Groundwater flow patterns in semi-arid mountainous alluvial aquifers: Hydrochemical and isotopic insights from the Central Bolivian Andes

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.

Groundwater contamination by arsenic (As) and per- and polyfluoroalkyl substances (PFAS) represents one of the most pressing environmental and public health challenges globally. While As contamination is predominantly geogenic and widespread in alluvial aquifers, PFAS contamination is largely anthropogenic, arising from industrial activities, landfill leachates, and aqueous film-forming foams (AFFF). Traditionally, scientific research has emphasized hydrogeological transport and contaminant chemistry; however, emerging evidence suggests that geomicrobiological processes play a critical role in regulating the mobility, transformation, and persistence of both contaminant classes. This emerging situation necessitates an integrated framework that explicitly links microbiology, geochemistry, and hydrology, particularly in vulnerable regions such as South Asia, including India and Bangladesh, where groundwater is the primary source of drinking water for millions of inhabitants.

Groundwater plays a vital role in sustaining agriculture and water security in semi-arid regions of India, where increasing extraction pressures necessitate a clear understanding of its variability. This study examines the spatio-temporal dynamics of groundwater levels in the alluvial aquifers of Anand district, Gujarat, over a 20-year period (2000–2020) using pre and post-monsoon data from India-WRIS. Geostatistical analysis through Ordinary Kriging within a GIS framework was employed to map and analyze groundwater distribution. Results indicate significant spatial and temporal variability, with pre-monsoon groundwater declining from 4.4-22.0 m below ground level (bgl) in 2000 to 5.6-26.1 m bgl in 2020, reflecting progressive depletion. Spatially, northern and western regions consistently showed shallower water levels, while southeastern areas remained critically deeper. Post-monsoon analysis revealed an initial recovery phase (2000–2008) due to effective recharge, followed by a reversal after 2012, with expansion of deeper zones exceeding 26 m bgl by 2020. The study highlights a shift from recovery to depletion driven by combined natural and anthropogenic factors, emphasizing the need for sustainable groundwater management strategies, including regulated extraction and improved recharge interventions.

An integrated transient Tier-3 human health risk assessment framework based on multispecies reactive transport modelling.

Abstract Understanding groundwater recharge pathways is essential for designing effective managed aquifer strategies, yet in heterogeneous alluvial aquifers, these pathways are often uncertain and difficult to resolve with sparse well networks. We apply gravity as an independent constraint to quantify groundwater storage dynamics and evaluate recharge strategies in the Ailiao River basin of southern Taiwan. Seven super‐hybrid gravity surveys, integrating absolute, relative, and superconducting gravimeters, were conducted between November 2023 and July 2025 under contrasting hydrological conditions. The surveys revealed distinct seasonal and spatial variability, with recharge driven primarily by rainfall and locally enhanced by river infiltration during wet periods. Stations within 1 km of the river recorded gravity increases up to 100 μGal, while sites beyond 3 km showed minimal (<10 μGal) or delayed responses, delineating a recharge front that attenuates with distance. Comparisons with groundwater levels demonstrated that thick unsaturated zones filter and delay recharge, complicating direct estimation of specific yield. In‐channel modifications provided localized, short‐term recharge but were constrained by unsaturated sediments, whereas stations near proposed recharge‐lake sites recorded stronger and more sustained wet‐season gains, suggesting that recharge lakes offer greater potential for long‐term aquifer replenishment. The super‐hybrid framework maintained referencing continuity despite instrument downtime and typhoon‐related damage, achieving precisions of 3–8 μGal. Beyond operational robustness, our results underscore the value of gravimetry as a non‐invasive complement to well monitoring, capable of constraining recharge fronts, aquifer heterogeneity, and the performance of managed aquifer strategies. These findings provide practical guidance for groundwater management in Taiwan and other alluvial plains worldwide.

Delineating major and trace element hotspots in groundwater through spatial autocorrelations and a double-clustering approach: The role of geologic and anthropogenic factors.

ABSTRACT Groundwater in mountain–valley systems plays a crucial role in sustaining river baseflows, irrigation, and water security, yet the mechanisms and seasonal variability of aquifer recharge remain poorly quantified. This study investigates the seasonal variability of groundwater recharge sources in the Ñuble–Perquilauquén basins of Central Chile, a representative Mediterranean mountain system. A process‐based hydrological model (SWAT) was complemented with stable isotope analysis (δ 18 O, δ 2 H) and a Bayesian mixing model (MixSIAR) to quantify the relative and absolute contributions of three recharge mechanisms: diffuse, focused, and mountain‐front recharge (MFR) derived from Cordilleran precipitation, springs and snowmelt. Results indicate that total recharge to the lowland alluvial aquifer varies seasonally from ~8.5 to ~27.2 mm/month in the dry and the rainy seasons, respectively. Diffuse precipitation‐driven recharge dominates in winter (45%–61%), focused recharge prevails in summer (60%–65%), and MFR provides a stable 11%–17% contribution throughout the year. Irrigation return flows contribute approximately 8% of total diffuse recharge in the valley zone, partially sustaining aquifer replenishment during dry months. The combined SWAT–MixSIAR framework reveals that the Andes Mountains act as a perennial source of groundwater from mountain‐front and, potentially, mountain‐block processes. These findings underscore not only the role of mountainous areas as a water reservoir for lowland aquifers, but also the relevance of integrating different approaches when estimating groundwater recharge.

