Isotopic Composition Of Groundwater Research Articles

Characterization of recharge sources of the Miocene Fluvial Moghra aquifer in the North Western Desert of Egypt

Study regionThe Miocene Fluvial Moghra Aquifer in the North Western Desert of Egypt Study focusThe study integrates stable isotope analyses with aeromagnetic and hydrogeological datasets to accomplish the following: (1) define the primary source(s) of recharge to the Moghra aquifer, (2) assess aquifer connectivity with the Nile River aquifer, and (3) investigate the vertical connectivity of the Moghra aquifer with the underlying Nubian Aquifer System (NAS) through subvertical faults. New hydrological insights for the regionThe findings reveal (1) significant heterogeneity in groundwater isotopic compositions (Group A: δ18O: 1.1–13.8 ‰; δD: 4.6–73.5 ‰, B: δ18O:−0.99 to 0.85; δD: −7.58 to 4.38 ‰; and C: δ18O:−1.1 to −3.4; δD: − 6.3 to −18.2 ‰) indicative of variability in recharge sources. (2) Groundwater compositions west (up to 30 km) of the Nile River (Group A) resemble enriched modern Nile waters following the Aswan High Dam (AHD) construction that give way further west to relatively depleted groundwater (Group B) resembling historical pre-AHD Nile water compositions. (3) Further west from Group B depleted Group C samples occur along intersections of multiple fault systems (NW and NE-oriented faults) interpreted as mixtures of rising highly-depleted paleo Nubian Aquifer System (NAS) waters and pre-AHD Nile waters. (4) The advocated structural control is reported in similar settings in Egypt, suggesting that intersections of multiple fault systems provide regional connections between deep and shallow aquifers in northeast Africa, recharge overlying shallow aquifers, and should be considered in groundwater management scenarios.

Linking recharge water sources to groundwater composition in the Hindon subbasin of the Ganges River, India

Groundwater resources of the densely populated Indo-Gangetic Basin are under increasing pressure, not only from extensive groundwater abstraction, but also from contamination. In this study we aim to better understand how different recharge sources affect the hydrochemical and isotope composition of groundwater. We used the Hindon subbasin in Northern India as a case study. Recharge water sources and groundwater were analysed for hydrochemical variables and stable isotopes along a 50 km transect between the Yamuna and Ganges rivers. Groundwater samples were statistically clustered based on hydrochemical variables, and the spatial variation of the groundwater clusters was compared with recharge sources.Groundwater quality could be linked to both recharge from irrigation canal water as well as recharge from polluted river water of the Hindon and its tributaries. We could not directly link groundwater outside these related zones to their recharge source. However, we suspect that shallow polluted groundwater (< 40 m depth) is affected by recharge from agricultural areas and infiltration of municipal wastewater, whereas deeper unpolluted groundwater (40–80 m depth) originates from recharge by rain and river water under more pristine conditions. Our findings show that human activities significantly impact the quality of groundwater, as we found vertical recharge of clean irrigation canal water and polluted municipal, agricultural and river surface water (up to 40 m depth). At one location, groundwater at 75 m depth shows increased Cl, NO3 and SO4 concentrations, suggesting accelerated downward displacement of polluted shallow groundwater by pumping. Limited horizontal displacement was found. We present a conceptual model demonstrating the evolution from a previously unpolluted groundwater system discharging to the river, to a contemporary system with infiltration dominance of polluted river, municipal and agricultural water and local clean irrigation canal water. This model may be relevant for large parts of the Indo-Gangetic Basin.

