Source Waters Research Articles

Fingerprinting Fluid Source in Calcite Veins: Combining LA-ICP-MS U-Pb Calcite Dating with Trace Elements and Clumped Isotope Palaeothermometry

Application of geochemical proxies to vein minerals - particularly calcite - can fingerprint the source of fluids controlling various important geological processes from seismicity to geothermal systems. Determining fluid source, e.g. meteoric, marine, magmatic or metamorphic waters, can be challenging when using only trace elements and stable isotopes as different fluids can have overlapping geochemical characteristics, such as δ18O. In this contribution we show that by combining the recently developed LA-ICP-MS U-Pb calcite geochronometer with stable isotopes (including clumped isotope palaeothermometry) and trace element analysis, the fluid source of veins can be more readily determined. Calcite veins hosted in the Devonian Montrose Volcanic Formation at Lunan Bay in the Midland Valley Terrane of Central Scotland were used as a case study. δD values of fluid inclusions in the calcite, and parent fluid δ18O values reconstructed from clumped isotope palaeothermometry, gave values which could represent a range of fluid sources: metamorphic or magmatic fluids, or surface waters which had undergone much fluid-rock interaction. Trace elements showed no particularly distinctive patterns. LA-ICP-MS U-Pb dating determined the vein calcite precipitation age – 318±30 Ma – indicating a metamorphic or magmatic fluid source was unlikely as there was no metamorphic or magmatic activity was occurring in the area at this time. The vein fluid source was therefore interpreted to be a surface water (meteoric based on paleogeographic reconstruction) which had undergone significant water-rock interaction. This study highlights the importance of combining the recently developed LA-ICP-MS U-Pb calcite geochronometer with stable isotopes and trace elements to help determine fluid sources of veins, and indeed any geological feature where calcite precipitated from a fluid that may have resided in the crust for a period of time (e.g. fault precipitates or cements).

A process‐based water stable isotope mixing model for plant water sourcing

AbstractStable isotopes of hydrogen and oxygen in water are common tools for investigating water uptake apportionment, but many of the existing methods rely on simple linear mixing approaches that do not mechanistically incorporate additional information about site physical properties and conditions. Here, we develop a ‘physically based root water uptake isotope mixing estimation’ model (PRIME) that combines a continuous and parametric probability density function for root water uptake with site physical data in a process‐based linear mixing framework. To demonstrate the application of PRIME, water uptake patterns of boreal forest Pinus banksiana trees were estimated on four dates in 2019. To aid in validation, estimates were compared with that of the Bayesian linear mixing model framework, MixSIAR. The two approaches provided similar results, but due to its continuous and parametric nature, PRIME provided estimates of superior resolution, certainty, and model parsimony. Although both models incorporate additional physical information into their mixing frameworks, PRIME does so in a mechanistic manner, thereby reflecting the relevant hydrological processes more effectively than the purely empirical approach taken by MixSIAR. Furthermore, because PRIME uses a continuous function to describe the predicted uptake pattern, it allows users to quantify water uptake with essentially infinite resolution, through integration over the desired depth ranges. These findings demonstrate the advantages of utilizing a continuous, parametric, and process‐based mixing model to estimate root water uptake apportionment, thus providing a relatively simple yet powerful tool with which to approach plant water sourcing.

Search for Natural Remedies for the Treatment of Salmonella Typhi Infections

Typhoid is a major public health threat, particularly in areas lacking access to clean water, proper nutrition, and hygienic practices. The emergence of antibiotic resistance, inaccessibility of vaccines to the global south, and asymptomatic carriers of Salmonella typhi (S. typhi) have made tackling the scourge of typhoid a major challenge. Plant-based antimicrobial compounds have opened a new avenue for effectively managing typhoid by minimizing drug-related side effects and targeting the resistant strains of typhoidal salmonella cost-effectively. This review article contains many such candidates, who can potentially modify the typhoid treatment guidelines like Azadirachta indica, Canarium schweinfurthii-derived scopoletin, and Punic granatum, and those that target the resistant strains of S. typhi, like Amaranthus hybridus, and Quercus infectoria and Phyllanthus emblica. The article will also shed light on the challenges and promises associated with phytocompound extraction and standardization as the researchers navigate the uncharted waters of alternative medicinal sources.