Groundwater in Al-Madinah Al-Munawwarah faces considerable challenges from high salinity, elevated TDS, and nitrate contamination, primarily due to urbanization and industrial activities, making ongoing monitoring and management essential for its sustainable use in both drinking water and agriculture. The assessment of groundwater quality was conducted on 44 wells tapping two major aquifers (basaltic and alluvial) in the region, utilizing various geochemical techniques, including ICP-MS, FAAS, and XRF, to evaluate hydrochemical characteristics and identify the primary controlling factors. Key physicochemical parameters, including total dissolved solids (TDSs), electrical conductivity (EC), pH, total hardness (TH), and major ion concentrations, were evaluated. The results indicate that several parameters exceed permissible limits established by Gulf and international standards, reflecting highly saline conditions that could adversely affect drinking water safety and agricultural practices. Elevated nitrate levels and other contaminants indicate a combination of geological processes, including mineral leaching, and anthropogenic activities, such as agricultural runoff. Correlations among various ions reveal complex interactions driven by both natural and human factors. High nitrate and potassium concentrations, particularly in the alluvial aquifer, combined with weak correlations with geogenic ions, indicate anthropogenic inputs. Heavy metals in groundwater were classified into two groups: those within permissible limits (Ag, Ba, Be, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Sb, and U) and those exceeding recommended limits (Zn, Al, As, Se, and Tl). Elevated metal concentrations are primarily attributed to water–rock interactions and the fertilizer use in surrounding agricultural areas. These findings highlight the urgent need for continuous monitoring and proactive groundwater to ensure sustainable and safe use of water resources.

Classifying groundwater management under different alternatives for Zanjan alluvial aquifer, northwestern Iran

Groundwater is the main water source for people, industries, and commerce in Varanasi, which is among the oldest cities in the world still inhabited. For the last several decades, the city has faced a drastic decline in the water table, which makes the problem even more serious for the hydrological balance and the city's economy. The present study is concerned with the diverse reasons and the major impacts of groundwater dropping in the Varanasi urban region. The researchers made use of a mix of continuous hydrogeological data, satellite images, and urban growth evaluation to point out unregulated urbanisation and high population density as the main depletion factors. The spread of impermeable surfaces has greatly limited the areas that naturally recharge, while the extraction rate is several times higher compared to the annual replenishment capacity of the underlying Gangetic alluvial aquifers. Plus, the change to water-intensive lifestyles and the absence of a centralised regulatory framework for private tube-well installations have increased the problem. The impacts of this depletion are very serious and show their face in water scarcity all over the place since shallow borewells fail, and it leads to deeper and more expensive drilling. The study also indicates a simultaneous problem of water quality deterioration, as the inward spread of contaminants and the increase of geogenic contaminants, namely arsenic and fluoride, frequently occur along with the decline of water levels. Land subsidence is also a potential consequence that would endanger the city’s ancient buildings because of the changes in the subsurface pore pressure. The article recommends an integrated water resource management (IWRM) plan as the way to conduct further research, and suggests that the enforced use of rainwater harvesting, the restoration of traditional urban ponds (kunds), and the development of a digital groundwater monitoring network be used to ensure the long-term sustainability of Varanasi’s water security.

ABSTRACT Inundation of a scroll-plain of the Darling River during 2010–2011 illustrate flood processes of an arid zone river with sediments dominated by sodic smectites. Initial inundation progressed as much by subsurface flow as by surface, with the subsurface flow extending some metres in front of the surface flow via pipes and cracks. Collapse into cavities was common. Swales were submerged to a depth of 2.5 m and a higher terrace <0.5 m. Microrelief determined surface flow direction which could be 180o from that in the main channel. Flow crossed the scroll-plain during the flood peak parallel to the trend of the channel belt. The fastest flows deposited ripples of fine sand and coarse silt of quartz and clay pellets. The clay-rich surface of the scroll-plain trapped pools of water in low points as the flood fell. Little or no recharge of the shallow alluvial aquifer by floodwaters occurred, and any recharge must be by lateral bank recharge. As the surface dried, it was broken up by shallow cracking. Deeper cracks are expected to form with drying of the subsoil.

