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Global Isotope Hydrogeology―Review

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Groundwater ¹⁸O/¹⁶O, ²H/¹H, ¹³C/¹²C, ³H, and ¹⁴C data can help quantify molecular movements and chemical reactions governing groundwater recharge, quality, storage, flow, and discharge. Here, commonly applied approaches to isotopic data analysis are reviewed, involving groundwater recharge seasonality, recharge elevations, groundwater ages, paleoclimate conditions, and groundwater discharge. Reviewed works confirm and quantify long held tenets: (i) that recharge derives disproportionately from wet season and winter precipitation; (ii) that modern groundwaters comprise little global groundwater; (iii) that “fossil” (>12,000‐year‐old) groundwaters dominate global aquifer storage; (iv) that fossil groundwaters capture late‐Pleistocene climate conditions; (v) that surface‐borne contaminants are more common in younger groundwaters; and (vi) that groundwater discharges generate substantial streamflow. Groundwater isotope data are disproportionately common to midlatitudes and sedimentary basins equipped for irrigated agriculture, but less plentiful across high latitudes, hyperarid deserts, and equatorial rainforests. Some of these underexplored aquifer systems may be suitable targets for future field testing.
Groundwater δ¹⁸O measurements made in spring and well waters around the globe. (a) Groundwater isotope measurement locations and δ¹⁸O values. High δ¹⁸O values are common at low latitudes and near coastlines; low δ¹⁸O values are common at high latitudes, high elevations, and continental interiors. Groundwater isotope data are relatively common throughout the contiguous United States, southern Canada, Europe, north and east Africa, north China, Bangladesh, the western islands of the Malay Archipelago, New Zealand, and populated regions of Australia. Groundwater isotope data are relatively sparse across Latin America, southwest Africa, central Asia, eastern Europe, central and southern China, Borneo, New Guinea, and areas of Australia. For example, relatively high‐density and nation‐wide groundwater δ¹⁸O data sets exist for Ireland (Regan et al., 2017), Costa Rica (Sánchez‐Murillo & Birkel, 2016), Uganda (Jasechko, 2014; samples collected with M. GebreEgziabher), Mexico (Wassenaar et al., 2009), India (Bhattacharya et al., 1985), South Africa (West et al., 2014), the United States of America, and Canada (Jasechko, Wassenaar & Mayer, 2017). (b) Groundwater (grey circles) and annual amount‐weighted precipitation (black squares) δ¹⁸O values and their variance with latitude. Precipitation isotope compositions are derived from the International Atomic Energy Agency (www‐naweb.iaea.org/napc/ih/IHS_resources_isohis.html), the United States Network for Isotopes in Precipitation (Welker, 2012) and the Canadian Network for Isotopes in Precipitation (e.g., Birks & Gibson, 2009; Delavau et al., 2011). Precipitation δ¹⁸O values plotted in panel b are ‘amount‐weighted’ over the entire period of record, meaning time‐steps during which more precipitation fell are weighted more than those during which less precipitation fell. Groundwater δ¹⁸O data presented here are derived from the United States' National Water Information System (data downloaded May 2018 from www.waterqualitydata.us) and from data sets compiled from the following n = 435 references (Al Faitouri & Sanford, 2015; Abid et al., 2010; Abid et al., 2011; Abid et al., 2012; Abouelmagd et al., 2014; Abu‐Jaber & Kharabsheh, 2008; Adams et al., 2001; Adiaffi et al., 2009; Adomako et al., 2011; Aeschbach‐Hertig et al., 2002; Aggarwal et al., 2000; Ahmad & Green, 1986; Ahmed et al., 2011; Ako Ako et al., 2012; Al‐Charideh & Abou‐Zakhem, 2010; Al‐Charideh, 2012; Al‐Charideh & Kattan, 2016; Al‐Charideh & Hasan, 2013; Alemayehu et al., 2011; Al‐Katheeri et al., 2009; Allen, 2003; Al‐Mashaikhi et al., 2012; Alpers & Whittemore, 1990; Al‐Ruwaih & Shehata, 2004; Alsaaran, 2005; Alyamani, 2001; Amer et al., 2012; Andre et al., 2005; Andrews et al., 1989; Andrews, Edmunds, et al., 1994; Andrews, Fontes, et al., 1994; Aravena et al., 2003; Arslan et al., 2013; Atkinson et al., 2014; Awad, 2011; Awad et al., 1994; Awad et al., 1997; Awad, 1997; Back et al., 1983; Bahati et al., 2005; Bajjali & Abu‐Jaber, 2001; Bajjali, 2006; Bakari, Aagaard, Vogt, Ruden, Brennwald, et al., 2012; Baker, 2009; Barbecot et al., 2000; Batista, Santiago, Frischkorn, Filho, & Forster, 1998; Bayari et al., 2009; Bennetts et al., 2006; Berg & Pearson, 2012; Beyerle et al., 1998; Beyerle et al., 2003; Bhatia et al., 2011; Bhattacharya et al., 1985; Blomqvist, 1999; Böhlke et al., 1998; Bouchaou et al., 2008; Bouchaou et al., 2009; Bouragba et al., 2011; Boutin, 2009; Bowen et al., 2012; Branchu & Bergonzini, 2004; Bretzler et al., 2011; Brown et al., 2011; Buck et al., 2005; Burg et al., 2013; Bwire Ojiambo et al., 2001; Calmels et al., 2008; Capaccioni et al., 2003; Carneiro et al., 1998; Carreira et al., 2011; Carreira, Marques & Nunes, 2014; Carrillo‐Rivera et al., 1992; Cartwright et al., 2012; Cartwight & Morgenstern, 2012; Cartwright & Weaver, 2005; Castany et al., 1974; 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SMOW = standard mean ocean water.
