Environmental Isotopes Research Articles

Inter-basin groundwater flow in the Ordos Basin: Evidence of environmental isotope and hydrological investigations

The Ordos Basin, located in arid and semi-arid region of China, is famous for its abundant groundwater resources and artesian features. The source of groundwater recharge, whether from local precipitation or external sources, has been debated. This study aims to elucidate the groundwater circulation mechanism in the Ordos Basin through scientific expedition, environmental isotope method, and hydrological drilling exploration, providing valuable insights for other artesian basins. Comprehensive analysis indicates that groundwater in the Ordos Basin is recharged by modern precipitation, primarily from high-elevation areas outside the basin. Deep groundwater from these external sources ascends to the aquifer through basement fault zones. Evidence from hydrogen and oxygen isotopes, hydraulic gradients, and water quantities suggests that the Tibetan Plateau is the most potential recharge source. Based on the distribution of Cenozoic basalt and data from seismic observation wells, we propose that leakage water from the Tibetan Plateau rift valley is transported to the Ordos Basin through fast channels, possibly lava tubes, and then upwelling through basement fault zones. This work provides a new perspective on the mechanism of inter-basin groundwater circulation.

Sources and Transformation of Nitrate in Shallow Groundwater in the Three Gorges Reservoir Area: Hydrogeochemistry and Isotopes

Nitrate is among the most widely occurring contaminants in groundwater on a global scale, posing a serious threat to drinking water supplies. With the advancement of urbanization and mountainous agriculture, the nitrate in the groundwater of Wanzhou District in the Three Gorges Reservoir Area has formed a complex combination of pollution sources. To more accurately identify the sources of nitrate in groundwater, this study integrates hydrochemical methods and environmental isotope techniques to analyze the sources and transformation processes in shallow groundwater nitrate under different land-use types. Furthermore, the Bayesian isotope mixing model (MixSAIR) is employed to calculate the contribution rates in various nitrate sources. The results indicate that nitrate is the primary form of inorganic nitrogen in shallow groundwater within the study area, with nitrate concentrations in cultivated groundwater generally higher than those in construction land and forest land. The transformation process of nitrate is predominantly nitrification, with little to no denitrification observed. In cultivated shallow groundwater, nitrate mainly originates from chemical fertilizers (36.3%), sewage and manure (35.4%), and soil organic nitrogen (24.7%); in forested areas, nitrate primarily comes from atmospheric precipitation (35.3%), chemical fertilizers (31.3%), and soil organic nitrogen (22.1%); while in constructed areas, nitrate mainly derives from chemical fertilizers (46.0%) and sewage and manure (32.2%). These results establish a scientific foundation for formulating groundwater pollution control and management strategies in the region and serve as a reference for identifying nitrate sources in groundwater in regions with comparable hydrogeological features and pollution profiles.

Evaluation of the source and mechanisms of groundwater recharge for the southern sections of the western Afar rift margin and associated rift floor

Study areaSouthern Sections of Western Afar rift margin and associated rift floor. Study focusThe purpose of this study is to identify the source and mechanisms of groundwater recharge based on geological, hydrochemical, and environmental isotope studies. New hydrogeological insightsThe investigation illustrate that the columnar jointed basalt was extended from the plateau to the rift margin. In addition, transverse structures trending NW-SE and NE-SW connect the plateau to the rift margin and the marginal grabens to the rift floor. The hydrochemical interpretations with EC and TDS shows that the groundwater from the plateau has evolved from Ca-dominated and slightly mineralized to Na-dominated and highly mineralized to the rift floor. Isotopically, the deep groundwater systems are depleted in the entire physiography in contrast to the shallow groundwater systems. Radon measurements are higher at the outlets of the marginal grabens and between the rift margin and the rift floor, suggesting groundwater feeds the river. A comprehensive analysis of the aforementioned results suggests that the deep circulating groundwater is recharged in the plateau and escarpment and chemically altered with increasing depth and along its preferential flow path into the rift floor. Therefore, the plateau area is the main source of recharge for the western Afar rift margin and associated rift floor, due to the presence of preferential pathways, mainly the columnar jointed basalts and cross-structures.

