Groundwater Model Research Articles

Hydraulic recharge and element dynamics during salinization in an overexploited coastal aquifer of the world's driest zone: Atacama Desert

The management of water resources in hyper-arid coastal regions is a challenging task because proper information regarding groundwater recharge and water budget is needed for maintaining the hydraulic balance in optimal conditions, avoiding salinization and seawater intrusion. Thus, this article deals with the estimation of the hydraulic recharge and the study of the effects of salinization on the dynamics of major and trace elements in an alluvial aquifer located in the world's driest zone, the northern Atacama Desert. The result of stable water isotopes (δD and δ18O) and tritium (3H) indicated that groundwater in the area is not recent, whereas 14C results estimated a groundwater residence time ranging between 11,628 and 16,067 yBP. The estimation of the artificial recharge coming from the urban water-supply-system leaks and wastewater/river-water/groundwater infiltration during irrigation was about 19.84 hm3/year, which represents an annual negative water balance of 177 hm3/year for the aquifer. The groundwater salinization triggered by seawater intrusion (up to 32.6 %) has caused the enrichment of Li, Rb, Ca, Ba, and Sr in groundwater by cationic exchange, where the excess of aqueous Na is exchanged by these elements in the aquifer sediments. Other elements such as B, Se, Si, and Sb are enriched in groundwater by ionic strength and/or anionic exchange during salinization. The heightened B concentrations derived from the B-rich alluvial sediments were higher than the limit suggested by international guidelines, representing a risk to consumers. Vanadium seems to be unaffected by salinization, whereas Pb, Mo, As, U, and Zr did not show a clear behavior during saline intrusion. Finally, this article highlights the consequences of conducting improper water management in coastal hyper-arid regions with exacerbated agriculture.

Where has hydrogeologic science been, and where is it going? Research trends in hydrogeology publishing over the past 60 years

Quantifying historical research trends in the field of hydrogeology is not only generally informative for hydrogeologists but is essential for fostering interdisciplinary collaboration and assessing the relationship between academic study and societal interests in hydrogeologic issues. To address this, a topic model was applied to over 37,000 academic abstracts published in over 20 journals between 1963–2022 in the field of hydrogeology to study the evolution of topic trends through time. Model results were fed into the popular large-language model ChatGPT to assign topic names, representing an unsupervised method. The results indicate that, historically, popular topics related to methodological development and analytical and numerical models analytical and numerical methods in groundwater flow modeling and well hydraulics have given way to topics related to more increasingly complex models (groundwater monitoring and uncertainty estimation and groundwater modeling calibration and simulation) as data and computational capability becomes increasingly available. An insight into the period of boom-and-bust in contaminant hydrogeology is reflected by a shift in focus from topics related to assessment and characterization of contaminant sources toward topics related to degradation and remediation methods. Topics of emerging prevalence (sustainable groundwater resource management, catchment hydrology and runoff processes) in the current period reflect an increasing focus on treating the surface-water/groundwater system as a single system. In addition, results suggest that topic distribution within the field of hydrogeology has become more varied as time has progressed.

Transforming Hydrology Python Packages into Web Application Programming Interfaces: A Comprehensive Workflow Using Modern Web Technologies

The accessibility and deployment of complex hydrological models remain significant challenges in water resource management and research. This study presents a comprehensive workflow for converting Python-based hydrological models into web APIs, addressing the need for more accessible and interoperable modeling tools. The workflow leverages modern web technologies and containerization to streamline the deployment process. The workflow was applied to three distinct models: a GRACE downscaling model, a synthetic time series generator, and a MODFLOW groundwater model. The implementation process for each model was completed in approximately 15 min with a reliable internet connection, demonstrating the efficiency of the approach. The resulting APIs provide standardized interfaces for model execution, progress tracking, and result retrieval, facilitating integration with various applications. This workflow significantly reduces barriers to model deployment and usage, potentially broadening the user base for sophisticated hydrological tools. The approach aligns hydrological modeling with contemporary software development practices, opening new avenues for collaboration and innovation. While challenges such as performance scaling and security considerations remain, this work provides a blueprint for making complex hydrological models more accessible and operational, paving the way for enhanced research and practical applications in hydrology.

