Groundwater Model Research Articles

Characterizing Offshore Freshened Groundwater Salinity Patterns Using Trans‐Dimensional Bayesian Inversion of Controlled Source Electromagnetic Data: A Case Study From the Canterbury Bight, New Zealand

AbstractThe study of offshore freshened groundwater (OFG) is gaining importance due to population growth and environmental pressure on coastal water resources. Marine controlled source electromagnetic (CSEM) methods can effectively map the spatial extent of OFG systems using electrical resistivity as a proxy. Integrating these resistivity models with sub‐surface properties, such as host‐rock porosity, allows for estimates of pore‐water salinity. However, evaluating the uncertainty in pore‐water salinity using resistivity models obtained from deterministic inversion approaches presents challenges, as they provide only one best‐fit model, with no associated estimate of uncertainty. To address this limitation, we employ trans‐dimensional Markov‐Chain Monte‐Carlo inversion on marine time‐domain CSEM data, acquired in the Canterbury Bight, New Zealand. We integrate resistivity posterior probability distributions with borehole and seismic reflection data to estimate pore‐water salinity with corresponding uncertainty estimates. The results highlight a low‐salinity groundwater body in the center of the survey area, hosted by consecutive silty‐ and fine‐sand layers approximately 20–60 km from the coast. The posterior probability distribution of resistivity models indicates freshening of the OFG body toward the shoreline within a permeable, coarse‐sand layer 40–150 m beneath the seafloor, suggesting an active connection between the OFG body and the terrestrial groundwater system. The approach demonstrates how Bayesian inversion constrains the uncertainties in resistivity models and subsequently in pore‐water salinity estimates. Our findings highlight the potential of Bayesian inversion to enhance our understanding of OFG systems and provide uncertainty constraints for hydrogeological modeling, thereby contributing to sustainable water resource development.

Coastal flowing artesian wells and submarine groundwater discharge driven by tidal variation at TaiCOAST site in Taoyuan, Taiwan

Study regionThis study was conducted at the TaiCOAST site, Taoyuan City, Taiwan. Study focusThree coastal flowing artesian wells (FAWs) were observed at the TaiCOAST site that do not match the well-known driving mechanisms of geological or topographical control. The observed phenomenon has not been discussed in the literature. This study collected hydraulic head data for the three coastal FAWs and one onshore well at the TaiCOAST site. A multi-phase (freshwater and seawater) model of groundwater in porous media was constructed to investigate the mechanism of the coastal FAWs and quantify submarine groundwater discharge (SGD). New hydrological insights for the regionThe rare observations of coastal FAWs at the TaiCOAST site were found to be neither geologically, topographically, nor recharge controlled. The constructed numerical model indicates that the coastal FAWs are driven by an upward flow due to the sharp interface between freshwater and seawater, which acts as a barrier. The depth profile of Darcy’s velocity was obtained from the calibrated model. The average Darcy’s velocity of SGD is (2.96 ± 0.61) × 10−6 m/s, which is similar to value reported in the literature. This study provides new insight into the mechanism of FAWs in coastal areas. The results can serve as an important reference for investigations of SGD.

Advances in the pilot point inverse method: Où En Sommes-Nous maintenant?

At a conference some years ago, one of the attendees came up to Ghislain de Marsily and asked, “Excuse me, but aren’t you the de Marsily who developed the Pilot Point Method? It is an honor to meet you.” His response highlights one of Ghislain’s greatest qualities, his humility: “Yes I am, thank you, you are very kind, but that was a very long time ago.” Since Ghislain de Marsily first developed the Pilot Point Method (PPM) in 1978, its development and use has grown significantly in applied decision-support modeling settings including hydrogeology, as well as in other industries, e.g., petroleum reservoir engineering. A technique that was once confined to academic realms, the PPM is now widely accepted as one of the industry pillars of inversion and uncertainty quantification for predictive groundwater modeling. Herein, we provide an update to de Marsily’s paper entitled “Four Decades of Inverse Problems in Hydrogeology” [De Marsily et al., 2000], but with a particular focus on the incredible adoption and advancement of de Marsily’s PPM and related inverse techniques over the last twenty years in the field of predictive groundwater modeling.

