Enhancing hydrological insight: isotopic methods revealing groundwater-surface water interactions in the Lower Quang Tri River Group, Vietnam.

The Lower Quang Tri River Group, situated in central Vietnam, faces a myriad of challenges, notably the decline in groundwater levels and the salinisation of both groundwater and surface water, significantly impacting water availability for domestic, agricultural, and industrial purposes. To address these pressing concerns, this study adopts a comprehensive methodology integrating hydrogeological measurements, isotopic techniques, and chemical analyses of various water sources, including local precipitation, surface water bodies, reservoirs, and groundwater samples. Utilising the deuterium and oxygen-18 signatures (δ2H and δ18O) in water molecules as environmental tracers for the assessment of base flow and water sources enables a nuanced understanding of the intricate interaction between surface water and groundwater. Research findings elucidate that during the dry season, groundwater recharge primarily stems from water in the reservoirs over approximately seven months. Base flow contributes between 80 and 85 % of streamflow during the rainy season, escalating to 100 % during the dry season. The mean travelling time of the base flow is estimated at 120 ± 10 days using the sine curve model developed by Rodgers et al. The insights gleaned from this study are poised to play a pivotal role in guiding the local water resources managers in licensing for the exploitation of a right quantities of groundwater as sustainable management strategies in the region.

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Precipitation in the plum rain period accounts for 40%-50% of annual precipitation in the monsoon region. To clarify the temporal variability of the isotopic composition of precipitation during the plum rain period from event to interannual time scale and identify the influencing factors, we analyzed the isotopic composition of precipitation and its influencing factors in Nanjing from 2015 to 2022. By using the Hybrid Single-particle Lagran-gian Integrated Trajectory (HYSPLIT) model with specific humidity analysis, we investigated the water vapor source and influencing factors. The results showed that 1) the isotopic abundance of atmospheric precipitation was depleted in the summer and enriched in winter. dx was lower in summer and higher in winter. The isotopic abundance of precipitation from the plum rain was depleted compared to mean value of the whole-year. 2) There was no significant correlation between δ2H and δ18O of the plum rain (precipitation) with local meteorological factors. However, dx was lower in light rain, reflecting the effect of sub-cloud evaporation. The average dx was higher during plum rain period in years with more total plum rain precipitation. 3) The low-latitude South China Sea and the western Pacific Ocean source area provided water vapor for the plum rain. The shift of moisture source region led to abrupt changes in precipitation isotopes. Our results could provide data support for studies on precipitation isotopes in the monsoon region, as well as a reference point for further understanding the precipitation mechanism of the plum rain and stu-dying the seasonal variability of atmospheric circulation in the East Asian monsoon region.

Limestone water mixing process and hydrogen and oxygen stable isotope fractionation response under mining action.

North China type coalfield are gradually mining deep, and the mixing of groundwater is intensified. Hydrogen and oxygen isotopes are important elements for tracing groundwater movement. The fractionation response mechanism under mining conditions is not clear. In this paper, combined with numerical simulation, MixSIAR isotope mixing model and other methods, according to the δD, δ18O and hydrochemical information of various water bodies, the impact of coal mining on hydrogen and oxygen isotope fractionation is analyzed from multiple perspectives. The results show that summer soil water is the main source of recharge for limestone water, accounting for 30.7%-41.5%, and the Zhan River is the main source of recharge for limestone water. Before groundwater recharge, evaporation leads to the increase of δ18O in surface water by 0.31‰-5.58‰, water loss by 1.81%-28.00%, the increase of δ18O in soil water by 0.47‰-6.33‰, and water loss by 2.74%-35.80%. Compared with the coal mining layer, the degree of hydrogen and oxygen isotope drift and water-rock interaction in the coal mine stopping layer are significantly improved. The results of numerical simulation show that the pumping activity reduces the 18O concentration in the mining layer. The ion ratio is used as a new variable to eliminate the influence of water-rock interaction when calculating the mixing ratio. The results show that the limestone water is in a state of receiving external recharge, and mixing effect increases the δ18O in limestone water by 0.86‰ on average, and the δD increases by 0.72‰ on average. The research results explain the controlled process of hydrogen and oxygen isotope fractionation under mining conditions, which is of great significance to coal mine safety production.