Distribution and drivers of co−hosts of antibiotic and metal(loid) resistance genes in the fresh−brackish−saline groundwater

Groundwater is an essential source of drinking water and agricultural irrigation water, and its protection has become a global goal for public health. However, knowledge about heavy metal(loid) resistance genes (MRGs) in groundwater and the potential co−selection of antibiotic resistance genes (ARGs) have seldom been developed. Here, during the wet and dry seasons, we collected 66 groundwater samples (total dissolved solids = 93.9−9530 mg/L) adjacent to Baiyangdian Lake in Northern China, which presented the few metal(loid) and antibiotic contamination. We identified 160 MRGs whose composition exhibited significant seasonal variation, and dissolved metal(loid)s (particularly Ba) played a determinative role in promoting the MRGs proliferation though with relatively low concentrations, suggesting the relatively vulnerable groundwater ecosystems. Moreover, 27.4% of MRG−carrying metagenome−assembled genomes (MAGs) simultaneously carried ARGs, with the most frequently detected MRG types of Cu, Hg, and As, and ARG types of multidrug and bacitracin. Physicochemical variables, variables related to total dissolved solids, metal(loid)s, and antibiotics synthetically shaped the variation of MRG−ARG hosts in groundwater. We found that the increase of MRG−ARG hosts was critically responsible for the spread of MRGs and ARGs in groundwater. Our findings revealed the widespread co−occurrence of MRGs and ARGs in few−contaminated groundwater and highlighted the crucial roles of salinity in their propagation and transmission.

Hydrogeochemical and isotopic investigations of groundwater in the reclaimed desert located between EL Nasr canal and Mariut Tableland, NW Coast, Egypt

A complete understanding of groundwater dynamics and its interaction with surface water under the impact of agricultural activities is vital for local agriculture, ecology, and residents of dry regions, which is not commonly recognized in arid areas. This research outlines the geochemical characteristics, recharge sources, and potential factors impacting groundwater quality in a new land reclamation located in the small basin of Abu Mina, which is part of the Western Nile Delta region.1 Thirty-one groundwater samples and two surface water samples were collected in 2021 to represent the Pleistocene aquifer and were subjected to multivariate statistical, hydrochemical, and stable isotope analyses. Data analysis demonstrates that Na+ > Ca2+ > Mg2+ > K+ and SO42– > Cl– > HCO3– > NO3– are the predominant cations and anions, respectively. Groundwater salinity ranged from 465.60 to 6455.18 mg/l, with slightly alkaline. Most of the water samples fall into one of three types of facies: Ca–Cl, Na–Cl, and Mixed Ca–Mg–Cl, in decreasing order. The meteoric genesis index (r2) indicates that deep meteoric water percolation dominates the Pleistocene aquifer. The aquiline diagrams, correlation matrix, and different ionic ratios indicate that evaporation, reverse ion exchange reactions, and the dissolution of carbonate and silicate minerals are the main processes governing groundwater chemistry. Factor analysis (FA) indicated that three factors explain groundwater hydrochemistry, accounting for 71.98% of the total variance. According to the rotating components matrix (F1–F3), the chemistry of the Quaternary aquifer is principally affected by evaporation, ion exchange reactions, and anthropogenic influences. Additionally, salinity increases due to the return flow of irrigation activities and mixing between old and recent water. The stable isotopes (δ18O and δ2H) indicate that the Quaternary aquifer receives groundwater recharge through the return flow of excess irrigation and canal seepage. Under desert reclamation conditions, groundwater salinization processes should be given special consideration. All groundwater samples are appropriate for agricultural irrigation based on the Sodium Adsorption Ratio (SAR), Permeability Index (PI), Percent Sodium (%Na), and Residual Sodium Carbonate (RSC).