Simulation and analysis of hydraulic transition process based on mechanical hydraulics.

With the scarcity of water resources in various regions, the pressure on water supply projects is also increasing, which has led to a sharp increase in the water hammer effect in water supply pump projects. In response to this issue, the study proposes to apply a unidirectional pressure regulating tower based on mechanical hydraulic technology to water hammer protection in water supply pumps. In addition, the study also optimizes the calculation method of water hammer and designs one simulation software, which is used to simulate and analyze the proposed water hammer protection measures. The study first determines through simulation software that the optimal initial water level of the unidirectional pressure regulating tower is 2m and the optimal diameter of the make-up water pipe is 600 mm. Afterwards, simulation analysis is conducted on different water hammer protection measures, and it is found that the maximum pressure of the water hammer under the proposed protection measures is the lowest, at 14.8m, which was lower than the comparison measures. In addition, according to expert ratings, the average protective effect rating of protective measure 1 by relevant technical personnel is 9.4 points, which is better than the comparative measure. The above results indicate that through the simulation analysis of hydraulic transition process based on mechanical hydraulics, it can be found that the water hammer protection measures proposed in the study have good protective functions and can effectively reduce the water hammer effect in water supply engineering.

Adsorption’s regularities of radionuclides on adsorbents from solutions

The object of research: The object of research is the uranium isotopes were found in the soil of the locations of numerous thermal and cold springs in the Kalbajar region of Azerbaijan in the same proportions as in natural uranium deposits.
 Investigated problem: Uranium isotopes were transferred from soil samples to solutions. İnvestigated problem of scientific work is study of regularities of adsorption of uranium isotopes from solutions on the surfaces of various adsorbents.
 The main scientific results: It is defined high adsorption properties of DOWEX HCR granular elastomer, activated carbon and expanded clay on the result of researches. The activation’s energy of desorption of uranium isotopes from the surface of adsorbents were determined. Relatively low values (22.1–26.3 kJ/mol) of the activation’s energy of desorption of uranium isotopes from the surface of adsorbents indicate the absence of the chemical nature of sorption and the presence of only weak van der Waals forces between the absorbed isotopes and the surface of the adsorbents.
 The area of practical use of the research results: The regularities of adsorption of uran isotopes on adsorbent masses from aqueous solutions can be used as an original cleaning method of the drain water of the physical stage of crude oil refining or other radiochemical processes to the norms for waste water.
 Innovative technological product: It was determined based on the research, that the concentration of radon in the waters of most springs of the “Istisu” deposit is below the regulatory permissible value (60 Bq/l), which indicates the suitability of these spring waters for use in medical treatment procedures.
 Scope of the innovative technological product: It is planned to resume (prevented more than 40 years ago due to global regional problems) the production of the original trademark – “Istisu” medicinal water, with the concentration of radon below the standard value. Considering the two-fold decrease in radon concentration in water over four days, the waters of the individual sources examined, with a 100-fold high radon content, after a certain time (several months) can also be used for use in medical procedures, which allows their transportation to large distances

Assessing the Influence of Flow Variability on Irrigated Agriculture in the Central Gharb Plain, Morocco

The lower main watercourse of the Sebou Basin in Mechra-Bel-Ksiri is experiencing adverse effects attributed to climate change, notably drought and rising temperatures. In this study, we evaluated the impact of these changes on the irrigation zone using a comprehensive methodology involving break, trend, and low-water analyses. Our assessment, incorporating Land Use and Land Cover Change (LULCC) considerations, reveals detrimental effects on the expansion of irrigated agriculture along the Sebou wadi and its Baht tributary. Consequently, farmers are compelled to tap into underground water sources to fulfill irrigation needs for their farms. This research sheds light on the pressing challenges posed by climate change in the region and the consequent adjustments farmers are forced to make in their water sourcing practices. Keywords: Flow variability; break; trend; low water; land use land cover.

