Isotopic Composition Of Groundwater Research Articles

High Plains groundwater isotopic composition in northeastern New Mexico (USA): relationship to recharge and hydrogeologic setting

In the High Plains (HP) region of northeastern New Mexico (NE NM), USA, underlying bedrock aquifers are utilized where the High Plains Aquifer is thin, absent, or unsaturated. These usage patterns, aquifer depletion, and increasing regional aridity imply that NE NM is a possible analogy for more easterly portions of the central HP. To examine the relationship between recharge, residence time, and hydrogeologic setting, 85 well and spring samples were analyzed for environmental tracers (δD, δ18O, δ13C, and limited tritium and carbon-14 activities). Approximately half of the wells were open to strata of the Dakota Group. δD was −105.0 to −41.7‰ (median −58.2‰) and δ18O was −13.7 to −4.4‰ (median −8.1‰). Overall, isotopic composition is correlated with elevation and influenced by hydrogeologic setting. Ten anomalously depleted waters, most near volcanic-capped mesas, may represent higher-elevation or winter-biased recharge, a different modern precipitation source, or recharge from a cooler climate. Recharge, estimated by chloride mass balance using groundwater chloride concentrations, averages 6 mm/year below 2,000-m elevation and 16 mm/year above 2,000 m. Tritium (nondetectable to 5.7 tritium units) and carbon-14 activities (modern carbon fraction 0.23–1.05) suggest that Holocene to modern waters occur, possibly as mixtures, and that alluvial channels and other surficial features promote recharge, likely at higher rates than regional averages. It is noteworthy that isotopically depleted waters in this study tended to be tritium-free. Additional residence time tracers and seasonal precipitation isotopic sampling could address recharge and the origin of depleted waters.

Identification of groundwater conditions in the recharge zone of regionally extended aquifer system with use of water chemistry and isotopes (Lwowek region, Poland)

Study regionThe studied recharge zone of regionally extended Quaternary aquifers - Lwówek region (Wielkpolska, Poland) is located in a region where Quaternary aquifers are the principal source of drinking water. A visible water quality deterioration trend was observed, manifested mainly by increase of nitrate concentration. Study focusThe main objective of the presented study is to examine groundwater flow patterns and vertical difference of groundwater chemistry using a multicomponent chemical tracers approach and groundwater isotopic compositions (stable isotopes, tritium and radiocarbon). New hydrological insights for the regionThis study documented the influence of water extraction on groundwater contamination. The contamination plume that moves to the deepest parts of the aquifer in the region, where water extraction is performed, was documented. As a consequence, in deep parts of the aquifer, nitrates appear, and the groundwater chemistry is very similar to that observed in the shallow part. Meanwhile, in regions where natural gradients exist (without water extraction) the contamination in the shallow part is also evident, but recently infiltrating water does not reach the deeper parts of the aquifer. A stagnation point exists in the deep part of the aquifer, with completely different groundwater chemistry in the deep and shallow parts of the aquifer. This work shown how groundwater chemistry can help to investigate groundwater flow systems.

Linking precipitation and groundwater isotopes in Ethiopia - Implications from local meteoric water lines and isoscapes

Sustainable water resource management requires a clear understanding of the spatial variation and seasonal recharge of groundwater and their links to local precipitation. A combined spatial analysis of the stable isotope composition of groundwater, surface water, and precipitation could help to understand groundwater sources and recharge processes. In this study, we collected 297 groundwater and stream water samples across Ethiopia and analyzed them for the oxygen and hydrogen isotope ratios. The isotope data were then interpolated into isoscapes for the entire country using the Cokriging method based on their spatial autocorrelation and covariance with elevation, annual mean precipitation, and temperature. These are the first groundwater and stream isoscapes for Ethiopia. By comparing the spatial isotopic distribution of the two water resources with local precipitation, we showed distinct groundwater recharge patterns in different regions in terms of their water sources, timing, seasonality, and the degree of surface and groundwater interaction. We found that the groundwater recharge was generally biased towards the main rainy season. However, in the relatively dry southeastern Ethiopia, groundwater could receive significant input from infrequent high-intensity rainfalls associated with tropical cyclones, but additional precipitation and groundwater isotope data are needed to further test our hypothesis in this data-poor region. Overall, this study has largely expanded the spatial coverage of water stable isotope data and filled significant data gaps in Ethiopia, which could be used to provide vital information in the arid and semi-arid parts of the country where water resources are critical in climate change adaptation strategies.