Conceptual Modflow Model Application with Boundary Condition Analysis

In this study, groundwater flow modeling of the aquifer in the Porsuk Basin, located in the Eskisehir province of Turkey, was conducted using the MODFLOW modeling method and the Groundwater Modeling System (GMS) software interface. The modeling was performed using the finite difference method under the assumption of steady flow, incorporating well data, aquifer boundaries, topographic elevations, riverbed data, and hydraulic parameters from the 1971 water year. This study provided insights into changes in groundwater over time and yielded a comprehensive water budget. By creating a three-dimensional solid model of the Porsuk Basin aquifer, general information about the region's hydrogeology was obtained. The results are expected to play a significant role in identifying potential drilling areas for groundwater extraction. Since the modeling utilizes objects from geographic information systems, it will enhance visualization of the regional structure for researchers, allowing them to observe the topographic features of the area in three dimensions. Additionally, it is anticipated that the groundwater conceptual model will inform drilling studies and serve as a foundation for research on groundwater transport and pollution.

Groundwater Study in Limestone Area using Geophysical Methods

Applied geophysics is important for groundwater investigation. Drilling groundwater boreholes without geophysical survey is probably unsuccessful. Meth district, Vientiane Province is the study site due to the most area coving with limestone area. The objective of this study is to define the groundwater model including location, depth and thickness of aquifer using 2D resistivity measurement, groundwater flow direction using self potential measurement. In addition, basic groundwater quality (pH, Ec and TDS) is also defended. 2D-Electrical Resistivity Tomography (2D-ERT) meauremnt using Wenner arrray was conduced for 2 lines with 300m of length and a varying from a=5m to 100m. Self Potential measurment using fixed-base method was performed for 3 lines with 300m of leng and 10m spacing of each measurming point. The results of the 2D-ERT show that low resistivity from 10 to 80 ohm-m at depth of 20-28m interpreted as aquifer layer. But high resistivity from 200 to 600 ohm-m is resulted from base rock. The volage from SP measurment varying ranging from -14 to 24 mV can be defined the flowdirection from NE-SW direction. It is correlating with the topography of the area. The result of physical properties from 3 samples show that pH=6.8±0.5, Ec=480±115 mS/cm, and TDS=343.3±80.2 mg/l. are under the standard values issued by Minstry of Health, Laos.The water qulity is good . The geophysical survey is effective usful methods for groundwater suvey due to low cost and rapid for survey. Geophysical result could be intributed to comunity, government sectors in order to high effective management the groundwtater resources in the future. Moreover, the data is also usfull for researcher and students.

Response of Antioxidant Enzymes of Winter Wheat (Triticum aestivum L.) to Shallow and Saline Groundwater Depths

ABSTRACTAntioxidant enzymes in plants are critical for protection against oxidative stress and for the overall health and resilience of plant systems. However, antioxidant responses of plants grown in shallow and saline groundwater are poorly understood. Therefore, understanding the biochemical responses of plants to shallow groundwater significantly affects food security and environmental conservation. With this aim, the present work was carried out for 2 years in drainable lysimeters to assess the effects of four different groundwater salinities (0.38, 2.0, 4.0 and 8.0 dS m−1) on the temporal changes in antioxidant enzymatic activity in wheat plants under three different groundwater depths (30, 55 and 80 cm). Superoxide dismutase (SOD), glutathione S‐transferase (GST) and catalase (CAT) activities varied significantly based on groundwater depth and salinity. CAT and GST enzyme levels increased curvilinearly with rising groundwater depth and salinity. Conversely, the GR enzyme activity showed no significant change with groundwater depths but increased linearly with higher salinity. SOD enzyme activity notably increased at a groundwater depth of 30 cm but decreased at a depth of 80 cm. Moreover, the peak activity of the GR enzyme was observed at a 6 dS m−1 groundwater salinity under groundwater depths. Additionally, the GST and CAT enzyme activities were inhibited more when the groundwater depth was <55 cm and the groundwater salinity was >4.20 dS m−1. Finally, identifying the peak levels of antioxidant enzymes could potentially serve as an indicator for determining the optimal timing for applying stress mitigation methods in areas with shallow and saline groundwater.

