Combining spectral analysis and geochemical tracers to investigate surface water-groundwater interactions: A case study in an intensive agricultural setting (southern Guatemala)

An increase in the frequency of severe hydrological events has highlighted the importance of sustainable water management in intensive agricultural regions. In a warming climate, improved understanding and stewardship of water resources are needed to guarantee water supply, ensure food security, and build resilience against extreme events. In this study, we evaluate a framework that combines spectral analysis and geochemical tracers as a potential tool for (1) gaining valuable insights into surface water (SW)-groundwater (GW) interactions, and (2) providing guidance for improved water management in an intensive agricultural basin in southern Guatemala. The framework proves to be useful in revealing important water dynamics, exposing key feedback mechanisms for water availability and quality. With the use of power density functions and hydrochemistry (T, pH, EC, and major ions), two specific interaction regimes (influent and effluent) were identified and delimited for the main watercourse. These segments are estimated to interact at high rates with the shallow aquifer in the river channel proximities and would lose influence towards the basin flanks. Furthermore, the δ2H and δ18O values indicate that regional groundwater flow systems play an essential role in the basin groundwater recharge. Lastly, we established three influence zones that depict the spatial extent of the SW-GW interactions within the basin. With these zones, we provide recommendations that will allow for further investigation and application into better water management strategies regulating groundwater development and land use activities within the agricultural context of the area.

Hydrogeochemical and isotopic evolution of groundwater in shallow and deep aquifers of the Kabul Plain, Afghanistan

Groundwater from shallow and deep aquifers are widely used for drinking, agricultural and industrial use in Kabul, the capital of Afghanistan. However, unplanned urbanization and rapid population growth has led to the installation of numerous unlicensed wells to meet the public demand. This has caused to extraction of huge amounts of groundwater from the subsurface and further deterioration of groundwater quality. Therefore, understanding the hydrogeochemical characteristics of groundwater in shallow aquifers and deep aquifers is imperative for sustainable management of the groundwater resource in Kabul Plain. Thus, in this study, we used a multi-parameter approach, involving hydrochemical and environmental isotopes to understand the geochemical evolution of entire groundwater system of the Kabul Plain including river and dam water. The results of this study show that shallow and deep aquifers are dominantly of Mg-(Ca)-HCO3 and Na-Cl water type, respectively. We observed that (1) water-rock interaction is the major contributing factor to the chemical compositions of groundwater in the Kabul Plain; (2) groundwater in deep aquifer is mainly influenced by silicate weathering, and dissolution of evaporitic and carbonate minerals and reverse cation exchange; (3) dissolution of carbonates and silicate weathering plays a pivotal role in the groundwater chemistry of shallow aquifer; (4) the stable isotopes of groundwater display that the shallow aquifer is principally recharged by river water and local precipitation; (5) the tritium analysis exhibited that groundwater of shallow aquifer was primarily recharged recently, whereas groundwater of deep aquifer is the mixture of pre 1953 with post 1953 groundwater. This study revealed that there are hydraulic interactions between the two aquifers and the deep aquifer is recharged through shallow aquifer. The findings of this study would be useful for Afghanistan's water authorities to develop an effective strategy for sustainable water resources management in the Kabul Basin.

Isotope hydrology tools in the assessment of arsenic contamination in groundwater: An overview

Groundwater is important for the survival of humanity and the demand for the same is drastically increasing globally. The precious water resources are under constant threat, either as a result of natural processes or due to the influence of the anthropogenic activities. Arsenic contamination of groundwater is one of those threats that have affected approximately over 500 million people in 107 countries globally. Although, many studies (∼1000 Nos.) have been carried out on arsenic hydrogeobiochemistry, only a few have reported, on the use of different isotopes in understanding the arsenic hydrochemistry, and its release mechanism and mobilization. Determination of the isotopic composition of a groundwater sample and its dissolved compounds enable a better insight into the hydrological processes that control the distribution and migration of arsenic in the subsurface hydrological system. The environmental isotopes of water molecules (δ18O and δ2H) have been widely used to assess the groundwater origin, its recharge mechanisms, the rock-water interactions and quality. The stable isotopes of dissolved compounds of water (δ34S, δ15N, δ13C, δ56Fe etc.) give better information on the reaction processes within these elements and thus act as a tracer for contaminants, while the radioactive isotopes, such as 14C, 3H, 81Kr, 36Cl, 39Ar etc., can be used to assess the residence time of groundwater and its renewability. This article reviews the different uses of environmental isotopes as tools for providing critical information on various hydrological processes in the arsenic contaminated regions that can't be obtained through conventional tools for better management of the groundwater resources.

Dataset for analyzing and modelling the eutrophication processes in groundwater-coastal lagoon systems: The La Pletera lagoons case study (NE Spain)

A comprehensive hydrogeological report was conducted to determine the origin, occurrence and processes affecting nitrogen in a Mediterranean coastal aquifer-lagoon system. Water levels, hydrochemical and isotopic data was gathered during a 4-year period in the La Pletera salt marsh area (NE Spain). They were collected from the alluvial aquifer, two natural lagoons and four other permanent lagoons excavated during a restoration process (in 2002 and 2016), two watercourses (the Ter River and the Ter Vell artificial channel), 21 wells (considering six of them for groundwater sampling) and the Mediterranean Sea. Potentiometric surveys were carried out seasonally, however twelve-monthly campaigns (from November 2014 to October 2015), and nine seasonal campaigns (from January 2016 to January 2018) were conducted for hydrochemical and environmental isotopes analyses. The evolution of the water table was analysed for each well, and potentiometric maps were plotted to determine the relationship between the aquifer and the lagoons, sea, watercourses, and groundwater flow. Hydrochemical data included physicochemical data measured in situ (temperature, pH, Eh, dissolved oxygen, and electrical conductivity), major and minor ions (HCO₃^-, CO₃^2-, Cl^-, SO₄^2-, F^-, Br^-, Ca²⁺, Mg²⁺, Na⁺, and K⁺), and nutrients (NO₂^-, NO₃^-, NH₄⁺, Total Nitrogen (TN), PO₄^3-, and Total Phosphorus (TP)). Environmental isotopes included stable water isotopes (δ¹⁸O and δD), nitrate (δ¹⁵N_NO3 and δ¹⁸O_NO3) and sulphate isotopes (δ³⁴S_SO4 and δ¹⁸O_SO4). Water isotopes were analysed for all campaigns, however, nitrate and sulphate isotopes water samples were only analysed in some particular surveys (November and December 2014; January, April, June, July and August 2015). Additionally, two more surveys for sulphate isotopes were conducted in April and October of 2016. The data generated through this research may be used as a starting point to analyse the evolution of these recently restored lagoons, and their future responses to global change. In addition, this dataset may be used to model the hydrological and hydrochemical behaviour of the aquifer.