Acid mine drainage from coal mines in the eastern Himalayan sub-region: Hydrogeochemical processes, seasonal variations and insights from hydrogen and oxygen stable isotopes.

Uncontrolled coal mining using non-scientific methods has presented a major threat to the quality of environment, particularly the water resources in eastern himalayan sub-region of India. Water bodies in the vicinity of mining areas are contaminated by acid mine drainage (AMD) that is released into streams and rivers. This study attempted to assess the impact of AMD, deciphering hydrogeochemical processes, seasonal fluctuations, and stable isotope features of water bodies flowing through and around coal mining areas. Self-organizing maps (SOMs) used to separate and categorize AMD, AMD-impacted and non-AMD impacted water from the different study locations for two sampling seasons revealed four clusters (C), with C1 and C2 impacted by AMD, C3 and C4 showing negligible to no impact of AMD. AMD impacted water was SO42- - Mg2+- Ca2+ hydrochemical type with sulphide oxidation and evaporation dominating water chemistry, followed by silicate weathering during both the sampling seasons. Water with negligible-to-no AMD-impact was Mg2+- Ca2+- SO42- to Ca2+ - HCO3- to mixed hydrochemical type with rock weathering and dissolution, followed by ion exchange as major factors controlling water chemistry during both the sampling seasons. Most of physicochemical parameters of C1 and C2 exceeded the prescribed limits, whereas in C3 and C4 water samples, parameters were found within the prescribed limits. Stable isotopes of hydrogen (δ2H) and oxygen (δ18O) during post-monsoon (PoM) varied between -41.04 ‰ and -29.98 ‰, and -6.60 ‰ to -3.94 ‰; and during pre-monsoon (PrM) varied between -58.18 ‰ and - 33.76 ‰ and -8.60 ‰ to -5.46 ‰. Deuterium excess (d-excess) ranged between 1.57 ‰ and 12.47 ‰ during PoM and 5.70 ‰ to 15.17 ‰ during PrM season. The stable isotopes analysis revealed that evaporation, mineral dissolution and mixing with rainwater are the key factors in study area.

Applicability of self-consistent reaction field (SCRF) method to DFT estimation of hydrogen isotope separation factors in reversible processes.

In earlier quantum chemical calculations of isotope effects, chemical species in the liquid phase were generally treated as existing in the gas phase. In recent years, however, advances in computational programs have made it easier for the self-consistent reaction field (SCRF) method to handle chemical species in the liquid phase, and as a result, it has become easier to apply the SCRF method to isotope effect calculations. This paper concerns the scope of application of the DFT-SCRF method to reversible processes for hydrogen isotope enrichment. It is found that the applicability of the method depends on the type of the intermolecular interaction in the liquid phase and the degree of hydrogen isotope effect (separation factor) on which the process is based. When the magnitude of the isotope effect of the separation system is greater than 10-1, the simple SCRF method is fully applicable; when the magnitude is around 10-2, SCRF with a dimer model, in which the monomer is replaced by a dimer, is applicable for the analysis of the liquid phase with relatively strong intermolecular interactions. Anharmonic correction to the separation factor calculated based on harmonic frequencies may be effective to systems with the liquid phase with weak intermolecular interactions.

Different responses of oxygen and hydrogen isotopes in leaf and tree-ring organic matter to lethal soil drought.