Water Geochemistry and Stable Isotope Changes Record Groundwater Mixing After a Regional Earthquake in Northeast India

AbstractRecorded earthquake‐induced changes in hydrogeological systems date back over 2,000 years. As a part of our ongoing hydrogeochemical monitoring effort to study such changes, we collected 406 groundwater samples twice a week between February 2021 and July 2023 from two bore wells in the Kopili fault zone of Northeast India. We analyzed stable isotope ratios (δ2H, δ18O) and dissolved element concentrations to obtain a 2.5‐year hydrogeochemical time series and responses to multiple regional earthquakes (Mw ≥ 3) within the monitored period. We find significant but transient anomalies in both the chemical and isotopic composition of groundwater at one of the observation wells (OW1) after the 2021 Assam Mw 6.4 earthquake, followed by prolonged alterations in the hydrochemistry at both wells. We do not identify any precursory changes. Using multivariate statistical techniques and analyzing compositional changes before and after the mainshock, we infer that the hydrochemical anomalies at OW1, representing an immediate response to the mainshock, can be attributed to the potential breach of a hydrological barrier. This, in turn, allowed the infiltration of new water into the OW1 aquifer, potentially sourced from the nearby Brahmaputra River. Subsequently, during the post‐anomaly period, the earthquake‐induced fracturing and the associated changes in permeability sustained a prolonged period of mixing between surface water and groundwater, resulting in newly formed hydrochemistry at both wells. Our findings highlight the dynamic nature of aquifer properties during earthquakes. Long‐term continuous evaluation of such changes may provide new insights into feedback between tectonics and fluid flow in the crust.

Stable Isotope Hydrology of Karst Groundwaters in Romania

In this article we present the first investigation of the stable isotope composition of groundwater in Romania, East-Central Europe, with a focus on the karst areas. Our aim is twofold: (1) to provide a countrywide map with the distribution of stable oxygen and hydrogen isotope ratios in groundwater, and (2) to assess the recharge patterns of karst water. We collected more than 600 water samples from springs and wells across Romania for stable isotope analyses and monitored in detail the stable isotope composition of the waters as they pass through five cave systems. Our data show a spatial distribution of the stable isotope composition of the groundwater with low values in the mountainous area and high values in the surrounding lowlands and the central Transylvanian Depression. However, waters in karst areas induce departures from this distribution, resulting from the fast (hours to days) transfer of waters from high (ponor) to low (spring) altitudes. Water emerging from the karst springs has generally lower δ values than before sinking through the ponors, thus indicating a substantial contribution of winter recharge through diffuse infiltration and seepage. This contribution results in overall dilution of the water entering through ponors, likely resulting in changes in the chemical composition of the water and diluting potential pollutants. Our data call for careful separation between karst and non-karst spring/well waters, as indiscriminate common treatment might lead to erroneous interpretations.

Hydrodynamics control for the well field of in-situ leaching of uranium

In this study, the groundwater hydrodynamics of two adjacent well sites were simulated under different pumping-injection ratios. The aim is to select an optimal pumping-injection ratio that can ensure the groundwater of the two well sites don't affect each other. In addition, the sulfur isotope composition of groundwater in the two well sites were analyzed to verify the simulated results. The results show that the flow velocity at different points outside the edge drilling hole decreases exponentially with the distance between the point and the edge hole. The streamline gradually extends outside of the borehole with the increase of leaching time. It is found that the optimal pumping-injection ratio is 1.003. In this case, the maximum distance between the moving front and the injection borehole is 28.44 m after leaching for 5 years. This indicates that the groundwater flow fields of the two well sites are well controlled. The significant difference in sulfur isotopes between the two well-sites further proves that the SO42− in the acid mining zone does not affect the groundwater in the zone leached by CO2+O2.