Nitrogen in surface aquifer - Coastal lagoons systems: Analyzing the origin of eutrophication processes

Coastal lagoons can act as sinks and sources of a wide range of substances, including nutrients, and pollutants. In these ecosystems, primary production is limited more by nitrogen than by phosphorus. For this reason, they are significantly vulnerable to nitrate pollution. In this study, a joined analysis of surface and ground water was conducted to determine the origin, occurrence and processes affecting nitrogen fate in a Mediterranean coastal aquifer-lagoon system. This included the analysis of water levels, as well as hydrochemical and isotopes data evolution for a 4-year period, which revealed two important insights of nitrogen cycling within the system. Firstly, we detected different origins for nitrate pollution (a mixture of sewage, manure and chemical fertilizers), as well as their nearly complete attenuation in the alluvial aquifer due to heterotrophic and autotrophic processes, favoured by the presence of organic matter and Fe-minerals in its sediments. Secondly, due to its rapid assimilation, inorganic nitrogen peaks in the lagoons were mainly detected after storm events. While nitrate peaks may be attributed to surface water runoff, ammonium peaks may be related to organic nutrient cycling. In contrast, we did not detect continuous and low N inputs, associated to groundwater flow. These results depict the need of a more integrated management strategy of these aquifer-lagoon systems.

Conceptual model of a semi-arid coastal aquifer using hydrogeochemical seasonal variation and isotopic fingerprints in Tamoios, Rio de Janeiro, Brazil

The present study focuses on the Tamoios aquifer (Rio de Janeiro State, Brazil), which is under pressure due to receiving a significant volume of urban runoff and sewage. The objective was based on a number of hydrogeochemical and isotope data to assess the aquifer functioning and establishing a conceptual model to evaluate the hydrogeochemical processes. The database consisted of groundwater samples (n = 20) and surface water samples (fluvial, lagoon, and seawater) (n = 4), analyzed for major and trace constituents plus ¹⁸O and ²H isotopes. Results demonstrate that most of the groundwater samples were classified as sodium-chloride type in the rainy season and magnesium-chloride type in the dry season. Ion ratios indicated the ion sources and chemical behavior. Groundwater remained with a relatively high salt content throughout the seasons, particularly in the samples from the southern portion of the aquifer. PHREEQC software simulations exposed dolomite and calcite in mostly undersaturated condition and halite subsaturated throughout the year. Hydrogeochemical behavior indicated the salt content in the groundwater was not related to a hypothetical saltwater intrusion and revealed a steady state condition for the groundwater interface. Groundwater samples have a similar isotopic signature and were likely influenced by evaporative effects, indicating a role for the existing ponds in aquifer recharge. Strong free surface evaporation effects, evapotranspiration, and drainage processes in the floodplains probably enhanced the high ionic concentration in the groundwater environment.

Identifying the characteristics and potential risk of seawater intrusion for southern China by the SBM-DEA model

Seawater intrusion (SWI) seriously affects the economic development of coastal areas in southern China, and understanding its mechanisms is the basis for effective control of SWI. Hydrogeochemical methods and slack-based measurement data envelopment analysis (SBM-DEA) are used to study the characteristics and potential risk of SWI in coastal cities of southern China. Types and distribution of SWI, coastal groundwater evolution, geological-geographic and economic threatens of SWI, potential SWI risk, and environmental management recommendations are explored. The results show that the intrusion areas of Zhejiang and Guangdong account for 94.1 % of the total intrusion area of southern China, and the intrusion degree in Zhejiang is the highest, followed by Guangdong and Fujian. SWI is prone to occur on the sandy and silty coasts of the plain area of southern China; it accelerates the groundwater evolution speed and shortens the evolution path. SBM-DEA can be well applied to evaluate the potential risk of SWI events, and the results indicate a noticeable difference in the environmental performance level of coastal cities in southern China. The low environmental performance level (<0.3) and severe SWI of Taizhou and Zhanjiang indicate that SWI gradually worsens with economic development. In contrast, the high environmental performance level (>0.7) and low SWI of Wenzhou, Fuzhou, Quanzhou, Shantou, and Beihai indicate that the potential risk of SWI is gradually decreasing. Moreover, this study confirms that the environmental Kuznets curve (EKC) phenomenon exists in SWI events for southern China, and SWI-EKC indicates that the urban development of south China is approaching maturity. The specific case of SWI and EPL in coastal cities of south China jointly indicates that optimizing industrial structure, implementing a resources management policy, and improving citizens' environmental awareness are fundamental measures to resolve the contradiction between economic development and environmental problems.

Surface water and groundwater interaction in the Vredefort Dome, South Africa: a stable isotope and multivariate statistical approach

The growing importance of groundwater as a freshwater supply in semi-arid areas such as the Vredefort Dome World Heritage Site (VDWHS) demands the judicious management and development of this vital resource. The increased demand for groundwater due to the contamination of surface water, coupled with the lack of information on hydrological interaction and associated water quality implications, present difficulties in establishing management strategies. An integrated study based on hydrochemistry and multivariate statistical techniques supplemented by environmental isotopes delineated discrete areas of surface water and groundwater interaction in a fractured-rock terrain. Surface water loss was observed in sections that exhibited declining groundwater levels, whereas limited baseflow was restricted to zones with stable groundwater levels. The multivariate statistical analysis revealed the combined effect of natural hydrochemical processes and anthropogenic sources as controlling factors of water composition, and highlighted zones of aquifer-river water mixing, where certain areas were found to be additionally polluted by human-derived contaminants. The stable isotope (¹⁸O and ²H) ratios confirm mixing between depleted groundwater and enriched river water, producing a composition that reflected an integration of the isotopic variations. The continuous wastewater discharge into the Vaal River combined with the increased groundwater exploitation may be prompting induced recharge conditions. The results suggest compartmentalization of the groundwater systems, where certain areas within 1 km of the channel were not influenced by river-induced contamination. This indicates that hydrological connectivity is governed by site-specific hydraulic properties. This study shows the usefulness of a multi-method approach by combining environmental isotopes, hydrochemistry, and multivariate statistics to characterize hydrological linkage in semi-arid regions.