Determination of soil-groundwater systems recharge mechanism in the middle Inner Mongolia Plateau by isotopic tracers

The steppe located in the middle Inner Mongolia Plateau (IMP) has scarce water resources and fragile ecosystems and has an arid-semiarid monsoon characterized by low and uneven seasonal precipitation and high evaporation. Until now, problems relating to the hydrologic processes in the area remain poorly understood, and thus strategies for the exploitation and utilization of the area’s local water resource remain limited. This study aimed to investigate the recharge and discharge processes of soil water and the source and circulation of groundwater through geochemical methods. The local meteoric water line (LMWI) of our study area in the middle IMP was first proposed and compared with the local meteoric water line of Baotou in Inner Mongolia and Ulan Bator. The δ18O and δD values of precipitation showed an increasing trend from June to August and decreasing trend from August to late October. The H and O isotopes of soil water varied with time, location, and soil profile depth. Besides the geographical and climatic factors, the Cl concentrations of soil water can be affected remarkably by land use type and land cover change. In our monitoring period, variations in the Cl concentrations and δ18O and δD values of shallow soil water indicated that the evaporation of soil water was in the sequence August > June > October. Furthermore, the detailed evaporation proportions of soil water were estimated. The isotope compositions of groundwater plotted on the LMWI and there could be a rapid recharge mechanism of the groundwater.

Source and Quality of Groundwater Surrounding the Qinghai Lake, NE Qinghai‐Tibet Plateau

Comprehensive studies on the spatial distribution, water quality, recharge source, and hydrochemical evolution of regional groundwater form the foundation of rational utilization of groundwater resources. In this study, we investigated the water levels, hydrochemistry, and stable isotope composition of groundwater in the vicinity of the Qinghai Lake in China to reveal its recharge sources, hydrochemical evolution, and water quality. The level of groundwater relative to the level of water in the Qinghai Lake ranged from -1.27 to 122.91 m, indicating most of the groundwater to be flowing into the lake. The local evaporation line (LEL) of groundwater was simulated as δ2 H=6.08 δ18 O-3.01. The groundwater surrounding the Qinghai Lake was primarily recharged through local precipitation at different altitudes. The hydrochemical type of most of the groundwater samples was Ca-Mg-HCO3 ; the hydrochemistry was primarily controlled by carbonate dissolution during runoff. At several locations, the ionic concentrations in groundwater exceeded the current drinking water standards making it unsuitable for drinking. The main source of nitrate in groundwater surrounding the Qinghai Lake was animal feces and sewage, suggesting that groundwater pollution should be mitigated in areas practicing animal husbandry in the Qinghai-Tibet Plateau, regardless of industrial and urbanization rates being relatively low in the region. The scientific planning, engineering, and management of livestock manure and wastewater discharge from animal husbandry practices is a crucial and is urgently required in the Tibetan Plateau.

Effects of Spatial Variability in the Groundwater Isotopic Composition on Hydrograph Separation Results for a Pre‐Alpine Headwater Catchment

AbstractIsotope hydrograph separation is a powerful tool to investigate catchment functioning. In most hydrograph separation studies, a pre‐event baseflow sample is used to represent the pre‐event water, and thus, baseflow is assumed to be a mixture of all the water that is stored in the catchment. However, baseflow may not be representative of all water stored in the catchment because some sources may not contribute to baseflow. This is problematic when the isotopic composition of the sources is highly variable. We quantified the effects of spatial variability in the shallow groundwater isotopic composition on pre‐event water characterization and hydrograph separation results. We compared the composition of groundwater sampled at 38 wells in a 0.2 km2 pre‐alpine catchment with stream water sampled before, during, and after three rainfall events. We estimated the number of groundwater samples needed to characterize the average groundwater composition in the catchment and its spatial variability and compared the results of two‐component hydrograph separations for different ways to characterize the pre‐event water. We found that differences in the calculated pre‐event water fractions and uncertainties were large and depended on which and how many samples were used to characterize the pre‐event water composition. Analyses based on a limited number of groundwater samples likely underestimate the real uncertainty and can give a false impression of accuracy. Our results highlight the importance of representing the variability in the pre‐event water composition when applying hydrograph separation analyses. We therefore recommend sampling pre‐event water at multiple locations or estimating the variability based on literature values.