The transmission of isotopic signals from precipitation to groundwater and its controls: An experimental study with soil cylinders of various soil textures and burial depths in a monsoon region

The use of stable isotopic composition in groundwater as a paleoclimatic proxy assumes that the isotopic values in groundwater are inherited from the corresponding values in the source precipitation. However, several studies have observed that groundwater δ18O (δ18Og) is not always in accordance with the average δ18O of precipitation (δ18Op). The cause of this inconsistency is still the subject of debate. Significant reduced isotopic variability and potential deviations from precipitation to groundwater because of the aquifer itself (e.g., groundwater buried depths and soil texture) and external precipitation characteristics (e.g., rainfall intensity and duration) have been poorly quantified in terms of identifying the dominant factors. To investigate the controls on the transmission of isotopic signals from precipitation to groundwater, we present new stable isotope data for precipitation and groundwater from an experimental site using eight soil cylinders of different soil textures and burial depths in a monsoon region. We collected data at inter-event sampling intervals (30 min) during and after two heavy precipitation events (daily amounts higher than 10 mm). Our results demonstrated a significant disparity between the average isotopic values of heavy rainfall and light rainfall, despite both exhibiting decreasing isotopic patterns throughout the course of the rainfall event. Various degrees of attenuation of isotopic variations were found in outflow waters with different groundwater burial depths (GBDs) and soil textures after rainfall events. The isotopic variations were not synchronized with the outflow changes. Specifically, there were evident outflow lags in response to heavy rainfall, especially in the lysimeters with GBDs deeper than 0.5 m, while the greatest isotopic variation was observed in the early infiltrating stage. Redundancy analysis indicated that precipitation amount was the most important contributor to the outflow variation (∼51 %, p < 0.05), while GBD was the key factor for isotopic temporal variation (∼52 %, p < 0.05). These results concurred with the clustering of principal component analysis results. The mixing of soil water and heavy rainfall event components was observed in four lysimeters with mixing proportions fP varying in the range of 7 %–51 %, indicating the effective recharge from heavy rainfall. Soil evaporation was highlighted with samples located parallel to the local meteoric water line. It is recommended that groundwater isotopes in paleoclimatic studies are used with additional constraints that consider disproportionate recharge from intense rainfall, past chaotic fluctuations in groundwater dynamics, and distortions introduced by evaporation.

Assessment of groundwater recharge and connectivity with surface water in a mountainous watershed using natural tracers in Daejeon, Korea

The water supply from headwater streams in mountainous regions is considered an important source for sustaining both water quality and quantity in lowland areas. The Korean terrain is characterized by mountainous regions, the hydrological environment is significantly impacted by seasonal weather conditions. This study focused on investigating the hydrochemistry and isotopic composition of groundwater and surface water to identify hydrological connectivity within a mountainous watershed area in Daejeon, Korea. The estimated recharge rate using water budget methods suggests that approximately 20% of the total precipitation contributes to groundwater recharge in this site. The δ18O–δ2H values of the water samples indicate a meteoric water source for groundwater recharge, while the isotope composition of surface water reveals altitude effects, implying that groundwater recharges at a higher altitude region. Additionally, water revealed altitude effects suggesting that the groundwater was inferred to recharge at a higher altitude region. The hydrochemical conservative components (87Sr/86Sr ratio and Cl−) indicate that this watershed undergoes temporary similar water–rock interactions along its flow path, but it is also impacted by anthropogenic contaminants from the surrounding public area. The results of the three-component endmember mixing analysis demonstrate that groundwater is predominantly influenced by surface water, indicating a close interrelationship among various water bodies in mountain hydrology. These findings provide a comprehensive approach to water resource management by combining recharge rate estimation and the assessment of water body connectivity using natural tracers.

Combining hydrochemistry and 13C analysis to reveal the sources and contributions of dissolved inorganic carbon in the groundwater of coal mining areas, in East China.