The oxygen and hydrogen isotopic composition (δ18O, δ2H) of plant tissues are key tools for the reconstruction of hydrological and plant physiological processes and may therefore be used to disentangle the reasons for tree mortality. However, how both elements respond to soil drought conditions before death has rarely been investigated. To test this, we performed a greenhouse study and determined predisposing fertilization and lethal soil drought effects on δ18O and δ2H values of organic matter in leaves and tree rings of living and dead saplings of five European tree species. For mechanistic insights, we additionally measured isotopic (i.e. δ18O and δ2H values of leaf and twig water), physiological (i.e. leaf water potential and gas-exchange) and metabolic traits (i.e. leaf and stem non-structural carbohydrate concentration, carbon-to-nitrogen ratios). Across all species, lethal soil drought generally caused a homogenous 2H-enrichment in leaf and tree-ring organic matter, but a low and heterogenous δ18O response in the same tissues. Unlike δ18O values, δ2H values of tree-ring organic matter were correlated with those of leaf and twig water and with plant physiological traits across treatments and species. The 2H-enrichment in plant organic matter also went along with a decrease in stem starch concentrations under soil drought compared with well-watered conditions. In contrast, the predisposing fertilization had generally no significant effect on any tested isotopic, physiological and metabolic traits. We propose that the 2H-enrichment in the dead trees is related to (i) the plant water isotopic composition, (ii) metabolic processes shaping leaf non-structural carbohydrates, (iii) the use of carbon reserves for growth and (iv) species-specific physiological adjustments. The homogenous stress imprint on δ2H but not on δ18O suggests that the former could be used as a proxy to reconstruct soil droughts and underlying processes of tree mortality.

A protocol for distilling animal body water from biological samples and measuring oxygen and hydrogen stable isotopes via cavity ring-down spectroscopy.

The application of stable isotope analysis (SIA) to the fields of ecology and animal biology has rapidly expanded over the past three decades, particularly with regards to water analysis. SIA now provides the opportunity to monitor migration patterns, examine food webs, and assess habitat changes in current and past study systems. While carbon and nitrogen SIA of biological samples have become common, analyses of oxygen or hydrogen are used more sparingly despite their promising utility for tracing water sources and animal metabolism. Common ecological applications of oxygen or hydrogen SIA require injecting enriched isotope tracers. As such, methods for processing and analyzing biological samples are tailored for enriched tracer techniques, which require lower precision than other techniques given the large signal-to-noise ratio of the data. However, instrumentation advancements are creating new opportunities to expand the applications of high-throughput oxygen and hydrogen SIA. To support these applications, we update methods to distill and measure water derived from biological samples with consistent precision equal to, or better than, ± 0.1 ‰ for δ17O, ± 0.3 ‰ for δ18O, ± 1 ‰ for δ2H, ± 2 ‰ for d-excess, and ± 15 per meg for Δ17O.

Seasonality of hydrogeochemical evolutions and isotopic variabilities (δ18O, δ2H and d-excess) in the surface water as well as groundwater from tropical central-south Mexico.

Due to adverse impact of the global warming on hydrological resources, we intended to document the hydrogeochemical evolutions of surface and groundwater at tropical central-south Mexico in terms of seasonality of rock-water interaction, precipitation/evaporation variation and moisture source by evaluating the major ion chemistry in Piper and Gibbs plots, Durov diagram and through estimation of the chloro-alkaline indices as well as assessing the stable isotope compositions (δ18O and δ2H) in samples from different seasons of a year. Surface water of the Lake Coatetelco shifted from mostly Ca-Mg-HCO3 facies in wet summer-autumn to Na-HCO3-Cl facies in the dry spring due to elevated Na, Cl and HCO3. Greater evaporation in spring led to a maximum δ18O enrichment of ca.7‰ compared to the other seasons, and much depleted deuterium excess (-40.92‰ to -39.20‰). Interaction of the lake water with subsurface carbonate lithologies, and comparable isotopic compositions reflected the enhanced interaction between the surface water body and aquifers in the wet autumn. Effect of seasonality, however, was unclear on the groundwater facies, and its heterogenous composition (Ca-Mg-HCO3, Na-HCO3-Cl and Na-HCO3) reflected the interactions with different lithologies. Fractionations in isotope compositions of the groundwater were caused from recharge at different elevations, seasonality of moisture sources and moisture recycling. The water-mineral saturation index was an efficient proxy of seasonality as the lake water and groundwater (avg SIcalcite > 0.5) of the dry autumn were saturated with calcite. This vital information about carbonate precipitation, pCO2 and chemical facies would be useful for the better interpretation of paleoclimate archives in this region.