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Global precipitation and groundwater isotope compositions. Panel (a) presents global precipitation δ¹⁸O and δ²H data (n = 68,382 samples). Panel (b) is a schematic of some of the processes that may alter stable O and H isotopic compositions of groundwaters. High‐temperature water‐rock interactions may increase δ¹⁸O values more so than δ²H values—yielding low deuterium excess values (Giggenbach, 1992). Low temperature water‐rock interactions may decrease δ¹⁸O values—in some cases leading to high deuterium excess values (examples limited mostly to deep crystalline basement brines; Kloppmann et al., 2002). Methanogensis may increase δ²H values with minimal impact on δ¹⁸O values—leading to high deuterium excess values. Partial evaporation may increase both δ²H and δ¹⁸O values along δ²H/δ¹⁸O slopes of ~3 to ~6—leading to low deuterium excess values. δ²H/δ¹⁸O slopes tend to be lower under low‐humidity conditions and where evaporation takes place from soils (e.g., δ²H/δ¹⁸O slopes of ~2 to ~5); δ²H/δ¹⁸O evaporation slopes tend to be higher for open water evaporation under humid condi‐tions (e.g., δ²H/δ¹⁸O slopes of ~5 to ~8). The great majority (90%) of compiled groundwater isotope compositions have deuterium excess values of between 0‰ and 20‰. A global regression of precipitation isotope compositions is labeled “meteoric waters” and follows δ²H = 8 × δ¹⁸O + 10 (the global meteoric water line; Craig, 1961). Deuterium excess (d) is calculated, following d = δ²H − 8 × δ¹⁸O (Dansgaard, 1964). Panel (b) is based partly on schematic presented by Horita (2005). Last, panel (c) presents compiled groundwater isotope compositions (n = 44,948 samples). A linear regression describing groundwater δ²H variations with groundwater δ¹⁸O is similar for the linear regression describing precipitation δ²H and δ¹⁸O values (from panel a). SMOW = standard mean ocean water.
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Some scenarios that may lead to the condition: groundwater δ¹⁸O < amount‐weighted annual precipitation δ¹⁸O. These include (a) groundwater aquifers replenished in part by precipitation that fell at higher elevations than the land surface at the location that the sample was collected (e.g., Gonfiantini et al., 1976; Payne & Yurtsever, 1974); (b) recharge of surface waters diverted for agricultural, domestic or industrial uses (e.g., Williams & Rodoni, 1997); (c) disproportionate recharge from intensive rainfall, in places where precipitation rates and δ¹⁸O values correlate inversely (e.g., Geirnaert et al., 1984; Vogel & Van Urk, 1975); (d) higher recharge/precipitation ratios for cold‐season precipitation relative to warm‐season precipitation (e.g., Brinkmann et al., 1963; Simpson et al., 1970); (e) retention of groundwater derived from precipitation during the late‐Pleistocene, when global atmospheric temperatures were as much as ~4°C cooler‐than present (e.g., Gonfiantini et al., 1974; Phillips et al., 1986; Vogel & Ehhalt, 1963); and (f) transport of waters from a place where precipitation δ¹⁸O values are relatively low to another place where precipitation δ¹⁸O values are relatively high before the water recharges (e.g., Liu & Yamanaka, 2012). These processes are not mutually exclusive; more than one may affect groundwater isotope compositions (e.g., Liu & Yamanaka, 2012). Decoupling these various processes can be challenging and requires consideration of local hydroclimate and hydrogeologic conditions (read, e.g., Uliana et al., 2007).
