Bound and mobile soil water isotope ratios are affected by soil texture and mineralogy,whereas extraction method influences their measurement

Questions persist about interpreting isotope ratios of bound and mobile soil water pools, particularly relative to clay content and extraction conditions. Interactions between pools and resulting extracted water isotope composition are presumably related to soil texture, yet few studies have manipulated the bound pool to understand its influence on soil water processes. Using a series of drying and spiking experiments, we effectively labelled bound and mobile water pools in soils with varying clay content. Soils were first vacuum dried to remove residual water, which was then replaced with heavy isotope-enriched water prior to oven drying and spiking with heavy isotope-depleted water. Water was extracted via centrifugation or cryogenic vacuum distillation (at four temperatures) and analysed for oxygen and hydrogen isotope ratios via isotope ratio mass spectrometry. Water from centrifuged samples fell along a mixing line between the two added waters but was more enriched in heavy isotopes than the depleted label, demonstrating that despite oven drying, a residual pool remains and mixes with the mobile water. Soils with higher clay + silt content appeared to have a larger bound pool. Water from vacuum distillation samples have a significant temperature effect, with high temperature extractions yielding progressively more heavy isotope-enriched values, suggesting that Rayleigh fractionation occurred at low temperatures in the vacuum line. By distinctly labelling bound and mobile soil water pools, we detected interactions between the two that were dependent on soil texture. Although neither extraction method appeared to completely extract the combined bound and mobile (total water) pool, centrifugation and high temperature cryogenic vacuum distillations were comparable for both δ²H and δ¹⁸O of soil water isotope ratios.     

Intercomparison of soil pore water extraction methods for stable isotope analysis and interpretation of hillslope runoff sources

Intercomparison of soil pore water extraction methods for stable isotope analysis has been a focus of recent studies in relation to plant source waters, which found a wide isotopic variance depending on the extraction method. Few studies have yet explored extraction effects for mobile pore waters that relate to hillslope runoff. This is because it is extremely difficult in natural systems to control the boundary conditions in order to assess and compare impacts of pore water extraction on resulting hillslope flow. With our new semicontrolled experiments on outdoor mini‐hillslopes, we studied mixing and runoff processes by means of stable isotopes of water and quantified relations between pore water extraction methods. We tested the null hypothesis that nondestructive and destructive pore water sampling methods sample the same soil water pool. Three hillslopes were mounted on load cells, filled with loamy sand textured soils from the Landscape Evolution Observatoryat Biosphere 2, equipped with soil moisture and temperature sensors, a bottom outflow, and a surface runoff gauge for isotope sampling. We followed the precipitation isotopic composition over and through the soil profile. One hillslope was instrumented with suction cups, on the second we installed sampling ports for in‐situ soil water vapour measurements, and the third hillslope was sampled destructively for applying the centrifugation and vapour equilibrium methods. All hillslopes were sampled at four depths (0–10, 10–20, 20–30, and 30–40 cm) at three different downslope positions. ²H and ¹⁸O analyses were performed via laser spectroscopy. We found no isotopic differences between rainfall, surface runoff, and bottom outflow. The in situ vapour ports' soil isotope data showed the widest spread over all hillslope positions and depths. Centrifugation's and suction cups' isotope results plotted closest to the local meteoric water line and within the range of hillslope runoff and bottom outflow data. Hillslope position did not influence the soil isotope results. These results suggest caution be used in the field when selecting an extraction technique for matching soil waters to runoff waters. Soil suction lysimeters and centrifugation appeared to be the most appropriate tools in this regard.     

