ABSTRACT Volcanic aquifers are vital groundwater resources in the Indonesian archipelago. This study investigates the southern slope of Mount Karang, a volcanic area with significant groundwater potential reflected in its abundant springs, to examine the influence of geological conditions on the physical and chemical characteristics of groundwater. The research combined geological mapping, observation of groundwater physical characteristics, and laboratory chemical analysis. Field observations identified 60 outcrops and 31 groundwater locations from springs and dug wells, with 12 samples collected for hydrochemical analysis. The results showed geological units comprising andesitic lava, lapilli tuff, volcanic breccia, and ash tuff, crossed by a northwest–southeast strike-slip fault zone. Based on lithology and fault patterns, groundwater was classified into six zones, with five facies identified: Ca-HCO₃, Ca.Mg-HCO₃, Na.K-Cl, Ca.Mg-SO₄, and Na.K-SO₄. Gibbs diagram and molar ratio analysis indicated mineral dissolution as the main factor controlling groundwater chemistry. Factor Analysis reduced nine groundwater characteristics into three principal components representing 86.54% of the variance, with Factor 1 dominated by calcium, potassium, pH, and total dissolved solids accounting for 51.42%. These findings demonstrate that geological factors significantly influence groundwater characteristics, providing important insights for the sustainable management of groundwater in volcanic areas.

In the Brazilian Semi-Arid region (SAB), water availability is a challenge due to the occurrence of intense rainfall, distributed spatially and temporally in an irregular manner, high evapotranspiration, and a strong dependence on surface and groundwater aquifers to guarantee human supply and agricultural development. Alluvial aquifers, common in stretches of intermittent rivers, have recharge derived from rainfall and surface runoff, influenced by the rate of water infiltration into the soil and groundwater storage. This work aimed to evaluate and zone the relative and absolute water potentials of an alluvial aquifer located in the Sumé Irrigated Perimeter (PIS), in the semi-arid region of Paraíba, Northeast Brazil. The methodology applied used the Analytic Hierarchy Process (AHP) to compare relevant factors, favoring the estimation of annual recharge from rainfall data, recharge volumes, and spatial representation through thematic maps for the chosen criteria, using Sentinel 2AB satellite images, MSI sensor, NIR and B4 channels, with low cloud cover, 10m resolution, and using QGIS 3.3.4 software. The results showed a predominance of moderate relative water potential, with areas of greater availability in the western and central regions of the PIS, in addition to identifying zones with higher absolute water potentials in the alluvium. It was possible to assess that the local hydrogeological dynamics constitute a useful tool to support the sustainable management of this alluvial aquifer, highlighting the importance of rational use and the need for water reuse as strategies for conserving groundwater resources in the PIS area. The implementation of a user registry and the granting of permits are necessary, even if no individual charges are levied on the use of raw water by rural producers, due to the small volume of water withdrawn.

Leakage from aging sewer and stormwater pipes into the subsurface poses significant environmental risks and threatens the integrity of urban infrastructure, primarily through the degradation of groundwater quality and the alteration of urban water balances. While the presence of defects within pipe networks is well-documented, accurately quantifying volumetric exchange fluxes remains a challenge due to the complex, nonlinear interactions between the pipe, the surrounding variably saturated soil, and the fluctuating groundwater level. Current modeling approaches often overlook the threshold behaviors of these systems, leading to potential inaccuracies in leakage estimation. In this study, we show for the first time that leaky pipes can become hydraulically disconnected from the underlying groundwater, a phenomenon analogous to well-known river-groundwater interactions that include disconnection. In a sewer-groundwater context, hydraulic disconnection is restricted to point sources and is strongly influenced by colmation/clogging. Through numerical modeling of a hypothetical case study, we show that the leakage flux from the pipe (in absolute terms) initially increases with declining groundwater levels, until a critical depth below the leaky pipe is reached. After this point, the hydraulic communication from the groundwater to the leaky pipe stops and the leakage flux can be considered constant. In a sensitivity analysis, we demonstrate the impact of the individual hydraulic parameters of the leaky-pipe-groundwater system on the hydraulic disconnection. We further modify the properties of the aquifer material, resulting in a hydraulic disconnection depth of 0.89 m, 1.77 m and 4.00 m below pipe for sand, loamy sand and sandy loam aquifers, respectively. This insight has important implications for leakage modeling: once hydraulic disconnection occurs, the leakage flux becomes independent of groundwater dynamics. The present study provides a proof-of-concept for the mechanism by which leaky sewers hydraulically disconnect from groundwater.

