Non-invasive determination of critical dissolved oxygen thresholds for stress physiology in fish using triple-oxygen stable isotopes and aquatic respirometry.

Understanding the critical thresholds of dissolved oxygen (O2) that trigger adaptive physiological responses in aquatic organisms is long hampered by a lack of robust, non-lethal or non-invasive methodologies. The isotope fractionation of triple O2 isotopes (18O/17O/16O) during respiration is linked to the amount of oxygen utilised, offering a potential avenue for new insights. Our experimental research involved measuring the oxygen isotope fractionation of dissolved O2 in closed-system aquatic respirometry experiments with wild sticklebacks (Gasterosteus aculeatus). These fish were either naturally adapted or experimentally acclimated to hypoxic and normoxic conditions. The aim was to observe their oxygen usage and isotope fractionation in response to increasingly severe hypoxia. Initial observations revealed a progressive 18O enrichment from the preferential uptake of 16O to a dissolved oxygen threshold of 3-5 mg O2 L-1, followed by an apparent reversal in oxygen isotope fractionation, which is mixing of 16O and 17O with the remaining O2 pool across all populations and indicative of a systematic change in oxygen metabolism among the fish. Unexpectedly, sticklebacks adapted to hypoxia but acclimated to normoxia exhibited stronger oxygen isotope fractionation compared to those adapted to normoxia and acclimated to hypoxia, contradicting the hypothesis that hypoxia adaptation would lead to reduced isotope discrimination due to more efficient oxygen uptake. These preliminary experimental results highlight the novel potential of using dissolved O2 isotopes as a non-invasive, non-lethal method to quantitatively assess metabolic thresholds in aquatic organisms. This approach could significantly improve our understanding of the critical oxygen responses and adaptation mechanisms in fish and other aquatic organisms across different oxygen environments, marking a significant step forward in aquatic ecological and physiological research.

Metabolic activity of Planktothrix rubescens and its consequences on oxygen dynamics in laboratory experiment: A stable isotope study.

Fluctuations in dissolved oxygen (DO) contents in natural waters can become intense during cyanobacteria blooms. In a reconnaissance study, we investigated DO concentrations and stable isotope dynamics during a laboratory experiment with the cyanobacterium Planktothrix rubescens in order to obtain insights into primary production under specific conditions. This observation was extended to sub-daily timescales with alternating light and dark phases. Dissolved oxygen concentrations and its isotopes (δ18ODO) ranged from 0.02 to 0.06 mmol · L-1 and from +9.6‰ to +23.4‰. The δ18ODO proved to be more sensitive than concentration measurements in response to metabolic variation and registered earlier shifts to dominance by respiration. Oxygen (O2) contents in the headspace and its isotopes (δ18OO2) ranged from 2.62 to 3.20 mmol · L-1 and from +9.8‰ to +21.9‰. Headspace samples showed less fluctuations in concentration and isotope trends because aquatic processes were hardly able to alter signals once the gas had reached the headspace. Headspace δ18OO2 values were corrected for gas-water equilibration and were determined to be higher than the mean δ18OH2O of -8.7‰. This finding suggests that counteracting respiration was important even during the highest photosynthetic activity. Additionally, headspace analyses led to the definition of a fractionation factor for respiration (αR) of this cyanobacterium with a value of 0.980. This value confirms the one commonly used for cyanobacteria. Our findings may become important for the management of water bodies where decreases in DO are caused by cyanobacteria.

Early Ultrastructural Changes in Biopsies From Patients With Symptomatic CKD of Uncertain Etiology.

Introduction: Although the investigation of chronic kidney disease of uncertain etiology (CKDu) has identified many possible influencing factors in recent years, the exact pathomechanism of this disease remains unclear. Methods: In this study, we collected 13 renal biopsies from patients with symptomatic CKDu (Sym-CKDu) from Sri Lanka with well-documented clinical and socioeconomic factors. We performed light microscopy and electron microscopic evaluation for ultrastructural analysis, which was compared with 100 biopsies from German patients with 20 different kidney diseases. Results: Of the 13 Sri Lankan patients, 12 were men (92.3%), frequently employed in agriculture (50%), and experienced symptoms such as feeling feverish (83.3%), dysuria (83.3%), and arthralgia (66.6%). Light microscopic evaluation using activity and chronicity score revealed that cases represented early stages of CKDu except for 2 biopsies, which showed additional signs of diabetes. Most glomeruli showed only mild changes, such as podocyte foot process effacement on electron microscopy. We found a spectrum of early tubulointerstitial changes including partial loss of brush border in proximal tubules, detachment of tubular cells, enlarged vacuoles, and mitochondrial swelling associated with loss of cristae and dysmorphic lysosomes with electron-dense aggregates. None of these changes occurred exclusively in Sym-CKDu; however, they were significantly more frequent in these cases than in the control cohort. Conclusion: In conclusion, our findings confirm the predominant and early alterations of tubular structure in CKDu that can occur without significant glomerular changes. The ultrastructural changes do not provide concrete evidence of the cause of CKDu but were significantly more frequent in Sym-CKDu than in the controls.