Decoupling anthropogenic vs. natural impacts at a wastewater treatment plant situated on acid sulfate soils

Decoupling natural and anthropogenic impacts on the subsurface environment can be difficult, particularly when it has been subject to a wide range of influences over time and space. In this work we show how the use of hydrogeochemical plotting tools, time-series analysis of key contaminants of concern, and targeted isotopic analysis can be used to better understand the contamination sources/processes in a complex environment - a Wastewater Treatment Plant (WWTP) located on coastal acid sulfate soils (ASS). Analysis of soil profiles for potential oxidisable sulfur, acid neutralising capacity (ANC), and pH_fox along with groundwater chemistry, revealed that oxidation of pyritic sediments, initially deposited during the mid-Holocene, have led to significant pH declines and the secondary mobilisation of metals into the groundwater environment. This is further complicated by historic anthropogenic inputs associated with the WWTP (e.g., effluent leakages) and the surrounding agricultural land uses. There is distinct separation between spatial and temporal trends in the nutrient and heavy metals data in groundwater, suggesting these reflect different contaminant sources and/or processes. Isotopic data indicate nutrients are largely derived from the WWTP, whereas time-series analysis of key contaminants of concern and hydrogeochemical plotting tools indicate metals are largely derived from the secondary mobilisation of ASS due to acidity generated during sulfide oxidation. This work highlights the importance of understanding the hydrogeological environment and need for careful planning and ongoing management of WWTP sites, particularly those constructed on potential acid sulfate soils (PASS), which, if disturbed or exposed, can lead to impacts beyond the area of ASS via groundwater discharge to nearby surface water bodies (in this case the site is adjacent to a Ramsar-listed wetland). The outcomes of this work have significant global application in the identification, assessment, and control of ASS, the practice of contaminant source attribution, and the siting and design of future WWTPs, which will continue to be sited in coastal areas to meet population needs.

Groundwater geochemistry evolution and geogenic contaminants in the Sulaimani-Warmawa Sub-basin, Sulaimani, Kurdistan Region, Iraq

Evolution of groundwater geochemistry in the Sulaimani-Warmawa Sub-basin in the Kurdistan Region of Iraq has been investigated using hydrogeochemical and isotopic methods. This is a semiarid region with seasonal precipitation in winter. Water chemistry generally evolves from Ca-HCO₃ groundwater type close to the basin boundaries towards Ca-Mg-HCO₃ groundwater type close to the Tanjero River along the axis of the basin. Some samples have increased concentrations of Na, Cl, and SO₄ as a consequence of dissolution of halite and gypsum embedded in carbonates. Values of pH are slightly alkaline or alkaline, and redox parameters indicate a moderately reducing environment. Isotopes δ²H and δ¹⁸O indicate recharge from winter precipitation with no evaporation. Values of dissolved ¹³C(DIC) correspond to equilibrium with carbonates and C4 plants as the source of CO₂. Values of ⁸⁷Sr/⁸⁶Sr in groundwater are in a good agreement with carbonate dissolution as a principal process. The principal geogenic contaminant is Ba with concentrations up to 0.383 mg/L. Dissolved concentrations of other geogenic contaminants such as As, F, Mn, and Cr are low or below the detection limit as expected based on their low contents in carbonate rocks. Inverse geochemical modeling on selected profiles calibrated using δ¹³C values provided mass transfer coefficients for possible geochemical reactions. Future work should focus on interactions in the hyporheic zone of the Tanjero River.

Unravelling 30 ka recharge history of an intensely exploited multi-tier aquifer system in North West India through isotopic tracers - Implications on deep groundwater sustainability

Northwest part of India is an agriculturally active region experiencing rapid rise in food production and steep decline in groundwater levels. The freshwater requirement is mostly met by regional aquifers which are inherently heterogeneous and undergoing extensive human inducted perturbations. These factors pose great challenge in planning sustainable groundwater management. In this study, environmental isotopes (²H, ¹⁸O, ¹³C, ³H and ¹⁴C) were applied to understand the regional recharge mechanism during the last 30 ka and hydrogeological controls impacting the aquifer dynamics and inter-aquifer connectivity of the Ghaggar River basin. Rayleigh distillation modeling indicates that major groundwater recharge is through monsoonal rains while rainfall during other seasons is lost either through evaporation or surface runoff. The evaporation loss is estimated to be 1.5 to 10% and more pronounced in the southern part of the study area. Regional recharge from Siwalik foothills contributes to groundwater up to a depth of 250 m below ground level (bgl). The lumped parameter modeling (LPM) using ³H data estimated groundwater ages 34.7 ± 12.1 and 95.8 ± 11.3 years for shallow and deep aquifers respectively. Radiocarbon dating indicates presence of paleogroundwater (0.4 to 28.6 ka before present, BP) in the deeper aquifer of central part of the study area. Interpretation of the paleowater and paleoprecipitation isotope data in conjunction with available paleogeomorphologic information suggests two different recharge phases. Phase I extending from ~28.6 to 10.1 ka, showed ~48-61% contribution from isotopically depleted perennial river system. Phase II spanning from ~12.5 to 0.4 ka BP showed insignificant contribution from river recharge, which can be attributed to the decreased strength of the perennial river flows. The research methodology proposed in this study will be beneficial in improving the understanding of groundwater storage and its variability with changes in regional climatic conditions.

Processes explaining the origin and evolution of groundwater composition in the Andean Precordillera and Altiplano of the Tarapacá Region of northern Chile

In the arid area of northern Chile, groundwater resources in the Andean formations are essential for native populations, ecological services, mining, and other human activities. Validated conceptual hydrogeological models are required for current and future water and land management. This work aims to explain the processes controlling the origin and distribution of recharge and groundwater composition in the Andean Precordillera and Altiplano of the Tarapacá Region of northern Chile, using major solutes in spring, river, and well water, and the stable and radioactive isotopes of water oxygen, hydrogen, and dissolved inorganic carbon. The waters are mainly of the Na-Ca-SO₄ type. Processes controlling the chemical evolution of waters are atmospheric dust contribution, evapo-concentration, and enhanced volcanic rock weathering, as well as halite dissolution in some locations. The isotopic composition of Precordillera eastern flank water samples follows an evaporation line, while those in the western flank, in the Altiplano, follow a line that is parallel to the local meteoric line, suggesting unsaturated zone evaporation processes of infiltrated rainfall. δ¹³C_DIC contents (-2 to -27‰) indicate mixing processes, volcanic CO₂ in the Altiplano, and calcite dissolution in some sectors. In the western depression, the only recharge is due to water infiltration in creek channels. In the highland areas, 5-25% of precipitation produces recharge. The estimated groundwater renewal time in the Precordillera was 3-14 kyr. The piezometric elevation in the Precordillera due to low-permeability intrusive rocks and local recharge prevents the east-west groundwater transfer from the Altiplano to the western depression and explains why the volcanic CO₂ in the Altiplano basins is not observed on the western flank. These results provide new insights for the evolution of water quality in volcanic aquifers in arid environments and provide considerations for estimating groundwater residence times using radiocarbon in areas influenced by volcanic CO₂.