The effect of beach slope variation on saltwater intrusion dynamics in the unconfined coastal aquifer (experimental and numerical)

Predicting groundwater level dynamics in shallow, unconfined aquifers remain a persistent challenge due to their high sensitivity to heterogeneous recharge, rapid anthropogenic extraction, and nonlinear interactions with climatic drivers conditions under which conventional models often fail to generalize. Here, we introduce two novel hybrid frameworks ANN-HBO and ANN-POA that couple artificial neural networks with metaheuristic optimization algorithms (Honey Badger Optimization and Pelican Optimization) to dynamically optimize network weights in data-scarce, non-stationary environments where deep learning architectures exhibit poor convergence and overfitting. The study is conducted in the Astaneh-Kuchesfahan aquifer, northern Iran, a shallow alluvial system limited storage capacity, and strong seasonal fluctuations driven by irrigation demand and evaporation. Hydraulic conductivity varies spatially from 10 to 35m/day, reflecting moderate to high transmissivity (approximately 200-600 m2/day), typical of well-drained fluvial systems. Using 256months of hydro-meteorological records, we identify the most informative predictors lagged groundwater levels (1-3months), temperature, precipitation, evaporation, and abstraction rates via the Minimum Redundancy Maximum Relevance (MRMR) algorithm, generating four optimized input scenarios. Compared to a standalone ANN, ANN-POA reduces RMSE by 29.3% and MAE by 24.1% on average across three observation wells, achieving test RMSE values of 0.213-0.287m and MAE of 0.260-0.437m. Lagged groundwater level (1-month) and monthly precipitation are identified as the dominant predictors, confirming the system's strong dependence on recent recharge. The superior robustness of ANN-POA over ANN-HBO and baseline models demonstrates its efficacy in navigating complex, high-dimensional parameter spaces without requiring extensive training data. This approach enables water managers in data-limited regions to transition from reactive to anticipatory aquifer management: identifying early signs of depletion, prioritizing managed recharge, and calibrating extraction quotas based on dynamic hydrological responses rather than static averages. Our framework offers a scalable, physics-informed, and low-cost solution for sustainable groundwater governance in vulnerable shallow aquifers worldwide.

This study focuses on the Umm Lajj region, situated along the eastern margin of the Red Sea, Northwest Saudi Arabia. The study integrates aeromagnetic and land gravity data to characterize subsurface geological structures influencing groundwater aquifers and their potential susceptibility to seawater intrusion. Data processing and interpretation were conducted employing a suite of filtering techniques to enhance data resolution and interpretability. A Butterworth filter was applied to distinguish between deep-seated and near-surface anomalies, while source edge detection and depth estimation methods were utilized to delineate structural features. The results reveal prominent NNW-, NW-, and NE-oriented fault systems, as well as associated grabens and horsts, which are structurally linked to Red Sea rifting processes. A significant structural basin was identified in the central coastal area and is interpreted as a potential groundwater aquifer. Gravity anomaly patterns also highlight low-density, fault-controlled zones that are likely conduits for seawater intrusion into the aquifer system. Conversely, NW-trending magmatic dikes and uplifted blocks along the western boundary may serve as partial barriers, restricting the lateral movement of seawater. The combined interpretation of magnetic and gravity data offers a detailed structural framework that enhances the understanding of regional hydrogeology and contributes to future groundwater resource management. These insights are particularly valuable for evaluating the risks of seawater intrusion and the vulnerability of groundwater systems in arid coastal settings.