The stable isotope hydrology of Sable Island, Nova Scotia, Canada with implications for evaluating the water budget of wild horses.

We investigated the stable isotope hydrology of Sable Island, Nova Scotia, Canada over a five year period from September, 2017 to August, 2022. The δ2H and δ18O values of integrated monthly precipitation were weakly seasonal and ranged from -66 to -15 ‰ and from -9.7 to -1.9 ‰, respectively. Fitting these monthly precipitation data resulted in a local meteoric water line (LMWL) defined by: δ2H = 7.22 ± 0.21 · δ18O + 7.50 ± 1.22 ‰. Amount-weighted annual precipitation had δ2H and δ18O values of -36 ± 11 ‰ and -6.1 ± 1.4 ‰, respectively. Deep groundwater had more negative δ2H and δ18O values than mean annual precipitation, suggesting recharge occurs mainly in the winter, while shallow groundwater had δ2H and δ18O values more consistent with mean annual precipitation or mixing of freshwater with local seawater. Surface waters had more positive values and showed evidence of isolation from the groundwater system. The stable isotopic compositions of plant (leaf) water, on the other hand, indicate plants use groundwater as their source. Fog had δ2H and δ18O values that were significantly more positive than those of local precipitation, yet had similar 17O-excess values. δ2H values of horsehair from 4 individuals lacked seasonality, but had variations typical to those of precipitation on the island. Differences in mean δ2H values of horsehair were statistically significant and suggest variations in water use may exist between spatially disparate horse communities. Our results establish an important initial framework for ongoing isotope studies of feral horses and other wildlife on Sable Island.

Topographic influence on ecohydrology in volcanic watersheds of the western Pacific monsoon area: evidence from water stable isotope composition of meteoric water, thermal water, and plants.

In Taiwanese volcanic watersheds, we investigated stable water isotopes in meteoric water, plants, and thermal water. Meteoric water exhibited a seasonal cycle, with heavier isotopes in winter and lighter ones in summer, especially in the southern region. The northern monsoon signal lagged the south by two weeks. In the Tatun mountains, young water fractions indicated prevalent old water sources. In the northern watershed, streamwater mainly came from the winter monsoon, while the southern one was influenced by alternating monsoons. Both indices indicated that winter plants depended on summer rainfall. Streamwater and plants had distinct sources in winter, supporting ecohydrological separation. Thermal spring water's d-excess helped identify water-rock interactions, with low d value signaling such interactions. The topographic wetness index showed a higher summer monsoon contribution to southern streamwater but a lower one to plants. The mean linear channel direction significantly affected the monsoon contribution fraction, with northeast-oriented channels vulnerable to northeastward winter monsoons. Finally, we developed a model illustrating hydrological processes on short and long timescales. Our findings enhance our understanding of hydrological disturbances' impact on water resources and ecosystems.

Cholesterol Stiffening of Lipid Membranes.

Biomembrane order, dynamics, and other essential physicochemical parameters are controlled by cholesterol, a major component of mammalian cell membranes. Although cholesterol is well known to exhibit a condensing effect on fluid lipid membranes, the extent of stiffening that occurs with different degrees of lipid acyl chain unsaturation remains an enigma. In this review, we show that cholesterol locally increases the bending rigidity of both unsaturated and saturated lipid membranes, suggesting there may be a length-scale dependence of the bending modulus. We review our published data that address the origin of the mechanical effects of cholesterol on unsaturated and polyunsaturated lipid membranes and their role in biomembrane functions. Through a combination of solid-state deuterium NMR spectroscopy and neutron spin-echo spectroscopy, we show that changes in molecular packing cause the universal effects of cholesterol on the membrane bending rigidity. Our findings have broad implications for the role of cholesterol in lipid-protein interactions as well as raft-like mixtures, drug delivery applications, and the effects of antimicrobial peptides on lipid membranes.