Spatial Assessment of Groundwater Salinity and Its Impact on Vegetation Cover Conditions in the Agricultural Lands of Al-Ahsa Oasis, Saudi Arabia

This study investigated the complex relationship among groundwater salinity, soil texture, and vegetation health in Al-Ahsa Oasis, Saudi Arabia. Utilizing vegetation condition index data from 5 years of satellite imagery, alongside ground measurements of groundwater salinity and soil analysis, the study unveiled significant spatial heterogeneity. The vegetation health and groundwater salinity in the study area ranged from 27 to 70% and from 1.7 to 9.6 dS m−1, respectively, and exhibited contrasting patterns, with a high vegetation condition index (healthy vegetation) in the north and east versus high salinity in the southwest. The applied cross-tabulation statistics revealed a strong negative correlation between the vegetation condition index and salinity, with “good” and “normal” vegetation health primarily coinciding with “slightly saline” groundwater. Soil texture further modulated the vegetation response, as “Silt loam” soils corresponded with “good” VCI, while “silty clay loam” only supported “dry” and “normal” conditions. The applied spatial autocorrelation analysis confirmed this, showing low vegetation clustering with “silt” texture and high salinity in the mid-west of the study area (p < 0.05). Outlier districts hint at local factors beyond simple salinity–vegetation health relationships. Finally, a conditional plot, applied as a spatial statistical approach, revealed that low vegetation health spatially correlates with high groundwater salinity (>5 dS m−1) on “silt loam” in the mid-west, while a higher vegetation condition index aligned with lower groundwater salinity in the east and north. These findings highlight the critical role of spatial variability and the need for advanced modeling and data integration to understand and predict the responses of vegetation to environmental challenges, and will be crucial for optimizing water management to ensure the long-term sustainability of Al-Ahsa Oasis.

Characterizing deep groundwater using evidence from oil and gas exploration wells in the Lower Kutai Basin of Indonesia

Groundwater at depths exceeding 500 m can be an important source of freshwater. However, the characteristics of deep groundwater in many regions of the world, including Indonesia’s sedimentary basins, remain vaguely defined. This study investigates the pressure regimes, hydraulic head distributions, salinity, and hydrochemical facies of deep groundwater using available evidence from oil and gas exploration wells in the Lower Kutai Basin of Indonesia. Pressure measurements and wireline log data reveal three pressure regimes within the basin: hydrostatic, overpressure, and underpressure. The top of the overpressure varies, from close to the surface onshore to depths of ~4.5 and ~3.8 km in the Mahakam Delta and offshore, respectively. Computed hydraulic heads at the top of the overpressure range from ~191 m above sea level onshore to ~71 m below sea level offshore, and are indicative of regional groundwater flow. The observed salinity of deep groundwater within the basin indicates predominantly brackish or saline conditions. Fresh (total dissolved solids < 1 g/L) groundwater to a depth of ~2 km is found at a small minority of wells onshore and in the delta; no fresh groundwater is found offshore. Four hydrochemical facies are observed: Na+/Cl–, Ca2+/Cl–, Na+/HCO3–, and Na+–Ca2+/HCO3–. This study indicates that deep fresh groundwater in the Lower Kutai Basin is of localized occurrence. Recharge from meteoric water may replenish deep fresh groundwater within the hydrostatic zone and sustain water supplies, whether brackish or fresh. Water produced from clay diagenesis is also cited as a possible process of freshening deep groundwater.