Groundwater renewal time by environmental tritium isotopes as a tracer for sustainable water resource management

Studying groundwater renewal time is a valuable tool to deepen the comprehension of sustainable groundwater resources specific to arid regions. Tritium is a radioactive isotope of hydrogen often used as an environmental tracer to study groundwater renewal time. The present work reports groundwater renewal times by the environmental tracer tritium to understand sustainable water resources in arid regions. First, groundwater samples were collected from wells in northeastern arid regions of Saudi Arabia. Then, an electrolysis process was employed to significantly increase the tritium level from twenty-five to thirty times the original concentration. Subsequently, the enriched water was analyzed using a liquid scintillation counter under optimized measurement conditions to determine the tritium concentration precisely. Two internationally recognized tritium laboratories conducted independent assessments to validate the estimated tritium levels. The verified tritium concentration was then used to estimate the groundwater renewal time using the Morgenstern and Pimenta curves. The results suggest that most of the monitored wells in the surveyed areas are more than a century old. Conversely, a few monitoring wells exhibit renewal times of several hundred years and may be considered nonrenewable water sources. These studies help to understand the geochemical characteristics of arid regions to ensure the sustainable management and protection of groundwater resources.

Identifying Groundwater Recharge Sources and Mechanisms using Hydrochemistry and Environmental Stable Isotopes in High Arsenic Holocene Aquifers of Bangladesh

Und erstanding the sources and mechanisms of aquifer recharge is critical for water resource management. So far, this is the first comprehensive study to explore the sources and mechanisms of groundwater recharge applying stable environmental isotopes and hydrogeochemical properties in Southwestern Bangladesh. Water samples from the different sources (groundwater, surface (pond), river and rain water) of high arsenic (As) Holocene aquifers were used to investigate the chemical tracers and environmental stable isotopes (δ18O and δ2H) to explore the hydrogeological features, groundwater recharge sources, mechanisms and processes. The analytical results showed high As concentration in water samples observed 590.7 μg/L. High As groundwater was identified by a low pH value, medium HCO3- and TDS with low concentration of NO3- and SO42-. However, depleted δ2H and δ18O signatures indicated that local vertical rainfall is the key source to recharge groundwater in study area. The recharge process from pond seepage and irrigation return flow also contribute in groundwater recharge. Isotopic signatures suggested the vertical connectivity amongst the aquifers and significant vertical recharge. The reduced heavier isotope reveals extra recharging from floods of mountains and high altitude. Due to the heterogeneous and anisotropic structure of the aquifer system the spatial variability of δ2H and δ18O shows limited lateral connectivity. The combined analyses of δ2H isotope and Cl- ion showed that evaporation is the primary cause for groundwater discharge. Based on the spatial and vertical recharge processes of groundwater, present study provides a crucial baseline knowledge for establishing a sustainable groundwater management strategy. The findings from this study can guide the development of targeted groundwater management strategies and improvement of understanding the groundwater recharge sources and processes.

Delineation of the Hydrogeological Functioning of a Karst Aquifer System Using a Combination of Environmental Isotopes and Artificial Tracers: The Case of the Sierra Seca Range (Andalucía, Spain)