TRITIUM CONCENTRATION IN MONTHLY PRECIPITATION NEAR THE FUSION TEST FACILITY IN JAPAN BEFORE AND AFTER THE DEUTERIUM PLASMA EXPERIMENT.

In Japan, the deuterium plasma experiment using the Large Helical Device was started at the National Institute for Fusion Science (NIFS) in March 2017 to investigate high-temperature plasma physics and hydrogen isotope effects in research leading towards the realisation of fusion energy. The deuterium plasma experiment produces small amount of tritium by fusion reactions. To understand any impacts by the experiment to the surrounding environment, monthly precipitation samples have been collected at the NIFS site since November 2013 to assess the relationship between isotope composition and chemical species in precipitation including tritium. By comparing data before and after the deuterium plasma experiment start, it was found that tritium released from the main stack of the fusion test facility had no impact on the environment surrounding NIFS.

LEVELS OF ATMOSPHERIC TRITIUM IN THE SITE OF FUSION TEST FACILITY.

In the deuterium plasma experiment using Large Helical Device at the National Institute for Fusion Science (NIFS), a small amount of tritium is produced by the D-D fusion reaction. Then, a part of produced tritium is discharged into the environment via a stack. Thus, the atmospheric tritium in the site of NIFS has been monitored before starting the deuterium plasma experiment. The atmospheric tritium concentrations at NIFS were indicated to be background levels in Japan. To investigate the impact of tritium discharged from the stack, the correlation between the atmospheric tritium concentration and the tritium concentration observed in the stack was evaluated, and no significant correlation was found. In addition, the atmospheric tritium concentration at NIFS ranged within the background levels in Japan. Therefore, the impact of discharged tritium from the stack would be negligible in the environment at NIFS.

DEUTERIUM TRANSFER ANALYSIS INCLUDING FOOD CHAIN FROM SEAWATER INTO ABALONE.

Tritium is released into the ocean from nuclear facilities located at coastal areas. In addition, tritiated water is decided to be released into the ocean from the Fukushima Dai-ichi Nuclear Power Plant. Although released tritium concentration would be strictly controlled, impact of tritium on the marine products is major concern for the public. In this study, deuterium transfers from seawater into seaweed (ulva) and abalone were measured. In addition, organically bound deuterium (OBD) transfer from ulva into abalone was measured. OBD concentrations in ulva were saturated in 2 weeks and those in abalone were saturated in 6 months. Ulva and abalone were exposed to seawater containing 0.2% (mol-D/mol-H) deuterium. Maximum OBD concentrations in ulva were ~0.1% (mol-D/mol-H) and those in abalone muscle were ~0.035% (mol-D/mol-H). Numerical deuterium transfer model was constructed. Obtained numerical model well represented the OBD-enriched ulva feeding experiment.

A novel deuterium-based model for measurement of exogenous surfactant using deuterium-depleted water.