Production and fluxes of inorganic carbon and alkalinity in a subarctic subterranean estuary

In this study, we focus on the biogeochemical processes that produce both dissolved inorganic carbon (DIC) and total alkalinity (TA) along a subarctic subterranean estuary (STE) located in the Gulf of St. Lawrence (Magdalen Island, Qc, Canada) in order to evaluate the DIC and TA fluxes as well as the buffering capacity of the exported groundwater to coastal waters. DIC and TA do not behave conservatively during mixing along the groundwater flow path and this implies the occurrence of internal redox reactions that control both their production and consumption. In addition, we show that the origin and composition of the organic carbon within the system alter the carbonate parameters by generating low pH conditions (5.9 - 7.2) and contributing to non-carbonate alkalinity (NCA) that accounts for more than 30% of TA. Whereas iron cycling plays a key role in the production of DIC in the fresh and low-salinity groundwaters, the precipitation of sulfide minerals neutralize the acidity produced by the metabolically produced CO2, in the saline groundwater where sulfate is available. The STE pCO2, computed from the DIC-pHNBS pair ranged from a few ppm to 16000 ppm that results in a CO2 evasion rate of up to 310 mol m−2d−1 to the atmosphere. Based on Darcy flow and the mean concentrations of DIC and carbonate alkalinity (Ac = TA - NCA) in the discharge zone, fluxes derived from submarine groundwater discharge were estimated at 1.43 and 0.70 mol m−2d−1 for DIC and Ac, respectively. Despite a major part of the metabolic CO2 being lost along the groundwater flow path, the SGD-derived DIC flux was still greater than the Ac flux, implying that groundwater discharge reduces the buffering capacity of the receiving coastal waters. This site-specific scale study demonstrates the importance of diagenetic reactions and organic matter remineralization processes on carbonate system parameters in STE. Our results highlight that subarctic STEs could be hot spots of CO2 evasion and a source of acidification to coastal waters that should be considered in carbon budgets.

Modeling the influence of climate on groundwater flow and heat regime in Brandenburg (Germany)

This study investigates the decades-long evolution of groundwater dynamics and thermal field in the North German Basin beneath Brandenburg (NE Germany) by coupling a distributed hydrologic model with a 3D groundwater model. We found that hydraulic gradients, acting as the main driver of the groundwater flow in the studied basin, are not exclusively influenced by present-day topographic gradients. Instead, structural dip and stratification of rock units and the presence of permeability contrasts and anisotropy are important co-players affecting the flow in deep seated saline aquifers at depths &amp;gt;500 m. In contrast, recharge variability and anthropogenic activities contribute to groundwater dynamics in the shallow (&amp;lt;500 m) freshwater Quaternary aquifers. Recharge fluxes, as derived from the hydrologic model and assigned to the parametrized regional groundwater model, reproduce magnitudes of recorded seasonal groundwater level changes. Nonetheless, observed instances of inter-annual fluctuations and a gradual decline of groundwater levels highlight the need to consider damping of the recharge signal and additional sinks, like pumping, in the model, in order to reconcile long-term groundwater level trends. Seasonal changes in near-surface groundwater temperature and the continuous warming due to conductive heat exchange with the atmosphere are locally enhanced by forced advection, especially in areas of high hydraulic gradients. The main factors controlling the depth of temperature disturbance include the magnitude of surface temperature variations, the subsurface permeability field, and the rate of recharge. Our results demonstrate the maximum depth extent and the response times of the groundwater system subjected to non-linear interactions between local geological variability and climate conditions.

Framework for comprehensive assessment of ecological water conveyance based on long-term evolution forecast of riparian forests in arid floodplains: synergistic consideration of flood inundation and groundwater