Intercomparison of soil pore water extraction methods for stable isotope analysis

Measurements of δ²H and δ¹⁸O composition of pore waters in saturated and unsaturated soil samples are routinely performed in hydrological studies. A variety of in‐situ and lab‐based pore water extraction methods for the analysis of the stable isotopes of water now exist. While some have been used for decades (e.g. cryogenic vacuum extraction) others are relatively new, such as direct vapour equilibration or the microwave extraction technique. Despite their broad range of application, a formal and comprehensive intercomparison of soil water extraction methods for stable isotope analysis is lacking and long overdue. Here we present an intercomparison among five commonly used lab‐based pore water extraction techniques (high pressure mechanical squeezing, centrifugation, direct vapour equilibration, microwave extraction, and cryogenic extraction). We applied these extraction methods to two physicochemically different soil types that were dried and rewetted with water of known isotopic composition at three different water contents. Our results showed that the extraction approach can have a significant effect on pore water isotopic composition as all methods exhibited significant deviations from the spiked reference water, depending secondarily on the soil type and soil water content. Most pronounced, cryogenic water extraction showed large deviations from the spiked reference water, whereas mechanical squeezing and centrifugation provided results closest to the spiked water for both soil types. We also compared results for each extraction method – where liquid water was obtained – on both an OA‐ICOS and IRMS. Differences between these two analytical instruments were negligible for these organic compound‐free waters. We suggest that users of soil water extraction approaches carefully choose an extraction technique that is suitable for the specific research question, adapted to the dominant soil type and water content of the study. Copyright © 2016 John Wiley & Sons, Ltd.     

Sampling soil water along the pF curve for δ2H and δ18O analysis

Soil water stable isotopes are widely used across disciplines (e.g., hydrology, ecology, soil science, and biogeochemistry). However, the full potential of stables isotopes as a tool for characterizing the origin, flow path, transport processes and residence times of water in different eco-, hydro-, and geological compartments has not yet been exploited. This is mainly due to the large variety of different methods for pore water extraction. While recent work has shown that matric potential affects the equilibrium fractionation, little work has examined how different water retention characteristics might affect the sampled water isotopic composition. Here, we present a simple laboratory experiment with two well-studied standard soils differing in their physico-chemical properties (e.g., clayey loam and silty sand). Samples were sieved, oven-dried and spiked with water of known isotopic composition to full saturation. For investigating the effect of water retention characteristics on the extracted water isotopic composition, we used pressure extractors to sample isotopically labelled soil water along the pF curve. After pressure extraction, we further extracted the soil samples via cryogenic vacuum extraction. The null hypothesis guiding our work was that water held at different tensions shows the same isotopic composition. Our results showed that the sampled soil water differed isotopically from the introduced isotopic label over time and sequentially along the pF curve. Our and previous studies suggest caution in interpreting isotope results of extracted soil water and a need to better characterize processes that govern isotope fractionation with respect to soil water retention characteristics. In the future, knowledge about soil water retention characteristics with respect to soil water isotopic composition could be applied to predict soil water fractionation effects under natural and non-stationary conditions. In this regard, isotope retention characteristics as an analog to water retention characteristics have been proposed as a way forward since matric potential affects the equilibrium fractionation between the bound water and the water vapour.     

Hydrogen and oxygen isotope ratios in the waters of the Ngawha hydrothermal area,North Auckland

The ratios of D/H and O¹⁸/O¹⁶ in natural waters from streams, boreholes, soda springs, hot pools, ponds and larger bodies of water in the Ngawha hydrothermal area were determined. The results are considered in relation to the isotopic changes known to occur in water subjected to evaporation. Where applicable chemical and other work was also considered. It is assumed that stream water isotope composition is the mean value for the isotopic composition of meteoric waters. Measurements on waters taken from boreholes drilled to 65 feet and 350 feet and from the other water sources mentioned, indicate that they were of meteoric origin as judged by stream isotope composition. The waters from the soda springs appeared to be isotopically the same as the stream water, a finding consistent with the absence of evaporative surface. These borehole waters were similar but slightly different in O¹⁸ due probably to exchange between rock and water. Heavy isotope enrichment of the ponds and larger bodies of water appeared to be due to non-equilibrium evaporation at ambient temperature. The hot pools in the Ngawha springs area proper were enriched in the heavier isotopes probably due to non-equilibrium evaporation at the usual hot pool temperature of about 40°C and also to exchange of O¹⁸ between water and rock. The water from a further borehole drilled to approximately 2,000 feet appeared also to be of meteoric origin but was changed in O¹⁸ content to an extent consistent with the assumption that oxygen isotope exchange with rock had taken place at approximately 230°C. The results are used to illustrate possibilities for the use of oxygen and hydrogen isotope measurements in hydrothermal investigations.     