Novel evaluations of sources and sinks of dissolved oxygen via stable isotopes in lentic water bodies.

Dissolved oxygen (DO) dynamics of a temperate drinking water reservoir in the Harz Mountains (Germany) were investigated over a time period of 18 months. Via depth profiles in a fortnightly sampling resolution we were able to trace DO and temperature dynamics including the formation and breakdown of a Metalimnetic Oxygen Minimum (MOM) by means of DO concentration, saturation patterns and stable isotope ratios of dissolved oxygen (expressed as δ18ODO). Over the evaluation period, 19.4 % of the samples collected had δ18ODO values compatible with atmospheric equilibration (+24.6 ‰ ± 0.4 ‰). With values smaller and larger than this threshold, the remaining δ18ODO values showed that 40.8 % of our samples were dominated by photosynthesis and 39.8 % by respiration. From December to April the reservoir was mixed and DO consumption by respiration exceeded production via photosynthesis. During stratification period, quantification of respiration/photosynthesis rates (R/P) confirmed the epilimnion as a photosynthetic (i.e. net-autotrophic) environment while the hypolimnion was heterotrophic and dominated by respiration at various degrees. Samples of the MOM zone showed the highest R/P ratios and had among the most positive δ18ODO signals caused by respiration. This study showed that combinations of DO concentrations and their isotope ratios are promising to quantify critical zones of respiration and photosynthesis in aquatic environments.

Dissolved oxygen isotope modelling refines metabolic state estimates of stream ecosystems with different land use background.

Dissolved oxygen (DO) is crucial for aerobic life in streams and rivers and mostly depends on photosynthesis (P), ecosystem respiration (R) and atmospheric gas exchange (G). However, climate and land use changes progressively disrupt metabolic balances in natural streams as sensitive reflectors of their catchments. Comprehensive methods for mapping fundamental ecosystem services become increasingly important in a rapidly changing environment. In this work we tested DO and its stable isotope (18O/16O) ratios as novel tools for the status of stream ecosystems. For this purpose, six diel sampling campaigns were performed at three low-order and mid-latitude European streams with different land use patterns. Modelling of diel DO and its stable isotopes combined with land use analyses showed lowest P rates at forested sites, with a minimum of 17.9 mg m-2 h-1. Due to high R rates between 230 and 341 mg m-2 h-1 five out of six study sites showed a general heterotrophic state with P:R:G ratios between 0.1:1.1:1 and 1:1.9:1. Only one site with agricultural and urban influences showed a high P rate of 417 mg m-2 h-1 with a P:R:G ratio of 1.9:1.5:1. Between all sites gross G rates varied between 148 and 298 mg m-2 h-1. In general, metabolic rates depend on the distance of sampling locations to river sources, light availability, nutrient concentrations and possible exchanges with groundwater. The presented modelling approach introduces a new and powerful tool to study effects of land use on stream health. Such approaches should be integrated into future ecological monitoring.

Exposure Assessment of Fluoride Intake Through Commercially Available Black Tea (Camellia sinensis L.) from Areas with High Incidences of Chronic Kidney Disease with Undetermined Origin (CKDu) in Sri Lanka.

Fluoride is a beneficial trace element for human health as its deficiency and excess levels can cause detrimental health effects. In Sri Lanka, dry zone regions can have excessive levels of fluoride in drinking water and can cause dental and skeletal fluorosis. In addition to drinking water, traditional habits of tea consumption can cause an additional intake of fluoride in the population. A total number of 39 locally blended black tea samples were collected from a village where chronic kidney disease with undetermined origin (CKDu) is prevalent. In addition, unblended tea samples were obtained from tea-producing factories. The fluoride contents in infusions of 2% weight per volume (w/v) were measured using calibrated ion-selective fluoride electrodes. The mean fluoride content was 2.68±1.03 mg/L in loose tea, 1.87±0.57mg/L in packed tea samples, and 1.14±0.55 mg/L in unblended tea. Repeated brewing of the same tea leaves showed that over 50% of fluoride leached into the solution in the first infusion. An estimate of the daily total average fluoride intake via tea consumption per person is 2.68 mg per day. With groundwater in many dry zone regions in Sri Lanka showing high fluoride levels that exceed 0.5 mg/L, the additional daily intake can rapidly exceed recommended thresholds of 2 mg/day. This can add to adverse health impacts that might also relate to CKDu.