Machine learning-based evaluation and dimensionality optimization of groundwater quality indices for drinking and irrigation in the Vietnamese Mekong Delta.

Groundwater serves as the primary supply source for socioeconomic activities in the Casamance region (south of Senegal), yet hydrogeochemical and water quality characteristics as well as flow patterns of groundwater at a regional scale are largely undocumented. This study aims to contribute to the understanding of hydrogeochemical processes of mineralization and to the hydraulic functioning of the Continental Terminal (CT) and Oligo-Miocene (OM) aquifer systems. Field campaigns were conducted in December 2021 (beginning of the dry season) and May 2024 (end of the dry season) to collect water samples from dug wells (n = 93), boreholes and piezometers (n = 46), and surface water (n = 3) for major and minor ions chemistry, water isotopes (δ2H, δ18O, 3H) and carbon isotopes (δ13C and 14C) analyses. The results reveal that silicate weathering and carbonate dissolution, together with ion exchange, are responsible for the groundwater mineralization in both aquifers with dominating Ca-HCO₃, mixed-HCO₃, and Na-Cl water types. The Comprehensive Pollution Index (CPI) method used to evaluate anthropogenic influence reveals that the pollutant levels in the CT aquifer are moderate to high, while the OM aquifer contains high-salinity (TDS = 2346mg/L) groundwater derived from residual marine waters. Stable isotope distributions in the OM aquifer reveal two distinct patterns: enriched values (from -6.68 to - 4.67 ‰ δ1⁸O) in the confined part of the aquifer and depleted values between - 6.85 and - 5.14 ‰ δ1⁸O in the unconfined part. Variations in stable isotope values indicate recharge under different climatic conditions and mixing with seawater, while 3H content indicates recent recharge in the CT aquifer and sub-modern water in the OM aquifer. δ13C and 14C correlation reveals a rise in carbon activity from the confined to the unconfined aquifer and an active isotopic exchange with the carbonate fraction of the OM aquifer matrix. These findings provide the first regional-scale insight into the functioning of the CT and OM aquifer systems. They will guide future interventions in water supply projects as well as in securing reliable and high-quality water for local communities and supporting water resource management in the Casamance region.

For a partially penetrating well, a Dirac pulse can be generated by sudden charge of water from the well end into the contiguous rock, and subsequently the pulse diffuses away in terms of slow P-wave. This process is described by the Green's function for the initial-value problem. The Green's functions subject to three boundary conditions (BC) are compared mutually, that is, no BC, zero Neumann BC and zero Dirichlet BC for infinitely far boundary, the confining unit and unconfined aquifer, respectively. The well end is set at 10 m below the boundary, and the mass of water suddenly injected is prescribed as 1 kg. Both intact Berea sandstone and fractured Berea sandstone are used for illustration. The spatial distribution and breakthrough curve of the fluid pressure disturbance (pf) are calculated. The results indicate that zero Neumann BC increases pf whereas zero Dirichlet BC decreases pf. For slow and fast P-waves in the regime of low frequency, it is rigorously shown that the ratios (between the confining pressure disturbance and fluid pressure disturbance) are lower than and higher than unity, respectively. This theoretical study suggests that the technique of Dirac pulse may be used for acquisition of small-scale permeability, thus helpful for resolving the heterogeneity of in situ permeability.