The Sierra Seca aquifer system is located in the northeast (NE) of the province of Granada, in the Prebetic Domain (Betic Cordillera). It is composed of different aquifer units hosted in the Lower Cretaceous and Upper Cretaceous limestones. The two aquifers are separated by a low permeability marl layer, which effectively acts as a barrier between them. To outline the behavior of the hydrogeological system, 407 samples of precipitation and 67 samples of groundwater were obtained from May 2020 to Oct. 2022 and isotopically (δ18O and δ2H) analyzed. For the estimation of the recharge elevation, a new methodology has been applied to estimate the isotopic content of recharge as a function of precipitation. This allowed the evaluation of the vertical gradient of both precipitation (∇Zδ18OP=−2.9 ‰/km) and aquifer recharge (−4.4 ‰/km≤∇Zδ18OR≤−2.9 ‰/km). Therefore, estimating (1) the recharge zone elevation associated with the aquifer system, which is comprised between 1500 and 1700 m a.s.l., and (2) the transit time of recharge to reach the outflow point of the aquifer system, which varies between 4 and 5 months, is possible. Additionally, three tracer tests were conducted to outline the hydrologic connection between the recharge and discharge zones of the aquifer system. The results show that the Fuente Alta spring drains the limestones of the Lower Cretaceous, while La Natividad spring does the same with the limestones of the Upper Cretaceous. In the case of the Enmedio spring, groundwater discharge is related to infiltration through the streambed of the watercourse fed by the Fuente Alta spring.

Enrichment mechanism of groundwater fluoride and its hydrogeological indications in the Badain Jaran Desert, northwest China

Fluoride (F−) enrichment in groundwater poses significant risks to drinking water safety; however, reports of high F− concentrations in groundwater in a vast desert and its hydrogeological indications are limited. In this study, we collected 275 groundwater samples from various depths in the Badain Jaran Desert (BJD) to investigate the distribution characteristics and enrichment mechanisms of F− in desert groundwater, utilizing hydrochemistry, environmental isotopes, and statistical methods. The results indicate that (1) the groundwater F− concentrations in the mountainous areas, desert areas, and downstream wetland areas are 0.10–9.45 mg/L (average of 3.05), 0.19–58.00 mg/L (average of 4.32), and 0.62–9.91 mg/L (average of 1.99), respectively, with corresponding exceedance rates (>1 mg/L) of 83%, 71%, and 80%. (2) In mountainous areas, F− enrichment mechanisms are attributed to ion exchange, evaporative concentration, and the dissolution of fluoride minerals. In desert areas, F− enrichment mechanisms involves multiple hydrogeochemical processes, with the dissolution of fluoride minerals being the most significant, followed by evaporative concentration and desorption. And evaporative concentration is particularly pronounced in shallow groundwater (depth <4 m). In wetland areas, F− enrichment mechanisms are mainly ion exchange and desorption. (3) The F− concentration characteristics (<2 mg/L) and the significant correlation between F− and HCO3− (R = 0.55, P < 0.01) in the deep groundwater of the desert suggest that groundwater does not originate from deep geothermal sources. (4) Based on the significant differences in groundwater F− concentrations, there is a weak hydrodynamic connection between the groundwater in the Quaternary aquifer and that in the underlying Cretaceous bedrock in desert areas. These findings not only facilitate the safe use of groundwater in desert environments but also provide new insights into the formation, circulation, and evolution of desert groundwater.

Conceptual modelling on water-rock reaction and genesis of high pH fluids in a typical granitoid geothermal reservoir: A case from Indus-Tsangpo Suture Zone, India

Two novel thermal springs are investigated along ITSZ of north-west Himalayas in Demchok geothermal belt. The area consists of deep-seated faults in LGB; despite being situated in ITSZ, fluid chemistry of these low-temperature springs differs significantly, specifically in terms of low TDS (161–168 mg/l) and high alkalinity, with neighbouring springs in Puga and Chumathang (∼2100 mg/l). Thermal waters are mixed type (Na–Cl–HCO3–SO4) with elemental composition influenced through silicate rock weathering and partial carbonate dissolution. Variable temperature speciation (25 °C–200 °C) indicates that in reservoir fluid, Na+ precedes over other cations, while sulfate and carbonate complexes being prominent for Mg and Ca, respectively. Aquifer boiling modelling suggests calcite scaling and silica mineral equilibration in reservoir. Environmental stable isotopes (δ18O and δD) suggest high-altitude recharge from Jamlung La (6156 m), with altitude-effect of 0.43 ‰ isotopic depletion per 100 m elevation in altitude. As only silica minerals equilibrate with thermal waters, silica, and gas geothermometers give most conservative estimate of reservoir temperature (125°−135 °C). The steep topography enables extensive lateral flow of hot fluids in outflow zone, leading to high mixing and a high water-rock ratio, which contribute to lower TDS. Conceptual modelling reveals that geothermal system is a low-enthalpic hydrothermal system controlled by joints and permeable fractures, having deep fluid circulation of meteoric waters of ∼1275 m at sub-surface, with anomalous geothermal gradient and steam migration as reservoir heat sources. These fluids closely resemble springs of Guerrero state, Mexico, exhibiting similarities in low-TDS, high-pH, and high concentration of dissolved silica.