Balance of carbon species combined with stable isotope ratios show critical switch towards bicarbonate uptake during cyanobacteria blooms.

Next to water quality deterioration, cyanobacteria blooms can affect turnover of aqueous carbon, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). We investigated interactions of these three phases and their stable isotopes in a freshwater pond with periodic cyanobacterial blooms over a period of 23 months. This helped to map turnover and sources of aqueous carbon before, during, and after bloom events. During bloom events POC isotope values (δ13CPOC) increased up to -17.4‰, after aqueous CO2 (CO2(aq)) fell below an atmospheric equilibration value of 412 µatm. Additionally, carbon isotope enrichment between CO2(aq) and POC (εCO2-phyto) ranged from 2.0 to 21.5‰ with lowest fractionations observed at pH values above 8.9. The increase of δ13CPOC and decrease of εCO2-phyto values at low pCO2 and high pH was most likely caused by the activation of the carbon concentrating mechanism (CCM). This mechanism correlated with prevalent assimilation of 13C-enriched HCO3-. Surprisingly, CO2(aq) still contributed more than 50% to the POC pool down to pCO2 values of around 150 µatm. Only after this threshold the reduced εCO2-phyto suggested incorporation of 13C-enriched HCO3-.

Extreme gradients in CO2 losses downstream of karstic springs.

Rivers are significant sources of CO2 to the atmosphere, and karstic watersheds are particularly important in this respect due to their large availability of inorganic carbon. This study examines characteristics of dissolved inorganic carbon (DIC) and excess partial pressures of CO2 (epCO2) in the source springs and headwaters of four watersheds in a Central European karstic region, via dissolved inorganic carbon concentration and stable carbon isotope measurements. Our results show the most 13C-depleted δ13CDIC values at the source springs, which become rapidly enriched downstream due to CO2 degassing. Concurrently, epCO2 values, while consistently in excess of atmospheric concentrations at the spring sources, show decreases of up to 92% within only 50 m downstream distance. In conjunction with the large observed flux estimates of up to 88 g C m-2 day-1, these findings suggest that karstic springs are major CO2 sources to the atmosphere. Because headwater streams constitute the bulk of the surface area of most watersheds, they may provide a disproportionately large contribution to CO2 effluxes in carbonate-dominated basins, in which source springs play a particularly important role.

Beware of effects on isotopes of dissolved oxygen during storage of natural iron-rich water samples: A technical note.

RATIONALE: Investigations of the isotope ratios of dissolved oxygen (δ18 ODO ) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe(II) may change pristine δ18 ODO values during storage and can lead to a misinterpretation. We sampled an Fe(II)-rich spring system and measured δ18 ODO values at various time intervals in order to determine influences of Fe-oxidation. METHODS: Water samples were collected from an Fe-rich spring and related stream and the δ18 ODO values were measured in fresh, 4- and 13-day-old samples with an isotope ratio mass spectrometer. Three replicates were measured for each sample with a 1σ of ± 0.2‰. On-site parameters and Fe(II) contents were also measured over the course of the spring system by multi-parameter probes and spectrophotometry. RESULTS: The δ18 ODO values over the course of the spring system in fresh, 4- and 13-day-old samples revealed differences of up to 8‰. We explain this increase by the consumption of DO by Fe(II)-oxidation. After a flow length of 85 m the differences in δ18 ODO values between fresh and older samples decreased because most of the Fe(II) was consumed. CONCLUSIONS: False interpretations of δ18 ODO values are possible if Fe-rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.

Geological CO2 quantified by high-temporal resolution stable isotope monitoring in a salt mine.