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... Third, mapping wells that tap fossil aquifers can enable a better understanding of the prevalence of communities that rely on fossil groundwater resources 13 . Although multiple studies have commented on the sustainability of fossil groundwater use [14][15][16][17][18] , the spatiotemporal patterns of fossil groundwater use and groundwater depletion remain unclear, partly because of a lack of geospatial data with locally relevant information for aquifer boundaries. ...
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Urban growth often results in changes in the urban hydrological cycle, causing impacts on water availability in densely populated regions. The water isotopologues can provide relevant information about the origin of water under different hydrogeological scenarios, aiding to implement better strategies for water conservation in coupled natural-urbanized environments. In this study, the isotopic compositions of multiple water sources were assessed in a pristine (Ipanema National Forest, FLONA) and an urbanized (Lavapés catchment, SOR) watershed located in the Sorocaba River basin (State of São Paulo, Southeastern Brazil), seeking to understand the causes of isotopic variability and to determine the relative contribution from different sources to streamflow, using the Bayesian mixing model approach. Differences in isotopic composition were observed, as FLONA yielded the most depleted water (ca. -7.5 ‰ [Formula: see text]18O for surface and groundwater and ca. + 11.0 ‰ d-excess), while SOR yielded the most enriched water (ca. -5.5‰ [Formula: see text]18O for surface and groundwater and -3.8‰ [Formula: see text]18O for the water supply system), with evidence of evaporation (ca. + 8.2 ‰ d-excess). The differences observed in isotopic compositions are related to a combination of different factors, such as geological framework, groundwater recharge, and evaporation associated with the Itupararanga water reservoir. Both in FLONA and SOR, groundwater discharge is the most important factor that regulates streamflow. However, in SOR, losses from the water supply system were almost constant along the year, representing an important contribution. The results presented here highlight the use of isotope hydrology techniques to solve problems related to urban hydrology.
... The chemical analyses provide direct indicators of water quality, while the isotopic compositions provide age and recharge indicators. Water isotopes provide unique fingerprints for different types of water (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997). Stable water isotopes (hydrogen: 1 H, 2 H; oxygen: 16 O, 18 O) have been used extensively to map the spatial distributions and physical characteristics (e.g., evaporation, mixing) of different water types (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997;Coplen, 1996). ...
... Water isotopes provide unique fingerprints for different types of water (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997). Stable water isotopes (hydrogen: 1 H, 2 H; oxygen: 16 O, 18 O) have been used extensively to map the spatial distributions and physical characteristics (e.g., evaporation, mixing) of different water types (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997;Coplen, 1996). Fossil waters, for example, have depleted isotopic compositions compared to meteoric waters. ...
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The availability and quality of limited groundwater resources in Egypt are negatively affected by both natural and anthropogenic forces. It is crucial to understand the recharge sources and mechanisms of country-scale aquifers in order to develop sustainable management scenarios for these groundwater resources. In this study, we compiled all historical hydrogen (δ²H) and oxygen (δ¹⁸O) stable isotopic compositions of 1618 ground- and surface-water samples collected for seven major aquifer and reservoir systems in Egypt. These data were then hosted in a public-domain web map application (https://bit.ly/3n9MtrM) that enables users to access, navigate, display, and analyze the isotopic composition of any water sample in Egypt. The developed application and customized web tools were then used to provide improved understanding of the country-wide water resources’ isotopic composition and to investigate the recharge sources and mechanisms of, and provide sustainable management scenarios for, the seven major Egyptian aquifer systems. Results indicate that (1) the alluvial and coastal aquifers are recharged from modern rainfall, floodwaters, and upward leakage from deep aquifers; (2) the Nile Valley and Delta, Moghra, and carbonate aquifers are receiving recharge from the Nile River, modern rainfall, irrigation canals and drains, and upward leakage from deep aquifers; (3) the Nubian receives modern recharge in areas of higher rainfall or close to surface water reservoirs; and (4) the fractured basement aquifers are recharged from modern rainfall and floodwaters. This cost-effective web application will guide locations and techniques for future field data collection; reduce time, efforts, and resources required for field activities; enable improved understanding the country-wide stable isotopic compositions of water resources; enhance understanding of recharge mechanisms nationwide; and enable the scientific community to address country-scale science questions in a unique way.
... The chemical analyses provide direct indicators of water quality, while the isotopic compositions provide age and recharge indicators. Water isotopes provide unique fingerprints for different types of water (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997). Stable water isotopes (hydrogen: 1 H, 2 H; oxygen: 16 O, 18 O) have been used extensively to map the spatial distributions and physical characteristics (e.g., evaporation, mixing) of different water types (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997;Coplen, 1996). ...