Ecological water conveyance (EWC) is a crucial measure to address ecological degradation in arid regions. To scientifically formulate ecological water conveyance pattern (WCP), and provide a basis for ecological flow management of reservoirs, this study proposes a framework to assess the effect of EWC, and applied it to the Irtysh River basin as a case study. Following the framework, we first developed a calculation method to assess the evolution of the reproduction and growth of riparian forests caused by hydrological variations. Additionally, simulations of surface water and groundwater conditions under different scenarios of water resources development were conducted based on hydrodynamic and groundwater models. The study further predicted the long-term evolution of riparian forest reproduction and growth based on the simulation results. The results indicated that when EWC was not carried out (reservoirs operating according to the designed rules), after 30 years, floodplain flood (FF) inundation area decreased by 47%, duration shortened by 73%, and groundwater levels in 80% of the region declined by 1.5–2 m, causing severe and moderate impacts on the reproduction and growth of riparian forests, respectively. Alternatively, adopting EWC (reservoirs operating according to ecological rules) led to a 24% decrease in FF inundation area, a 63% reduction in its duration, and a groundwater level decline of 0.5–1 m in 87% of the region, and these changes caused severe impacts on riparian forest reproduction, while growth remained unaffected. These findings indicate that the current EWC effectively safeguards the growth of riparian forests but fall short of mitigating the impacts of water conservancy project construction and operation on the reproduction of riparian forests. Given the current trajectory, riparian forests are anticipated to undergo a gradual decline. Consequently, a sustainable EWC should ensure that the water volume discharged into the region exceeds 53% of the natural inflow and urgently requires measures to extend the duration of FF inundation. This study can provide new insights for the early evaluation, comparison and optimization of various WCPs before their implementation.

Modelling of potential vegetation identifies diverging expectable outcomes of river floodplain widening

River re-naturalisations are at the forefront of conservation efforts. The hope is that these interventions will benefit both local ecosystems and facilitate flood mitigation. While hydrological modelling has been a standard procedure in assessing the outcomes of river re-naturalisations, vegetation modelling has not always been performed as part of these assessments. We hypothesised that the use of potential vegetation modelling, i.e. the modelling of self-sustainable vegetation that can survive after the intervention, can provide insight into vegetation outcome of river re-naturalisation and can thus support vegetation restoration and conservation planning. We investigated the utility of potential vegetation modelling under a specific, widespread element of river re-naturalisation: river floodplain widening at three study sites along the Tisza River in Hungary. We applied potential vegetation modelling, in addition to hydrological and groundwater modelling, to assess the expected vegetation outcome of the river floodplain widening. Flood frequency and duration were assessed by the HEC-RAS hydrological model. Based on the output this hydrological model provided, expected values of the water-related explanatory variables (including groundwater level) were calculated. Statistical relationships encompassed by the existing mulitple potential vegetation (MPV) models of Hungary were applied to the environmental variable sets corresponding to conditions before and after floodplain widening (pre- and post-treatment, respectively), including water-related and other explanatory variables. This resulted in predicted potential vegetation distribution for pre-treatment and post-treatment conditions, which were then compared via ordinations and PERMANOVA. At two of the study sites, post-treatment potential vegetation prediction typically showed vegetation types requiring wetter and less saline conditions when compared to the pre-treatment potential vegetation distribution. This pattern corresponds to general expectations given river floodplain widening. However, at one of the sites, the potentiality of saline and non-saline steppe vegetation was actually more pronounced under the expected post-treatment conditions than those before widening. The strengthening of the potentiality of dry vegetation types can be explained by the minor environmental differences related to the microrelief. As the studied sites were all located in the lowlands where geomorphological variation is small, the effect of these minor geomorphological differences on post-treatment potential vegetation would have remained hidden without applying MPV models. In conclusion, scenarios that employ MPV models help predict river restoration outcomes more accurately and can help identify factors that might otherwise be overlooked. Thus, when combined with physical modelling of river flow, their use can aid in the restoration and landscape planning decisions in river re-naturalisation projects.

A three-dimensional numerical model for variably saturated groundwater flow using meshless weak-strong form method

Meshless numerical models have attracted much attention due to the circumvention of troublesome mesh generation. Current meshless numerical groundwater (GW) models either focus on only pumping in fully saturated zone or merely simulate variably saturated GW flow without pumping. However, these two components are both essential for a GW model to represent practical real-world conditions. This gap is bridged in this study by developing a three-dimensional model, CCHE3D-GW-Meshless, using the meshless weak-strong form method. The new model code was verified with three representative cases, and good agreement was found with the analytical/numerical solutions in all cases. CCHE3D-GW-Meshless was then applied to a field case of a pumping well near a meandering river located at Shellmound, Mississippi, USA. The hydrological properties of the site were obtained through calibration which yielded a generally good match with observed data considering the incompletely known heterogeneity in aquifer properties, indicating the applicability of the model.