Appraising the factors favouring uranium mobilization and associated health risk assessment in groundwaters of north-western India

An attempt has been made in this study to evaluate the factors favoring the uranium mobilization into the groundwater of Northwest India using uranium isotope activity ratio (²³⁴U/²³⁸U), radon (²²²Rn) and environmental isotopes of water (²H, ¹⁸O and ³H). The values range from 23 - 597 µg/L for total uranium and 634-3210 Bq/m³ for radon and the corresponding annual effective dose is estimated to be 18.9-490 µSv/a and 6.2-31.5 μSv/a respectively. Uranium activity ratio (UAR) varies from 0.68 - 1.17 and maximum samples indicate secular equilibrium. Environmental isotopic data indicates that the source to groundwater is vertical percolation of rainwater in the case of shallow zone while regional flows from outcrop areas recharge the deep groundwater. A wide scatter is noticed in environmental ³H content (0.23-6.62 TU) indicating both fast and sluggish water flows. The UAR phase diagram suggests that leaching process controls the uranium mobilization into the groundwater. The correlations among UAR, uranium and U_excess further indicate oxidative nature of leaching process. Statistical treatment of the obtained data along with available geochemical and isotope evidences suggest that source of uranium is common but the driving processes are different for shallow and deep zone. Influences of root zone CO₂, oxic species from irrigation return flows and water level fluctuations are also evaluated. Low uranium, low UAR, low ³H and high ²²²Rn activity in deep zone suggest uranium being released from the roll front as well as transported from outcrop regions. This study highlights the application of uranium isotope ratio, radon and environmental isotopes in assessing vulnerability of alluvial aquifers towards uranium contamination.

Isotopic content in high mountain karst aquifers as a proxy for climate change impact in Mediterranean zones: The Port del Comte karst aquifer (SE Pyrenees, Catalonia, Spain)

The objective of this work is to characterize the impact of climate change in the karst aquifer of the Port del Comte Massif (PCM). Six regional climate models (RCMs) from CLYM'PY Project are used to analyse the magnitude and trends of changes on precipitation and temperature (RCP4.5 and RCP8.5 scenarios) and how these changes propagate through the hydrogeological system as groundwater resources availability and the associated water isotopic content. The study uses the RCMs climate change forcings as input data to a combination of (1) a semi-distributed hydrological model for simulating the hydrodynamical response of the aquifer, and (2) a lumped parameter model for simulating the isotopic content in groundwater at the outlet of the aquifer. A mean decrease of 2.6% and 1.9% in yearly precipitation and a mean increase of 1.9 and 3.1 °C in average temperature is expected in PCM at the end of the 21st century in the RCP4.5 and RCP8.5 scenarios, respectively. This climate signal entering the hydrogeological system results in a mean decrease in recharge of 3.9% and 0.5% from rainfall and of 59.3% and 76.1% from snowmelt, and a decrease of 7.6% and 4.5% in total system discharge, but also generates an isotopic enrichment in groundwater discharge (δ¹⁸O_GW) of 0.50‰ and 0.84‰, respectively. Moreover, from a long-term (2010-2100) perspective, the mean trend in δ¹⁸O_GW is 0.7‰/100 yr and 1.2‰/100 yr for RCP4.5 and RCP8.5, respectively, resulting in easily measurable annual lapse rates with the current analytical methods.

Influence of regional climatic on the hydrogeochemistry of a tropical river basin-a study from the Walawe river basin of Sri Lanka

The Walawe river basin is one of the important watersheds in Sri Lanka subjected to water scarcity due to intensive exploitation for irrigation and domestic purposes. The groundwater resource in the basin is under-explored for its capacity to sustain a continuous supply of water for future demand while facing the growing climate change challenges. The objective of this study was to identify the behavior of groundwater in the Walawe river basin that flows through two major climatic zones in Sri Lanka. The study approach includes hydrogeochemical and stable isotope analysis in order to differentiate the geochemical evolution of groundwater in the basin with respect to climatic factors. Water samples from thirty-eight (38) deep wells (> 20 m), 25 shallow wells, and 14 surface water bodies were collected and measured for their major ions, and isotope ratios of δ²H and δ¹⁸O. The results indicated a clear difference in the geochemistry of groundwater between the two climatic zones of the basin. The dry zone area was characterized by a higher content of dissolved minerals as compared to that in the wet zone area. Silicate weathering, calcite dissolution, and ion exchange processes were found to be the main control of groundwater geochemistry in the basin. The Ca-HCO₃-type water was found to be the predominant water type. The isotope data suggested that the groundwater in the study area is recharged mainly from the northeast monsoon rain. Isotope characteristics also suggested that direct infiltration is prominent in the wet zone regions, whereas modifications of shallow groundwater by evaporation were dominated in the dry zone areas. The findings of the study suggest that water quality management in the dry zone areas of the basin is critical for the future sustainability of the water resource of the basin.

Saline groundwater in the Buffels River catchment, Namaqualand, South Africa: A new look at an old problem

Namaqualand, South Africa, is a global biodiversity hotspot but local populations are affected by challenging economic conditions largely because of poor access to water. In this study groundwater types are characterised and sources of salts and salinisation processes are identified using hydrochemistry and δ¹⁸O, δ²H and ⁸⁷Sr/⁸⁶Sr data. Analysis of δ¹⁸O and δ²H data suggests that evaporation does not play a major role in salinisation of the groundwater. However, major ion chemistry and ⁸⁷Sr/⁸⁶Sr ratios indicate that salts present in the groundwater are linked to dry deposition of marine aerosols and ion-exchange reactions in soils in the alluvial aquifer systems. The hydrochemical variability of the groundwater in the basement aquifer system suggests that there are strong local controls linked to weathering processes in individual basement rock types. The region is also notable for the high density of heuweltjies, biophysical features associated with increased nutrient levels, associated with termite activity. Electromagnetic scanning as well as measurement of water-soluble soil electrical conductivity values on and off heuweltjies, show that heuweltjies are saline with salinity increasing with depth. The level of groundwater salinity correlates with the level of heuweltjie salinity. Precipitation records from the last 150 years provide support for the hypothesis that accumulated salts, and in particular, heuweltjie salts are flushed into the groundwater system during sporadic large volume precipitation events. Thus, heuweltjies and hence termite activity, could potentially represent a previously unrecognized contributor to groundwater salinisation across Namaqualand and in other parts of the world.

Processes controlling groundwater salinity in coastal wetlands of the southern edge of South America

The Argentine Atlantic coast constitutes an extensive area where numerous wetlands develop under humid, semi-arid and arid conditions, in which there are also variations in relation to tidal influence with estuarine, mixing and marine areas. The aim of this work is to conduct a comparative study on the processes controlling the groundwater salinity in medium to high latitudinal coastal wetlands of four natural reserves with contrasting hydrological and climatic conditions. In each study area a monitoring network was established where the content of CO₃^2-, HCO₃^-, Cl^-, SO₄^2-, Ca²⁺, Mg²⁺, Na⁺, K⁺, δ²H and δ¹⁸O of the water were determined. The results show a saline groundwater increase along a latitudinal gradient with electrical conductivities varying from 0.3 mS/cm at 34°47' S to 154 mS/cm at 42° 25' S. The results obtained show that the ionic contents in groundwater are partially controlled by the salinity of the tidal flood water whose electrical conductivity varies from 0.3 mS/cm in the Río de la Plata estuary to 52 mS/cm in the sea water of the southern study area. In the southern wetlands, where an increase of aridity is also registered, there is a clear increase in groundwater ionic concentrations, which occurs without isotopic enrichment indicating processes of salts dissolution of the sediments. The evaporites precipitation occurs due to the total evaporation of the tidal water that floods the wetlands in spring high tides. The salinization of groundwater responds to natural processes inherent to the hydrological, climatic and lithological characteristics of each wetland. Given that the areas studied correspond to natural reserves, the results generate databases that will allow the identification of future changes in salinity associated with anthropic influences or changes in hydrological and/or climatic conditions as a result of climate change.