Application of a hybrid-fruit-peel (HFP) coagulant in low carbon source wastewater treatment as an external carbon source.

The application of a hybrid-fruit-peel (HFP) coagulant used as an external carbon source (ECS) in both simulated water and real sewage having a low carbon source treated with sequencing batch reactor (SBR) was studied, compared with that of sodium acetate (NaAc). The impact of HFP on sludge properties (such as extracellular polymer substance (EPS), dehydrogenase activity (DHA), charged property, size, microscopic images and bacteria phase) was characterized. The results showed that as an ECS, HFP basically gave similar nitrogen removal to NaAc and also gave a similar developing trend of both dissolved oxygen (DO) and pH. HFP promoted more proliferation of microorganisms and posed higher levels of protein (PN) and polysaccharide (PS) than NaAc, but gave slightly lower DHA than NaAc. After HFP was added as an ECS, the types and quantities of microorganisms increased significantly, the effluent qualities were improved and the sludge size and extensibility became larger, which was conducive to direct contact and remove pollutants. HFP played a similar role to NaAc as ECS and can be used as a quality and slow-releasing ECS for low carbon source wastewaters.

Effects of Cellulosic Carbon Addition on Nitrogen Removal from Simulated Dry Land Drainage, and Its Environmental Effects

Agricultural non-point source pollution has emerged as a significant driver of declining global water quality in recent years. Ditch systems hold considerable promise for trapping and purifying pollutants. However, the persistent challenge has been the limited availability of carbon sources in drainage water, which significantly hinders nitrogen (N) removal in ditches. This study investigated the dynamic changes in ammonia (NH4+) and nitrate (NO3−) levels caused by three cellulosic carbon additions (rice straw, coir, and sawdust) during both winter and summer seasons. Water column devices were used as containers, and the impacts on environmental factors and water denitrification rates were explored. Results demonstrated that the addition of straw exhibited the most effective N removal in winter and summer, and significantly enhanced water denitrification rates in a short timeframe, with the maximum denitrification rate reaching 1482.42 μmol·L−1·h−1. However, there was an observed accumulation of NH4-N and chemical oxygen demand (COD) in summer. Also, the addition of sawdust resulted in a notable increase in greenhouse gas emissions during the summer test. In conclusion, during the cooler seasons of winter and spring when temperatures are not as high, the combined use of various cellulosic carbon sources has the potential to enhance water denitrification and mitigate adverse environmental impacts, offering valuable applications for water quality improvement.

Characteristics of geothermal waters in eastern Wugongshan based on hydrogen, oxygen, and strontium isotopes

It is crucial to investigate the features of the geothermal waters for sustainable exploitation and utilization. It is rich in low-medium temperature geothermal waters in the Wugongshan area in Jiangxi Province, which is urgent to find out the characteristics of the geothermal waters. This study intends to reveal the genetical model of geothermal waters in the eastern Wugongshan area including recharge source, cyclic evolution, and heat source, based on the methodical analysis of hydrogen, oxygen, and strontium isotopic data. Results show that the precipitation at recharge elevations between 1500 and 2200 m is the main recharge source of geothermal waters in the study area. Precipitation infiltrates deep into the area near the crust and is heated into geothermal fluid. Water-rock interactions occur during deep circulation. In the upwelling process, geothermal waters are mixed with shallow groundwaters at different mixing ratios (17%–74%). Finally, a conceptual model of the geothermal waters is proposed. It can provide some guidance for the development of geothermal waters in the eastern Wugongshan area.