Alluvial valley aquifers are important sources of water supply in many areas but effects of co-located oil and gas development on these resources have not been widely reported, especially in settings where recharge is dominated by stream infiltration. Interpreting the presence of geochemical indicators in the context of hydrology, geology, and other factors provides a more complete understanding of the relations between groundwater and sources of oil-field fluids and aids in identifying risks associated with oil and gas development. Groundwater and Salinas River water samples were collected in an alluvial valley near the San Ardo Oil Field in Monterey County, California and analyzed for a wide range of dissolved chemical, gas, and isotopic constituents to determine if oil-field fluids (water and gas from oil-producing and non-producing zones) have mixed with fresh groundwater used for supply. Hydraulic gradients, age-dating tracers, and other geochemical indicators showed that recharge from the Salinas River has the potential to dilute oil-field fluids that might migrate or seep into the aquifer. Groundwater and Salinas River water collected downgradient of the San Ardo Oil Field showed little or no evidence of mixing with oil-field fluids. Some samples within the oil field contained trace amounts of hydrocarbons or elevated temperatures, indicating that any potential effects from oil-field activities are minor or have been diluted by recharge from the Salinas River. The two samples with the most geochemical evidence of potential mixing with oil-field fluids (SP-18 and GW-17) were collected west of or along the Los Lobos fault, where naturally occurring hydrocarbons are near the land surface. Those samples were also collected near active or inactive oil-field wells, and so anthropogenic activities and pathways could not be ruled out as a cause of trace detections of hydrocarbons and elevated temperatures in the aquifer.

The Mediterranean region is highly vulnerable to climate variability, with profound implications for water security. This study assesses the impact of four decades (1983-2023) of climatic fluctuations on land use and groundwater resources in the Wadi Fekan sub-watershed, northwest Algeria. Using standardized indices (SPI, CMI) and break detection tests on data from 11 rainfall stations, we identified a pivotal climatic shift between 1999 and 2006. This break initiated a wetter regime, leading to a 31.7% increase in rainfall. The resulting increase in runoff led to a measurable expansion of the main riverbed (+0.045% in land-use class). Concurrently, the alluvial aquifer experienced substantial recharge, with volumes rising from 17.997 km3 (2003-2013) to 25.615 km3 (2013-2023). However, spatial analysis revealed a paradox: despite overall wetter conditions, aridity intensified in the basin’s center due to a Foehn effect, and the aquifer’s net water balance remained negative over the study period. This indicates that groundwater overexploitation during prior droughts created a hydrologic deficit that recent rainfall has not fully offset. These findings demonstrate the persistent vulnerability of semi-arid aquifers to climatic stress and anthropogenic pressure, underscoring the critical need for sustainable management strategies that address both climate variability and historical overuse to mitigate future water crises.

Abstract Understanding the mixing of groundwater age and of nonpoint source (NPS) pollutants in water samples is crucial for interpreting age tracer and NPS pollutant data from production wells. Traditionally, diffusion and mechanical dispersion have been key mixing processes embedded in physical models for simulating and interpreting age tracer and NPS pollutant data. Also, a large body of literature highlights mixing due to aquifer heterogeneity across scales. Importantly, the collection of water samples through wells introduces additional mixing across the internal volume of the well screen. Here, we investigate and quantify the magnitude of mixing due to these four processes—diffusion, mechanical dispersion, aquifer heterogeneity, and in‐well mixing—through a Monte Carlo‐based modeling framework and sensitivity study. We consider wells in a typical unconsolidated alluvial aquifer system. We find that in‐well mixing and aquifer heterogeneity dominate the mixing process in larger production wells. In small production wells and in monitoring wells, diffusion and (sub‐grid scale) mechanical dispersion add significantly to age/pollutant mixing. Across an ensemble of larger production wells (e.g., in regional planning), the range of age (or pollutant) mixing observed is dominated by in‐well mixing, with aquifer heterogeneity not significantly changing the age mixing distribution. The depth of the well screen has some impact on age mixing only in small (monitoring) wells. Our work suggests that ensemble age (or mixed NPS pollutant concentration) distributions across large sets of production wells can be satisfactorily estimated from well construction information and by considering advective transport in equivalent homogeneous media.