Groundwater Salinity Across India: Predicting Occurrences and Controls by Field-Observations and Machine Learning Modeling.

Elevated groundwater salinity is unsuitable for drinking and harmful to crop production. Thus, it is crucial to determine groundwater salinity distribution, especially where drinking and agricultural water requirements are largely supported by groundwater. This study used field observation (n = 20,994)-based machine learning models to determine the probabilistic distribution of elevated groundwater salinity (electrical conductivity as a proxy, >2000 μS/cm) at 1 km2 across parts of India for near groundwater-table conditions. The final predictions were made by using the best-performing random forest model. The validation performance also demonstrated the robustness of the model (with 77% accuracy). About 29% of the study area (including 25% of entire cropland areas) was estimated to have elevated salinity, dominantly in northwestern and peninsular India. Also, parts of the northwestern and southeastern coasts, adjoining the Arabian Sea and the Bay of Bengal, were assessed with elevated salinity. The climate was delineated as the dominant factor influencing groundwater salinity occurrence, followed by distance from the coast, geology (lithology), and depth of groundwater. Consequently, ∼330 million people, including ∼109 million coastal populations, were estimated to be potentially exposed to elevated groundwater salinity through groundwater-sourced drinking water, thus substantially limiting clean water access.

A numerical modelling approach to investigate the fate of brine reject of farm scale desalination plants on groundwater aquifers in arid environments

Farm-scale desalination units are gaining popularity for agricultural irrigation in arid countries, such as Kuwait to meet freshwater demands. However, less attention has been given to the management of environmentally hazardous brine reject water they produce. In this study we investigated the fate of brine water produced by the inland desalination units on the underlying aquifers using numerical modelling and field investigations. The methodology involved developing groundwater flow and solute transport models using Flex VMF-SEAWAT to simulate the movement of reject brine. The field investigations included collecting 150 water samples and conducting pumping tests on newly drilled wells. This numerical simulation considered advection, dispersion, and adsorption processes with variable groundwater density following rigorous validation and calibration of the developed numerical models. The results show that the RO reject brine will significantly increase groundwater salinity, exceeding 10,000 mg/L when accounting for advection, dispersion, and adsorption processes. The sustainable yield of the aquifer, with a salinity of <10,000 mg/L, averages 500 Mm3 but is expected to be depleted within 16 years with the current extraction rate. The resulting hydraulic properties are favourable with K about 100 m/d, T > 1000 m2/day, and Sy just >0.1. The adopted values for dispersivity and adsorption coefficients for chloride and sulphate salts in the aquifer were 10 m and 1 × 10−7 [mg/L]−1 respectively. Chemical and numerical analyses indicate a mixing ratio between the reject brine and groundwater in the study area of approximately 10 %. Uncontrolled groundwater extraction, combined with the surface disposal of RO reject brine, has led to a significant decline in groundwater levels and an increase in the salinity. The adsorption ratio of simulated brine plume was 13 %. The authors recommend to dispose the RO reject water in a safe location or transfer it to the nearest wastewater treatment plant for proper treatment and reuse.

Time-Lapse Electromagnetic Conductivity Imaging for Soil Salinity Monitoring in Salt-Affected Agricultural Regions

In this study, the temporal variation in soil salinity dynamics was monitored and analyzed using electromagnetic induction (EMI) in an agricultural area in Port Said, Egypt, which is at risk of soil salinization. To assess soil salinity, repeated soil apparent electrical conductivity (ECa) measurements were taken using an electromagnetic conductivity meter (CMD2) and inverted (using a time-lapse inversion algorithm) to generate electromagnetic conductivity images (EMCIs), representing soil electrical conductivity (σ) distribution. This process involved converting EMCI data into salinity cross-sections using a site-specific calibration equation that correlates σ with the electrical conductivity of saturated soil paste extract (ECe) for the collected soil samples. The study was performed from August 2021 to April 2023, involving six surveys during two agriculture seasons. The results demonstrated accurate prediction ability of soil salinity with an R2 value of 0.81. The soil salinity cross-sections generated on different dates observed changes in the soil salinity distribution. These changes can be attributed to shifts in irrigation water salinity resulting from canal lining, winter rainfall events, and variations in groundwater salinity. This approach is effective for evaluating agricultural management strategies in irrigated areas where it is necessary to continuously track soil salinity to avoid soil fertility degradation and a decrease in agricultural production and farmers’ income.