Investigation of the Source of Iceland Basin Freshening: Virtual Particle Tracking with Satellite-Derived Geostrophic Surface Velocities

In the 2010s, a large freshening event similar to past Great Salinity Anomalies occurred in the Iceland Basin that has since propagated into the Irminger Sea. The source waters of this fresh anomaly were hypothesized to have come from an eastward diversion of the Labrador Current, a finding that has since been supported by recent modeling studies. In this study, we investigate the pathways of the freshwater anomaly using a purely observational approach: particle tracking using satellite altimetry-derived surface velocity fields. Particle trajectories originating in the Labrador Current and integrated forward in time entered the Iceland Basin during the freshening event at nearly twice the frequency observed prior to 2009, suggesting an increased presence of Labrador Current-origin water in the Iceland Basin and Rockall Trough during the freshening. We observe a distinct regime change in 2009, similar to the timing found in the previous modeling papers. These spatial shifts were accompanied by faster transit times along the pathways which led to along-stream convergence and more particles arriving to the eastern subpolar gyre. These findings support the hypothesis that a diversion of relatively fresh Labrador Current waters eastward from the Grand Banks can explain the unprecedented freshening in the Iceland Basin.

Organic matter biofiltration performance modeling: Influence of influent water quality, operating conditions, and biomass

The impact of source water dissolved organic matter (DOM) origin, empty bed contact time (EBCT), temperature, and pretreatment methods on biofiltration performance was evaluated and predictive models based on experimental data were developed. Three DOM source water types, terrestrial, microbial, and treated wastewater (WW) effluent, were utilized. A model was developed to predict biofilter performance for dissolved organic carbon (DOC) removal based on the influent biodegradable DOC (BDOC) fraction, a single active biomass measurement from the top of the filter and the filter EBCT. A biomass distribution model was developed to predict total active biomass throughout the filter based on a single biomass measurement from the top of the filter. The measured BDOC fractions were 21 % for the nonWW impacted source waters, 36 % for the WW effluents and 62 % for the ozonated WW effluents. At an EBCT of 15 min, biofilters removed between 7 and 21 % of the DOC (19 to 50 % for BDOC) depending on the DOM type and use of ozonation. When the EBCT decreased to 5 min DOC removal decreased by 40 % and when increased to 30 min removal increased by 42 %. When the temperature decreased from 22 °C to 6 °C DOC removal was 33 % lower and when increased to 28 °C removal was 42 % higher. ATP values were found to be a function of temperature and DOM origin, as the average ATP values from the WW effluent biofilters were almost double that of the non-WW impacted sources and pre-ozonation of the WW effluent yielded values three times higher. The model was applied to the results of 27 different biofilter runs at three EBCTs yielding one distinct rate constant for the non-WW impacted source waters and one rate constant for the WW effluents. The model was successfully applied to the results of 19 filter runs from the literature and to those from a pilot plant over 6 months of operation.

Variability in precipitation influences the water sourcing and adaptive strategies of key plant species within the desert steppe ecosystem

The composition and species diversity of desert plant communities exhibit high sensitivity to changes in precipitation. From the perspective of plant community dynamics, alterations in precipitation levels result in increased complexity in water absorption and utilization strategies, consequently leading to diversified adaptation strategies. Focusing on the desert steppe within the transitional zone between agriculture and animal husbandry in northern China, this study investigates the mechanisms through which variations in precipitation impact plant species' water absorption and utilization strategies in desert steppes. In this study, a controlled precipitation experiment was conducted on site in a natural desert steppe setting with three precipitation treatments: 50 % reduction in precipitation (PE), normal precipitation (CK), and 50 % increase in precipitation (PD). By analysing the δ2H and δ18O values of plants, soil, and precipitation, as well as indicators such as plant species composition, biomass, specific leaf area (SLA), and δ13C values, the changes in plant community species and the main plant species' water sources and strategies under different precipitation conditions were studied. The results show that (1) Within each treatment, the variability of soil δ2H and δ18O in the 0–20 cm soil layer was greater than that in the 20–60 cm soil layer. The slope of the soil δ2H and δ18O fitting line in the PE treatment was larger than that in the PD and CK treatments. (2) Except for Leymus secalinus, the dominant plant species, including Lespedeza potaninii, Polygala tenuifolia, Convolvulus ammannii, Kali collinum, and Stipa breviflora, exhibited an increase in water absorption from deeper soil layers as precipitation decreased. The diversity in water sources among the dominant plant species decreased with increasing precipitation, while the overlap index of soil moisture utilization across different soil layers initially increased and then decreased with increasing precipitation levels. (3) The root-to-shoot ratio (RS) of L. potaninii and P. sibirica shows an extremely significant negative correlation with SWC, while the RS of C. ammannii and K. collinum exhibits a significant negative correlation with SWC. The δ13C values of S. breviflora demonstrate an extremely significant negative correlation with SWC, while those of L. secalinus, C. ammannii, and S. breviflora show a significant negative correlation. In contrast, the δ13C values of K. collinum exhibit an extremely significant positive correlation with SWC. The SLA of all six plants exhibits an extremely significant positive correlation with SWC. The water source diversity of L. potaninii, C. ammannii, K. collinum, and S. breviflora demonstrates an extremely significant positive correlation with SWC, while that of P. sibirica and L. secalinus shows a significant positive correlation with SWC. Conclusion: As precipitation decreases, soil water evaporation becomes more pronounced. The primary plant species within the community exhibit flexibility in absorbing moisture from different soil layers in response to changes in precipitation levels. Differential water utilization strategies among different plants have led to hydrological niche separation, intensifying competition for soil moisture, particularly during extremely arid conditions. Plant water utilization strategies tend to become more conservative as precipitation decreases, and they demonstrate the ability to adapt by altering their water use strategies in response to varying precipitation levels.