Environmental isotopes (δ18O, δ2H) and the spatio-temporal variation of Fe, Mn and P in surface and groundwaters in a coal extraction area and thermoelectric power plant, Candiota, South Brazil

Environmental isotopes (δ18O, δ2H) and the spatio-temporal variation of Fe, Mn and P in surface and groundwaters in a coal extraction area and thermoelectric power plant, Candiota, South Brazil

Tracing contaminants of emerging concern in the Awash River basin, Ethiopia

Study regionAwash River basin, Central Ethiopia Study focusThe study focuses on characterization of Emerging Organic Contaminants (EOCs) in the Awash River basin. Characterization of the EOCs was supplemented by chemical analysis of samples from river, boreholes, tap water, and surface water reservoirs. Analyses of environmental isotopes (δ 2 H, δ 18 O, and 222Rn) were used to investigate the exchange of contaminants between surface and groundwater supply sources. New hydrological insights for the regionMore than 100 EOCs are identified in all water supply sources. The EOCs are linked to agricultural applications, urban, and industrial sources. Based on the analysis of chemical and environmental tracers, the deep groundwater has greater protection from contamination than the river and the shallow groundwater. The heavy metal analysis prevails the same. The shallow aquifers are affected by urban, industrial, and agricultural pollutants. Attributed to the different contaminant sources, distinct variations in terms of compound types were observed at different locations. Water supply sources located upstream are dominated by urban and industrial contaminants while compounds from agricultural applications dominate the downstream sites. Artificial infrastructures serve as attenuation points for urban and industrial sourced compounds. Characterization of EOCs showed new contaminant loads in the water supply sources, which haven’t been tested before, with potential impact on human and wider environmental health, and may necessitate a revision of the customary water quality test and monitoring practices.

Experimental assessment of elemental analyzer isotope ratio mass spectrometry normalization methodologies for environmental stable isotopes.

In stable isotope mass spectrometry, isotope delta values are normalized to internationally recognized reference scales using a combination of certified and in-house isotope reference materials. Numerous techniques exist for performing this normalization, but these methodologies need to be experimentally assessed to compare their effect on reproducibility of isotope results. We tested normalization methods by the number of reference materials used, their matrix, their isotope range, and whether normalization required extrapolating beyond the isotope range. We analyzed eight commercially available isotope reference materials on a ThermoFinnigan Delta-V isotope ratio mass spectrometer (IRMS) and an Elementar VisION IRMS for nitrogen and carbon isotope composition via solid combustion with an elemental analyzer and computed every possible isotope normalization (n = 612). Additionally, we assessed how sample matrix affected linearity effects on both instruments using five in-house reference materials. Normalizations exhibited the best performance when the reference materials spanning an isotope range of at least 20‰ were matrix matched with the samples and did not require extrapolation beyond the calibration curve. When these conditions were not met, the number of reference materials used had a significant effect on accuracy, with normalizations composed of two reference materials exhibiting particularly inconsistent performance at isotope ranges below 20‰. Linearity effects were found to exceed instrument precision by two orders of magnitude irrespective of matrix type and were not predicted by working gas diagnostics. Interlaboratory comparability of isotope results is improved when operators of elemental analyzer isotope ratio mass spectrometry (EAIRMS) systems select reference materials spanning an isotope range of at least 20‰. Additionally, using three or more isotopic reference materials, avoiding extrapolation beyond the range of the normalization curve, and matching the matrix of the reference materials to the samples improve normalizations.