Using stable isotopes to assess river water dynamics and groundwater input in the largest European Arctic river (Severnaya Dvina).

Rivers play a key role in the water cycle on the earth via integrating all hydrological channels to return terrestrial precipitations back to the oceans. In addition, rivers, together with groundwater, are powerful transformers of the surface lithosphere, responsible for chemical weathering of rocks and the removal of solute into the ocean. Tracing the dynamics of surface water and groundwater versus atmospheric feeding of rivers presents important issues in Arctic regions due to the ongoing change of the structure of hydrological runoff. In this study, stable water isotopes were used to reveal the temporal dynamics of water sources and to predict their possible change under the conditions of ongoing climate warming of the largest European Arctic river, the Severnaya Dvina, and adjacent groundwater. The isotopic composition of the river waters of the studied region is formed by the mixing of atmospheric precipitation with groundwater. The isotopic depletion in the springtime is mainly due to the recharge of thawed snow waters. A less pronounced effect in the autumn-winter period is provided by the discharge of groundwater into rivers, including the meltwater of the Last Glacial Period. This depletion is partially offset due to discharge of isotopically light thawed snow waters and is linked to evaporation in headwater streams, reservoirs, and wetlands. The isotopic composition of groundwater with low mineralization was formed throughout the Holocene and to a large extent depends on paleoclimatic conditions in the study area. In addition to fresh groundwater, brackish groundwater also takes part in the river's recharge. These brackish waters are associated with ancient and modern marine transgressions on the estuarine site and with the dissolution of Ca sulfate rocks in a karst region located in the middle reaches of the river. According to isotope data, the average annual input of the underground source to the total river flow is 25%. The results of this work will serve as the basis for continuing monitoring of the isotopic composition of river waters with an assessment of hydrological processes and observation of short as well as long-term climatic and anthropogenic impacts.

Hydrogeochemical characteristics of underground waters in the suture zone of the Ufa brachyanticline and the Pre-Urals foreland basin

During the period of long-term research, since the end of the XIX century, a large hydrogeochemical material has been accumulated within the suture zone of the Ufa brachyanticline and the Pre-Urals foreland basin. There are very few works on studying the conditions of natural water formation from the perspective of modern methodologies based on the study of variations in the isotope ratios of hydrogen and oxygen within the study area. The main goal of the work is to establish the conditions for the formation of the isotopic and chemical composition of groundwater of the Lower Perm aquifer complex, which is distributed within the suture zone of the Ufa brachyanticline and the Pre-Urals foreland basin, which belongs to the Kishert karst region. The features of the formation of the chemical and isotopic composition of groundwater in the suture zone of the Ufa brachyanticline and the Pre-Urals foreland basin in the Kishert region are analyzed based on data obtained in the period 2017–2018. In the Western part of the region, there is a watered zone associated with tectonic disturbances, where karst water is discharged. The chemical composition of the karst waters of the Kishert region has been studied quite fully. Data on the isotopic composition (deuterium and oxygen–18 content), which has been studied to a lesser extent, were used for a detailed study of the conditions for the formation of groundwater composition. In addition, the chemical and isotopic composition of precipitation and river water is considered. It is shown that the source at the Nizkoe village, discharging near the contact of carbonate-sulfate and terrigenous rocks, is characterized by the НСО3 –SO4 –Ca–Mg composition and mineralization 550,0–738,0 mg/dm3 , the source at the Zuevo village, which unloads within distribution of terrigenous rocks of the Kungurian stage, has a SO4 –Ca composition and high mineralization of 1623,0-2110,0 mg/dm3 . The isotopic composition of the two sources varies within rather narrow limits and is lighter than the weighted average value of atmospheric precipitation, which indicates that these sources are primarily recharge by light snow waters of the winter and spring periods. The waters of the lakes Molebnoe and Proval mainly НСО3 –Ca composition, they are characterized by vertical hydrochemical zonality associated with a change in chemical parameters. An increase in the mineralization of lake water indicates a groundwater inflow from sulfate (Proval lake) and carbonate (Molebnoe lake) sediments of the Kungurian stage. Lake waters have the heaviest and relatively stable composition due to the processes of evaporation from the surface and discharge of deep waters at the bottom of the lakes. The relationship between atmospheric precipitation, river waters and the waters of Proval lake and the source of the village of Nizkoe was established, and for the Zuevsky source only with river waters. A weak correlation was revealed for the Molebnoye lake with precipitation (r = 0,25) and river waters (r = 0,12).