... Water isotopes provide unique fingerprints for different types of water (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997). Stable water isotopes (hydrogen: 1 H, 2 H; oxygen: 16 O, 18 O) have been used extensively to map the spatial distributions and physical characteristics (e.g., evaporation, mixing) of different water types (Jasechko, 2019;Cook and Herczeg, 2000;Clark and Fritz, 1997;Coplen, 1996). Fossil waters, for example, have depleted isotopic compositions compared to meteoric waters. ...
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Groundwater resources in the Kingdom of Saudi Arabia (KSA) have high levels of natural radioactivity. Within the northwestern KSA, gross alpha (α) and gross beta (β) levels exceed national and international drinking-water limits. In this study, we developed and used an automated machine learning (AML) approach to quantify relationships between gross α and gross β activities and different geological, hydrogeological, and geochemical conditions. Two AML model groups (group I for gross α; group II for gross β) were constructed, using water samples collected from 360 irrigation and water supply wells, to define a robust model that explains the spatial variability in gross α and gross β activities, as well as variables that control the gross activities. Each group contained four model families: deep neural network (DNN), gradient boosting machine (GBM), generalized linear model (GLM), and distributed random forest (DRF). Model inputs include chemical compositions as well as geological and hydrogeological conditions. Three performance metrics were used to evaluate the models during training and testing: normalized root mean square error (NRMSE), Pearson's correlation coefficient (r), and Nash-Sutcliff efficiency (NSE) coefficient. Results indicate that (1) the GBM model outperformed (training: NRMSE: 0.37 ± 0.10; r: 0.92 ± 0.05; NSE: 0.85 ± 0.09; testing: NRMSE: 0.71 ± 0.08; r: 0.72 ± 0.08; NSE: 0.49 ± 0.12) the DNN, DRF, and GLM models when modelling gross α activities; (2) gross α activities are controlled by pH, stream density, nitrate, manganese, and vegetation index; (3) the DRF model outperformed (training: NRMSE: 0.41 ± 0.05; r: 0.92 ± 0.02; NSE: 0.83 ± 0.04; testing: NRMSE: 0.67 ± 0.09; r: 0.77 ± 0.07; NSE: 0.54 ± 0.12) the GBM, DNN, and GLM models when modelling gross β activities; (4) input variables that affect the gross β actives are pH, temperature, stream density, lithology, and nitrate; and (5) no single model could be used to model both gross α and gross β activities—instead, a combination of AML models should be used. Our computationally efficient approach provides a framework and insights for using AML techniques in water quality investigations and promotes more and improved use of different geological, hydrogeological, and geochemical datasets by the scientific community and decision makers to develop guidelines for mitigation.
... ) (Rozanski et al., 1993;Vuille et al., 2003;Jasechko, 2019). The lack of a temperature effect in tropical regions, where the temperature ranges from 20°C to 30°C, is often attributed to the dominance of the monthly amount effect (Rozanski et al., 1993) and the seasonal changes of the locations of storm origin. ...
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Precipitation isotope ratios (O and H) record the history of water phase transitions and fractionation processes during moisture transport and rainfall formation. Here, we evaluated the isotopic composition of precipitation over the central-southeastern region of Brazil at different timescales. Monthly isotopic compositions were associated with classical effects (rainfall amount, seasonality, and continentality), demonstrating the importance of vapor recirculation processes and different regional atmospheric systems (South American Convergence Zone-SACZ and Cold Fronts-CF). While moisture recycling and regional atmospheric processes may also be observed on a daily timescale, classical effects such as the amount effect were not strongly correlated (δ18O-precipitation rate r ≼ −0.37). Daily variability revealed specific climatic features, such as δ18O depleted values (∼6‰ to −8‰) during the wet season were associated with strong convective activity and large moisture availability. Daily isotopic analysis revealed the role of different moisture sources and transport effects. Isotope ratios combined with d-excess explain how atmospheric recirculation processes interact with convective activity during rainfall formation processes. Our findings provide a new understanding of rainfall sampling timescales and highlight the importance of water isotopes to decipher key hydrometeorological processes in a complex spatial and temporal context in central-southeastern Brazil.
... The expression "environmental tracer" is given to those isotopes, naturally present in the environment, which can be used to trace a process, a cycle, or a mechanism of transfer that occurs in the environment [6]. In this context, the following may be included: all the processes related to the hydrogeological cycle (i.e., recharge processes and groundwater-surface water interaction, connection among river/lakes) [22,23], geochemical and bio-geochemical transformations [24][25][26][27][28], potential connections among source of contamination and groundwater or surface water [29,30]. In general, a tracer is defined as a substance that is naturally present in the environment (natural tracer) or can be artificially introduced (artificial tracer) in a known concentration, generally with the aim of marking the pathway of surface water or groundwater, to label the water flow [31,32]. ...