Taking a coal mine in the Weibei Mining Area of Shaanxi Province as the engineering background, and focusing on the floor water inrush problem encountered in the mining of narrow strip working faces above the Ordovician limestone confined aquifer, this study effectively overcomes the limitations of the traditional high-cost surface large-scale aquifer grouting reconstruction technology and the discontinuous underground short-hole aquifer grouting reconstruction technology, and proposes a continuous underground directional long-borehole grouting reinforcement technology targeting the aquifuge. A comprehensive research methodology integrating theoretical analysis, numerical simulation, and field measurement is adopted to systematically investigate the pore structure characteristics, mining-induced failure mechanism, water richness and grouting reinforcement effect of the coal floor. The results show that the permeability of quartz sandstone ranges from 0.033 to 0.059 mD, and its porosity varies from 1.926% to 7.776%. Notably, scanning electron microscope (SEM) observations reveal that abundant micropores, mesopores and distinct fractures are developed inside the rock, endowing it with certain seepage capacity. Under the condition of narrow strip mining, the maximum failure depth of the floor is approximately 5.74m, with shear failure as the main failure mode. The rock strata within the range of -6.5m to -15.5m below the floor are basically unaffected by mining activities and exhibit favorable water-resisting performance. Therefore, based on the floor failure depth, water-resisting performance, and pore structure characteristics, the grouting reconstruction horizon is arranged within the quartz sandstone layer. Finally, grouting reconstruction of the target horizon was carried out using underground directional long boreholes, which significantly improved the apparent resistivity of the rock mass. This confirms that the continuous grouting technology can effectively seal fractures and pores and enhance the performance of the target aquifuge. The research results provide a new approach for the prevention and control of water hazards in mine floors under similar conditions.

Well P&A constitutes a critical stage within the decommissioning process in oil exploration and production projects, reflecting the industry’s commitment to sustainability, environmental protection, and social responsibility. The correct and timely execution of permanent abandonment is essential to prevent adverse environmental impacts, such as hydrocarbon leaks and contamination of underground aquifers, thereby protecting marine and terrestrial ecosystems. According to the IOGP’s Decommissioning Insights Report 2023, well abandonment campaigns can represent 40% to 60% of the total budget for decommissioning programs. Established economic analysis concepts are applied to estimate the impacts of postponing well abandonment for up to five years relative to the schedule set out in Petrobras’ Strategic Planning (2025–2029). To this end, a comparative analysis of Total Economic Value (TEV) is performed across three scheduling scenarios, considering five cost components (all discounted to present value): (i) direct abandonment costs, (ii) regulatory costs associated with non-compliance with SGIP commitments, (iii) expected monetary value (EMV) of well integrity loss/blowout events, (iv) decommissioning obligations costs (non-COGO), and (v) opportunity costs related to revitalization projects. The methodology is applied to a portfolio of 150 interventions, selected based on the availability of well integrity failure probability functions required to compute EMV. The results indicate that performing abandonment at the appropriate time avoids costs significantly higher than the financial disbursement allocated in the Strategic Plan. In conclusion, beyond being environmentally responsible and a concession obligation, well abandonment is decisive for the sustainability of the oil exploration and production business.

Machine learning-based estimation of specific yield in single-well unconfined aquifers

Groundwater quality index prediction and aquifer failure risk analysis using metaheuristic-tuned artificial neural networks.

Dissolved noble gases have been recognized for decades as ideal artificial hydro(geo)logical tracers, as they are chemically inert, invisible, and non-toxic. However, their widespread adoption has historically been limited by the difficulty of tracer injection, sampling, and analysis procedures. Developments in portable, high-resolution dissolved gas measurement technology over the last two decades have rekindled interest in the use of gas tracer methods for routine hydrogeological investigations, such as well-to-well tracer tests, intra-well tests, or studies of river infiltration towards alluvial aquifers. The application of gases in aqueous environments still poses unique challenges compared to other tracer methods, as potential exsolution and degassing need to be accounted for, and, if possible, avoided during tracer injection. Here, we present a simple and efficient methodology that addresses these challenges and allows the efficient, on-site preparation and injection of highly concentrated tracer solutions with controlled dissolved gas concentrations. We applied the method in a large drinking water wellfield and performed well-to-well tracer tests in an unconfined aquifer using helium-4 (4He), neon-20 (20Ne) and krypton-84 (84Kr). Known tracer quantities were injected together with fluorescent dyes into an observation well upgradient of a pumping well. Gas tracer breakthrough was monitored in the pumping well with a portable mass spectrometer. Breakthrough curves of 4He and 84Kr compared favorably with fluorescent dye tracers, and enabled reliable estimates of groundwater flow velocities, travel times, and tracer recovery. These findings illustrate how noble gases can substitute or complement other artificial tracer methods, even in large-scale settings. The methodology can be extended to other gases (e.g., neon-22, xenon isotopes, light hydrocarbons), significantly expanding the range of artificial tracers available for routine hydrogeological investigations.