Coupling of WetSpass-M and MODFLOW models for groundwater flow assessment

Recharge is considered a key parameter in groundwater models for sustainable management of aquifers, which is influenced by factors such as land use, soil and weather. The present study was conducted to couple WetSpass-M and MODFLOW models in the Neyshabur–Rokh Basin. To this aim, the simulated recharge by the WetSpass-M model was applied as an input into MODFLOW to assess the groundwater balance. The hydrodynamic coefficients were determined by calibrating the model, and the model sensitivity to the hydraulic conductivity coefficient, specific yield and recharge was evaluated. The results indicated that the annual average of surface runoff, actual evapotranspiration, interception and recharge during 1991–2017 equalled 18, 36, 7.6 and 42.6% of the average annual precipitation in the basin, respectively. The accurate alignment of simulated and observed water levels, along with the achievement of suitable evaluation criteria values in both steady and transient states, demonstrates the WetSpass-M model's precision in estimating recharge and successfully integrating the two models. The groundwater balance assessment revealed a significant deficit in the aquifer, with the model demonstrating greater sensitivity to the hydraulic conductivity coefficient and providing valuable insights for the sustainable management of the Neyshabur aquifer. Thematic collection: This article is part of the Monitoring the aquifers collection available at: https://www.lyellcollection.org/topic/collections/monitoring-the-aquifers

Mercury speciation transformation mediated by thiolated biochar in high salinity groundwater: Interfacial processes, influencing factors, and mechanisms

Groundwater, a vital freshwater source, faces mercury contamination threats, impacting ecosystems and human health. Removing low concentrations of mercury from high-salinity groundwater has always been a challenge. This study demonstrates the effectiveness of thiol-modified biochar (BCS) in decontaminating mercury from saline groundwater by mediating mercury speciation transformation. Initially, mercury species (e.g., HgClOH, Hg(OH)2, HgCl2, HgOHCO3–) selectively attach to thiol groups, leading to an increase in surface zeta potential and a reduction in electrostatic attraction to calcium species (e.g., CaCO3 and CaSO4). Then, cations bound to thiol groups (e.g., Na+, Ca2+, and CaHCO3+) undergo ion exchange with mercury species (e.g., Hg2+, HgOH+, and HgCl+). Finally, mercury species (e.g., Hg(OH)2, HgSO4, and HgS) gradually precipitate or co-precipitate with calcium minerals (e.g., calcite, aragonite, and vaterite). Compared with biochar (BC), BCS exhibits a larger mercury adsorption capacity (qm = 4.30 ± 0.13 mg g−1) and a faster initial reaction rate (h2 = 4.12 × 10–2 mg g−1 min−1). BCS withstands pH variations, small organic molecules, and coexisting heavy metal ions but exhibits slightly reduced performance in higher ionic strength and with large organic molecules. Additionally, BCS serves as an effective permeable reactive barrier against flowing mercury-contaminated groundwater, the Adams-Bohart and Yan models effectively describe the breakthrough curves. These findings provide scientific support for BCS’s application in remediating high-salinity and trace mercury-contaminated groundwater.