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Environmental isotopes are essential in hydrogeological studies, thanks to their contribution to the understanding of aquifers dynamics, vulnerability, water resources assessment, and management issues. The environmental isotopic approach plays a vital role in tracing the hydrological cycle and identifying various sources of contamination in the environment and gives independent information concerning what can be determined by a traditional hydrogeological study. Even in the framework of COP-26, isotopes have been indicated as fingerprints of climate change and therefore suitable for the evaluation of water balance and assessment of processes involved therein; in pollution studies they are used as fundamental support of traditional geochemical measures. Tritium, in particular, has been used since the 1960s to identify potential leaks in the containment walls of waste disposal sites, since its presence in the leachate (at very high levels in some cases) depends on the incorrect waste disposal of some peculiar items. Its use as a tracer of pollution by landfills is highlighted and emphasized by the very low concentrations of tritium in the natural environment. By comparing tritium content of leachate to that of water downflow from the waste disposal site, it is therefore possible to establish with a good success rate whether leachate have migrated or not out of the landfill, in the surrounding environment. An additional potential of tritium is to give a prompt indication of pollution risk in the environment indicating leaching even before the chemical indicator of pollution can be detected. This article wants to provide a contribution to the scientific community, collecting all the existing research in this field and providing data and benchmarks about this method, in particular stressing the role of tritium as an indicator of leachate transfer out of waste disposal sites.
... The stable hydrogen and oxygen isotopes in precipitation are widely used to understand the atmospheric (Yao et al., 2013;Baker et al., 2019), hydrological Jasechko, 2019) and ecological (Pederzani and Britton, 2019;Bowen et al., 2019) processes. Falling raindrops in the atmosphere usually experience evaporation to some degree, which is also called sub-cloud evaporation or below-cloud evaporation, and then the mass and volume of raindrops reaching the ground are logically smaller than those in the cloud (Stewart, 1975;Peng et al., 2007). ...
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In arid northwest China where the precipitation intensity is relatively low, the stable hydrogen and oxygen isotopes (δ²H and δ¹⁸O) in precipitation are usually impacted by the sub-cloud evaporation. To understand the sub-cloud evaporation effect under an arid climate, we used the hourly meteorological data at 14 stations in the Kaxgar-Yarkant River Oasis, a rainshadow oasis of northwest China, and estimated the monthly and hourly isotopic change in falling drops from the cloud base to the ground. The results showed that the hourly meteorological observations are an effective way to assess the spatiotemporal pattern of sub-cloud evaporation effect. Across the 14 stations, the annual mean changes in deuterium excess below cloud base ranged from −19.1‰ to −6.8‰. The impacts of sub-cloud evaporation during spring and autumn are larger than those during winter and weaker than those during summer. The exponential regression (R² = 0.96), instead of linear regression (R² = 0.85), is better at describing the relationship between the raindrop remaining fraction and the isotopic changes from the cloud base to the ground. The sensitivity analysis of isotopic changes to different relative humidity scenarios shows that the drying scenario may lead to slightly larger sensitivity than the wetting scenario. The impact of sub-cloud evaporation on stable isotope composition in precipitation may be underestimated when low precipitation events (especially less than 1 mm/h) are not sampled.
... As stated in Section 2.1, graphical hydrograph separation analysis is more accurate when the hydrograph data span annual or longer time scales (Barlow et al., 2014). Considering that surface water and groundwater processes often occur over months to decades (Price, 2011;Jasechko, 2019), we sought to extend our time scale beyond a decade to analyze current conditions. Our choice of 14 years allowed us to achieve that goal and also maximize the number of sites included in our analysis of current relationships across the precipitation gradient. ...