The relevance of CO2 emissions from geological sources to the atmospheric carbon budget is becoming increasingly recognized. Although geogenic gas migration along faults and in volcanic zones is generally well studied, short-term dynamics of diffusive geogenic CO2 emissions are mostly unknown. While geogenic CO2 is considered a challenging threat for underground mining operations, mines provide an extraordinary opportunity to observe geogenic degassing and dynamics close to its source. Stable carbon isotope monitoring of CO2 allows partitioning geogenic from anthropogenic contributions. High temporal-resolution enables the recognition of temporal and interdependent dynamics, easily missed by discrete sampling. Here, data is presented from an active underground salt mine in central Germany, collected on-site utilizing a field-deployed laser isotope spectrometer. Throughout the 34-day measurement period, total CO2 concentrations varied between 805 ppmV (5th percentile) and 1370 ppmV (95th percentile). With a 400-ppm atmospheric background concentration, an isotope mixing model allows the separation of geogenic (16-27%) from highly dynamic anthropogenic combustion-related contributions (21-54%). The geogenic fraction is inversely correlated to established CO2 concentrations that were driven by anthropogenic CO2 emissions within the mine. The described approach is applicable to other environments, including different types of underground mines, natural caves, and soils.

Dominance of in situ produced particulate organic carbon in a subtropical reservoir inferred from carbon stable isotopes.

Sources of particulate organic carbon (POC) play important roles in aqueous carbon cycling because internal production can provide labile material that can easily be turned into CO2. On the other hand, more recalcitrant external POC inputs can cause increased loads to sedimentary organic matter that may ultimately cause CH4 release. In order to differentiate sources, stable isotopes offer a useful tool. We present a study on the Itupararanga Reservoir (Brazil) where origins of POC were explored by comparing its isotope ratios (δ13CPOC) to those of dissolved inorganic carbon (δ13CDIC). The δ13CPOC averaged around - 25.1‰ in near-surface waters, which indicates higher primary production inferred from a fractionation model that takes into account carbon transfer with a combined evaluation of δ13CPOC, δ13CDIC and aqueous CO2. However, δ13CPOC values for water depths from 3 to 15 m decreased to - 35.6‰ and indicated different carbon sources. Accordingly, the δ13CDIC values of the reservoir averaged around + 0.6‰ in the top 3 m of the water column. This indicates CO2 degassing and photosynthesis. Below this depth, DIC isotope values of as low as - 10.1‰ showed stronger influences of respiration. A fractionation model with both isotope parameters revealed that 24% of the POC in the reservoir originated from detritus outside the reservoir and 76% of it was produced internally by aqueous CO2 fixation.

Influences of seawater intrusion and anthropogenic activities on shallow coastal aquifers in Sri Lanka: evidence from hydrogeochemical and stable isotope data.

Water supplies in coastal aquifers throughout the world are often threatened by salinization due to seawater intrusion and anthropogenic activities. In the Kalpitiya Peninsula in Sri Lanka, agricultural and domestic water supplies entirely depend on groundwater resources extracted from unconfined Holocene sandy aquifers. To differentiate the effects of seawater intrusion and agriculture on the coastal aquifers of this 160 km2 peninsula, 43 groundwater samples were collected. These samples were analyzed for major ions, trace elements, and stable isotopes of water (δ18O and δ2H). The solute compositions were dominated by Cl-, [Formula: see text], and [Formula: see text], which were mostly balanced by Ca2+, Na+, and Mg2+. Among the four main water types, Na+-Cl- and Ca2+-[Formula: see text] classifications were predominant in the investigated aquifers. Modifications of the groundwater due to evaporation during irrigation activities, but also due to seawater intrusion seem most plausible as indicated by the correlation of δ18O with δ2H (δ2H = 5.51 * Î´18O-3.08, r = 0.93) deviating from the local meteoric water line. Particularly in the southern part of the peninsula, Mg2+/Ca2+ ratios and stable isotopes of water attributed salinization of groundwater to agricultural activities. However, especially in the north, seawater intrusions were also evident. Established mass balance calculations revealed that local groundwater had seawater admixtures of up to 12%. Our results indicate that integrated water management is essential and water resources should critically monitor in the Kalpitiya Peninsula in order to avoid over-exploitation and further seawater inflows.

Direct oxygen isotope effect identifies the rate-determining step of electrocatalytic OER at an oxidic surface.

Understanding the mechanism of water oxidation to dioxygen represents the bottleneck towards the design of efficient energy storage schemes based on water splitting. The investigation of kinetic isotope effects has long been established for mechanistic studies of various such reactions. However, so far natural isotope abundance determination of O2 produced at solid electrode surfaces has not been applied. Here, we demonstrate that such measurements are possible. Moreover, they are experimentally simple and sufficiently accurate to observe significant effects. Our measured kinetic isotope effects depend strongly on the electrode material and on the applied electrode potential. They suggest that in the case of iron oxide as the electrode material, the oxygen evolution reaction occurs via a rate-determining O-O bond formation via nucleophilic water attack on a ferryl unit.