Effects of grid resolution on regional modelled groundwater salinity and salt fluxes to surface water

Coastal fresh groundwaters face growing threats from rising withdrawals and climate change, with salinization of surface and groundwater as notable impacts. Recent developments in parallel variable-density modelling enable studying these threats at unexplored resolutions and extents. To improve the understanding of ground- and surface water salinization processes, we designed an experiment to measure how spatial resolution affects both groundwater salinity distribution and salt loads to surface waters. An existing parallelized coupled variable-density groundwater flow and salt transport (VD-GWT) model is applied to a region in the Netherlands whereby the surface water drainage network is parameterized at grid resolutions between 10 and 250 m. The results show a strong dependency between resolution and the salinity distribution in shallow groundwater while simulated salt loads are affected by imperfect flux scaling. The computational resources needed for this test suggest that regional high-resolution VD-GWT models are feasible using HPC environments, albeit not practical.

Quality and health impact of groundwater in a coastal region: a case study from west coast of southern India.

Seawater intrusion seriously threatens the quality of coastal groundwater, affecting nearly 40% of the world's population in coastal areas. A study was conducted in the Kamini watershed situated in the Udupi district of Karnataka to assess the groundwater quality and extent of seawater intrusion. During the pre-monsoon period, 57 groundwater and 3 surface water samples were analyzed to understand the impact of seawater on the groundwater and surface water. The analysis revealed that the groundwater in the study area is slightly alkaline. The weighted overlay analysis map indicated that 11% of the study area is unsuitable for drinking water due to the influence of seawater. The Piper plot analysis revealed that the groundwater is predominantly CaMgCl facies. The hydrogeochemical facies evolution diagram (HFED) showed that 62% of the groundwater is affected by seawater. The HFED and Piper plots also indicate that the surface water is also affected by seawater. These results are also supported by various molar ratios such as Cl- vs. Cl⁻/HCO3⁻, Cl⁻ vs. Na⁺/Cl⁻, Cl- vs. SO42-/Cl-, and Cl⁻/HCO3- vs. Mg2+/Ca2+, suggesting that the majority of the water sample has been affected by seawater. The saturation indices indicated that mineral dissolution has significantly contributed to groundwater salinization. The correlation between sulfate concentration and calcite and dolomite dissolution suggested the influence of seawater intrusion in the coastal aquifer. The process of reverse ion exchange mainly influences the groundwater chemistry according to chloroalkali indices. The total hazard index (THI) values of nitrate and fluoride exceeded limits, posing health risks to adults and children. Studies suggest that with time and space, seawater intrusion is increasing in some pockets of the study area, especially along the west coast.

Analysis of the Spatial Distribution of Seawater Intrusion Using a Hydrogeochemical Approach: A Study of the Coastal Geology of Kebumen Regency, Indonesia

This study examines seawater intrusion in the southern part of Kebumen District, focusing on the impact of various geological formations on groundwater salinity. Groundwater samples were systematically collected from 11 locations representing alluvial, volcanic, coastal sediment, and karst geological conditions. The samples were analyzed for major ions, including calcium (Ca²⁺), magnesium (Mg²⁺), chloride (Cl-), and bicarbonate (HCO₃-), as well as Total Dissolved Solids (TDS). Ion ratios such as Mg²⁺/Ca²⁺, Cl⁻/HCO₃⁻, and Mg²⁺/(Mg²⁺+Ca²⁺), along with the Fraction of Seawater (fsea), were calculated to assess the extent of seawater intrusion. The study employed the Gibbs diagram method to identify the primary processes controlling groundwater chemistry, revealing that water-rock interactions are dominant in most samples, particularly in those affected by seawater intrusion. The results indicate significant seawater intrusion in alluvial and karst regions, especially at sampling points TP 2, TP 4, and TP 10, while volcanic and coastal sediment areas show minimal intrusion. These findings underscore the critical influence of geological conditions on seawater intrusion and highlight the need for targeted groundwater management strategies. Further research focusing on long-term monitoring is recommended to better understand and mitigate the impacts of seawater intrusion in this region.