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Study region Regional precipitation gradient across Kansas, USA. Study focus As precipitation increases, baseflow and surface runoff generally increase, but it is unclear whether they increase proportionally and how proportions respond to climate and land use changes. This study examined variation in streamflow components of perennial streams across the study region and its relationships with watershed properties. We evaluated streamflow components with hydrograph separation and used Spearman’s rank correlation tests and principal component analysis (PCA) to assess spatial trends (28 sites) and Mann-Kendall and Sen’s Slope tests to assess temporal relationships (9 sites, 1960–2018). New hydrological insights for region Runoff and baseflow both increase eastward with precipitation but the increase is greater for runoff. As such, baseflow index (BFI, baseflow/streamflow) decreases with increasing precipitation, potentially reflecting the limits of infiltration on recharge/runoff partitioning. Spatial patterns in variables that influence infiltration (land use and soil texture) also vary with precipitation, consistent with long-term influences of climate on landscapes. Since 1960, the watersheds included in our temporal analysis experienced small, mainly insignificant increases in precipitation and temperature and large, significant increases in irrigation. During this time, BFI increased significantly only in semi-arid, agriculture-dominated catchments overlaying higher permeability deposits. These findings underscore the importance of watershed characteristics as controls on current spatial patterns in streamflow and BFI and also the sensitivity of streamflow and BFI to climate and land use changes over time.
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The water present within trees when sugars and cellulose are formed is the source of hydrogen and oxygen atoms that are incorporated into tree-ring cellulose (see Chaps. 10.1007/978-3-030-92698-4_10 and 10.1007/978-3-030-92698-4_11 ). However, the isotope composition of relevant water pools is often unknown when trying to interpret δ ¹⁸ O and δ ² H isotopic records in tree rings. This chapter focuses on the factors that can influence the O and H isotope ratios of source waters for trees. Trees generally use water that originated as precipitation, but this does not mean that the isotope ratios of water used by trees—predominantly taken up by roots from soils—and incorporated in cellulose exactly matches precipitation isotope ratios. Precipitation isotope ratios vary in space and time, and only a fraction of all precipitation infiltrates soils, reaches roots, and is ultimately taken up by trees. Considering species, soils, and climates may allow for predicting which fraction of water resides in the root-zone during the growing seasons, and how its isotope ratios deviate from that of average precipitation. Here we provide an overview of the terrestrial water cycle and the associated transport and fractionation processes that influence the stable isotope ratios of water used by trees. We highlight obstacles and opportunities to be considered, towards more accurately interpreting the records of O and H isotope ratios in tree cellulose.
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The present study was conducted to delineate the pollution vulnerability of the Quaternary aquifer in two areas, Imbaba and Shobra El-Khima, near Cairo, Egypt. Environmental isotopes combined with hydro-chemistry were used for this purpose. The groundwater in the Imbaba area (average total dissolved solids about 900 mg/L; sodium/chloride, sulfate, and bicarbonate water types) is more mineralized than groundwater in the Shobra El-Khima area (average total dissolved solids 500 mg/L; calcium and sodium/bicarbonate water type). A high nitrate content and significant mineralization in the groundwater are probably due to contamination of recharge to the aquifer by irrigation drainage, deteriorated sewage networks, and septic tanks. The deuterium and oxygen-18 compositions of the groundwater are depleted compared to Nile River water, which is the main source of aquifer recharge. This less isotopically enriched water probably represents older Nile water recharge that flooded the region before construction of the Aswan High Dam in 1963, or it is a mixture of a young water and originally deposited paleowater that was in deeper horizons at a time of cooler and more humid climate. Intensive pumping has moved the paleowater higher in the aquifer. Groundwater in the Shobra El-Khima area has higher residence time, based on the tritium concentration , than groundwater in the Imbaba area. The percentage of the isotopically depleted water equals 75% in the Shobra El-Khima and 35% in Imbaba, and the thickness of the clay cap above the aquifer is 38 m in Shobra El-Khima and 20 m in Imbaba. These factors are indicative of the rate of recharge to the aquifer and were used to evaluate the pollution vulnerability in the two areas.
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Identification of paleowater in aquifers tapped for the public water supply is important both for sound water management, because paleowater is likely to be withdrawn unsustainably in high demand regions in the absence of large scale artificial recharge operations, and for contamination susceptibility assessment, because paleowater is typically isolated from contamination sources. In this study, paleowater, which herein includes pre-Holocene groundwater and also water that recharged well before the onset of significant human alteration of the hydrologic system in California, is identified using three key isotopic indicators of groundwater residence time: tritium (³H), radiocarbon (¹⁴C), and radiogenic helium-4 (⁴Herad). We compared results of these three tracers from a unique data set of more than 2000 wells that are predominantly long-screened drinking water production wells. Although considerable uncertainty is associated with calculated apparent ages for individual samples, non-parametric statistical tests indicate that the composite set of isotopic indicators support classification of samples into categories that allow identification of wells most likely to produce paleowater. Approximately 7% of the wells included in the study show strong evidence for producing paleowater, with screens extending greater than 146 m below ground surface, in which ³H activity is less than 1 pCi/L, ¹⁴C activity is less than 40.9 pmC, and ⁴Herad concentration exceeds 7.4 × 10⁻⁸ cm³STP/gwater. An additional 22% of wells produce mixed-age water with a component of paleowater, with screens extending greater than 95 m below ground surface, in which ³H is less than 5 pCi/L, and ¹⁴C activity is less than 95.91 pmC. Wells in desert basins of southeastern California and wells in the southwestern quadrant of the Central Valley are most likely to produce paleowater that is pre-Holocene in age. Very few wells in the northwestern portion of the state, the foothills and Sierra Nevada regions, and coastal basins with intensive artificial recharge activities are categorized as producing paleowater. Climate is the primary control on paleowater occurrence, with arid portions of the state that were wetter during the Pleistocene having the largest number of wells categorized as producing paleowater. Secondarily, paleowater is found at the end of very long flow paths in confined aquifers, e.g., in the center of the Northern Sacramento Valley. In contrast, paleowater may be masked in areas where unconfined or semi-confined conditions allow substantial mixing between modern recharge and paleowater. Modern, artificially recharged water has replaced very old groundwater on a large scale in urban coastal basins.