Stable isotope tracers, like 13 C, can be used for the measurement of the partition between the endogenous and exogenous pulmonary disaturated-phosphatidylcholine (DSPC). Deuterium labeling methods are still not fully explored. Our aim was to investigate the feasibility of using deuterium-depleted water (DDW) and deuterium-enriched water (DEW) to measure endogenous and exogenous pulmonary DSPC in a rabbit model of surfactant depletion. Data obtained from the 13 C dilution method were used as a reference. We studied 9 adult rabbits: 4 drank DDW and 5 DEW for 5 days. Lung surfactant depletion was induced at Day 5 by repeated saline bronchoalveolar lavages (BAL), which were stored as a pool (BAL pool). After endogenous surfactant depletion, rabbits received exogenous surfactant followed by a second BAL depletion procedure (End-Experiment Pool). DSPC quantity, and palmitic acid (PA)-DSPC 2 H/1 H (δ2 H) and 13 C/12 C ratios (δ13 C) of exogenous surfactant batches and of BAL pools were measured by High-Resolution Mass Spectrometry. The amount of exogenous surfactant recovered from the lungs ranged from 45% to 81% and, it was highly correlated with those obtained with the use of the 13 C (r = 0.9844, p < 0.0001). We demonstrated that commercially available purified DDW and even low doses of DEW can be used to modify the deuterium background of endogenous surfactants with the purpose of measuring the contribution of exogenous surfactants to the endogenous alveolar surfactant pool.

Comparative cryogenic extraction rehydration experiments reveal isotope fractionation during root water uptake in Gramineae.

Determining whether isotope fractionation occurs during root water uptake is a prerequisite for using stem or xylem water isotopes to trace water sources. However, it is unclear whether isotope fractionation occurs during root water uptake in gramineous crops. We conducted prevalidation experiments to estimate the isotope measurement bias associated with cryogenic vacuum distillation (CVD). Next, we assessed isotope fractionation during root water uptake in two common agronomic crops, wheat (Triticum aestivum L.) and maize (Zea mays L.), under flooding after postdrought stress conditions. Cryogenic vacuum distillation caused significant depletion of 2 H but negligible effects on 18 O for both soil and stem water. Surprisingly CVD caused depletion of 2 H and enrichment of 18 O in root water. Stem and root water δ18 O were more than soil water δ18 O, even considering the uncertainty of CVD. Soil water 18 O was depleted compared with irrigation water 18 O in the pots with plants but enriched relative to irrigation water 18 O in the pots without plants. These results indicate that isotope fractionation occurred during wheat and maize root water uptake after full irrigation and led to a heavy isotope enrichment in stem water. Therefore, the xylem/stem water isotope approach widely used to trace water sources should be carefully evaluated.

First use of triply labelled water analysis for energy expenditure measurements in mice.

The doubly labelled water (DLW) method is widely used to determine energy expenditure. In this work, we demonstrate the addition of the third stable isotope, 17O, to turn it into triply labelled water (TLW), using the three isotopes measurement of optical spectrometry. We performed TLW (2H, 18O and17O) measurements for the analysis of the CO2 production (rCO2) of mice on different diets for the first time. Triply highly enriched water was injected into mice, and the isotope enrichments of the distilled blood samples of one initial and two finals were measured by an off-axis integrated cavity output spectroscopy instrument. We evaluated the impact of different calculation protocols and the values of evaporative water loss fraction. We found that the dilution space and turnover rates of 17O and 18O were equal for the same mice group, and that values of rCO2 calculated based on 18O-2H, or on 17O-2H agreed very well. This increases the reliability and redundancy of the measurements and it lowers the uncertainty in the calculated rCO2 to 3% when taking the average of two DLW methods. However, the TLW method overestimated the rCO2 compared to the indirect calorimetry measurements that we also performed, much more for the mice on a high-fat diet than for low-fat. We hypothesize an extra loss or exchange mechanism with a high fractionation for 2H to explain this difference.

Formula Assignment Algorithm for Deuterium-Labeled Ultrahigh-Resolution Mass Spectrometry: Implications of the Formation Mechanism of Halogenated Disinfection Byproducts.