East China is a highly aggregated coal-grain composite area where coal mining and agricultural production activities are both flourishing. At present, the geochemical characteristics of dissolved inorganic carbon (DIC) in groundwater in coal mining areas are still unclear. This study combined hydrochemical and carbon isotope methods to explore the sources and factors influencing DIC in the groundwater of different active areas in coal mining areas. Moreover, the 13C isotope method was used to calculate the contribution rates of various sources to DIC in groundwater. The results showed that the hydrochemical types of groundwater were HCO3-Ca·Na and HCO3-Na. The main water‒rock interactions were silicate and carbonate rock weathering. Agricultural areas were mainly affected by the participation of HNO3 produced by chemical fertilizer in the weathering of carbonate rocks. Soil CO2 and carbonate rock weathering were the major sources of DIC in the groundwater. Groundwater in residential areas was primarily affected by CO2 from the degradation of organic matter from anthropogenic inputs. Sulfate produced by gypsum dissolution, coal gangue accumulation leaching and mine drainage participated in carbonate weathering under acidic conditions, which was an important factor controlling the DIC and isotopic composition of groundwater in coal production areas. The contribution rates of groundwater carbonate weathering to groundwater DIC in agricultural areas and coal production areas ranged from 57.46 to 66.18% and from 54.29 to 62.16%, respectively. In residential areas, the contribution rates of soil CO2 to groundwater DIC ranged from 51.48 to 61.84%. The results will help clarify the sources and circulation of DIC in groundwater under the influence of anthropogenic activities and provide a theoretical reference for water resource management.

Origin of old saline groundwater in the deep coastal formations of the Atacama Desert region: Consideration of lithium, boron, strontium and uranium isotopes contents

This paper presents a multi-isotopic study to investigate the origin of saline groundwater in the Cordillera de la Costa, in the arid Atacama Desert region in northern Chile. Understanding the origin of deep saline groundwater can be challenging using only hydrogeochemical and common isotope methods. New 87Sr/86Sr, δ11B, δ7Li, and 234U/238U values of dissolved salts have been added to previously known chemical and isotopic compositions of groundwater, inorganic dissolved carbon, and sulfate. In this extremely arid area, the chemical and isotopic compositions of coastal springs primarily reflect the concentration of salts from atmospheric wet and dry deposition, which accumulate in the topsoil and dissolve during the occasional heavy rainfall events. These events recharge the aquifer with sodium-chloride type, high salinity water, with rock weathering contributing minimally to the chemical composition of groundwater. However, high salinity water of the calcium chloride and sodium-calcium chloride types in the deep fractured aquifer have a significantly different chemical and isotopic composition. In this investigation, it has been concluded that the salinity of these last waters is due to remnants of ancient marine water, mineral alteration and interaction with predominantly andesitic volcanic rocks in the Cordillera de la Costa. The deep saline groundwater has δ11B values between 17.3‰ and 20.7‰, and 87Sr/86Sr values below 0.706, suggesting mixing with recharge groundwater. The δ7Li composition of the deep groundwater varies between 23.5‰ and 31.5‰, pointing to a contribution of salts of marine origin and mineral alteration. However, the low Li concentrations relative to B in recharge water and in deep groundwater shows a preferential Li removal from pore water. Uranium and uranium isotopes data are limited to shallow groundwater due to the very low U concentrations in deep groundwater following reduction of U, and point to enhanced water–rock interaction in a rock-dominated environment. The dissolution of old buried saline deposits at shallow depth as a source of salinity is ruled out.

Paleohydrogeology of the Horonobe area, Northern Hokkaido, Japan: Groundwater flow conditions during glacial and postglacial periods estimated from chemical and isotopic data for fracture and pore water