A comparison of the toluene distillation and vacuum/heat methods for extracting soil water for stable isotopic analysis

Hanford Loam, from Richland, Washington, was used as a test soil to determine the precision, accuracy and nature of two methods to extract soil water for stable isotopic analysis: azeotropic distillation using toluene, and simple heating under vacuum. The soil was oven dried, rehydrated with water of known stable isotopic compositions, and the introduced water was then extracted.

Compared with the introduced water, initial aliquots of evolved water taken during a toluene extraction were as much as 30 ‰ more depleted in D and 2.7 ‰ more depleted in ¹⁸O, whereas final aliquots were as much as 40 ‰ more enriched in D and 14.3 ‰ more enriched in ¹⁸O. Initial aliquots collected during the vacuum/heat extraction were as much as 64 ‰ more depleted in D and 8.4 ‰ more depleted in ¹⁸O than was the introduced water, whereas the final aliquots were as much as 139 ‰ more enriched in D, and 20.8 ‰ more enriched in ¹⁸O. Neither method appears quantitative; however, the difference in stable isotopic composition between the first and last aliquots of water extracted by the toluene method is less than that from the vacuum/heat method. This is attributed to the smaller fractionation factors involved with the higher average temperatures of distillation of the toluene. The average stable isotopic compositions of the extracted water varied from that of the introduced water by up to 1.4 ‰ in δD and 4.2 ‰ in δ¹⁸O with the toluene method, and by 11.0 ‰ in δD and 1.8 ‰ in δ¹⁸O for the vacuum/heat method.

The lack of accuracy of the extraction methods is thought to be due to isotopic fractionation associated with water being weakly bound (not released below 110°C) in the soil. The isotopic effect of this heat-labile water is larger at low water contents (3.6 and 5.2% water by weight) as the water bound in the soil is a commensurately larger fraction of the total. With larger soilwater contents the small volume of water bound with an associated fractionation is not enough to affect the remaining unbound introduced soil water. Pretreatment of the soil to equilibrate the heat-labile water to the test water produced good results for the toluene distillation but not the vacuum/heat extraction method.

Vapors collected over the soils also show stable isotopic variations related to soilwater content. These vapors also appear to be in closer equilibrium with the free water, as extracted by the toluene method, than with the originally introduced water; thus, the soil vapors do not appear to be isotopically affected by the heat-labile water.

The toluene method appears to be better for extracting soil water for stable isotopic analysis because it allows more precise temperature control and excludes the extraction of heat-labile water which is isotopically fractionated. The bound nature of this heat-labile water limits association with the hydrologically active soil water; thus, the exclusion of this water from the soil water attained by toluene distillation may be advantageous. However, the azeotropic nature of toluene distillation affords no benefit and the extraction procedure must continue to completion.     

Evidence for variations in cryogenic extraction deuterium biases of plant xylem water across foundational northeastern US trees

Measurements of oxygen and hydrogen isotopes in plant xylem water (²H, ¹⁸O) have helped to redefine conceptual and numerical models of the hydrological cycle and understand how plants compete for subsurface water. Recent experiments have shown that Cryogenic Vacuum Extraction (CVE) of plant xylem water can result in a δ²H bias. We tested if CVE δ²H-biases varied significantly across seven foundational northeastern US forest trees with a series of tree core rehydration experiments. Our analysis demonstrated that CVE δ²H-biases were well predicted by sample gravimetric water content and varied significantly with tree species identity. We show that species-level δ²H-bias corrections can result in substantially different understandings of plant water uptake and transpiration versus uncorrected data or generic bias corrections. This research demonstrates an urgent need for the critical evaluation of CVE for plant water extraction. In the absence of a stronger understanding of CVE δ²H-biases, we recommend that xylem water δ²H observations should not be used in plant water uptake studies.     