Tracking nitrate contamination in groundwater from a residential Peri-urban area in Mar del Plata city (Argentina) using isotopic, hydrogeochemical and microbiological tracers.

Groundwater recharge is critical because it restores the underground aquifers that supply billions of people. Over-abstraction of groundwater without recharge can lead to declining groundwater levels, land subsidence, and long-term ecological damage. As natural recharge is slow in many areas, particularly in arid and semi-arid climates, active management is required to ensure sustainability. Accurate estimation of groundwater recharge is crucial for sustainable water resource management, especially on islands where surface water resources are limited and vulnerable to climate variability. In this study, annual groundwater recharge in the island aquifer was estimated using two different approaches, the chloride mass balance (CMB) method and the numerical modeling tool HYDRUS-1D, using Bozcaada in the northern Aegean Sea, Türkiye. The CMB method is based on chloride concentrations measured in precipitation and groundwater samples, while the HYDRUS-1D model simulates water movement within the unsaturated zone over time. In this context, the study focuses specifically on recharge estimation challenges in small Mediterranean islands, where hydrogeological heterogeneity, limited monitoring networks, and marine influence complicate the reliable quantification of groundwater recharge. The analysis showed that the average annual recharge on the island was calculated to be 39 mm using the CMB method. In contrast, the HYDRUS-1D model provided more variable results for the different observation wells, with annual recharge estimated at 270 mm in the RK6 region, 75 mm in the RK12 region, and 165 mm in the RK13 region. The results indicate a higher recharge potential, especially in highly permeable formations such as the Kirazlı Formation, which includes sandstone. However, the study shows that the limitations of the CMB method become apparent on small islands and in areas at risk of seawater intrusion, leading to conflicting results between the methods. Consequently, it is emphasized that the combined use of approaches such as CMB and HYDRUS can provide a more comprehensive and reliable assessment, even under limited data conditions.

Groundwater ammonium (NH4+) enrichment in agricultural plains is often attributed to anthropogenic pollution, yet geogenic contributions remain poorly understood. This study investigates NH4+ distribution and genesis in the North Henan Plain, China, using hydrochemical, isotopic, and sediment analyses. Results show that NH4+ concentrations increase with depth in the unconfined aquifer, while confined aquifers exhibit localized enrichment within clay lenses. Dual-nitrate isotopes (δ15N-NO3 and δ18O-NO3) reveal significant anthropogenic nitrogen inputs that are effectively attenuated by denitrification under reducing conditions. Critically, δ15N-NH4 values (+ 1.3‰- + 7.2‰) provide strong evidence that NH4+ primarily originates from the mineralization of buried organic nitrogen rather than fertilizers. The correlation between Fe2+ and NH4+ further supports a coupled release mechanism during organic matter degradation. Sediment leaching experiments demonstrate that clay layers, characterized by high adsorption and bound water retention, act as primary "ammonium reservoirs". In contrast, sandy strata facilitate migration without significant accumulation. This research emphasizes that elevated NH4+ levels are predominantly shaped by specific sedimentary environments and lithological structures. The findings offer novel insights into the geogenic origins of nitrogen in agricultural alluvial-fluvial systems, highlighting the necessity of integrating geological controls into groundwater management frameworks.

