisture storage (SMS), and rainfall, in the GAB, influence the extent of surface water at five spring supergroups (Eulo, Barcaldine, Flinders, Springsure, and Springvale). We used satellite observations (2002–2017) to assess ET, SMS, rainfall, the normalized difference vegetation index (NDVI) and the modified normalized difference water index (mNDWI) for observing surface–groundwater interactions. NDVI responded positively to the GWS variation over the GAB and varies from sub-basin to sub- basin, with higher correlations in the Carpentaria sub-basin and some parts of Central and Western Eromanga. GWS variations was correlated with ET, SMS, rainfall, NDVI, mNDWI and surface water level (SWL). After a strong La Niña began in 2010, we uncovered relatively higher linear relationships between different components (ET, SMS, rainfall, NDVI, mNDWI, and SWL) and GWS variation (R2 > 0.50) than before the La Niña (R2 < 0.50), with the Flinders spring supergroup being the exception. NDVI and SMS are found to be the most significant predictor variables among ET, rainfall, SWL, and mNDWI components to influence GWS. This study provides improved understanding of surface-ground water interaction in spring wetlands and the influence of different hydrological components on variation in spring wetland extent in the GAB region.

The use of stable isotopes to identify surface water–groundwater interaction in the Kruger National Park, South Africa

The role of groundwater, in general, is often overlooked in freshwater ecosystem management policies and in the management of South Africa’s flagship conservation area, the Kruger National Park (KNP). To address this gap, a generalised conceptual model of surface water–ground water (sw–gw) interactions in the southern and central regions of the KNP was developed. To do this, stable isotope ratios (d18O and d2H) of groundwater, rainfall and surface water were used to determine the extent to which the base flow of perennial, seasonal and ephemeral streams on different geologies (granite vs. basalt) is driven by rainfall or groundwater. These results show that the δ18O and δ2H ratios of perennial rivers are similar to that of groundwater, while seasonal and ephemeral rivers on basalts have values closer to rainfall. On granite substrates, however, the isotope ratios of the seasonal and ephemeral rivers have values closer to groundwater than rainfall. The larger seasonal Mbyamiti River had similar isotope ratios to that of groundwater, and the highly ephemeral Nwaswitsontso had episodic interaction with groundwater (i.e. isotopic ratios overlap occasionally). These results show that decisions necessary for the sustainable management of groundwater resources are better informed when the natural interaction, movement, and exchange between groundwater and rivers are understood. This has particular relevance for large conservation areas in southern Africa that are expected to experience more variable climates in the future with both increases in drought and rainfall intensities.