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The study of stable isotopes in the precipitation of Ceará was conducted on monthly rainfall samples between 2011 and 2019 and on daily rainfall samples between 2018 and 2019 in order to support the global knowledge of hydrological processes in semi-arid regions given the scarcity of such data in the Northeast of Brazil. Results showed that the frequency of rainfall sampling in semi-arid regions may induce an isotopic bias, due to the partial evaporation from the water collectors during monthly storage causing isotopic fractionation. In the absence of adequate data processing, this bias may lead to a wrong definition of the Local Meteoric Water Line (LMWL), which can have major consequences on the understanding of groundwater recharge processes. The processing of the monthly isotopic data (elimination of the evaporated data) and the analysis of daily data highlighted that the LMWL of Quixeramobim is characterized by a y-intercept value greater than +12 ‰, indicating that rainwater is composed of vapours from marine origin, but also from continental recycling. Analysis of δ18O monthly variations in relation to climatic factors suggested the importance of convective systems and relative humidity in the control of the isotopic composition of precipitation in a semi-arid tropical region.

Mesa de Los Santos is an elevated plateau located in the Eastern Cordillera of Colombia (Andean region) in the Department of Santander (altimetric variation 300 to 1800 m a.s.l.). The region is marked by a shortage of potable surface water. The isotopic study of the rain and its relation to Los Santos Formation phreatic and fractured aquifer was carried out. Four rain stations were installed on top of the plateau (1275 to 1684 m a.s.l.) to collect monthly rainwater samples for stable isotopes. The rainwater stations recorded 1469 to 764 mm from North to South, and the seasonal patterns of stable isotopes were similar in all stations. The preliminary Local Meteoric Water Line is δ2H = 8.22×δ18O + 13.9, slightly above the GMWL with an intercept of +13.9 ‰, possibly indicating continental vapor recycling. The rain-weighted annual isotope means showed more depleted values in the southmost rain station (δ18O = −8.99 ‰, 1275 m a.s.l.). From the phreatic and fractured aquifer, 35 groundwater points were monitored bimonthly for stable isotopes (n=134) and three trends were differentiated in the isotopic content value of δ18O. Large temporal variability characterized 10 groundwater points (amplitudes greater than 1 ‰ in δ18O for each point) consistently with the seasonal behaviour of the rain, indicating flows with rapid transit on a monthly scale. 21 groundwater points showed a low temporal variability with year-round differences under 1 ‰ and presenting a spatial distribution of δ18O with enriched values towards the north (between −6.71 ‰ and −6.00 ‰) and depleted values towards the south (−8.97 ‰ to −8.14 ‰), which tends to be also consistent with the rainwater isotope distribution observed. The stability of year-round groundwater isotopic values is a sign of efficient mixing of groundwater and a slower transit. Finally, 4 groundwater points presented signs of evaporation, showing d-excess values between −6.4 ‰ and +4.3 ‰, in connection with surface water bodies that undergo evaporation before infiltration.

The determination of the origin of salinity in the superficial aquifer of Mornag (NE Tunisia), and the understanding of its hydrological and geochemical behaviours related to severe natural and anthropogenic constraints, were approached by the combined study of chemical elements and stable isotopes (2H and 18O). This study indicates that: (1) the high salinities of the superficial aquifer of Mornag are mainly explained by the dissolution/precipitation processes of evaporite minerals in the aquifer formation, (2) the present-day recharge during rainwater infiltration brings downward a high content of nitrates and other dissolved salts, (3) infiltration of untreated sewage from the main urban areas contaminates the aquifer, (4) two other sources of dissolved salts in groundwater exist, favoured by the intensive exploitation of the phreatic aquifer. The first one is due to mineralised water infiltration from Meliane Wadi where activities, mainly a cement factory, discharge their wastewater. Intrusion of marine saltwater is the second source of salinity caused by aquifer over-exploitation. This hypothesis is supported by the high chloride concentration (>122 m e.q. L−1), Br/Cl ratios (>1.8 ‰) and the piezometric level lower than sea level. On the other hand, the artificial recharge with low mineralization waters by the Mejerda-Cap Bon Canal and the natural recharge in the valley of wadi El Hma, contribute to a dilution of groundwater. The freshwater/saltwater mixing causes geochemical interactions modifying the water chemistry: cationic exchange, precipitation phenomena. Isotopic tools (2H and 18O) show that the water of this aquifer system has been recently recharged by direct infiltration from the boundary and in the valley of wadis.