INSULARITY, ENVIRONMENTAL VULNERABILITIES AND ADAPTATION STRATEGIES IN THE BLISS KASSA ISLANDS IN CASAMANCE (SENEGAL)

The relationship between the environment and societies is a complex one in terms of the way in which human communities interact with the environment. This relationship is characteristic of the different uses made of the areas in which they live. A segmented approach is therefore needed to understand how these areas are changing, hence the choice in this research to focus on small island areas in the Ziguinchor region of southern Senegal. The article looks at three main challenges identified in this area, including waste production and management, sea encroachment and salinisation. A qualitative approach was adopted in order to gather peoples perceptions and experiences of spatial change over time in relation to natural and anthropogenic dynamics, the consequences of these changes and related adaptation strategies. The results of the semi-structured interviews and focus groups show: firstly, a change in lifestyles and an excessive accumulation of plastic waste that popular recycling practices are unable to contain secondly, a loss of freshwater resources with the accelerated salinisation of groundwater and surface water thirdly, a decline in socio-economic living spaces with the advance of the sea, which also contributes to salinisation. The findings show that the people of Bliss Kassa are increasingly vulnerable, while the public systems in place to deal with these issues remain embryonic and largely outstripped by the pace of change.

Simultaneous identification of groundwater contamination source information, model parameters, and boundary conditions under an unknown boundary mode

Simultaneous identification of groundwater contamination source information, model parameters, and boundary conditions under an unknown boundary mode

Will there be water? Climate change, housing needs, and future water demand in California

Climate change in California is expected to alter future water availability, impacting water supplies needed to support future housing growth and agriculture demand. In groundwater-dependent regions like California's Central Coast, new land-use related water demand and decreasing recharge is already stressing depleted groundwater basins. We developed a spatially explicit state-and-transition simulation model that integrates climate, land-use change, water demand, and groundwater gain-loss to examine the impact of future climate and land use change on groundwater balance and water demand in five counties along the Central Coast from 2010 to 2060. The model incorporated downscaled groundwater recharge projections based on a Warm/Wet and a Hot/Dry climate future from a spatially explicit hydrological process-based model. Two urbanization projections from a parcel-based, regional urban growth model representing 1) recent historical and 2) state-mandated housing growth projections were used as alternative spatial targets for future urban growth. Agricultural projections were based on recent historical trends from remote sensing data. Annual projected changes in groundwater balance were calculated as the difference between land-use related water demand, based on historical estimates, and climate-driven recharge plus agriculture return flows. Results indicate that future changes in climate-driven groundwater recharge, coupled with cumulative increases in agricultural water demand, result in overall declines in future groundwater balance, with a Hot/Dry future resulting in cumulative groundwater decline in all but Santa Cruz County. Cumulative declines by 2060 are especially prominent in San Luis Obispo (−2.9 to −5.1 Bm3) and Monterey counties (−6.5 to −8.7 Bm3), despite limited changes in agricultural water demand over the model period. These two counties show declining groundwater reserves in a Warm/Wet future as well, while San Benito and Santa Barbara County barely reach equilibrium. These results suggest future groundwater supplies may not be able to keep pace with regional demand and declining climate-driven recharge, resulting in a potential reduction in water security in the region. However, our county-scale projections showed new housing and associated water demand does not conflict with California's groundwater sustainability goals. Rather, future climate coupled with increasing agricultural groundwater demand may reduce water security in some counties, potentially limiting available groundwater supplies for new housing.