Incorporating interpretation uncertainties from deterministic 3D hydrostratigraphic models in groundwater models

Abstract. Many 3D hydrostratigraphic models of the subsurface are interpreted as deterministic models, where an experienced modeler combines relevant geophysical and geological information with background geological knowledge. Depending on the quality of the information from the input data, the interpretation phase will typically be accompanied by an estimated qualitative interpretation uncertainty. Given the qualitative nature of uncertainty, it is difficult to propagate the uncertainty to groundwater models. In this study, a stochastic-simulation-based methodology to characterize interpretation uncertainty within a manual-interpretation-based layer model is applied in a groundwater modeling setting. Three scenarios with different levels of interpretation uncertainty are generated, and three locations representing different geological structures are analyzed in the models. The impact of interpretation uncertainty on predictions of capture zone area and median travel time is compared to the impact of parameter uncertainty in the groundwater model. The main result is that in areas with thick and large aquifers and low geological uncertainty, the impact of interpretation uncertainty is negligible compared to the hydrogeological parameterization, while it may introduce a significant contribution in areas with thinner and smaller aquifers with high geologic uncertainty. The influence of the interpretation uncertainties is thus dependent on the geological setting as well as the confidence of the interpreter. In areas with thick aquifers, this study confirms existing evidence that if the conceptual model is well defined, interpretation uncertainties within the conceptual model have limited impact on groundwater model predictions.

Quantitative Groundwater Modelling under Data Scarcity: The Example of the Wadi El Bey Coastal Aquifer (Tunisia)

The Grombalia aquifer constitutes a complex aquifer system formed by shallow, unconfined, semi-deep, and deep aquifers at different exploitation levels. In this study, we focused on the upper aquifer, the Wadi El Bey coastal aquifer. To assess natural aquifer recharge, we used a novel physiography-based method that uses soil texture-dependent potential infiltration coefficients and monthly rainfall data. The developed transient flow model was then applied to compute the temporal variation in the groundwater level in 34 observation wells from 1973 to 2020, taking into account the time series of spatially variable groundwater recharge, artificial groundwater recharge from 5 surface infiltration basins, pumping rates on 740 wells, and internal prescribed head cells to mimic water exchange between the wadis and aquifer. The quantified deviations in the computed hydraulic heads from measured water levels are acceptable because the database used to construct a scientifically sound and reliable groundwater model was limited. Further work is required to collect field data to quantitatively assess the local inflow and outflow rates between surface water and groundwater. The simulation of 12 climate scenarios highlighted a bi-structured north‒south behaviour in the hydraulic heads: an increase in the north and a depletion in the south. A further increase in the pumping rate would, thus, be severe for the southern part of the Wadi El Bey aquifer.

Linking Transpiration to Reef Nitrogen Supply on a Tropical Coral Island

AbstractCoral reef islands are biodiversity hotspots with high conservation value, but we have a poor understanding of how island vegetation, through transpiration, influences groundwater nutrient supply to adjacent reef systems. Here we combine stable isotope tracing, geophysical surveys, and satellite analysis to unravel the links between transpiration and the discharge of groundwater nitrate to the waters surrounding a coral reef island (Lady Elliot Island in the Great Barrier Reef). Over a 2‐year study period (2020–2021) there was a net loss of freshwater from the island, that is, evapotranspiration (ET) exceeded rainfall, largely caused by high rates of transpiration (73% of ET). Transpiration was higher in forested areas, and groundwater salinity appeared to be higher there. By tracing the nitrogen stable isotope signature of groundwater nitrate into reef organism tissue we were able to map groundwater nitrogen discharge spatially and temporally. Groundwater nitrogen discharge was focussed on one side of the island and did not vary seasonally as expected, despite reduced rainfall and seabird guano inputs over the austral winter/spring. Based on our results, we propose that transpiration by island vegetation slows groundwater flow and concentrates nitrate in the groundwater being released to the surrounding reef system. High concentrations of nitrate in groundwater (up to 27 mmol L−) were observed on Lady Elliot Island and these seem to have increased since 2014, but further work is required to understand if this is a normal scenario for tropical coral islands or the result of the islands' revegetation program.