Understanding the difference in groundwater flow between glacial and interglacial periods is crucial for predicting the impact of future climate changes on groundwater movement. This study assesses the difference in groundwater flow between the last glacial period (LGP) and the postglacial period (PGP) in fractured mudstones of the Horonobe area, Japan, by combining the data for stable isotopes (δD and δ18O) and Cl− concentration of fracture and pore waters with radiocarbon (14C) age. The isotopic compositions of fractures and pore waters indicate that groundwater at 28–250 m deep in a borehole closest to the recharge area comprises meteoric water, recharged under the same climates as the present. The fracture water has isotopic compositions more similar to meteoric water than the matrix pore water near the fracture. The 14C age of fracture water suggests meteoric water recharge during the PGP. At greater depths in the borehole and sampling points in other boreholes, the isotopic compositions indicate the mixing of glacial meteoric and altered connate water, with the fracture water having comparable isotopic compositions with the matrix pore water. The recharge timing of meteoric water is inferred to be the LGP or before based on 14C dating. These results suggest that the meteoric water recharged during the PGP flows at a shallow depth, whereas the meteoric water recharged during the LGP intruded to greater depths. This result is consistent with previous inferences from surface geophysical and geological surveys that the depths of local valleys during the LGP were greater by < 50 m than the present ones and enhanced the downward hydraulic gradient. Combining the chemical and isotopic compositions of groundwater with 14C age helps assess the groundwater flow during the LGP and PGP in fractured rocks.

Isotope composition of groundwater and surface waters in the area of the Dombrovsky quarry of Kalush-Golinsk deposit of potassium salts

Enrichment of heavy isotopes of hydrogen (D, 3H) and oxygen (18О) is observed in water samples collected from open brine storage of Kalush-Golinsky deposit. The brine storage facilities were formed during the operation of the Kalush-Golynsky deposit of potassium salts and after its decommissioning. Enrichment of isotopes is also observed in groundwater samples, collected from wells located along with downstream groundwater from brine storage facilities. The analysis of levels of influence of possible sources of chemical pollution of groundwater (waters of the Dombrovsky quarry, tailings and sludge storage, salt dumps, saline soils) and correlation relationships between isotopes and groundwater mineralization have been determined by statistical processing of geochemical data.

Statistical and isotopic analysis of sources and evolution of groundwater

Investigating sources and evolution of groundwater in the Black Volta River basin is vital for understanding aquifer chemical configuration. Though resettlement has made groundwater in the region an important water supply source, its hydrochemical origin is poorly understood. Therefore, hydrochemistry was coupled with stable isotope (δ2H and δ18O) techniques to improve knowledge for effective groundwater resource management. For this purpose, 23 boreholes were sampled in the Black Volta River basin of Ghana. Major ion dominance is in the order: Na+ > Ca2+ > Mg2+ >K+ and HCO3− > Cl− > SO42− > NO3− respectively. Hydrochemical facies indicates the presence of soluble silicate minerals within aquifer. Hierarchical cluster analysis showed higher ionic concentration within aquifers which decreases towards discharge areas. This is attributed to longer residence time at lower elevations leading to interaction of water with silicate and accessory minerals. Oxygen-18 spatial plot shows recharge occurs along subsurface fractured zones which is confirmed by the water table contour of the study area. Natural geochemical processes, anthropogenic influences and the dissolution of silicate/albite components were extracted using Principal component analysis/factor analysis. These two components which explains the hydrochemistry constituted 79.5% of the total variance in the hydrochemical data and have eigen values > 1. Isotopic composition of groundwater indicates is of meteoric origin and the aquifers were recharged by precipitation. The outcome of this study provides theoretical support for the monitoring and management of groundwater, and its implication on the migration of people in arid/semi-arid regions.

Precipitation Method for Determination of Carbon and Oxygen Isotopes to Detect Groundwater Contamination Near a Municipal Landfill

The paper emphasizes the importance of isotope studies as a unique tool for detecting groundwater contamination with the landfill leachate. The aim of the study was to present an additional and useful method for detecting groundwater contamination, based on stable isotope analysis. The proposed method relies on the interpretation of measured δ13CDIC and δ18O levels (in precipitated carbonates during preparation of a water sample). According to this method, two zones with different isotope composition of groundwater were identified: the first zone with natural groundwater and with low δ13CDIC levels (from −20.6 to −12.4‰) and high δ18O levels (from −13.6 to −8.0‰), and the second zone with leachate-contaminated groundwater rich in δ13CDIC (from −10.9 to + 3.6‰) and high level of δ18O (from −9.8 to −7.1‰). Measuring the isotopic composition of oxygen alone, is insufficient to delimit the contaminated zone due a partial overlap of natural levels with those of the contaminated with groundwater leachates. Determination of δ13CDIC and δ18O in the landfill leachate-contaminated water can provide an effective tool to detect groundwater contamination near municipal landfills, and it can help to minimize the number of samples collected for the analysis of conventional parameters. This proven method may offer an easy-to-use solution for detecting groundwater contamination.