A comprehensive monitoring approach for a naturally anoxic aquifer beneath a controlled landfill

The processes leading to high levels of arsenic (As), iron (Fe), and manganese (Mn) in groundwater, in a naturally reducing aquifer at a controlled municipal landfill site, are investigated. The challenge is to distinguish the natural water-rock interaction processes, that allow these substances to dissolve in groundwater, from direct pollution or enhanced dissolution of hydroxides as undesired consequences of the anthropic activities above. Ordinary groundwater monitoring of physical-chemical parameters and inorganic compounds (major and trace elements) was complemented by environmental isotopes of groundwater (tritium, deuterium, oxygen-18 and carbon-13) and dissolved gases (carbon-13 of methane and carbon dioxide and carbon-14 of methane). Pearson/Spearman correlation indices, as well as Principal Component Analysis (PCA), were used to determine the main correlations among variables. The concurrent presence of As, Fe and CH4, as reported in similar anoxic environments, suggests that anaerobic oxidation of methane could drive the reductive dissolution of As-rich Fe(III)(hydro)oxides. Manganese is more sensitive to carbon dioxide, possibly due to a decrease in pH which accelerates the dissolution of Mn-oxides. Finally, we found that tritium and deuterium, which have been used for decades as leachate tracer in groundwater, may be subject to false positives due to the reuse of water recovered from leachate treatment (which has the same isotopic signature of leachate) within the plants, to comply with the requirements of the circular economy. The integration of the environmental isotope analysis into the traditional monitoring approach can effectively support the comprehension of processes. However, this strategy needs to be complemented by a good conceptual hydrogeological model and expert evaluation to avoid misinterpretations.

Hydrogeomorphology and recharge mechanism of a coastal aquifer in tectonic-controlled watersheds: A multi-proxy approach based on remote sensing and environmental isotopes

Coastal aquifers worldwide can be considered an essential water source required for preservation of the coastal ecosystems. However, these aquifers are vulnerable to seawater intrusions and over-pumping due to their proximity to the sea and human activities, respectively. Therefore, the investigation of recharge mechanism has special importance in the regions of tectonic rift activities where they exhibit complex geological structures and hydrogeological characteristics. In the current research, an integrated approach of remote sensing and environmental isotopes together with field investigation and subsurface datasets (aeromagnetic, gravity and well logs) are employed to identify the nature and factors affecting groundwater recharge of the Miocene coastal aquifer along the Red Sea-Gulf of Suez western margin (continental rift basin). The research findings reveal that: (1) The coastal Miocene aquifer consists of both clastic (sandstone, sand and gravels) and carbonate rocks (limestone and dolomite) with subsurface thickness ranges between 100 and 200 m. (2) Its groundwater shows contrast salinity values (expressed by total dissolved solids, TDS) between 2755 and 10,996 mg/l, due to the variation in recharge rates and the lithologic dissimilarity of the water bearing formations. (3) The environmental isotopes indicate that the Miocene groundwater has a mixed isotopic signature between modern meteoric (rainfall) and fossil waters. (4) The isotopic data of the Miocene aquifer and the enriched Gulf of Suez verifies that no seawater intrusions are affecting this aquifer with existing hydraulic barriers (clogged/sealed faults or impermeable massive blocks). (5) The hydrogeomorphological investigations, aeromagnetic and gravity data reveal an existence of two morphotectonic depressions (water collectors) which have paleo and recent recharge opportunities for the subsurface sedimentary layers (2–4 km thick). From the applied viewpoint, these two depressions have potential prospects for future groundwater exploration, which has significant impact on food security and land reclamation.