Using water quality and isotope studies to inform research in chronic kidney disease of unknown aetiology endemic areas in Sri Lanka

Chronic Kidney Disease of unknown aetiology (CKDu) is a major public health concern in dry climatic, agricultural regions of Sri Lanka. The chemistry of groundwater (the main source of drinking water) in the area has been studied extensively, in relation to the occurrence of CKDu. This paper investigates water quality studies published in CKDu affected areas of Sri Lanka and also presents a new data set of 27 hydrochemical and isotopic samples collected from groundwater wells in selected CKDu endemic areas in Sri Lanka. The study outcomes do not provide evidence of pollutants such as heavy metals in groundwater. However, the study identifies elevated concentrations of silica which requires further investigation. Two groups of groundwater have been identified based on the isotopic results suggesting different sources or origins. The available water quality data, including the data from this study, is not sufficient to answer questions on whether the chemistry of groundwater is related to the CKDu occurrence. However, this study identifies the importance of detailed investigation into degradation products of agrochemicals, the organic matter content and the influence of elevate silica concentration in groundwater. The study also provides research directions in the form of isotopic tracers and the frequency of sampling that is needed to capture potential pollutants in future groundwater quality studies in CKDu endemic areas in Sri Lanka.

Combining environmental isotopes with Contaminants of Emerging Concern (CECs) to characterise wastewater derived impacts on groundwater quality

The potential for Wastewater Treatment Plants (WWTPs) to cause adverse impacts to groundwater quality is a major global environmental challenge. Robust and sensitive techniques are required to characterise these impacts, particularly in settings with multiple potential contaminant sources (e.g. agricultural vs. site-derived). Stable (δ²H_H2O, δ¹⁸O_H2O, δ¹⁵N_NO3, δ¹⁸O_NO3 and δ¹³C_DIC) and radioactive (³H and ¹⁴C) isotopes were used in conjunction with three Contaminants of Emerging Concern (CECs) - carbamazepine, simazine and sulfamethoxazole - to discriminate between multiple potential contamination sources at an Australian WWTP. The radioactive isotope tritium provided a sensitive indicator of recent (post-1990s) leakage, with groundwater activities between 0.68 and 1.83 TU, suggesting WWTP infrastructure (activities between 1.65 and 2.41) acted as a recharge 'window', inputting treated or partially treated effluent to the underlying groundwater system. This was corroborated by water stable isotopes, which showed clear demarcation between δ¹⁸O_H2O and δ²H_H2O in background groundwater (δ¹⁸O_H2O and δ²H_H2O values of approximately -5 and -28‰, respectively) and those associated with on-site wastewater (median δ¹⁸O_H2O and δ²H_H2O values of -1.2 and -7.6‰, respectively), with groundwater down-gradient of the plant plotting on a mixing line between these values. The CECs, particularly the carbamazepine:simazine ratio, provided a means to further distinguish wastewater impacts from other sources, with groundwater down-gradient of the plant reporting elevated ratios (median of 0.98) compared to those up-gradient (median of 0.11). Distinctive CEC ratios in impacted groundwater close to the WWTP (∼3.0) and further down-gradient (2.7-9.3) are interpreted to represent a change in composition over time (i.e., recent vs. legacy contamination), consistent with the site development timeline and possible changes in effluent composition resulting from infrastructure upgrades over time. The data indicate a complex set of co-mingled plumes, reflecting different inputs (in terms of both quantity and concentration) over time. Our approach provides a means to better characterise the nature and timing of wastewater derived impacts on groundwater systems, with significant global implications for site management, potentially allowing more targeted monitoring, management and remedial actions to be undertaken.

Evaluation of Environmental Pollution and Microbial Treatment of Shallow Groundwater in El Omayed Area, Egypt

In the last few years, the northwestern coastal zone of Egypt has been affected by many stresses that impact its water resources, leading to undesirable consequences related to water quantity and quality. This paper aims to evaluate the possible contamination sources of shallow aquifers in the El-Omayed area, Egypt, that are causing its deterioration; this was achieved through an integration of hydrochemical and isotopic techniques, an assessment of the undesirable consequences of potential toxic metals (PTMs) on human health risk as a result of direct water consumption, and finally, the application of biological treatment in the remediation of some metal contamination. The chemical composition, environmental isotopes (δ¹⁸ O and δ² H), and microbial analyses were analyzed from 13 collected water samples. Approximately 20% of the groundwater samples were classified as fresh water, and the rest were classified as slightly saline to moderately saline. Stable isotopes proved the contribution of the El Sheikh Zoied canal (which is mainly recharged from the Nile system) as a recharge source. The toxicity of PTMs (Cd, Cu, Co, Cr, Pb, Zn, B, and Fe) was evaluated on the basis of their exceedance values. It was proven that the majority of the groundwater samples were contaminated, which might be attributed to natural and anthropogenic actions in the study area; however, according to human health risk exposure assessment calculations, there is no human cancer risk posed via ingestion of drinking groundwater. The total bacterial count was determined for all water samples; autochthonous microorganisms were capable of removing heavy metals in the polluted water sample. The bacterial strain M52, which was identified by 16S rRNA sequencing as Stenotrophomonas rhizophila, showed the best results, by removing 75% and 65% of the initial concentrations of Fe and B, respectively. The results indicate that this bacterial strain may be useful and represents an environmentally friendly method to remove pollutants and heavy metals from contaminated water. Integr Environ Assess Manag 2020;16:461-471. © 2020 SETAC.