Groundwater recharge in the Akaki catchment,central Ethiopia: evidence from environmental isotopes (δ18O, δ2H and 3H) and chloride mass balance

Recharge patterns, possible flow paths and the relative age of groundwater in the Akaki catchment in central Ethiopia have been investigated using stable environmental isotopes δ¹⁸O and δ²H and radioactive tritium (³H) coupled with conservative chloride measurements. Stable isotopic signatures are encoded in the groundwater solely from summer rainfall. Thus, groundwater recharge occurs predominantly in the summer months from late June to early September during the major Ethiopian rainy season. Winter recharge is lost through high evaporation–evapotranspiration within the unsaturated zone after relatively long dry periods of high accumulated soil moisture deficits. Chloride mass balance coupled with the isotope results demonstrates the presence of both preferential and piston flow groundwater recharge mechanisms. The stable and radioactive isotope measurements further revealed that groundwater in the Akaki catchment is found to be compartmentalized into zones. Groundwater mixing following the flow paths and topography is complicated by the lithologic complexity. An uncommon, highly depleted stable isotope and zero‐³H groundwater, observed in a nearly east–west stretch through the central sector of the catchment, is coincident with the Filwoha Fault zone. Here, deep circulating meteoric water has lost its isotopic content through exchange reactions with CO₂ originating at deeper sources or it has been recharged with precipitation from a different rainfall regime with a depleted isotopic content. Copyright © 2006 John Wiley & Sons, Ltd.     

Evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site Yucca Mountain,Nevada, USA,revealed by the isotope composition of fluid-inclusion water

Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (δD and δ¹⁸O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the δD values were measured using a continuous-flow isotope-ratio mass spectrometry method. The δ¹⁸O values were calculated from the δ¹⁸O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry.The δD values measured in all samples range between ? 110 and ? 90‰, similar to Holocene meteoric water. Coupled δ¹⁸O–δD values plot significantly, 2 to 8‰, to the right of the meteoric water line. Among the various processes operating at the topographic surface and/or in the unsaturated zone only two processes, evaporation and water–rock exchange, could alter the isotope composition of percolating water. Our analysis indicates, however, that none of these processes could produce the observed large positive δ¹⁸O-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (>100 °C), which is only possible in the deep-seated hydrothermal environment. The stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at Yucca Mountain precipitated; instead they point to the deep-seated provenance of the mineral-forming waters and their introduction into the unsaturated zone from below, i.e. a hypogene origin.     

Storage,mixing, and fluxes of water in the critical zone across northern environments inferred by stable isotopes of soil water