• Deep learning can simulate daily groundwater level accurately in diverse locations. • Multistep model configuration may deteriorate accuracy under certain conditions. • Integrate groundwater signatures into model evaluation process for broader insights. Simulating groundwater level accurately ahead of time is critical for various practical reasons. In this study, we simulate daily groundwater levels using meteorological data at 1-day to 7-day lead times across 249 unconfined wells in the contiguous United States. Our objectives include: 1) comparing LSTM-based Seq2Seq and Seq2One models for our simulation tasks, 2) investigating the efficacy of addressing failed models (where Nash Sutcliffe-Efficiency (NSE) < 0) by using alternative training-validation splits, and 3) integrating groundwater signatures for a more thorough model evaluation process. Our results demonstrate satisfactory to good performance with our best performing models achieving median NSE scores of 0.744 and 0.603 for 1-day and 7-day lead simulations, respectively. Notably, we show that LSTM-based Seq2One model outperforms the Seq2Seq model at 1-day lead simulation, however the difference in their performance is not statistically significant for 4-day and 7-day lead simulation scenarios. We found that 14% of our models exhibit subpar performance, which may be attributed to modeling complex underlying groundwater systems without key anthropogenic forcing variables. And we show that simply changing the training-validation splits is generally not sufficient to address these failed models. Our analysis with groundwater signatures, which are statistical aggregates of groundwater level hydrographs, reveals that most groundwater dynamics are well captured by the model, and notable correlations between model NSE and groundwater signatures are found. These findings position us towards better interpreting the capabilities and limitations of our groundwater models

Volatile organic compounds (VOCs) such as trichloroethene (TCE) and tetrachloroethene (PCE) are commonly detected in urban environments with legacy contamination. Pathways of indoor VOC exposure through sewer infrastructure remain underexplored, particularly in the context of rising groundwater driven by seasonal rainfall and climate change in coastal settings. This study investigates how seasonal groundwater fluctuations influence VOC concentrations in sewers in the San Francisco Bay Area at a site characterized by shallow, unconfined groundwater and vulnerable sewer infrastructure in a setting with soil known to be contaminated by TCE/PCE. Passive air sampling was conducted across three time periods: one in the dry season and two during the wet season, defined by precipitation totals and differences in depth to groundwater. 8 samples were analyzed using Wilcoxon rank-sum tests and results indicate significantly elevated concentrations of TCE/PCE in sewer air during wetter conditions, with PCE showing a marginally significant wet season increase (p=0.057). No remarkable detections were observed in corresponding indoor or ambient air samples, suggesting that well-maintained plumbing seals in older buildings are critical for limiting indoor exposure to VOCs from contaminated sewer systems. These findings demonstrate that seasonal hydrological dynamics can influence VOC transport in sewers in coastal settings. With sea-level rise and extreme precipitation events intensifying nationally, similar risks will emerge in other coastal cities with legacy contaminants, aging underground infrastructure, and aging buildings. This study highlights the need for increased investigations of sewer systems as preferential pathways for vapor intrusion where groundwater levels are changing and underscores the importance of integrating hydrological and climatic variables into risk assessments for contaminated coastal environments.

The multi-aquifer system of the Dakhla region, in the hyper-arid Atlantic Sahara, was investigated to understand the primary recharge mechanisms and identify the sources contributing to the groundwater salinization. Groundwater samples were collected and analyzed for their chemical and stable isotopes (δ2H and δ18 O) composition. The results show that the groundwater quality ranges from brackish (up to 4700 µS/cm) to saline (15,810 µS/cm). Ionic ratios and saturation indices indicate that evaporite dissolution is the primary process driving groundwater mineralization, whereas the Cl-/Br- molar ratio demonstrates that saline intrusion is negligible. Water isotopes reveal two distinct groundwater types: paleo-recharged water in the deeper aquifers and modern meteoric water in the shallow aquifers. Overall, salinization is mainly governed by water-rock interactions under thermal effect, coupled with ion exchange and rock weathering across all aquifers. Additionally, groundwater in the Plio-Quaternary aquifer shows signs of evaporation before infiltration. These processes collectively degrade the groundwater quality, rendering it unsuitable for domestic and agricultural uses. This study presents the first hydrogeochemical model in the region, establishing a scientific foundation for sustainable groundwater management and protection against overexploitation and pollution in the Dakhla coastal area.