Inter-laboratory test for oxygen and hydrogen stable isotope analyses of geothermal fluids: Assessment of reservoir fluid compositions.

RATIONALE: Knowledge of the accuracy and precision for oxygen (δ18 O values) and hydrogen (δ2 H values) stable isotope analyses of geothermal fluid samples is important to understand geothermal reservoir processes, such as partial boiling-condensation and encroachment of cold and reinjected waters. The challenging aspects of the analytical techniques for this specific matrix include memory effects and higher scatter of delta values with increasing total dissolved solids (TDS) concentrations, deterioration of Pt-catalysts by dissolved/gaseous H2 S for hydrogen isotope equilibration measurements and isotope salt effects that offset isotope ratios determined by gas equilibration techniques. METHODS: An inter-laboratory comparison exercise for the determination of the δ18 O and δ2 H values of nine geothermal fluid samples was conducted among eleven laboratories from eight countries (CeMIEGeo2017). The delta values were measured by dual inlet isotope ratio mass spectrometry (DI-IRMS), continuous flow IRMS (CF-IRMS) and/or laser absorption spectroscopy (LAS). Moreover, five of these laboratories analyzed an additional sample set at least one month after the analysis period of the first set. Statistical evaluation of all the results was performed to obtain the expected isotope ratios of each sample, which were then subsequently used in deep reservoir fluid composition calculations. RESULTS: The overall analytical precisions of the measurements were ± 0.2‰ for δ18 O values and ± 2.0‰ for δ2 H values within the 95% confidence interval. CONCLUSIONS: The measured and calculated δ18 O and δ2 H values of water sampled at the weir box, separator and wellhead of geothermal wells suggest the existence of hydrogen and oxygen isotope-exchange equilibrium between the liquid and vapor phases at all sampling points in the well. Thus, both procedures for calculating the isotopic compositions of the deep geothermal reservoir fluid - using either the analytical data of the liquid phase at the weir box together with those of vapor at the separator or the analytical data of liquid and vapor phases at the separator -are equally valid.

Arsenic-rich shallow groundwater in sandy aquifer systems buffered by rising carbonate waters: A geochemical case study from Mannar Island, Sri Lanka.

Major ion, trace elements, and stable isotope analyses were performed on groundwater samples collected from Mannar Island in the northern Indian Ocean. Arsenic concentrations up to 34µg/L have been observed in groundwater samples from the island. In addition, 23% of extensively used shallow drinking water wells showed higher arsenic contents than the recommended value by the World Health Organization (10µg/L). Groundwater in the island showed pH values between 6.9 and 8.9 and was dominated by Na+, K+, Ca2+, Mg2+, HCO3-, Cl- and SO42-. The δ18OH2O and δ2HH2O composition of most groundwater plotted very close to the local meteoric waterline, however, some wells showed enriched isotope compositions that are most likely due to evaporation. Sea water intrusion in this island was likely of minor importance as indicated by the major ion composition. An approximated mass balance calculation using chloride concentrations indicated that out of the 35 investigated wells only 6 near-shore wells were influenced by sea water intrusion up to about 15%. Even though this is a sandy aquifer, groundwaters were characterized with higher contents of dissolved inorganic carbon (DIC) (2.11-10.9mmol/L). The corresponding δ13CDIC values varied from -19.4‰ to -6.5‰. Except for a few samples with values approaching -20‰, these isotope values are more typical for carbonate dissolution and equilibration of CO2 in the aquifer. This study shows that the underlying carbonate system may buffer the aqueous geochemistry of the groundwater on the island. The high arsenic content in groundwater may have been mobilized through reductive dissolution of Fe-Mn oxides and oxy-hydroxides that are coated on sandy aquifer materials. The lower content of DOC (0.2-1.5mmol/L) provides evidence for the restricted formation of pyrite in the aquifer.

River recharge versus O2 supply from the unsaturated zone in shallow riparian groundwater: A case study from the Selke River (Germany).