Modeling evaluation of the impact of residual source material on remedial time frame at a former uranium mill site

Groundwater contamination at legacy uranium processing sites is an ongoing global challenge. Plumes at many uranium-contaminated sites are more persistent than originally predicted by groundwater modeling. Previous investigations of uranium plume persistence identified residual and secondary sources that contribute to plume longevity, but there is a remaining need to revise forecasted cleanup times using information about these ongoing sources. The purpose of this study is to investigate the quantitative impact of residual vadose zone sources of uranium on groundwater remediation time frame. This objective was approached by applying numerical uranium transport simulations and uncertainty analysis to a former uranium mill site in the southwestern United States. Information from recent site investigations provided details about the distribution and release characteristics of uranium accumulations in the vadose zone. The residual uranium characteristics were incorporated as decaying source terms in the transport model. A stochastic approach using an iterative ensemble smoother was applied for history matching, and the transport model was used to assess the impact of multiple remedial alternatives on forecasted time frame. The forecasted time frame to achieve the groundwater remediation goal for uranium by monitored natural attenuation is on the order of thousands of years, and treatment of the dissolved plume does not reduce the projected time frame. The large proportion of residual uranium mass remaining in the vadose zone and the gradual leaching rate due to the site's semiarid climate create a long-lived source that can sustain a dissolved plume for thousands of years despite an estimated 99% mass removal achieved during mill tailings disposal. Residual uranium in vadose zone sediments beneath former tailings impoundments could present comparable uranium plume persistence and remediation challenges at other legacy uranium mill sites in semiarid climates. Other remaining uranium-impacted sites are similarly complex, and forecasted remedial time frames are needed to effectively achieve compliance, manage risk, assess the benefits of additional treatment, manage and project costs, and support beneficial site reuse.

Multi-tracer approach to constrain groundwater flow and geochemical baseline assessments for CO2 sequestration in deep sedimentary basins

Geological storage of gases will be necessary in the push to net zero and the energy transition to reduce carbon emissions to atmosphere. These include CO2 geological storage in suitable sandstone reservoirs. Understanding groundwater flow, connectivity and hydrogeochemical processes in aquifer and storage systems is vital to prevent risk and protect important water resources, such as the Great Artesian Basin. Here, we provide a ‘tool-box’ of geochemical assessment methods to provide information on flow patterns through the basin's aquifers (changes in chemistry along flow path), stagnant versus flowing conditions (cosmogenic isotopes and noble gases), inter-aquifer connectivity and seal properties (major ions, Sr and stable isotopes), water quality (major ions and metals) and general assessments on residence times of groundwater (cosmogenic isotopes and noble gases). This information can be used with reservoir and groundwater models to inform on possible changes in the above-mentioned processes and serve as input parameters for CO2 injection impact modelling. We demonstrate the use and interpretation on an example of a potential CO2 storage geological sequestration site in the Surat Basin, part of the Great Artesian Basin, and the aquifers that overly the reservoir. The stable water isotopes are depleted compared to average rainfall and most likely indicate greater contributions from monsoonal rain events from the northern monsoonal troughs, where amount and rainout effects lead to the depletion rather than colder recharge climates. This is supported by the modern recharge temperatures from noble gases. Inter-aquifer mixing between the Precipice Sandstone reservoir and the Hutton Sandstone aquifer seems unlikely as the Sr isotope ratios are distinctly different suggesting that the Evergreen Formation is a seal in the locations sampled. Mixing, however, occurs on the edges of the basin, especially in the south-east and east where the Surat Basin transitions into the Clarence-Moreton Basin. Groundwater flow appears to be to the south in the Precipice Sandstone, with a component of flow east to the Clarence-Morton Basin. The cosmogenic isotopes and noble gases strongly indicate very long residence times of groundwater in the central south Precipice Sandstone around a proposed storage site. 14C values below analytical uncertainty, R36Cl ratios at secular equilibrium as well as high He concentrations and high 40Ar/36Ar ratios support the argument that groundwater flow in this area is extremely slow or groundwater is stagnant. The results of this study reflect the geological and hydrogeological complexities of sedimentary basins and that baseline studies, such as this one, are paramount for management strategies.