Groundwater isotope ratios reflect convective and stratiform (paleo)precipitation fractions in Brazil

The stable isotope ratios of oxygen and hydrogen in shallow groundwater reflect the long-term amount-weighted mean isotopic composition of precipitation. Here, we report the isotopic composition of more than 1000 shallow groundwater samples (age < ∼5,000 yr) across continental Brazil. Recent studies of global precipitation have shown a strong negative correlation between the isotopic compositions (e.g., δ18O values) and stratiform fractions of Brazilian precipitation, with geospatial correspondence to the South Atlantic convergence zone pattern. This isotope-precipitation type geospatial correlation is also reflected in the isotopic compositions of shallow groundwaters, and speleothem records from the region show small variations in δ18O values, implying general climatic stability in this region during the past few millennia. We propose that shallow groundwater isotopic compositions reflect bulk proportions of convective and stratiform fractions in precipitation and could thus be useful in conjunction with groundwater dating as proxy archives to characterize past changes in the proportions of convective and stratiform precipitation in tropical and subtropical latitudes. This groundwater isotope-precipitation type relationship could also have broader implications when considered for paleoclimatic interpretation of oxygen isotope records in dated speleothems from other locations.

Temporal Variations in Scrubbing of Magmatic Gases at the Summit of Kīlauea Volcano, Hawai‘i

AbstractMeasurements of gas compositions and emission rates play a major role in monitoring restless volcanoes. However, thermodynamic calculations imply that scrubbing by groundwater will prevent most HCl and significant SO2 emissions until dry pathways are established, thus leading to underestimates of gas released from magma and magma volumes. Despite the significance, direct evidence for scrubbing is mostly lacking. Based on 50 water samples collected between 2003 and 2011 from the deep NSF Well at the summit of Kīlauea Volcano, we show that the chemical and stable isotope compositions of groundwater were modified by magmatic gas condensation. Temporal variations of dissolved SO42− and Cl− in the water coincided with changes in magmatic and volcanic activities. The temporal variations in water level and chemical and isotopic compositions support prior inferences of magmatic SO2 and HCl scrubbing by groundwater at Kīlauea Volcano.

Hydrogeochemical and isotopic characteristics of groundwater in Xinchang preselected site and their implications.

The safety disposal of high-level radioactive waste (HLW) has become an important issue for nuclear energy and environmental protection. Water chemistry and environmental isotope are accepted as feasible ways to trace groundwater circulation; it can effectively reveal the conversion relationship between different groundwater of the disposal site. Geochemical and isotopic tracers were used to constrain origins and chemical evolution of groundwater in the arid fissure system of the Xinchang preselected site for high-level radioactive waste geological disposal in China. Groundwater level, water temperature, and water chemistry information at different depths were obtained by multi-layer groundwater monitoring. The results show that the chemical and isotopic composition of groundwater in this fissure system is mainly controlled by evaporation, the water chemistry type of the shallow groundwater is mainly Na-Cl-SO4 or Na-Ca-Cl-SO4, and the deep groundwater is mainly Na-Cl-SO4. Based on the values of monitoring data in deep borehole, the fluctuation of groundwater level is less than 0.40m with weak hydrodynamic condition, and the geothermal gradient is 1.91°C/100m. The isotope analyses indicate that the groundwater in the system recharged by local atmospheric precipitation, and the deep groundwater recharged capacity of the site is weak and with no deep cycle.

Regional isotopic investigation of evaporation and water-rock interaction in mine pit lakes in Nevada, USA

The isotopic geochemistry of mine-impacted waters may reveal water-budget relationships and geochemical mechanisms of water-rock interaction. In this study, stable isotopic analysis of water (δ2H and δ18OH2O) and sulfate (δ34S and δ18OSO4) were completed and cataloged in 15 mine pit lakes in Nevada, USA, representing the most spatially distributed and largest dataset related to the isotopic composition of pit lakes yet presented in the literature. The isotopic composition of groundwater and surface springs was also included at several sites to more fully characterize the connected groundwater-surface water systems. Results of δ2H and δ18OH2O analysis indicate that the pit lakes are highly evaporated (more so than natural lakes in the same region), although the strength of evaporative fractionation differs spatially. Pit lakes also vary in the degree to which they are enriched in heavy isotopes relative to influent groundwater, which gives insight into the basin-scale water budget for these areas. Isotopes of sulfate (δ34S and δ18OSO4) and water (δ18OH2O) were used to test several commonly-held models of sulfide oxidation, and results suggest that none of the applied models can fully explain the isotopic composition of pit-lake waters. Most sulfur isotope data suggest a mixed source of aqueous sulfate including the oxidation of pyrite and dissolution of sulfate salts.