Surface water and groundwater interaction in the Kosi River alluvial fan of the Himalayan Foreland.

We report the isotopic composition of the surface water and groundwater of the Kosi River fan on the Himalayan Foreland, India. We have collected 65 water samples from surface water (Kosi River (n = 2), streams (n = 9), waterlogging (n = 29), and canal (n = 4)), and groundwater (n = 21) for δ18O and δ2H analysis during December 2019. We obtained groundwater level data measured at the observation wells from the Central Groundwater Board, India, for 1996 and 2017. The groundwater level varies from 1.0 to 8.1m below ground level (bgl) and from 0.5 to 9.0m bgl during 1996 and 2017, respectively. We have used water table fluctuation approach to estimate the recharge rate. The recharge rate in the Kosi Fan varies from 0.7 to 21.4mm/year from 1996 to 2017. Further, we have used δ18O and δ2H values of water samples to identify the source and the interaction between surface water and groundwater. The δ18O value of groundwater shows a wide variation (from -9.3‰ to -5.6‰) compared to the surface water, i.e., streams (-7.8‰ to -6.4‰) and canals (-6.9‰ to -6.0‰), suggesting mixing in groundwater during recharge processes. Furthermore, we have used a two-component mixing model to assess the fraction contribution from streams and precipitation to groundwater. The estimated fraction contribution from stream water to groundwater ranges from 45 to 83%. We also suggest higher recharge is limited up to the depth of 6m bgl. We suggest precipitation and surface water actively recharge groundwater. We conclude that marked spatial variation in the isotopic composition of groundwater is mainly due to the local recharge sources and interaction between surface water and groundwater.

How do geomorphic characteristics affect the source of tree water uptake in restored river floodplains?

AbstractAlpine rivers and their floodplains have been highly modified by human activities during the last decades. River restoration projects aim to counteract these negative impacts and to restore ecosystem services provided by riparian habitats. We studied two recently restored river sites in the Ahr/Aurino and Mareit/Mareta Rivers (Italian Alps) to investigate how geomorphic conditions, soil moisture, and groundwater level affect the source of water used by grey alder (Alnus incana (L.) Moench). We compared the isotopic composition (δ2H) of tree sap at different locations (low terraces formed during bed incision and recent floodplains formed after restoration) with that of potential water sources, that is, groundwater, soil water, and rainfall. The monthly variation in the isotopic composition of rainfall was reflected in both shallow and deeper soil water, as well as in the isotopic composition of sap. The redistribution of precipitation and groundwater in the soil differed between the post‐restoration floodplain sites and the post‐incision terraces, leading to a different relation between the sap water, soil water, and groundwater isotopic composition. The results show that transpiration of A. incana trees growing on recent floodplains is mostly supported by stream‐fed soil water, whereas trees growing on terraces mainly use precipitation‐fed soil water. These marked, morphology‐related differences in the source of transpiration water of grey alder highlight how channel degradation still affects the ecohydrological processes in Alpine fluvial corridors. Nonetheless, large restoration interventions—in terms of channel widening—can enable the self‐formation of new floodplain areas characterized by stream water‐fed riparian ecosystems.