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Human water use, climate change and land conversion have created a water crisis for billions of individuals and many ecosystems worldwide. Global water stocks and fluxes are estimated empirically and with computer models, but this information is conveyed to policymakers and researchers through water cycle diagrams. Here we compiled a synthesis of the global water cycle, which we compared with 464 water cycle diagrams from around the world. Although human freshwater appropriation now equals half of global river discharge, only 15% of the water cycle diagrams depicted human interaction with water. Only 2% of the diagrams showed climate change or water pollution—two of the central causes of the global water crisis—which effectively conveys a false sense of water security. A single catchment was depicted in 95% of the diagrams, which precludes the representation of teleconnections such as ocean–land interactions and continental moisture recycling. These inaccuracies correspond with specific dimensions of water mismanagement, which suggest that flaws in water diagrams reflect and reinforce the misunderstanding of global hydrology by policymakers, researchers and the public. Correct depictions of the water cycle will not solve the global water crisis, but reconceiving this symbol is an important step towards equitable water governance, sustainable development and planetary thinking in the Anthropocene.
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Eastern Beringia is one of the few Western Arctic regions where full Holocene climate reconstructions are possible. However, most full Holocene reconstructions in Eastern Beringia are based either on pollen or midges, which show conflicting early Holocene summer temperature histories. This discrepancy precludes understanding the factors that drove past (and potentially future) climate change and calls for independent proxies to advance the debate. We present a ~13.6 ka summer temperature reconstruction in central Yukon, part of Eastern Beringia, using precipitation isotopes in syngenetic permafrost. The reconstruction shows that early Holocene summers were consistently warmer than the Holocene mean, as supported by midges, and a thermal maximum at ~7.6–6.6 ka BP. This maximum was followed by a ~6 ka cooling, and later abruptly reversed by industrial-era warming leading to a modern climate that is unprecedented in the Holocene context and exceeds the Holocene thermal maximum by +1.7 ± 0.7 °C.
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To enhance our understanding of the regional hydroclimate in the Central Tibetan Plateau, different types of water samples were collected across the Naqu River basin in the summer (July, August) and winter (January, December) of 2017 for isotopic analysis. With Cuona Lake as the demarcation point, the δ18O values of the river water increased initially and then decreased from upstream to downstream along the river’s mainstream. In the Naqu River system, a general decrease of δ18O values in the trunk stream of the lower reaches (from the head of Cuona Lake) was revealed owing to the gradual dilution of increased isotopically-depleted tributary inflow. Lakes play an important role in regulating runoff and changes in the levels of stable isotopes in rivers or streams. Additionally, the decrease of δ18O is controlled by processes involved in the ‘isotopic altitude effect’. Larger contributions of winter precipitation in surface runoff at higher elevations would produce higher deuterium excess in stream water. On the regional scale, with Cuona Lake as the demarcation point, one of the clearest findings was that the river/stream’s deuterium excess value decreased first and then increased from the south to the north in the summer; but there was a positive linear increase in the winter. From geographical and climatological perspectives, the changes of deuterium excess could result from increasing effects of summertime, and the generation of continental/local recycled and monsoonal water vapor in the surface runoff northward. The study area is at the critical transition between the Indian monsoon system in the South and the Northern belt of the westerlies, as revealed by the intermediate deuterium excess values.