Contamination and natural attenuation characteristics of petroleum hydrocarbons in a fractured karst aquifer, North China

A rare super-large fractured karst aquifer located in Zibo city, Shandong Province of Northern China was polluted by petroleum hydrocarbons from a petrochemical company. Over the last 30 years, it has been the focus of several remediation efforts. In this study, the contamination and natural attenuation characteristics of the petroleum hydrocarbons were elucidated using hydrogeochemical indicators (DO, DOC, Cl^-, HCO₃^-, pH, NO₃^-, and SO₄^2-), petroleum hydrocarbons elements and environmental isotopes (δ¹⁵N_NO3, δ¹⁸O_NO3, δ¹³C_DIC, and δ¹³C_DOC). With the aid of GIS, statistical analyses, as well as first-order decay model and electron-acceptor-limited kinetic model, the spatio-temporal evolution characteristics of the petroleum hydrocarbons were modeled. Results showed a positive natural attenuation trend over the last 3 decades where intrinsic biodegradation mechanism was found to be the most important factor driving the degradation of hydrocarbons in the aquifer system. The hydrogeochemical association between the indicators and petroleum hydrocarbons provided the evidences of biodegradation and also served as markers, highlighting the occurrence of anaerobic respiration without methanogenic activities within the heterogenous karst media. Furthermore, the mean natural attenuation rate of petroleum hydrocarbons was calculated to be 3.76 × 10^-3/day whereby the current highest petroleum hydrocarbons concentration (361.13 μg/L) is estimated to be degraded completely in 6 years under the present hydrogeological and environmental conditions.

The origin of solutes in groundwater in a hyper-arid environment: A chemical and multi-isotope approach in the Atacama Desert, Chile

The major ion and the multi-isotopic composition (⁸⁷Sr/⁸⁶Sr, δ¹¹B, δ³⁴S(SO₄) and δ¹⁸O(SO₄)) of groundwater from the Central Depression in northern Chile is investigated to identify the origin of groundwater solutes in the hyper-arid core of the Atacama Desert. The study area is between the Cordillera de Domeyko and the Central Depression, at latitudes 24-25°S, and is characterized by near-zero air moisture conditions, rare precipitation and very limited runoff. Groundwater composition varies from Ca-HCO₃ to Ca, Na-SO₄ type below elevations of 3400 m a.s.l. The rCl/rBr ratio of meteoric waters and groundwater overlap, but significantly increase in the aquifer as salinity goes up due to evapoconcentration far from the Domeyko Cordillera. The wind-displaced dust originating in the Central Depression (⁸⁷Sr/⁸⁶Sr: 0.706558-0.710645; δ³⁴S(SO₄): 0 to +4‰) affects the precipitation composition in the highest parts of the Domeyko Cordillera (⁸⁷Sr/⁸⁶Sr: 0.706746-0.709511; δ³⁴S(SO₄): +1 to +6‰), whose δ³⁴S(SO₄) and δ¹¹B values are greatly different from marine aerosols, discarding its contribution to dust at this distance inland. Sr and S isotopic values in groundwater indicate a strong relation with three main geological units: i) Paleozoic rocks contribute high radiogenic strontium isotope ratios to groundwater (0.707011-0.714862), while sulphate isotopic composition is probably acquired from atmospheric dust (>- 1.4‰), ii) Jurassic marine limestones contribute low-radiogenic strontium isotopic ratios to groundwater (<0.70784), while sulphate can be related to oxidized sulphides that change the isotopic signatures of sulphur (<-1.2‰), and iii) mixed salts in the Atacama Gravels contribute lower radiogenic strontium isotopic ratios and sulphate to groundwater (⁸⁷Sr/⁸⁶Sr: <0.707324; δ³⁴S(SO₄): +0.1 to +7.7). These three processes reflect water-rock interactions. The δ¹¹B of groundwater generally up to +13‰, does not increase along the regional groundwater flow path, discarding fractionation by interaction with clays. These results improve the understanding of the groundwater evolution in hyper-arid systems through a new conceptual model.

Geochemical mechanisms controlling the isotopic and chemical composition of groundwater and surface water in a sector of the Pampean plain (Argentina)

The Samborombón River basin, which has eminently rural characteristics, develops within the Pampean plain along with other basins with serious contamination problems due to the growth of the industrial activities and the absence of a proper groundwater management. Considering that the knowledge of the natural baseline quality is important to measure an imposed environmental change, the aim of this work was to study the mechanisms that control the natural composition of groundwater and superficial water in the Samborombón River basin. In order to achieve this, detailed analysis of the minerals forming the aquifer matrix along with the data obtained from isotopes, major and trace elements were performed. Geochemical processes that define the chemical characteristics and contribute to the evolution of both surface and groundwater are associated with rainwater infiltration, carbonate dissolution, cation exchange and evaporation. Additionally, the low concentration of heavy metals may respond to natural background levels. Comprehending the hydrochemical processes governing groundwater and superficial water quality, particularly in a rural area where water supply is vital for the development of human activities, is essential to prevent environmental deterioration and thus, enhance socio-economic growth.

Complex hydrochemical characteristics of the Middle-Upper Pleistocene aquifer in Soc Trang Province, Southern Vietnam

Environmental isotope techniques were applied to study the hydrochemical characteristics of groundwater in Soc Trang Province, Southern Vietnam, in frame of the project Improvement of Groundwater Protection in Vietnam (IGPVN). Groundwater samples were collected from various monitoring wells (newly drilled by the IGPVN project), national monitoring wells, private tube wells and production wells. Surface water samples were collected from rivers, ponds or canals. The aquifer system is more complex than presumed as the hydrochemical and stable isotope compositions of groundwater samples in the Middle-Upper Pleistocene (qp_2-3) aquifer differ significantly in lateral direction. Furthermore, observed changing redox reactions within the target aquifer from dry to wet season make it probable that some interaction with overlying aquifers exists. The stable isotope signatures of the qp_2-3 groundwater samples can be divided into two distinct groups which, respectively, originated from paleo-meteoric water and either was located in paleo-salinized areas of the qp_2-3 aquifer or resulted from evaporation effect of recharging water prior or during infiltration process. In fact, individual parts of "the same" aquifer seem not to be hydraulically connected to each other. The environmental isotope data provided neither evidences of hydraulic connection between the rivers and the qp_2-3 aquifer nor of recent groundwater recharge in the Province. As a result, saltwater from the sea intruded inland to some extent via the Hau River during the dry season, but it did not affect the target aquifer. Any recharge from surface water to the qp_2-3 aquifer in Soc Trang should occur outside the boundaries of Soc Trang Province. Considering the low groundwater transit velocities roughly estimated in this study (3.6 m/year and 7.8 m/year), it may take several ten thousands to hundred thousands of years for recharging water from beyond the Vietnam's national border to reach the qp_2-3 aquifer in Soc Trang Province. Consequently, natural recharge cannot help to reduce groundwater declining in the short-to-middle term.