Quantifying soil water storage, mixing, and release via recharge, transpiration, and evaporation is essential for a better understanding of critical zone processes. Here, we integrate stable isotope (²H and ¹⁸O of soil water, precipitation, and groundwater) and hydrometric (soil moisture) data from 5 long‐term experimental catchments along a hydroclimatic gradient across northern latitudes: Dry Creek (USA), Bruntland Burn (Scotland), Dorset (Canada), Krycklan (Sweden), and Wolf Creek (Canada). Within each catchment, 6 to 11 isotope sampling campaigns occurred at 2 to 4 sampling locations over at least 1 year. Analysis for ²H and ¹⁸O in the bulk pore water was done for >2,500 soil samples either by cryogenic extraction (Dry Creek) or by direct equilibration (other sites). The results showed a similar general pattern that soil water isotope variability reflected the seasonality of the precipitation input signal. However, pronounced differences among sampling locations occurred regarding the isotopic fractionation due to evaporation. We found that antecedent precipitation volumes mainly governed the fractionation signal, temperature and evaporation rates were of secondary importance, and soil moisture played only a minor role in the variability of soil water evaporation fractionation across the hydroclimatic gradient. We further observed that soil waters beneath conifer trees were more fractionated than beneath heather shrubs or red oak trees, indicating higher soil evaporation rates in coniferous forests. Sampling locations closer to streams were more damped and depleted in their stable isotopic composition than hillslope sites, revealing increased subsurface mixing towards the saturated zone and a preferential recharge of winter precipitation. Bulk soil waters generally comprised a high share of waters older than 14 days, which indicates that the water in soil pores are usually not fully replaced by recent infiltration events. The presented stable isotope data of soil water were, thus, a useful tool to track the spatial variability of water fluxes within and from the critical zone. Such data provide invaluable information to improve the representation of critical zone processes in spatially distributed hydrological models.     

Oxygen isotope fractionation in decarbonation metamorphism: the Mottled Zone event

Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the “Mottled Zone Event” is a low-pressure (1–25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ¹⁸O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ¹⁸O in silicates can be modelled as a Rayleigh distillation of CO₂ approximately 16‰ enriched in ¹⁸O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C.     

Recharge processes during snowmelt: An isotopic and hydrometric investigation

Results from hydrometric and isotopic investigations of unsaturated flow during snowmelt are presented for a hillslope underlain by well-sorted sands. Passage of melt and rainwater through the vadose zone was detected from temporal changes in soil water ²H concentrations obtained from sequential soil cores. Bypassing flow was indicated during the initial snowmelt phase, but was confined to the near-surface zone. Recharge below this zone was via translatory flow, as meltwater inputs displaced premelt soil water. Estimates of premelt water fluxes indicate that up to 19 per cent of the premelt soil water may have been immobile. Average water particle velocities during snowmelt ranged from 6.2 × 10^?7 to 1.1 × 10^?6 ms^?1, suggesting that direct groundwater recharge by meltwater during snowmelt was confined to areas where the premelt water table was within 1 m of the ground surface. Soil water ²H signatures showed a rapid response to isotopically-heavy rain-on-snow inputs late in the melt. In addition, spatial variations in soil moisture content at a given depth induced a pronounced lateral component to the predominantly vertical transport of water. Both factors may complicate isotopic profiles in the vadose zone, and should be considered when employing environmental isotopes to infer recharge processes during snowmelt.     

Experimental hydrogen isotope studies,II. Fractionations in the systems epidote-NaCl-H2O,epidote-CaCl2-H2O and epidote-seawater,and the hydrogen isotope composition of natural epidotes

The hydrogen isotope fractionation factors between epidote and aqueous 1 M and 4 M NaCl, 1 M CaCl₂ solutions, and between epidote and seawater, have been measured over the temperature range 250–550°C over which the degree of dissociation of dissolved species varies significantly. Measured fractionations at 350°C are decreased by up to 12‰, 9‰ and 7‰ relative to pure water in seawater, 1 M CaCl₂ and 1 M NaCl respectively, while above 500°C fractionations are not measurably dependent on fluid composition. Water—solution fractionation factors are derived which are generally applicable to the correction of mineral—water hydrogen isotope fractionations for the composition of the fluid phase.The hydrogen isotope compositions of natural epidotes are interpreted in the light of experimental fractionation data for situations where temperature, δD (fluid), and, in some cases, fluid chemistry, are independently known. Epidotes from active geothermal systems have hydrogen isotope quench temperatures consistent with or close to measured well temperatures unless the measured temperature has declined substantially since epidote formation or there is uncertainty in the D/H ratio of the water associated with the epidote because of isotopic heterogeneity in the well waters. Hydrothermal and metamorphic epidotes show closure temperatures of 175–225°C and 200–250°C. There is no evidence that retrograde metamorphic fluids, if present, are isotopically different from prograde fluids.The water-solution fractionations indicate strong solute-solvent interactions between 250 and 450°C and imply that both dissociated and associated species contribute to the fractionation effects through modification of the orientations and structure of the water molecules. Solute-solvent interactions become negligible at temperatures around 550°C.     