Groundwater recharge estimation in Mediterranean mountain environments by isotope profiles – Partitioning of macropore and matrix flow

Developing and continuously improving effective and feasible methods to estimate groundwater recharge is essential to ensure sustainable resource assessment. Mountain environments represent key areas for the regional water balance. In this study, the isotope profile method was used to investigate water fluxes through the vadose zone and subsequent groundwater recharge in a Mediterranean mountain environment. Six high-resolution soil water isotope profiles were taken at two different locations in the Troodos Massif (Cyprus) in the eastern Mediterranean, focusing on estimating recharge in the soil of hillslopes. Analysis of the isotope profiles yielded recharge rates ranging from 30–70 mm/year, leading to a re-evaluation of previous studies on groundwater recharge. The comparison of results obtained by the peak-shift method and mathematical modeling was used to partition matrix and macropore flow, reaching up to 39 %. Potential methodological limits of the environmental isotope profile method in skeleton-rich soils were found.

Use of environmental isotopes of water and geochemistry to determine nitrate sources in south of Cuernavaca aquifer

El acuífero Cuernavaca es la principal fuente de agua dulce permanente que sustenta el desarrollo económico y el suministro a la población de la capital del estado de Morelos. En los últimos 30 años, los nitratos en el sur del acuífero Cuernavaca se han incrementado, degradando la calidad del agua subterránea. La identificación del origen del nitrato es fundamental para implementar políticas públicas que controlen y reviertan dicha contaminación. Con base en las características propias de la región, como son tipo de suelo y geología, incremento acelerado de la población, y actividades industriales, agrícolas y recreativas, entre ellas la práctica del golf, se plantean cuatro posibles fuentes de nitrato en la zona de estudio: 1) nitrato del suelo, 2) infiltración de aguas residuales, 3) fertilizantes provenientes de zonas agrícolas, y 4) fertilizantes provenientes de campos de golf. En la porción sur del acuífero Cuernavaca, los datos geoquímicos y los isótopos ambientales del agua (δ18O, δ2H, δ3H) indican que la contaminación por nitrato proviene principalmente de aguas residuales. Asimismo, indican que el flujo profundo aporta arsénico a algunos aprovechamientos subterráneos. Los resultados también muestran correlación entre las concentraciones del nitrato y del uranio en el acuífero. El registro periódico de los datos isotópicos permitió también determinar la línea de agua meteórica de la zona del acuífero Cuernavaca.

Combining groundwater budget, hydrochemistry and environmental isotopes to identify the groundwater flow in carbonate aquifers located in Campania Region (Southern Italy)

Study regionCarbonate mountains of Mt. Maggiore and Mt. Tifata, Campania Region, southern-central Italy, Mediterranean basin. Study focusThe hydrogeological relationship between the carbonate massifs of Mt. Maggiore and Mt. Tifata is investigated. Archival and newly acquired data on groundwater availability, hydrochemical and isotopic data were considered. Their combined use led to the proposal of new hypotheses regarding the connection between these aquifers. The exchange of groundwater through this connection would be induced by the strong groundwater withdrawals from the well fields at Mt. Tifata; the area of possible connection was also identified. A mineralization model of some local springs showing high CO2 and TDS values is also proposed. New hydrological insights for the regionThe carbonate rocks are widely outcropping with a mountainous morphology and host important groundwater resources in the studied region. The springs related to these carbonate aquifers have excellent chemical characteristics and, for these reasons, the major aqueducts in Campania Region rely on these groundwater resources. The well fields of the Mt. Maggiore and Mt. Tifata supply part of the metropolitan area of Naples, with 3.8 million inhabitants. The quantitative evaluation of groundwater resources and the proposed groundwater circulation scheme can support a sustainable and diversified use of the resource taking into account the presence of waters already used for drinking purposes and waters with high TDS values.

Understanding arsenic behavior in alluvial aquifers: Evidence from sediment geochemistry, solute chemistry and environmental isotopes