Delayed nitrate dispersion within a coastal aquifer provides constraints on land-use evolution and nitrate contamination in the past

Identifying sources of anthropogenic pollution, and assessing the fate and residence time of pollutants in aquifers is important for the management of groundwater resources, and the ecological health of groundwater dependent ecosystems. This study investigates anthropogenic contamination in the shallow alluvial aquifer of the Marana-Casinca, hydraulically connected to the Biguglia lagoon (Corsica, France). A multi-tracer approach, combining geochemical and environmental isotopic data (δ¹⁸O-H₂O, δ²H-H₂O, ³H, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹¹B), and groundwater residence-time tracers (³H and CFCs) was carried out in 2016, and integrated with a study of land use evolution in the catchment during the last century. Groundwater NO₃^- concentrations, ranged between 2 mg/L and up to 30 mg/L, displaying the degradation of groundwater quality induced by anthropogenic activities (agricultural activities). Comparatively high δ¹⁵N-NO₃^- values (up to 19.7‰) in combination with δ¹¹B values that were significantly lower (between 23‰ and 26‰) than the seawater background are indicative of sewage contamination. The ongoing deterioration of groundwater quality can be attributed to the uncontrolled urbanization development all over the alluvial plain, with numerous sewage leakages from the sanitation network and private sewage systems. Integration of contaminant and water-residence time data revealed a progressive accumulation of pollutants with time in the groundwater, particularly in areas with major anthropogenic pressure and slow dynamic groundwater flow. Our approach provides time-dependent insight into nitrogen pollution in the studied aquifer over the past decades, revealing a systematic change in the dominant NO₃^- source, from agricultural to sewage contamination. Yet, today's low groundwater quality is to large parts due to legacy pollution from land-use practices several decades ago, underlining the poor self-remediating capacity of this hydrosystem. Our results can be taken as warning that groundwater pollution that happened in the recent past, or today, may have dire impacts on the quality of groundwater-dependent ecosystems in the future.

Hydrochemical and environmental isotope analysis of groundwater and surface water in a dry mountain region in Northern Chile

This case study examines the geological imprint and land use practices on water quality in the arid Huasco Valley against the backdrop of ongoing water conflicts surrounding competing demands for agriculture and mining. The study is based on a detailed analysis of spatial and temporal variations of monthly surface and bi-monthly groundwater quality samples measured during the Chilean summer of 2015/16. Additional information on source regions and river-groundwater interactions were collected using stable water isotopes. Regarding the geological impact on water quality, high concentrations of Ca²⁺, SO₄^2- and HCO₃^- indicate a strong influence of magmatic rocks, which constitute this high mountain basin, on the hydrochemistry. Piper and Gibbs-diagrams revealed that all samples show a homogenous distribution dominated by rock-water interactions. Measured NO₃^- concentrations in surface water are generally low. However, groundwater aquifers exhibit higher concentrations. Mn is the only heavy metal with elevated concentrations in surface water, which are possibly related to mining activities. The results illustrate that both surface and groundwater can be classified as suitable for irrigation. In addition, groundwater has been found to be suitable as drinking water. High similarities in isotopic signatures indicate a strong connection between surface and groundwater. Isotopic analyses suggest a strong influence of evaporation. This combined approach of hydrogeochemical and isotopic analysis proved to be a helpful tool in characterizing the catchment and can serve as a basis for future sustainable water management.

Hydrogeochemical and isotopic signature of surface and groundwater in a highly industrialized sector of the Rio de la Plata coastal plain (Argentina)

The coastal plain of the middle estuary of the Río de la Plata is a highly industrialized area and is densely populated by sectors. The main human activity in the sector encompassed between the cities of Ensenada and Berisso is associated with the petrochemical industry. In this work, hydrogeochemical and isotopic characteristics of surface and groundwater in the impacted area are analyzed and the results are contrasted with those obtained in an undisturbed protected area. Major and trace elements were determined using standardized methods while the stable isotopes δ¹⁸O y δ²H were analyzed by mass spectroscopy. Human impact is evidenced by the occurrence of large variations in the major chemical composition of water, and also by the elevated concentrations of some trace elements that are not contributed from natural sources. These results may contribute to the understanding of chemical processes and pollutants distribution in highly industrialized coastal plain areas.

Hydrodynamic influence on reservoir sustainability in semi-arid climate: A physicochemical and environmental isotopic study

Water scarcity and increasing water demand require the development of water management plans such as establishing artificial lakes and dams. Plans to meet water needs are faced by uprising challenges to improve water quality and to ensure the sustainability of hydro-projects. Environmental isotopes coupled to water physicochemical characteristics were investigated over a biennial cycle to assess both geomorphological and environmental impacts on the water quality of a reservoir situated in an intensively used agricultural watershed under a Mediterranean semi-arid climate. The particularity of the semi-arid climate and the diverse topography generate a continental and orographic rain effect on the isotopic composition of precipitation and the water recharged sources. The studied reservoir responds quickly to land-use activities and climatic changes as reflected by temporal and spatial variations of water chemistry and isotopic composition. Increasing changes in precipitation rate and dry periods significantly modified the water isotopic composition in the reservoir. During the first year, hydrogen (δD) and oxygen (δ¹⁸O) isotopes are depleted by 6 and 2‰ between dry and wet season, respectively. While a shift of -2‰ for δD and -1‰ for δ¹⁸O was detected during the second annual cycle. Environmental isotopic compositions demonstrate for the first time the occurrence of groundwater inflow to the central (Cz) and dam (Dz) zones of the Qaraaoun reservoir. The Cz and Dz can be considered as open water bodies subjected to dilution by groundwater inflow, which induces vertical mixing and reverse isotopic stratification of the water column. In the contrary, the river mouth zone acts as a closed system without groundwater intrusion, where heavy water accumulates and may act as a sink for contaminants during dry season. Groundwater influx acts as a dilution factor that renews the hypolimnion, and minimizes the perturbations induced by both internal biogeochemical reactions and external hydrological variations. Attention should be devoted to the hydrogeological location of planned reservoirs, which should take into account the vicinity of shallow water table to insure good water quality and water sustainability.

Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial aquifers of southwest Punjab

Groundwater samples collected from the alluvial aquifers of southwest Punjab, both shallow and deep zones were measured for environmental tritium (³H) and stable isotopes (²H and ¹⁸O) to evaluate the source of recharge and aquifer dynamics. The shallow groundwater shows wide variation in isotopic signature (δ¹⁸O: -11.3 to -5.0‰) reflecting multiple sources of recharge. The average isotopic signature of shallow groundwaters (δ¹⁸O: -6.73 ± 1.03‰) is similar to that of local precipitation (-6.98 ± 1.66‰) indicating local precipitation contributes to a large extent compared to other sources. Other sources have isotopically distinct signatures due to either high altitude recharge (canal sources) or evaporative enrichment (irrigation return flow). Deep groundwater shows relatively depleted isotopic signature (δ¹⁸O: -8.6‰) and doesn't show any evaporation effect as compared to shallow zone indicating recharge from precipitation occurring at relatively higher altitudes. Environmental tritium indicates that both shallow (³H: 5 - 10 T.U.) and deeper zone (³H: 1.5 - 2.5 T.U.) groundwaters are modern. In general the inter-aquifer connections seem to be unlikely except a few places. Environmental isotope data suggests that shallow groundwater is dynamic, local and prone to changes in land use patterns while deep zone water is derived from distant sources, less dynamic and not impacted by surface manifestations. A conceptual groundwater flow diagram is presented.