A 35ka record of groundwater recharge in south-west Australia using stable water isotopes.

The isotopic composition of groundwater can be a useful indicator of recharge conditions and may be used as an archive to infer past climate variability. Groundwater from two largely confined aquifers in south-west Australia, recharged at the northernmost extent of the westerly wind belt, can help constrain the palaeoclimate record in this region. We demonstrate that radiocarbon age measurements of dissolved inorganic carbon are appropriate for dating groundwater from the Leederville aquifer and Yarragadee aquifer within the Perth Basin. Variations in groundwater δ18O values with mean residence time were examined using regional and flow line data sets, which were compared. The trends in the regional groundwater data are consistent with the groundwater flow line data supporting the hypothesis that groundwater δ18O is a robust proxy for palaeo-recharge in the Perth Basin. A comparison between modern groundwater and rainfall water isotopes indicates that recharge is biased to months with high volume and/or intense rainfall from the westerly wind circulation and that this has been the case for the last 35ka. Lower stable water isotope values are interpreted to represent recharge from higher volume and/or more intense rainfall from 35ka through the Last Glacial Maximum period although potentially modulated by changes in recharge thresholds. The Southern Perth Basin groundwater isotopic record also indicates a trend towards higher volume and/or intense rainfall during the Mid- to Late Holocene. The long-term stable water isotope record provides an understanding of groundwater palaeo-recharge. Knowledge of recharge dynamics over long time scales can be used to improve current water sharing plans and future groundwater model predictions.

The effect of geochemical processes on groundwater in the Velenje coal basin, Slovenia: insights from mineralogy, trace elements and isotopes signatures

This study investigated the mineralogical and isotopic composition of groundwater and precipitation to identify and constrain geochemical processes within stacked Pliocene and Triassic aquifers in the Velenje coal basin. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopic analysis revealed that suspended matter in the Pliocene aquifer consists of feldspars and quartz, while dolomite, calcite and feldspars are present in the aquifer dewatering Triassic strata. The concentrations of trace elements in Triassic and Pliocene aquifers range from highest to lowest Zn > Fe > Ni > Al > Ba > Mn > B>Li > Mo > As with the majority of trace element concentrations below international drinking water health guidelines. Multivariate principal component analysis indicated that concentrations of Mn, Ba, Eu, Cs, Y, Li and T, pH, conductivity and dissolved oxygen in samples were the best chemical parameter for distinguishing the two aquifers. A significant positive correlation (p < 0.05) was found between Ni, Mn, Co, Zn, As and Mo. Groundwater in the Pliocene aquifer likely has an external source of carbon based on the δ13CCO2 values (− 12.3 to − 3.6‰). The groundwater also has detectable levels of dissolved methane with isotopic values (− 77.7 to − 51.4‰ δ13CCH4; − 247 to − 162‰ δ2HCH4) consistent with microbial methanogenesis. The groundwater in the Triassic aquifer has tritium values (up to 4.1 TU 3H) characteristic of modern recharge (< 50 years), while the lack of detectable 3H (0 TU) in the Pliocene aquifer is consistent with longer residence times.

Evolution of the chemical and isotopic composition of groundwater under a semi-arid climate; the case of the Cenomano-Turonian aquifer within the Essaouira basin (Morocco)

This study evaluated how the chemical and isotopic composition of the groundwater in the Cenomano-Turonian aquifer of the Essaouira basin from Morocco has changed in response to low annual rainfall associated with climate change during the period 1995–2018. The study of the chemical composition of groundwater shows a Cl–Ca–Mg type facies which was maintained during the observation period (1995–2018). The electrical conductivity (EC) and the Cl− content of groundwater increased during the study period in response to low rainfall with a mean values range from 1214 (1995) to 2482 µs/cm (2018) for EC and from 11.36 (1995) to 14 meq/L (2018) for Cl−. Stable isotope measurements indicate that the Cenomano-Turonian aquifer is recharged by rainfall of Atlantic origin. Levels of 18O during the period 2007 and 2016 were generally stable with slight fluctuations (≈ 1‰) that may be due to analytical errors, suggesting that climate change has no effect on the isotopic signature of groundwater in the study area. Tritium levels in groundwater decreased between 2007 and 2016, possibly due to low recharge rates associated with low annual rainfall during this period.