The hydro-geochemistry and isotopic variations in groundwater, coupled with sediment geochemistry, were investigated in the Middle Gangetic Plain, India, to better understand the aquifer dynamics that influence the arsenic (As) evolution and mobilization. Eighty-four groundwater samples, thirteen River water samples, and two sediment cores (33 mbgl) were studied. The samples were analyzed for major ions and trace metals, including As and stable isotopic variability (δ2H, δ18O, and δ13C). The study area was categorized into older and younger alluvium based on existing geomorphological differences. Younger alluvium exhibits higher As enrichment in sediment and groundwater, ranging of 2.59–31.52 mg/kg and bdl to 0.62 mg/L. Groundwater samples were thermodynamically more stable with As(OH)3 species ranging from 88.5% to 91.4% and FeOOH from 69% to 81%, respectively. PHREEQC and mineralogical analysis suggested goethite and siderite act as a source and sink for As. However, statistical analysis suggested reductive dissolution as the primary mechanism for As mobilization in the study area. Spatio-temporal analysis revealed elevated concentrations of As in the central and northeastern regions of the study area. Stable isotope (δ2H and δ18O) analysis inferred active recharge conditions primarily driven by precipitation. The depleted d-excess value and enriched δ18O in the groundwater of younger alluvium indicate the effect of groundwater recharge with significant evaporation enrichment. Groundwater recharge potentially decreased the quantity of arsenic in groundwater, whereas evaporation enrichment increased it. Rainwater infiltration during recharge introduces oxygenated water into the aquifer, leading to changes in the redox conditions and facilitating biogeochemical reactions. The carbon isotope (δ13C) results suggest that high microbial activity in younger alluvium promotes As leaching from sediment into the groundwater.

The Use of Environmental Isotopes in Hydrogeology

As a consequence of the population increase, there is a growing need for groundwater use worldwide, leading to the intensification of agricultural practices alongside the growth of industrial development [...]

Responses of underground air and drip water geochemistry to meteorological factors: A multi-parameter approach in the Rull Cave (Spain)

Our research aims to assess the complex interactions between the elements that constitute and influence a cave system through the analysis of an extensive dataset of climatic and environmental parameters (222Rn, CO2, drip rates, chemical composition, and environmental isotopes) measured in air, water, and solid in the Rull Cave (southeastern Spain). Of particular importance is understanding the effect of rainfall and temperature on water and gas transport through the epikarst and the involved processes. Our results show that the cave gaseous concentration patterns do not only depend on the temperature-caused movement of air masses, but they can also be affected by abundant rainfall. The δ18O and δD composition of cave water also relies on such precipitations for the effective transfer of the rainfall signal into the cave, which can take between 3 and 7 days. The elemental ratios (Sr/Ca and Mg/Ca) show high responsiveness to the water drip rate, hinting that enhanced prior calcite precipitation (PCP) occurs at slower drip rates. Despite this, and regardless of drip rates, calcite saturation indices follow a seasonal variation pattern inversely proportional to the cave air CO2 concentration, while δ13C-DIC is proportional. Our results show how the interlinkage between these multiple components defines the dynamics of the atmosphere-soil-cave system. Cave monitoring is then essential to understand the karstic vadose zone, which is highly sensitive to climatic influence and its changes.

Hydrogeochemical and environmental isotope study of Topusko thermal waters, Croatia

Thermal waters in Topusko (Croatia), with temperatures of up to 65 °C, have been used for heating, health, and recreational tourism for the past fifty years. Hydrogeochemical monitoring can provide insights into deeper geological processes and indicate system changes from baseline levels. It helps to identify potential anthropogenic impacts, as well as natural changes. Hydrogeochemical, geothermometrical, and environmental isotope studies of thermal waters in Topusko were conducted to improve the existing conceptual model of the Topusko hydrothermal system (THS), providing a baseline for continuous monitoring of the thermal resource. 2-year thermal springs and precipitation monitoring took place from March 2021 until March 2023. Major anions and cations, stable and radioactive isotopes (i.e. 18O, 2H, SO42−, 3H and 14C) and geothermometers were used to assess the origin of thermal waters in Topusko and their interaction with thermal aquifer. The results indicate the meteoric origin of thermal water, which was recharged in colder climatic conditions around the late Pleistocene–Early Holocene. Thermal water was last in contact with the atmosphere before approximately 9.5 kyr. Ca-HCO3 hydrochemical facies suggests carbonate dissolution as the dominant process driving the solute content. Geothermometrical results indicate an equilibrium temperature in the reservoir of 90 °C.