Besides gas-water-exchange in surface waters, respiratory consumption of dissolved oxygen (DO) in adjacent riparian groundwater may trigger the addition of so far hardly explored sources from the unsaturated zone. These processes also systematically influence stable isotope ratios of DO and were investigated together with Cl- as a conservative tracer for water mixing in a near-river riparian groundwater system. The study focused on a losing stream section of the Selke River at the foot of the Harz Mountains (Germany). The study area exposed steep DO gradients between the stream water and riparian groundwater between April 2016 and May 2017. Our results indicated dominant influences of microbial community respiration with observed DO concentration gradients. These observations can be explained by DO from the river that is subject to fractionation by microbial respiration with a typical fractionation factor (αr) of 0.982. However, with such respiration dominance, we expected a simultaneous enrichment of δ18ODO towards values that are more positive than the well-known atmospheric O2 signal of +23.9‰ versus the Vienna Standard Mean Ocean Water standard (VSMOW). Surprisingly, our measurements revealed much lower δ18ODO values between +22‰ and +18‰ in the near-river groundwater. Mass balance calculations revealed that the DO pool in the shallow and unconfined aquifer receives contributions of up to about 80% by diffusion of oxygen from the vadose zone with a distinctly lower isotope value than the one of the atmosphere. This finding about additional oxygen sources from the unsaturated zone has numerous ramifications for oxygen related processes in near-river environments including the oxidation of pollutants, nutrients and ecosystem health.

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon - implications for tracing carbon turnover in reservoirs.

The aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L-1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ13CDIC) with the most negative value of -18.4 ‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of -28.5 ‰ from DOC and -23.4, -31.8 and -30.7 ‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ13CDIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.

From Global to Local and Vice Versa: On the Importance of the 'Globalization' Agenda in Continental Groundwater Research and Policy-Making.

Groundwater is one of the most important environmental resources and its use continuously rises globally for industrial, agricultural, and drinking water supply purposes. Because of its importance, more knowledge about the volume of usable groundwater is necessary to satisfy the global demand. Due to the challenges in quantifying the volume of available global groundwater, studies which aim to assess its magnitude are limited in number. They are further restricted in scope and depth of analysis as, in most cases, they do not explain how the estimates of global groundwater resources have been obtained, what methods have been used to generate the figures and what levels of uncertainty exist. This article reviews the estimates of global groundwater resources. It finds that the level of uncertainty attached to existing numbers often exceeds 100 % and strives to establish the reasons for discrepancy. The outcome of this study outlines the need for a new agenda in water research with a more pronounced focus on groundwater. This new research agenda should aim at enhancing the quality and quantity of data provision on local and regional groundwater stocks and flows. This knowledge enhancement can serve as a basis to improve policy-making on groundwater resources globally. Research-informed policies will facilitate more effective groundwater management practices to ensure a more rapid progress of the global water sector towards the goal of sustainability.

Controls of evaporative irrigation return flows in comparison to seawater intrusion in coastal karstic aquifers in northern Sri Lanka: Evidence from solutes and stable isotopes.

Groundwater in Miocene karstic aquifers in the Jaffna Peninsula of Sri Lanka is an important resource since no other fresh water sources are available in the region. The subsurface is characterized by highly productive limestone aquifers that are used for drinking and agriculture purposes. A comprehensive hydrogeochemical study was carried out to reveal the processes affecting the groundwater quality in this region. Major and trace element composition and environmental isotope ratios of oxygen and hydrogen (δ(18)OH2O and δ(2)HH2O) were determined in 35 groundwater samples for this investigation. The ion abundance of groundwater in the region was characterized by an anion sequence order with HCO3->Cl->SO4->NO3-. For cations, average Na(+)+K(+) contents in groundwater exceeded those of Ca(2+)+Mg(2+) in most cases. Ionic relationships of major solutes indicated open system calcite dissolution while seawater intrusions are also evident but only close to the coast. The solute contents are enriched by agricultural irrigation returns and associated evaporation. This was confirmed by the stable isotope composition of groundwater that deviated from the local meteoric water line (LMWL) and formed its own regression line denoted as the local evaporation line (LEL). The latter can be described by δ(2)HH2O=5.8×δ(18)OH2O -- 2.9. Increased contents of nitrate-N (up to 5mg/L), sulfate (up to 430mg/L) and fluoride (up to 1.5mg/L) provided evidences for anthropogenic inputs of solutes, most likely from agriculture activities. Among trace elements Ba, Sr, As and Se levels in the Jaffna groundwater were higher compared to that of the dry zone metamorphic aquifers in Sri Lanka. Solute geochemistry and stable isotope evidences from the region indicates that groundwater in the area is mainly derived from local modern precipitation but modified heavily by progressive evaporative concentration rather than seawater intrusion.