Quantitative contribution of cryogenic vacuum extraction and radial water transport to xylem-source water deuterium offsets

The positions and magnitudes of deuterium offsets between bulk xylem and corresponding source waters are under debate and quantifying them is essential for isotope-based ecohydrological investigations. In this study, stable isotopes (δ2H, δ18O, and δ13C), iteration method, and rehydration experiments were combined to quantitatively determine the magnitude of cryogenic vacuum extraction (CVE)- and radial water transport (RWT)-induced deuterium offsets using one riparian tree species Salix babylonica L. A modified potential water source line (MPWL) was proposed to identify the total δ2H offsets between bulk xylem and source waters. The relationships between δ2H offsets induced by CVE or RWT and plant water content, leaf δ13C values, soil water content (SWC), and the depth to the water table (WTD) were investigated. Results showed that the bulk xylem waters in different tissue positions of S. babylonica showed −7.0 ‰ to −4.0 ‰ deuterium depletion relative to MPWL at four different sites (p < 0.01). The isotopic compositions of sap water coincided well with MPWL on the dual-isotope plot at the four sites. The CVE- and RWT-induced δ2H offsets accounted for 75.1 % and 24.9 % of the total δ2H offsets, respectively. The CVE-induced δ2H offsets were significantly negatively correlated with plant water content. In comparison, the RWT-induced δ2H offsets were negatively related to plant leaf δ13C values, trunk water content, and SWC, but positively correlated with WTD. This study provides a quantitative contribution of two major sources of deuterium offsets. The results provide critical insights into isotope-based plant water source identification and evapotranspiration partitioning.

Ocean Deserts Could Help Capture CO&lt;sub&gt;2&lt;/sub&gt; and Mitigate Global Warming

Various nutrient sources in the upper waters of oceanic subtropical gyres, which are the Earth's largest oligotrophic ecosystems, play a crucial role in governing the sequestration of atmospheric CO2.

Evaluation of Different Bias Correction Methods for Dynamical Downscaled Future Projections of the California Current Upwelling System

AbstractBiases in global Earth System Models (ESMs) are an important source of errors when used to obtain boundary conditions for regional models. Here we examine historical and future conditions in the California Current System (CCS) using three different methods to force the regional model: (a) interpolation of ESM output to the regional grid with no bias correction; (b) a “seasonally‐varying” delta method that obtains a season‐dependent mean climate change signal from the ESM for a 30‐year future period; and (c) a “time‐varying” delta method that includes the interannual variability of the ESM over the 1980–2100 period. To compare these methods, we use a high‐resolution (0.1°) physical‐biogeochemical regional model to dynamically downscale an ESM projection under the RCP8.5 emission scenario. Using different downscaling methods, the sign of future changes agrees for most of the physical and ecosystem variables, but the spatial patterns and magnitudes of these changes differ, with the seasonal‐ and time‐varying delta simulations showing more similar changes. Not correcting the ESM forcing leads to amplification of biases in some ecosystem variables as well as misrepresentation of the California Undercurrent and CCS source waters. In the non‐bias corrected and time‐varying delta simulations, most of the ecosystem variables inherit trends and decadal variability from the ESM, while in the seasonally‐varying delta simulation the future variability reflects the observed historical variability (1980–2010). Our results demonstrate that bias correcting the forcing prior to downscaling improves historical simulations, and that the bias correction method may impact the spatial and temporal variability of the future projections.