Hydrodynamic Model of the Area of the Żelazny Most Mining Waste Storage Facility to Reconstruct the Migration of Saline Groundwater

This paper presents the construction of a numerical three-dimensional model of the area of the Żelazny Most Mining Waste Storage Facility (MWSF). In the study area, the difficult geological conditions associated with glaciotectonics are accompanied by a complex hydrotechnical system of sediment deposition and sedimentary water drainage. In order to effectively reflect the water flow paths, a detailed schematization was carried out, using 700,000 boreholes and more than 300 hydrogeological cross-sections. In addition, numerous drainage sections, streams, and ditches were included to reliably assess the amount of saline water entering the underlying aquifers. This research was supported by magnetic resonance sounding (MRS) studies of the reservoir’s sediments. The MWSF is currently being expanded, so the work primarily focuses on illustrating changes in the hydrodynamic field resulting from the inclusion of the new southern section. Models of similar facilities have been implemented before, but in the current one, the combination of meticulous analysis of the hydro-structural system, the water balance, a significant amount of data, the size of the facility, and the use of an unstructured discretization grid in the calculations is undoubtedly innovative and will be an important contribution to the development of analogous solutions around the world.

Aboveground Carbon Stocks across a Hydrological Gradient: Ghost Forests to Non-Tidal Freshwater Forested Wetlands

Upper estuarine forested wetlands (UEFWs) play an important role in the sequestration of atmospheric carbon (C), which is facilitated by their position at the boundary of terrestrial and maritime environments but threatened by sea level rise. This study assessed the change in aboveground C stocks along the estuarine–riverine hydrogeomorphic gradient spanning salt-impacted freshwater tidal forested wetlands to freshwater forested wetlands in seasonally tidal and nontidal landscape positions. Standing stocks of C in forested wetlands were measured along two major coastal river systems, the Winyah Bay in South Carolina and the Savannah River in Georgia (USA), replicating and expanding a previous study to allow the assessment of change over time. Aboveground C stocks on these systems averaged 172.9 Mg C ha−1, comparable to those found in UEFWs across the globe and distinct from the terrestrial forested ecosystems they are often considered to be a part of during large-scale C inventory efforts. Groundwater salinity conditions as low as 1.3 ppt were observed in conjunction with losses of aboveground C. When viewed in context alongside expected sea level rise and corresponding saltwater intrusion estimates, these data suggest a marked decrease in aboveground C stocks in forested wetlands situated in and around tidal estuaries.

Source and transport enrichment mechanism of iodine in shallow saline groundwater in Tianjin coastal area

Source and transport enrichment mechanism of iodine in shallow saline groundwater in Tianjin coastal area

Direct and Remote Sensing Monitoring of Plant Salinity Stress in a Coastal Back-Barrier Environment: Mediterranean Pine Forest Stress and Mortality as a Case Study

The increase in atmospheric and soil temperatures in recent decades has led to unfavorable conditions for plants in many Mediterranean coastal environments. A typical example can be found along the coast of the Campania region in Italy, within the “Volturno Licola Falciano Natural Reserve”, where a pine forest suffered a dramatic loss of trees in 2021. New pines were planted in 2023 to replace the dead ones, with a larger tree layout and interspersed with Mediterranean bushes to replace the dead pine forest. A direct (in situ) monitoring program was planned to analyze the determinants of the pine salinity stress, coupled with Sentinel-2 L2A data; in particular, multispectral indices NDVI and NDMI were provided by the EU Copernicus service for plant status and water stress level information. Both the vadose zone and shallow groundwater were monitored with continuous logging probes. Vadose zone monitoring indicated that salinity peaked at a 30 cm soil depth, with values up to 1.9 g/L. These harsh conditions, combined with air temperatures reaching peaks of more than 40 °C, created severe difficulties for pine growth. The results of the shallow groundwater monitoring showed that the groundwater salinity was low (0.35–0.4 g/L) near the shoreline since the dune environment allowed rapid rainwater infiltration, preventing seawater intrusion. Meanwhile, salinity increased inland, reaching a peak at the end of the summer, with values up to 2.8 g/L. In November 2023, salts from storm-borne aerosols (“sea spray”) deposited on the soil caused the sea-facing portion of the newly planted pines to dry out. Differently, the pioneer vegetation of the Mediterranean dunes, directly facing the sea, was not affected by the massive deposition of sea spray. The NDMI and NDVI data were useful in distinguishing the old pine trees suffering from increasing stress and final death but were not accurate in detecting the stress conditions of newly planted, still rather short pine trees because their spectral reflectance largely interfered with the adjacent shrub growth. The proposed coupling of direct and remote sensing monitoring was successful and could be applied to detect the main drivers of plant stress in many other Mediterranean coastal environments.