Population growth and agricultural intensification are ever more demanding in water resources, and threaten their quality and quantity, especially in semi-arid and arid areas. The Sahara desert is typical of these multiple constraints and rapid changes making sustainable management of water resources a major issue. The wide regional aquifers have nearly fossil water. In contrast, alluvial aquifers associated with small wadis are recharged nearly each year by the rare floods. In the case of Wadi Tamanrasset, in southern Algeria, the alluvial aquifer is exploited intensively because of its accessibility and its good chemical quality. The physical conditions commanding flow and recharge of groundwater, and especially the transfer from surface to groundwater, were studied by combining hydrodynamics and isotopic tracers (18O, 2H). Monthly monitoring of groundwater level in around thirty wells was complemented by automatic recorders with hourly time steps since March 2016 in three wells, along a 600 m transect perpendicular to the wadi bed. The slow response of the water table is visible only two months after the first flood; its maximum rise was 1 m in 2016. Isotopic analyses of rainwater on a daily time scale, of the wadi water on a 1 h time scale during runoff and of groundwater (two complete campaigns in the dry and wet seasons and a specific monitoring of the transect every month) have provided additional information on surface-groundwater transfer.

In semi-arid areas, groundwater (GW) represents the most reliable resource for water supply. In the semi-arid region of Ceará (Northeastern Brazil), GW of the crystalline fractured aquifers has a salinity higher than expected in this type of environment. This study implemented a multidisciplinary approach based on hydrodynamic (piezometric) and hydrogeochemical (18O, 2H, 3H, 14C, CFC, SF6, Electrical Conductivity, major ions) data to understand the salinization processes. Results demonstrate that GW is generally recent and recharged by meteoric waters mainly through indirect infiltration from evaporated surface water. The study suggests that GW, originally bicarbonated, becomes progressively enriched in chloride due to the dissolution and leaching of salts, during the wet season, which come from meteoric inputs and have precipitated in the unsaturated zone and pond sediments during dryer periods.

A shallow ice core was extracted at the summit of Mera Peak at 6376 m a.s.l. in the southern flank of the Nepalese Himalaya range. From this core, we reconstructed the seasonal deposition fluxes of dust and refractory black carbon (rBC) since 1999. This archive presents well preserved seasonal cycles based on a monsoonal precipitation pattern. According to the seasonal precipitation regime in which 80% of annual precipitation falls between June and September, we estimated changes in the concentrations of these aerosols in surface snow. The analyses revealed that mass fluxes are a few orders of magnitude higher for dust (10.4 ± 2.8 g m−2 yr−1 than for rBC (7.9 ± 2.8 mg m−2 yr−1). The relative lack of seasonality in the dust record may reflect a high background level of dust inputs, whether from local or regional sources. Over the 10-year record, no deposition flux trends were detected for any of the species of interest. The data were then used to simulate changes in the surface snow albedo over time and the potential melting caused by these impurities. Mean potential melting caused by dust and rBC combined was 713 kg m−2 yr−1, and for rBC alone, 342 kg m−2 yr−1 for rBC under certain assumptions. Compared to the melting rate measured using the mass and energy balance at 5360 m a.s.l. on Mera Glacier between November 2009 and October 2010, i.e. 3000 kg m−2 yr−1 and 3690 kg m−2 yr−1 respectively, the impact of rBC represents less than 16% of annual potential melting while the contribution of dust and rBC combined to surface melting represents a maximum of 26%. Over the 10-year period, rBC variability in the ice core signal primarily reflected variability of the monsoon signal rather than variations in the intensity of emissions.

We investigated past climate variability and the zonal short and long-range transport of air masses in tropical South America using chemical, isotopic and palynological signals from a 42 m-long ice core recovered in 2003 from the saddle of the Nevado Coropuna, southern Peru (72°39´ W; 15°32´ S; 6080 m a.s.l.). We found that precipitation at this site depends mainly on the easterly circulation of air masses originated from the tropical Atlantic Ocean. Nevertheless, sporadic Pacific air masses arrivals, and strong cold waves coming from southern South America reach this altitude site. In spite of post-depositional effects, we were able to identify two strong ENSO (El Niño-Southern Oscillation) event signatures (1982–1983 and 1992) and the eruptive activity of the nearby Sabancaya volcano (1994).