Mechanism of anomalous manifestations δ13C CO2 underground water on Tashkent geodynamic range

The results of long-term isotope and gas measurements of various groundwater sources in the Tashkent artesian basin are presented. In addition to areal studies, the results of regime observations of selected wells at the Tashkent geodynamic test site are also presented to identify prognostic features as a precursor to strong earthquakes. The first results were obtained related to the manifestation of the Nazarbek earthquake on December 11, 1980, with a magnitude of 5.3, a source depth of 10 km. On the basis of this earthquake, the behavior of isotopic and gas precursors at the Ulugbek well, Tashkent geodynamic test site, was analyzed. A quantitative analysis was made of the anomalous manifestation of the gas and isotopic composition of groundwater in the Ulugbek borehole during the preparation and execution of the Nazarbek earthquake. Determination of CO2 release from rock samples recovered from deep wells of the landfill made it possible to quantify the ratio of “background” CO2 mixing from carbonate decomposition to anomalous. Judging by the obtained ratio, the rocks during the preparation of the earthquake should emit 2 times more carbon dioxide than the background CO2. This was confirmed by observations of the CO2 content in the well, when an increase in its relative concentration of 250% was recorded. The analysis of the δ13C time series, obtained as a result of long-term regime observations of the carbon isotopic composition of CO2 in the groundwater of the Tashkent geodynamic test site, made it possible to identify significant anomalies in the carbon content.

The triple argon isotope composition of groundwater on ten-thousand-year timescales

Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas 14C of dissolved inorganic carbon and dissolved 4He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic 40Ar in groundwater acquired from decay of 40K in aquifer minerals should be insensitive to some processes that impact 14C and 4He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic 40Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100 ka). Here we present the first high-precision (<1‰) measurements of triple Ar isotopes (40Ar, 38Ar, 36Ar) in groundwater. We introduce a model that distinguishes radiogenic 40Ar from atmospheric 40Ar by using the non-radiogenic Ar isotopes (36Ar, 38Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic 40Ar excess (Δ40Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ40Ar ranges from ~0‰ (the expected minimum value) to +4.2‰ across three study areas near Fresno, San Diego, and the western Mojave Desert. Based on measurements from a network of 23 scientific monitoring wells in San Diego, we find evidence for a strong dependence of Δ40Ar on aquifer lithology. We suggest that Δ40Ar is fundamentally controlled by the weathering of old K-bearing minerals and thus reflects both the degree of groundwater-rock interaction, which is related to groundwater age, and the integrated flow through different geological formations. Future studies of Late Pleistocene groundwater may benefit from high-precision triple Ar isotope measurements as a new tool to better interpret 14C- and 4He-based constraints on groundwater age and flow.

Estimation of seasonal base flow contribution to a tropical river using stable isotope analysis

Increase in water demand within the Cauvery River Basin is a cause of serious concern over sustainable utilization and managment of water resources. Sustainable, efficent and equitable managment of Cauvery riverwater requires a thorough understanding of the contributing sources and hydrological processes influencing avaliability of water resources. The study investigates the seasonal contribution of surface runoff and baseflow to the Cauvery streamflow using isotope mass balance approach. Stable Isotope measurements (δ2H, δ18O) of Cauvery river water along with groundwater are carried during seasonal time intervals spanning from 2014 − 2016, covering pre-monsoon (PM), south-west monsoon (SWM) and north-east monsoon (NEM) seasons. Field campaigns yielded seasonal datasets of Cauvery riverwater and groundwater isotopic composition. Stable isotope analysis showed a seasonal shift in the river water isotopic composition (δ2H 8‰, δ18O 0.95‰) between pre- monsoon (PM) and south-west monsoon (SWM) seasons. Isotopically heavier values are recorded during the pre-monsoon season; coinciding with the period of low flow condition, whereas the monsoon season is characterized by high discharge and recorded isotopically lighter values. The Krishna Raja Sagar (KRS) Reservoir controls the isotopic composition of riverwater by evaporative process, during the pre-monsoon season resulting in river water becoming isotopically lighter. We utilized two-component mixing model to estimate the seasonal base flow contribution to the Cauvery River, for which the river is segmented into three sectors. The average base flow contribution to river flow during pre-monsoon season in sector I is 85 ± 5%, whereas in sector II and III the average groundwater contribution drops to 59 ± 3% and 39 ± 3% respectively. Similarly, the average base flow contribution to the river flow during south-west monsoon season drops to 32 ± 7% in sector I, whereas in sector II and III the average base flow contribution is estimated at 42 ± 7% and 47 ± 7% respectively. The base flow contribution is appreciably high in the upper Cauvery Basin comprising the Western Ghats, which plays a crucial role in sustaining stream flow during the pre-monsoon season. Assessment of spatial and temporal variability of surface water and groundwater isotopic composition within the Cauvery River Basin provides an important dataset for developing a sustainable river management plan in this tropical mesoscale riverine system.