Hydrochemical and isotopes studies in a hypersaline wetland to define the hydrogeological conceptual model: Fuente de Piedra Lake (Malaga, Spain)

The Fuente de Piedra lake is a hypersaline wetland of great extension (13.5km²) and rich in aquatic birds and other species. It became therefore the third Spanish wetland to be included in the Ramsar convention and has been a "nature reserve" since 1984. The lake has an endorheic basin (150km²) with variable-density flows dominated by complex hydrogeological conditions. The traditional conceptualization of endorheic basins in semiarid climates considered that the brine in this hydric system was exclusively of evaporative origin and was placed only in the lake and its surrounding discharge area in the basin. Previous geophysical and hydrochemical studies identified different types of waters and brines. In this work, natural tracers (Cl^-, Br^-, Na⁺, Mg²⁺) and environmental isotopes (¹⁸O, ²H, ¹⁴C, ¹³C and ³H) were employed to a) discriminate different types of brines according to their degree of evaporation and genesis, and b) to estimate residence times of brine waters and identify recharge areas of the different flow subsystems. A conceptual model of the hydrogeological system of the lake basin and its links to a regional karst system is proposed.

Correlation of the seasonal isotopic amplitude of precipitation with annual evaporation and altitude in alpine regions

The time series of stable water isotope composition relative to IAEA-GNIP meteorological stations located in alpine zones are analyzed in order to study how the amplitude of the seasonal isotopic composition of precipitation (Aδ) varies along a vertical transect. A clear relationship between Aδ and local evaporation is obtained, with slopes of -0.87 ‰/100mm/yr and -7.3 ‰/100mm/yr for Aδ(18)O and Aδ(2)H, respectively. When all sampling points of the vertical transect receive the same moisture sources, then a linear relationship between Aδ and elevation is obtained, with vertical gradients of 0.16 ‰/100mm/yr and 1.46 ‰/100mm/yr forAδ(18)O and Aδ(2)H, respectively.

Hydrogeochemistry of co-occurring geogenic arsenic, fluoride and iodine in groundwater at Datong Basin, northern China

Abnormal levels of co-occurring arsenic (As), fluorine (F) and iodine (I) in groundwater at Datong Basin, northern China are geochemically unique. Hydrochemical, (18)O and (2)H characteristics of groundwater were analyzed to elucidate their mobilization processes. Aqueous As, F and I ranged from 5.6 to 2680 μg/L, 0.40 to 3.32 mg/L and 10.1 to 186 μg/L, respectively. High As, F and I groundwater was characterized by moderately alkaline, high HCO3(-), Fe(II), HS(-) and DOC concentrations with H3AsO3, F(-) and I(-) as the dominant species. The plots of δ(18)O values and Cl/Br ratios versus Cl(-) concentration demonstrate build-up of more oxidizing conditions and precipitation of carbonate minerals induced by vertical recharge and intensive evaporation facilitate As retention to Fe (hydr) oxides, but enhance F and I mobilization from host minerals. Under reducing conditions, As and I can be simultaneously released via reductive dissolution of Fe (hydr) oxides and reduction of As(V) and I(V) while F migration may be retarded due to effects of dissolution-precipitation equilibria between carbonate minerals and fluorite. With the prevalence of sulfate-reducing condition and lowering of HCO3(-) concentration, As and I may be sequestered by Fe(II) sulfides and F is retained to fluorite and on clay mineral surfaces.

Isotopic and hydrogeochemical characterization of high-altitude karst aquifers in complex geological settings. The Ordesa and Monte Perdido National Park (Northern Spain) case study

The Ordesa and Monte Perdido National Park, located in the Southern Pyrenees, constitutes the highest karst system in Western Europe. No previous studies regarding its geochemical and isotopic groundwater characterization are available in this area. This work presents the results of field and sampling campaigns carried out between July 2007 and September 2013. The groundwater presents high calcium bicarbonate contents due to the occurrence of upper Cretaceous and lower Paleocene-Eocene carbonate materials in the studied area. Other relevant processes include dissolution of anhydrite and/or gypsum and incongruent dissolution of Mg-limestone and dolomite. The water stable isotopes (δ(18)O, δ(2)H) show that the oceanic fronts from the Atlantic Ocean are responsible for the high levels of precipitation. In autumn, winter, and spring, a deuterium excess is found in the recharge water, which could be related to local atmospheric transport of low-altitude snow sublimation vapour and its later condensation on the snow surface at higher altitude, where recharge is mostly produced. The recharge zones are mainly between 2500m and 3200ma.s.l. The tritium content of the water suggests short groundwater transit times. The isotopic composition of dissolved sulphate points to the existence of regional fluxes mixed with local discharge in some of the springs. This work highlights the major role played by the altitude difference between the recharge and discharge zones in controlling the chemistry and the vertical variability of the isotopic composition in high-altitude karst aquifers.

Characterization of the Gacka River basin karst aquifer (Croatia): hydrochemistry, stable isotopes and tritium-based mean residence times

The Gacka River basin aquifer is a highly-developed karst system, located in the Croatian Dinarides. It is mostly composed of permeable Jurassic and Cretaceous carbonate rocks, and clastic sedimentary rocks of Paleogene age. Gacka River provides high quality water for the town of Otočac and several villages; together with the neighboring Lika River, the water is used for the Hydroelectric Power Plant at Senj on the coast. About 10 perennial and over 20 seasonal springs are located at 450 to 460 ma.s.l. (above sea level). Three major springs (Pećina, Majerovo and Tonkovića) provide 57% of the mean annual river flow. Similarities between the average groundwater temperatures as well as between the average specific electrical conductivity values (9.0°C-328 μS/cm, 9.6°C-350 μS/cm and 8.9°C-312 μS/cm) of the springs imply that they are fed from aquifers with similar mean residence times (MRTs). The mean δ(18)O contents of Majerovo, Tonkovića, and Pećina are around -10.1‰, -9.2‰ and -8.9‰, respectively, revealing differences in the mean recharge area elevations. Compared to the temporal amplitude of the(18)O signal of precipitation, the (18)O signal variations of the springs are substantially attenuated because the recharges occurring at different times are well mixed within the aquifers. This indicates MRTs of more than just a few years. The average tritium contents of Pećina, Majerovo and Tonkovića are 5.48 TU, 6.13 TU and 6.17 TU, respectively. Serially connected exponential-plug type unsteady lumped-parameter models run on an annual time scale resulted in rather satisfactory matches between the observed and calculated tritium contents for all studied springs. The models revealed similar MRTs (and corresponding reservoir volumes) for Pećina, Tonkovića and Majerovo of 12 years (470 Mm(3)), 12 years (1,190 Mm(3)), and 12.2 years (1,210 Mm(3)), respectively. Plug flow conditions dominate in about 90% of the total aquifer volumes.