Calcium oxalate minerals of the general formula CaC 2 O 4 . x H 2 O are widely present in nature and usually associated with pathological calcifications, constituting up to 70-80% of the mineral component of renal calculi. The monohydrate phase (CaC 2 O 4 . H 2 O, COM) is the most stable form, accounting for the majority of the hydrated calcium oxalates found. These mineral phases have been studied extensively via X-ray diffraction and IR spectroscopy and, to a lesser extent, using 1 H, 13 C, and 43 Ca solid-state NMR spectroscopy. However, several aspects of their structure and reactivity are still unclear, such as the evolution from low- to high-temperature COM structures (LT-COM and HT-COM, respectively) and the involvement of water molecules in this phase transition. Here, we report for the first time a 17 O and 2 H solid-state NMR investigation of the local structure and dynamics of water in the COM phase. A new procedure for the selective 17 O- and 2 H-isotopic enrichment of water molecules within the COM mineral is presented using mechanochemistry, which employs only microliter quantities of enriched water and leads to exchange yields up to ∼30%. 17 O NMR allows both crystallographically inequivalent water molecules in the LT-COM structure to be resolved, while 2 H NMR studies provide unambiguous evidence that these water molecules are undergoing different types of motions at high temperatures without exchanging with one another. Dynamics appear to be essential for water molecules in these structures, which have not been accounted for in previous structural studies on the HT-COM structure due to lack of available tools, highlighting the importance of such NMR investigations for refining the overall knowledge on biologically relevant minerals like calcium oxalates., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Sustainable groundwater management implies a good knowledge of recharge processes, especially in areas with water deficit, like the semi-arid region of Banabuiú watershed (Ceará State, Northeast of Brazil). In this zone, phreatic aquifers consist of Precambrian crystalline fractured reservoirs characterised by a high spatial anisotropy, both in terms of hydrodynamics and water quality. This study implemented a multi-tracer approach ( 18 O, 2 H, 14 C, 3 H, CFC, SF 6 ), combined with hydrodynamic data (i.e. groundwater levels) to identify the groundwater recharge origin and the recharge mechanisms, and to estimate the groundwater residence time. At the basin scale, hydrodynamic data and local observations indicated the high reactivity of aquifers to precipitation and suggested that infiltration processes occur mostly through preferential infiltration zones. Stable isotope data showed a major contribution of evaporated surface water in the recharge process from many artificial or natural ponds. Groundwater residence time determination highlighted the spatio-temporal heterogeneity of flow path organisation within aquifers, with variable contributions between fast vertical flow (present-day end-member; 15-85 %) and a slower horizontal flow (old end-member <1960), underlining the vulnerability of aquifers to present-day environmental stress or pollution.

Water isotopes from plant xylem and surrounding environment are increasingly used in eco-hydrological studies. Carrière et al. [1] analyzed a dataset of water isotopes in (i) the xylem of three different tree species, (ii) the surrounding soil and drainage water and (iii) the underlying karst groundwater, to understand tree water uptake during drought in two different Mediterranean forests on karst setting. The xylem and soil water were extracted by cryogenic distillation. The full dataset was obtained with Isotope Ratio Mass Spectrometry (IRMS) and Isotope Ratio Infrared Spectrometer (IRIS), and included 219 measurements of δ 2 H and δ 18 O. Prompted by unexpected isotopic data characterized by a very negative deuterium excess, a subsample of 46 xylem samples and 9 soil water samples were double checked with both analytical techniques. IRMS and IRIS analyses yielded similar data. Therefore, the results reveal that laser spectrometry allows an accurate estimation of xylem and soil water isotopes. The dataset highlights a strong 2 H depletion in xylem water for all species. Deuterium does not seem adequate to interpret ecological processes in this dataset given the strong fractionation., (© 2020 The Author(s).)

Karst environments are unusual because their dry, stony and shallow soils seem to be unfavorable to vegetation, and yet they are often covered with forests. How can trees survive in these environments? Where do they find the water that allows them to survive? This study uses midday and predawn water potentials and xylem water isotopes of branches to assess tree water status and the origin of transpired water. Monitoring was conducted during the summers of 2014 and 2015 in two dissimilar plots of Mediterranean forest located in karst environments. The results show that the three monitored tree species (Abies alba Mill, Fagus sylvatica L, and Quercus ilex L.) use deep water resources present in the karst vadose zone (unsaturated zone) more intensively during drier years. Quercus ilex, a species well- adapted to water stress, which grows at the drier site, uses the deep water resource very early in the summer season. Conversely, the two other species exploit the deep water resource only during severe drought. These results open up new perspectives to a better understanding of ecohydrological equilibrium and to improved water balance modeling in karst forest settings., (Copyright © 2019 Elsevier B.V. All rights reserved.)