Hydrogeologic and geochemical survey of aquifers based on chemical and isotopic characterisation of groundwater and rain waters: a case study in the Sisseb el Alem Basin (central-east Tunisia)

Chemical and isotopic composition of groundwaters and rain waters in the Sisseb el Alem basin (Tunisia) was monitored to obtain a hydrologic and geochemical characterization of the aquifers. The studied aquifers are the only water resource for domestic and irrigation use in the basin. Two aquifers have been identified by hydrogeologic data, one, shallower, flowing in Plio-Quaternary sediments, the other, deeper, in Miocenic–Oligocenic sediments. Although this area is intensively farmed with widespread fertiliser use and settlements are unsewered, groundwater in the area is relatively uncontaminated by land-use activities. The chemical and isotopic compositions of the sampled waters, which have a meteoric origin, are largely controlled by both the geological signatures of their drainage area and by evaporative processes and water–rock interaction (reverse cationic exchange) phenomena. Geochemical data suggest, on the basis of the Na/Cl, Cl/SO4 ratios and concentrations of Li, Sr, and Mn, that a hydraulic connection exists between two distinctive aquifers. One strongly influenced by Lake Saadine water characterised by Na/Cl > 1, high Cl/SO4 and Li, Sr, and Mn > 0.1 meq/l, the other, deeper, characterised by Na/Cl < 1, low and near constant Cl/SO4 and Li, Sr, and Mn < 0.1 meq/l. Despite relatively high salinity and SAR values, all sampled waters are suitable for irrigation in well-draining soils. The isotopic composition of the rain waters highlights the importance of evaporation in controlling the composition of the groundwater.

High-Frequency Water Isotopic Analysis Using an Automatic Water Sampling System in Rice-Based Cropping Systems

High-resolution data on a field scale is very important for improving our understanding of hydrological processes. This is particularly the case for water-demanding agricultural production systems such as rice paddies, for which water-saving strategies need to be developed. Here we report on the application of an in situ, automatic sampling system for high-resolution data on stable isotopes of water (18O and 2H). We investigate multiple rice-based cropping systems consisting of wet rice, dry rice and maize, with a single, but distributed analytical system on a sub-hourly basis. Results show that under dry conditions, there is a clear and distinguishable crop effect on isotopic composition in groundwater. The least evaporative affected groundwater source is that of maize, followed by both rice varieties. Groundwater is primarily a mixture of irrigation and rainwater, where the main driver is irrigation water during the dry season and rainwater during the wet season. Stable isotopes of groundwater under dry season maize react rapidly on irrigation, indicating preferential flow processes via cracks and deep roots. The groundwater during the dry season under wet and dry rice fields is dominated at the beginning of the growing season mainly by the input of rainwater; later, the groundwater is more and more replenished by irrigation water. Overall, based on our data, we estimate significantly higher evaporation (63–77%) during the dry season as compared to the wet season (27–36%). We also find, for the first time, significant sub-daily isotopic variation in groundwater and surface ponded water, with an isotopic enrichment during the daytime. High correlations with relative humidity and temperature, explain part of this variability. Furthermore, the day-night isotopic difference in surface water is driven by the temperature and relative humidity; however, in groundwater, it is neither driven by these factors.