Unique ocean circulation pathways reshape the Indian Ocean oxygen minimum zone with warming

Abstract. The global ocean is losing oxygen with warming. Observations and Earth system model projections, however, suggest that this global ocean deoxygenation does not equate to a simple and systematic expansion of tropical oxygen minimum zones (OMZs). Previous studies have focused on the Pacific Ocean; they showed that the outer OMZ deoxygenates and expands as oxygen supply by advective transport weakens, the OMZ core oxygenates and contracts due to a shift in the composition of the source waters supplied by slow mixing, and in between these two regimes oxygen is redistributed with little effect on OMZ volume. Here, we examine the OMZ response to warming in the Indian Ocean using an ensemble of Earth system model high-emissions scenario experiments from the Coupled Model Intercomparison Project Phase 6. We find a similar expansion–redistribution–contraction response but show that the unique ocean circulation pathways of the Indian Ocean lead to far more prominent OMZ contraction and redistribution regimes than in the Pacific Ocean. As a result, only the outermost volumes (oxygen&gt;180 µmol kg−1) expand. The Indian Ocean experiences a broad oxygenation in the southwest driven by a reduction in waters supplied by the Indonesian Throughflow in favor of high-oxygen waters supplied from the southern Indian Ocean gyre. Models also project a strong localized deoxygenation in the northern Arabian Sea due to the rapid warming and shoaling of marginal sea outflows (Red Sea and Persian Gulf) and increases in local stratification with warming. We extend the existing conceptual framework used to explain the Pacific OMZ response to interpret the response in the Indian Ocean.

222Rn in selected waters sources from Quang Nam - Da Nang region - Central part of Vietnam

This paper presents the radon concentrations of forty-eight samples collected from lakes, streams and rivers as surface water, from dug well as ground water and from boreholes and thermal waters as underground waters in the Da Nang – Quang Nam, central part of Vietnam. The measured radon concentrations varied from 0.265 (in lake water) to 107 Bq L−1 (in underground water) with the following trend from the lowest to the highest: 222Rn in lakes<222Rn in rivers<222Rn in stream<222Rn in dug wells<222Rn in thermal waters<222Rn in drilled wells. Overall, the average 222Rn concentration in groundwater is twelve times higher than that in the surface water.Assuming that each person drinks 2.10−3 m3 d−1 and uses six cubic meters for washing purposes per month, the estimated total annual effective dose due to ingestion and inhalation of radon from the studied water samples ranges from 1.3 to 541 μSv y−1 for adults; from 1.2 to 479 μSv y−1 for children; and from 2.1 to 838 μSv y−1 for infants.

Coordination of rooting, xylem, and stomatal strategies explains the response of conifer forest stands to multi-year drought in the southern Sierra Nevada of California

Abstract. Extreme droughts are a major determinant of ecosystem disturbance that impacts plant communities and feeds back into climate change through changes in plant functioning. However, the complex relationships between aboveground and belowground plant hydraulic traits and their role in governing plant responses to drought are not fully understood. In this study, we use a model, the Functionally Assembled Terrestrial Ecosystem Simulator in a configuration that includes plant hydraulics (FATES-Hydro), to investigate ecosystem responses to the 2012–2015 California drought in comparison with observations at a site in the southern Sierra Nevada that experienced widespread tree mortality during this drought. We conduct a sensitivity analysis to explore how different plant water sourcing and hydraulic strategies lead to differential responses during normal and drought conditions. The analysis shows the following. Deep roots that sustain productivity through the dry season are needed for the model to capture observed seasonal cycles of evapotranspiration (ET) and gross primary productivity (GPP) in normal years, and deep-rooted strategies are nonetheless subject to large reductions in ET and GPP when the deep soil reservoir is depleted during extreme droughts, in agreement with observations. Risky stomatal strategies lead to greater productivity during normal years as compared to safer stomatal control, but they also lead to a high risk of xylem embolism during the 2012–2015 drought. For a given stand density, stomatal and xylem traits have a stronger impact on plant water status than on ecosystem-level fluxes. Our study highlights the significance of resolving plant water sourcing strategies to represent drought impacts on plants and consequent feedbacks in models.