Ultrahigh-resolution mass spectrometry (UHR-MS) coupled with isotope labeling has attracted significant attention in elucidating the mechanisms of the transformation of dissolved organic matter (DOM). Herein, we developed a novel formula assignment algorithm based on deuterium (D)-labeled UHR-MS, namely, FTMSDeu, for the first time. This algorithm was employed to determine the precursor molecules of halogenated disinfection byproducts (Xn-DBPs) and to evaluate the relative contribution of electrophilic addition and substitution reactions in Xn-DBP formation according to the H/D exchange of DOM molecules. Further, tandem mass spectrometry with homologous-based network analysis was used to validate the formula assignment accuracy of FTMSDeu in the identification of iodinated disinfection byproducts. Electrophilic substitution accounted for 82-98, 71-89, and 43-45% of the formation for Cl-, Br-, and I-containing Xn-DBPs, respectively, indicating the dominant role of the electrophilic substitution in chlorinated disinfection byproducts with low Br and I concentrations. The absence of putative precursors in some Xn-DBPs also suggests that Xn-DBP formation includes secondary reactions (e.g., oxidation and hydrolysis) in addition to the electrophilic addition and/or substitution of halogens. These findings highlight the significance of isotopically labeled UHR-MS techniques in revealing the transformation of DOM in natural and engineered systems.

Deuterium-depleted water: A new tracer to label pulmonary surfactant lipids in adult rabbits.

Stable isotope tracing can be safely used for metabolic studies in animals and humans. The endogenous biosynthesis of lipids (lipogenesis) is a key process throughout the entire life but especially during brain and lung growth. Adequate synthesis of pulmonary surfactant lipids is indispensable for life. With this study, we report the use of deuterium-depleted water (DDW), suitable for human consumption, as metabolic precursor for lipogenesis. We studied 13 adult rabbits for 5 days. Four rabbits drank tap water (TW) and served as controls; in four animals, DDW was substituted to drinking water, whereas five drank deuterium-enriched water (DEW). After 5 days, a blood sample and a bronchoalveolar lavage (BAL) sample were collected. The 2 H/1 H (δ2 H) of BAL palmitic acid (PA) desaturated phosphatidylcholine (DSPC), the major phospholipid of pulmonary surfactant, and of plasma water was determined by high-resolution mass spectrometry. We found that the δ2 H values of DDW, DEW and TW were -984 ± 2‰, +757 ± 2‰ and -58 ± 1‰, respectively. After 5 days, plasma water values were -467 ± 87‰, +377 ± 56‰ and -53 ± 6‰, and BAL DSPC-PA was -401 ± 27‰, -96 ± 38‰ and -249 ± 9‰ in the DDW, DEW and TW, respectively. With this preliminary study, we demonstrated the feasibility of using DDW to label pulmonary surfactant lipids. This novel approach can be used in animals and in humans, and we speculate that it could be associated with more favourable study compliance than DEW in human studies.

Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval.

RATIONALE: New methods to measure stable isotopes of soil and tree water directly in the field enable us to increase the temporal resolution of obtained data and advance our knowledge on the dynamics of soil and plant water fluxes. Only few field applications exist. However, these are needed to further improve novel methods and hence exploit their full potential. METHODS: We tested the borehole equilibration method in the field and collected in situ and destructive samples of stable isotopes of soil, trunk and root xylem water over a 2.5-month experiment in a tropical dry forest under natural abundance conditions and following labelled irrigation. Water from destructive samples was extracted using cryogenic vacuum extraction. Isotope ratios were determined with IRIS instruments using cavity ring-down spectroscopy both in the field and in the laboratory. RESULTS: In general, timelines of both methods agreed well for both soil and xylem samples. Irrigation labelled with heavy hydrogen isotopes clearly impacted the isotope composition of soil water and one of the two studied tree species. Inter-method deviations increased in consequence of labelling, which revealed their different capabilities to cover spatial and temporal heterogeneities. CONCLUSIONS: We applied the novel borehole equilibration method in a remote field location. Our experiment reinforced the potential of this in situ method for measuring xylem water isotopes in both tree trunks and roots and confirmed the reliability of gas permeable soil probes. However, in situ xylem measurements should be further developed to reduce the uncertainty within the range of natural abundance and hence enable their full potential.