Spatially-Resolved Integrated Precipitation-Surface-Groundwater Water Isotope Mapping From Crowd Sourcing: Toward Understanding Water Cycling Across a Post-glacial Landscape

Isotopic analyses of δ18O and δ2H of water in the context of the hydrologic cycle have allowed hydrologists to better understand the portioning of water between the different water domains. Isoscapes on a large spatial scale have been created to show isotopic variation in waters as a function of elevation, temperature, distance to coast, and water vapor source. We present the spatial and temporal isotopic results of precipitation, surface water, and groundwater of an ongoing study across Massachusetts, USA in order to establish an isotopic baseline for the region. This represents one of the most comprehensive and detailed isotopic studies of water across a 10,000 sq mi area that has exhaustively sampled important components of the terrestrial hydrologic cycle (precipitation, groundwater, and surface waters). We leverage the support of volunteers and citizen scientists to crowd source samples for isotopic analysis. The database consists of water samples from 14 precipitation sites, 409 ground water sites and 516 surface water sites across the state of Massachusetts, USA. The results indicate that groundwater isotopic composition ranges from δ18O −11 to −4‰ surface water ranges from δ18O −13 to −3.84‰ and precipitation ranges from δ18O −17.88 to −2.89‰. On a first order, the small bias of mean groundwater (−8.7‰) and surface water (−8.0‰) isotopes compared to precipitation δ18O (−7.6‰) supports that groundwater recharge and surface water storage effects through the hydrologic year impact the isotopic composition of surface and groundwater. While differences are distinct, they are larger than previously reported values, but still suggest more importance of summer precipitation than previously acknowledged. On average seasonal amplitudes of precipitation (2.7‰), surface water (1.13‰), and groundwater (~0‰) of the region demonstrate young water fractions of surface water to be 40% with groundwater ~0%. Results demonstrate that mean δ18O in precipitation, surface water and groundwaters are more enriched in heavy isotopes in areas near the coast, than the interior and western portion of Massachusetts. The hope is for this dataset to become an important tool for water management and water resource assessment across the region.

An Overview of Aquifer Physiognomies and the δ18O and δ2H Distribution in the South African Groundwaters

A comprehensive assessment of the stable isotope distribution in the groundwater systems of South Africa was conducted in relation to the diversity in the aquifer lithology and corresponding hydraulic characteristics. The stable isotopes of oxygen (18O) and hydrogen (2H) in groundwater show distinct spatial variation owing to the recharge source and possibly mixing effect in the aquifers with the existing water, where aquifers are characterized by diverse hydraulic conductivity and transmissivity values. When the shallow aquifer that receives direct recharge from rainfall shows a similar isotopic signature, it implies less mixing effect, while in the case of deep groundwater interaction between recharging water and the resident water intensifies, which could change the isotope signature. As aquifer depth increases the effect of mixing tends to be minimal. In most cases, the isotopic composition of recharging water shows depletion in the interior areas and western arid zones which is attributed to the depleted isotopic composition of the moisture source. The variations in the stable isotope composition of groundwater in the region are primarily controlled by the isotope composition of the rainfall, which shows variable isotope composition as it was observed from the local meteoric water lines, in addition to the evaporation, recharge and mixing effects.