Watershed Development and Eutrophying Potable Source-Water Reservoirs in a Warming Temperate/Subtropical Region

Reservoirs are increasingly valuable worldwide as potable source waters, yet in many geographic regions, their limnology and trophic status are poorly known. We characterized 14 drinking water reservoirs and their watersheds across the warming temperate/subtropical southeastern USA. Selected reservoirs had at least three years of accessible summer water quality data during 2010–2020, including Secchi depth, nutrients, and algal biomass as chlorophyll a, and depth profiles for temperature and dissolved oxygen. Most watersheds, including lands within a 10-km radius of the reservoirs, had sustained substantial urbanization and/or intensive industrialized animal production, in some cases including the discharge of partially treated human sewage or livestock slaughterhouse wastes near or into the reservoirs. Five reservoirs were assessed as mesotrophic; the others were eutrophic. Most were stratified, but ephemeral near-surface thermoclines were common, and many were too shallow (median depth 5.0 m) to maintain uniform temperatures in the relatively warm hypolimnia. Bottom-water hypoxia/anoxia occurred throughout the summers but, surprisingly, in 8 of 14 reservoirs hypoxia commonly extended to surface waters. In the Southeast as in many regions, drinking water reservoirs are poorly protected and degrading as livestock production and/or urban development increasingly characterize their watersheds. The eutrophication trajectory of these valuable resources should be used as an indicator of ecosystem health and water quality in developing more protective management and policy actions.

Improving the accuracy of δ18O and δ17O values of O2 measured by continuous‐flow isotope‐ratio mass spectrometry with a multipoint isotope‐ratio calibration

RationaleStable isotope analysis of O2 is a valuable tool to identify O2‐consuming processes in the environment; however, reference materials for O2 isotope analysis are lacking. Consequently, a one‐point calibration with O2 from ambient air is often applied, which can lead to substantial measurement uncertainties. Our goals were to develop a simple multipoint isotope‐ratio calibration approach and to determine measurement errors of δ18O and δ17O values of O2 associated with a one‐point calibration.MethodsWe produced O2 photosynthetically with extracted spinach thylakoids from source waters with δ18O values of −56‰ to +95‰ and δ17O values of −30‰ to +46‰. Photosynthesis was chosen because this process does not cause isotopic fractionation, so that the O isotopic composition of the produced O2 will be identical to that of the source water. The δ18O and δ17O values of the produced O2 were measured by gas chromatography coupled with isotope‐ratio mass spectrometry (GC/IRMS), applying a common one‐point calibration.ResultsLinear regressions between δ18O or δ17O values of the produced O2 and those of the corresponding source waters resulted in slopes of 0.99 ± 0.01 and 0.92 ± 0.10, respectively. In the tested δ range, a one‐point calibration thus introduced maximum errors of 0.8‰ and 3.3‰ for δ18O and δ17O, respectively. Triple oxygen isotopic measurements of O2 during consumption by Fe2+ resulted in a δ18O–δ17O relationship (λ) of 0.49 ± 0.01 without δ scale correction, slightly lower than expected for mass‐dependent O isotopic fractionation.ConclusionsNo significant bias is introduced on the δ18O scale when applying a one‐point calibration with O2 from ambient air during O2 isotope analysis. Both O2 formation and consumption experiments, however, indicate a δ17O scale compression. Consequently, δ17O values cannot be measured accurately by GC/IRMS with a one‐point calibration without determining the δ17O scale correction factor, e.g. with the O2 formation experiments described here.