Groundwater and surface water connectivity within the recharge area of Guarani aquifer system during El Niño 2014–2016

AbstractRecharge areas of the Guarani Aquifer System (GAS) are particularly sensitive and vulnerable to climate variability; therefore, the understanding of infiltration mechanisms for aquifer recharge and surface run‐off generation represent a relevant issue for water resources management in the southeastern portion of the Brazilian territory, particularly in the Jacaré‐Pepira River watershed. The main purpose of this study is to understand the interactions between precipitation, surface water, and groundwater using stable isotopes during the strong 2014–2016 El Niño Southern Oscillation event. The large variation in the isotopic composition of precipitation (from −9.26‰ to +0.02‰ for δ18O and from −63.3‰ to +17.6‰ for δ2H), mainly associated with regional climatic features, was not reflected in the isotopic composition of surface water (from −7.84‰ to −5.83‰ for δ18O and from −49.7‰ to +33.6‰ for δ2H), mainly due to the monthly sampling frequency, and groundwater (from −7.04‰ to −7.76‰ for δ18O and from −49.5‰ to −44.7‰ for δ2H), which exhibited less variation throughout the year. However, variations in deuterium excess (d‐excess) in groundwater and surface water suggest the occurrence of strong secondary evaporation during the infiltration process, corresponding with groundwater level recovery. Similar isotopic composition in groundwater and surface water, as well as the same temporal variations in d‐excess and line‐conditioned excess denote the strong connectivity between these two reservoirs during baseflow recession periods. Isotopic mass balance modelling and hydrograph separation estimate that the groundwater contribution varied between 70% and 80%, however, during peak flows, the isotopic mass balance tends to overestimate the groundwater contribution when compared with the other hydrograph separation methods. Our findings indicate that the application of isotopic mass balance methods for ungauged rivers draining large groundwater reservoirs, such as the GAS outcrop, could provide a powerful tool for hydrological studies in the future, helping in the identification of flow contributions to river discharge draining these areas.

Can groundwater sampling techniques used in monitoring wells influence methane concentrations and isotopes?

Methane concentrations and isotopic composition in groundwater are the focus of a growing number of studies. However, concerns are often expressed regarding the integrity of samples, as methane is very volatile and may partially exsolve during sample lifting in the well and transfer to sampling containers. While issues concerning bottle-filling techniques have already been documented, this paper documents a comparison of methane concentration and isotopic composition obtained with three devices commonly used to retrieve water samples from dedicated observation wells. This work lies within the framework of a larger project carried out in the Saint-Édouard area (southern Québec, Canada), whose objective was to assess the risk to shallow groundwater quality related to potential shale gas exploitation. The selected sampling devices, which were tested on ten wells during three sampling campaigns, consist of an impeller pump, a bladder pump, and disposable sampling bags (HydraSleeve). The sampling bags were used both before and after pumping, to verify the appropriateness of a no-purge approach, compared to the low-flow approach involving pumping until stabilization of field physicochemical parameters. Results show that methane concentrations obtained with the selected sampling techniques are usually similar and that there is no systematic bias related to a specific technique. Nonetheless, concentrations can sometimes vary quite significantly (up to 3.5 times) for a given well and sampling event. Methane isotopic composition obtained with all sampling techniques is very similar, except in some cases where sampling bags were used before pumping (no-purge approach), in wells where multiple groundwater sources enter the borehole.

Hydrogeochemical and isotopic evaluation of groundwater with elevated arsenic in alkaline aquifers in Eastern Punjab, Pakistan

Geochemical investigation was carried out for delineating factors responsible for the mobilization of arsenic (As) from aquifer material into the groundwater. Four sites along Ravi River, (Samada, Sarai Chimba, Kot Maiga and Chah Fatehwala), were selected based on the blanket survey. Groundwater-rock interaction and evaporation were the key phenomena controlling groundwater chemistry, as shown by the hydrogeochemical data. Groundwater was predominantly Na-Cl type, with other principle facies being Na-HCO3, Na-Ca-HCO3 and Ca-Mg-Cl. The groundwater As concentration ranged between below detection level (2 μg/L) to 548 μg/L with 59% samples exceeding the World Health Organization (WHO) guidelines for As in drinking water (10 μg/L) and 31% having higher concentrations than the National Environmental Quality Standard (NEQS, 50 μg/L). Moderate to high concentrations of SO4−2 averaged at 244 mg/L and moderate NO3− concentrations averaged at 8 mg/L, together with alkaline pH (7.3–8.8) and high Eh values (113–402 mV) suggest partial oxidizing nature of the aquifers. The values for δ 18O and δ 2H in groundwater varied between −9.14 and −5.51‰, and −56.57 to −39.5‰ respectively, and suggests meteoric origin of the groundwater with some evaporative loss. This effect could be partly responsible for elevated levels of pH and salinity in groundwater. Based on geochemical and isotopic composition of groundwater, desorption of As from metal surfaces under alkaline environment might be the factor causing As enrichment in study area.