Spatio‐temporal heterogeneity in soil water stable isotopic composition and its ecohydrologic implications in semiarid ecosystems

Spatio‐temporal heterogeneity in soil water content is recognized as a common phenomenon, but heterogeneity in the hydrogen and oxygen isotope composition of soil water, which can reveal processes of water cycling within soils, has not been well studied. New advances are being driven by measurement approaches allowing sampling with high density in both space and time. Using in situ soil water vapour probe techniques, combined with conventional soil and plant water vacuum distillation extraction, we monitored the hydrogen and oxygen stable isotopic composition of soil and plant waters at paired sites dominated by grasses and Gambel's oak (Quercus gambelii) within a semiarid montane ecosystem over the course of a growing season. We found that sites spaced only 20 m apart had profoundly different soil water isotopic and volumetric conditions. We document patterns of depth‐ and time‐explicit variation in soil water isotopic conditions at these sites and consider mechanisms for the observed heterogeneity. We found that soil water content and isotopic variability were damped under Q. gambelii, perhaps due in part to hydraulic redistribution of deep soil water or groundwater by Q. gambelii in these soils relative to the grass‐dominated site. We also found some support for H isotope discrimination effects during water uptake by Q. gambelii. In this ecosystem, the soil water content was higher than that at the neighbouring Grass site, and thus, 25% more water was available for transpiration by Q. gambelii compared with the Grass site. This work highlights the role of plants in governing soil water variation and demonstrates that they can also strongly influence the isotope ratios of soil water. The resulting fine‐scale heterogeneity has implications for the use of isotope tracers to study soil hydrology and evaporation and transpiration fluxes to improve understanding of water cycling through the soil–plant–atmosphere continuum.     

A δ18O and δ2H stable water isotope analysis of subalpine forest water sources under seasonal and hydrological stress in the Canadian Rocky Mountains

Subalpine forests are hydrologically important to the function and health of mountain basins. Identifying the specific water sources and the proportions used by subalpine forests is necessary to understand potential impacts to these forests under a changing climate. The recent “Two Water Worlds” hypothesis suggests that trees can favour tightly bound soil water instead of readily available free-flowing soil water. Little is known about the specific sources of water used by subalpine trees Abies lasiocarpa (Subalpine fir) and Picea engelmannii (Engelmann spruce) in the Canadian Rocky Mountains. In this study, stable water isotope (δ¹⁸O and δ²H) samples were obtained from S. fir and Engelmann spruce trees at three points of the growing season in combination with water sources available at time of sampling (snow, vadose zone water, saturated zone water, precipitation). Using the Bayesian Mixing Model, MixSIAR, relative source water proportions were calculated. In the drought summer examined, there was a net loss of water via evapotranspiration from the system. Results highlighted the importance of tightly vadose zone, or bound soil water, to subalpine forests, providing insights of future health under sustained years of drought and net loss in summer growing seasons. This work builds upon concepts from the “Two Water Worlds” hypothesis, showing that subalpine trees can draw from different water sources depending on season and availability. In our case, water use was largely driven by a tension gradient within the soil allowing trees to utilize vadose zone water and saturated zone water at differing points of the growing season.     