Unraveling microbial community variation along a salinity gradient and indicative significance to groundwater salinization in the coastal aquifer

Saltwater intrusion poses a global environmental challenge in coastal regions, profoundly influencing groundwater microbial communities and biogeochemical processes. This study aims to elucidate the origin of groundwater salinity and understand the response of microbial interactions to groundwater salination through stable isotopes and 16S rDNA gene sequencing. The findings reveal a significant increase in groundwater TDS from inland to the coastline, ranging from 0.5 g/L to 144.6 g/L. Ionic relationship and isotopic fractionation indicate that the high salinity in groundwater originates not only from paleo–seawater but also from the dissolution of halite. Salty groundwater exhibits lower species richness and diversity than fresh groundwater. TDS, SO42–, and NO2– are the pivotal factors shaping the microbial community of salty groundwater, while NO3– and H2SiO3 are dominant variables influencing fresh groundwater community. This study identifies the order Alteromonadales, family Alteromonadaceae, and genus Marinobacter as potential biomarkers of seawater or paleo–seawater sources. Phylogenetic molecular network analysis of microbial communities illustrates that groundwater salinization results in a more compact microbial network but enhances microbial interactions. Predicted functional genes indicate that sulfate reduction genes (sat, met3, aprA, aprB, dsrA, and dsrB) are more abundant in salty groundwater than in fresh groundwater, suggesting a high metabolic potential for sulfate. These results elucidate the pivotal role of groundwater salinization in microbial community evolution, shaping specific functional zones in coastal groundwater.

Regolith-Hosted Rare Earth Element Mineralization in the Esperance Region, Western Australia: Major Characteristics and Potential Controls

A number of regolith-hosted REE occurrences have recently been discovered in the Esperance region in southern Western Australia. This paper summarizes major characteristics of REE mineralization and discusses contributing factors and potential controls. The main aim is to explain why there is a lack of highly sought-after ion-adsorption-clay-type REE deposits across the region despite the presence of the regolith-hosted REE mineralization on a regional scale. Local mineralization mostly occurs as continuous flat-lying enrichment “blankets” within the residual regolith developed over Archaean–Proterozoic granite gneisses and granitoids with elevated REE content. The enriched horizon is commonly located in the lower saprolite and saprock and is accompanied by an overlying REE-depleted zone. This distribution pattern, together with the data on HREE fractionation and the presence of the supergene REE minerals, indicates chemogenic type enrichment formed by supergene REE mobilization into groundwater, downward transport, and accumulation in the lower part of the weathering profile. Residual REE accumulation processes due to bulk rock volume and mass reduction during weathering also contribute to mineralization. It is proposed that climate and groundwater chemistry are the critical regional controls on the distribution of REEs in the weathering profile and on their speciation in the enrichment zone. Cenozoic aridification of climate in southwest Australia heavily overprinted pre-existing REE distributions in the weathering profile. Acidic (pH < 4), highly saline groundwaters intensely leached away any relatively weakly bound, adsorbed or colloidal REE forms, moving them downward. Dissolved REEs precipitated as secondary phosphates in neutral to alkaline environment at lower Eh near the base of the weathering profile forming the supergene enrichment zone. Low denudation rates, characteristic of areas of low relief under the arid climate, are favourable for the preservation of the existing weathering profiles with REE mineralization.