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Rain recharges soil water storages and either percolates downward into aquifers and streams or is returned to the atmosphere through evapotranspiration. Although it is commonly assumed that summer rainfall recharges plant-available water during the growing season, the seasonal origins of water used by plants have not been systematically explored. We characterize the seasonal origins of waters in soils and trees by comparing their midsummer isotopic signatures (δ2H) to seasonal isotopic cycles in precipitation, using a new seasonal origin index. Across 182 Swiss forest sites, xylem water isotopic signatures show that summer rain was not the predominant water source for midsummer transpiration in any of the three sampled tree species. Beech and oak mostly used winter precipitation, whereas spruce used water of more diverse seasonal origins. Even in the same plots, beech consistently used more winter precipitation than spruce, demonstrating consistent niche partitioning in the rhizosphere. All three species' xylem water isotopes indicate that trees used more winter precipitation in drier regions, potentially mitigating their vulnerability to summer droughts. The widespread occurrence of winter isotopic signatures in midsummer xylem implies that growing-season rainfall may have minimally recharged the soil water storages that supply tree growth, even across diverse humid climates (690–2068 mm annual precipitation). These results challenge common assumptions concerning how water flows through soils and is accessed by trees. Beyond these ecological and hydrological implications, our findings also imply that stable isotopes of δ18O and δ2H in plant tissues, which are often used in climate reconstructions, may not reflect water from growing-season climates.
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The recharge mechanism of groundwater in the Badain Jaran Desert, North China has been a focus of research and still disputable in the past two decades. In this study, the chemical and hydrogen (H) and oxygen (O) isotopic characteristics of shallow groundwater, lake water and local precipitation in the Badain Jaran Desert and neighboring areas were investigated to reveal the relationships between various water bodies and the recharge source of shallow groundwater. Isotopic and hydrogeochemical results show that (1) shallow groundwater was associated with local precipitation in the Ayouqi and Yabulai regions, (2) lake water was mainly recharged by groundwater in the desert hinterland, (3) shallow groundwater of the desert hinterland, Yabulai Mountain and Gurinai Grassland had a common recharge source. Shallow groundwater of the desert hinterland had a mean recharge elevation of 1869 m a.s.l. on the basis of the isotope-altitude relationship and thus originated chiefly from lateral infiltration of precipitation in the Yabulai Mountain. It is further concluded that shallow groundwater flowed towards the Gurinai Grassland according to the groundwater table contour map. Along the flow pathway, the H-O isotopic variations were primarily caused by the evaporation effect but chemical variations of shallow groundwater were affected by multiple factors, e.g., evaporation effect, dilution effect of occasional heavy-precipitation and dissolution of aquifer evaporites. Our findings provide new insight into the groundwater cycle and benefit the management of the limited water resources in the arid desert area.
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The recharge sources and groundwater age in the Songnen Plain, Northeast China, were confirmed using environmental isotopes. The isotopic signatures of the unconfined aquifers in the southeast elevated plain and the north and west piedmont, cluster along local meteoric water lines (LMWLs) with a slope of about 5. The signature of source water was obtained by the intersection of these LMWLs with the regional meteoric water line (RMWL). This finding provides evidence that the recharge water for these areas originate from the Changbai Mountains and the Low and High Hingan Mountains, respectively. Groundwater in the unconfined aquifer in the low plain yields a LMWL with a slope of 4.4; its nitrate concentration indicates the admixture of irrigation return flow. The δ-values of the unconfined aquifer in the east elevated plain plot along the RMWL, reflecting recharge by local precipitation. The mean residence time of groundwater in these aquifers is less than 50 years. However, the ¹⁴C age of the groundwater in the confined Quaternary aquifer ranges from modern to 19,500 years, and in the Tertiary confined aquifer from 3,100 to 24,900 years. Modern groundwater is mainly recharged to the Quaternary confined aquifer on the piedmont by local precipitation and lateral subsurface flow.
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The recharge and origin of groundwater and its residence time were studied using environmental isotopic measurements in samples from the Heihe River Basin, China. δ18O and δD values of both river water and groundwater were within the same ranges as those found in the alluvial fan zone, and lay slightly above the local meteoric water line (δD=6.87δ18O+3.54). This finding indicated that mountain rivers substantially and rapidly contribute to the water resources in the southern and northern sub-basins. δ18O and δD values of groundwater in the unconfined aquifers of these sub-basins were close to each other. There was evidence of enrichment of heavy isotopes in groundwater due to evaporation. The most pronounced increase in the δ18O value occurred in agricultural areas, reflecting the admixture of irrigation return flow. Tritium results in groundwater samples from the unconfined aquifers gave evidence for ongoing recharge, with mean residence times of: less than 36 years in the alluvial fan zone; about 12–16 years in agricultural areas; and about 26 years in the Ejina oasis. In contrast, groundwater in the confined aquifers had 14C ages between 0 and 10 ka BP.