Irrigation is essential to intensive agriculture worldwide, particularly in semi-arid climates where rainfall is unreliable. The use of surface and groundwater for crop irrigation can alter aquifer dynamics by lowering water tables and degrading water quality through the leaching of agrochemicals, nutrients, and organic carbon. These changes pose significant risks to the integrity of groundwater ecosystems. To assess these impacts, we conducted a spatial and temporal study of groundwater chemistry and biota (using environmental DNA (eDNA)) in a shallow alluvial aquifer adjacent to an irrigated cotton farm in northern NSW, Australia. We sampled a series of wells in a transect stretching from an irrigated cotton farm toward the Namoi River, over an annual crop cycle. Groundwater levels declined over the year. Groundwater quality, particularly electrical conductivity and ammonia concentrations, varied with distance from the field. Water chemistry varied more between wells than over time, with wells close to the field showing distinctly different chemical characteristics to those further away. Drainage from the irrigated field was evident in the chemistry and isotopic signature of the groundwater, and also in biota. Taxa typical of surface waters and soils, and cotton DNA were more prominent in wells under the effect of irrigation. Biota displayed a potential 'ecological memory', where communities showed legacy effects related to past agricultural activities and hydraulic conditions. These results indicate the spatial extent of impacts of irrigation on shallow aquifers and confirm the direct and indirect effects of agricultural practices on groundwater levels, quality, and biotic communities within a short distance from an irrigated field. These findings have widespread relevance for the management of off-site impacts of irrigation activities.

Evaluation of arsenic and nitrate contamination in groundwater from alluvium aquifers: Hydrogeochemical features, water quality and health risk assessment

Chlorinated solvents such as vinyl chloride (VC) and trichloroethylene (TCE) represent a persistent threat to groundwater-derived drinking-water supplies, including riverbank filtration well fields in alluvial aquifers. This work presents a laboratory-scale evaluation of an electrochemical barrier concept for targeted VC and TCE removal performed using synthetic groundwater representative of a riverbank filtration setting in the Danube River basin. Experiments were conducted in a covered batch reactor equipped with Ti/IrO2–RuO2 mixed-metal-oxide anodes and Ti cathodes, systematically varying current intensity (10–60 mA), treatment time (0–60 min), active anode surface area (12–48 cm2), and inter-electrode distance (0.5–2.5 cm). At 60 mA, VC and TCE removals of 97% and 95%, respectively, were achieved within 20 min, while prolonged treatment to 60 min increased removal to about 99% for VC and 98.5% for TCE. Multivariate analysis (PCA) and correlation assessment identified applied current as the dominant control parameter, particularly for TCE removal, whereas electrode configuration and spacing played secondary roles within the investigated range. For the most cost-effective treatments meeting Serbian drinking-water criteria, estimated electricity costs were 0.39 €/m3 for VC and 0.10 €/m3 for TCE. Overall, the results demonstrate the technical feasibility and promising cost-effectiveness of electrochemical barriers as a proactive measure to protect riverbank filtration systems from future VC and TCE contamination n urban environments, while highlighting the need for follow-up studies on by-product formation and long-term performance.

This study investigates groundwater levels (GWLs) in the alluvial aquifer of the Sava River valley, located in the north-western part of Croatia. It provides the first quantitative assessment of groundwater levels using machine learning in this part of Europe. Groundwater levels from 1998 to 2017 were predicted using support vector regression (SVR). The input variables were initially monthly data on two basic elements that influence groundwater dynamics (precipitation and the Sava River levels). Later, GWLs from the previous month (GWL-1) were added as an additional predictor. Results demonstrated that the SVR model effectively predicts groundwater levels. Introducing GWL-1 reduced RMSE and MAE values by more than 47% and 46%, respectively, while increasing the R2 value by over 36%. The improvement was more pronounced farther from the Sava River, since GWLs near the river are more directly influenced by river stage fluctuations, diminishing the impact of GWL-1. Compared to the existing regional numerical model (NM), the SVR model outperformed the NM with improvements of approximately 12% to 76% across performance indicators. Our findings suggest that the SVR model provides a reliable method for predicting groundwater levels at specific observation wells, making it a valuable tool for applications such as forecasting groundwater availability for farmers during dry periods and flood risk assessment during periods of heavy rainfall.

The Goulmima-Tadighoust region faces significant challenges with its alluvial aquifer, mainly due to intensive groundwater exploitation driven by the Green Morocco plan. This study, conducted in 2019 and 2021, investigates the deterioration of groundwater quality in the area, mapping the Quaternary aquifer using physicochemical parameters. Geographic Information System (GIS) was employed to create thematic maps for spatial analysis. Groundwater in the region exhibits two distinct chemical facies, impacted by salinization processes, leading to elevated electrical conductivity. Water quality indicators suggest unsuitability for irrigation due to high salinity and alkalinity. Nitrate pollution is observed, posing a moderate health risk. Factors contributing to water quality degradation include natural contamination, untreated wastewater discharge, improper solid waste disposal, and geological factors. A considerable portion of the study area exhibits a Water Quality Index (WQI) exceeding 200, indicating poor water quality, particularly south of Tadighoust and east of Goulmima oasis.