Influences of forest canopy on snowpack accumulation and isotope ratios

The stable water isotopes, ²H and ¹⁸O, can be useful environmental tracers for quantifying snow contributions to streams and aquifers, but characterizing the isotopic signatures of bulk snowpacks is challenging because they can be highly variable across the catchment landscape. In this study, we investigate one major source of isotopic heterogeneity in snowpacks: the influence of canopy cover. We measured amounts and isotopic compositions of bulk snowpack, throughfall, and open precipitation during seven campaigns in mid-winter 2018 along forest-grassland transects at three different elevations (1196, 1297, and 1434 m above sea level) in a pre-Alpine catchment in Switzerland. Snowpack storages under forest canopies were 67 to 93% less than in adjacent open grasslands. On average, the water isotope ratios were higher in the snowpacks under forest canopy than in open grasslands (by 13.4 ‰ in δ²H and 2.3 ‰ in δ¹⁸O). This isotopic difference mirrored the higher isotope values in throughfall compared with open snowfall (by 13.5 ‰ in δ²H and 2.2 ‰ in δ¹⁸O). Although this may suggest that most of the isotopic differences in snowpacks under forests versus in open grasslands were attributable to canopy interception effects, the temporal evolution of snowpack isotope ratios indicated preferential effluxes of lighter isotopes as energy inputs increased and the snowpack ripened and melted. Understanding these effects of forest canopy on bulk snowpack snow water equivalent and isotopic composition are useful when using isotopes to infer snowmelt processes in landscapes with varying forest cover.     

Temporal fluctuations of water repellency in forest soils of Galicia,NW Spain. Do soil samples dried at laboratory reflect the potential soil water repellency?

Water repellency is a widespread property of Pinus pinaster and Eucalyptus globulus forest soils in NW Spain and is particularly severe during the summer dry conditions. The aim of this work was to compare actual water repellency at field‐moist samples with potential water repellency after drying at 25 and 105 °C in samples collected at different times of year under four forest soils. Also, we investigated whether drying at 25 or 105 °C led to repellency values comparable to the highest levels reached under field conditions in the summer with a view to developing an appropriate sampling protocol towards estimating the maximum possible water repellency of a given soil as a key to establishing its environmental effects. The actual and potential water repellency was determined by using the water drop penetration time (WDPT) and molarity of an ethanol drop (MED) tests. Clear seasonal patterns of water repellency were observed from the results for the four forest soils, peaking in the dry period and disappearing after prolonged wet periods. Water repellency lasts longer in sandy loam soils than in more finely textured soils, and also under eucalyptus than under pine forests. Drying soil samples at 25 or 105 °C increased water repellency, as measured with the WDPT method, in the four soils, but especially in the non‐repellent samples collected during the wet period. The increase was more marked in the sandy loam soils than in the more finely textured soils, and also after drying at 105 °C than at 25 °C. MED measurements exposed a common trait in the four soils; thus, the water repellency values obtained under field conditions in summer invariably exceeded those obtained after drying at 25 or 105 °C. In addition, the repellency values for dried samples collected in the wet period were never comparable to the maximum levels observed under field conditions in the summer. Copyright © 2011 John Wiley & Sons, Ltd.     

Strontium isotope geochemistry of the Lemachang independent silver ore deposit, northeastern Yunnan, China

Sr isotope geochemical studies (the 87Sr/86Sr and ?18O-87Sr/86Sr systems) on the wall rocks and ores from the Lemachang independent Ag deposit in northeastern Yunnan provide strong evidence that the ore-forming fluids had flown through radiogenetically Sr-enriched rocks or strata prior to their entry into the locus of ore precipitation, and water-rock interaction is the main mechanism of Ag ore precipitation. The radiogenetically Sr-enriched source region may be the Proterozoic basement (the Kunyang and Hekou groups). Moreover, the theoretical modeling of the Sr isotopic system indicates that the ore-forming fluids contain as much as 3×10?6 Sr with isotopic composition of Sr being 0.750 and that of oxygen 7.0‰. The ore-forming temperatures were estimated at 150-250℃ for the carbonate rock-type ores and at 200-260℃ for the clastic rock-type.     

Thermodynamics of the oxygen isotope fractionation involving plagioclase

The thermodynamic relationship between the oxygen isotope fractionation properties of plagioclase and its composition has been derived by treating plagioclase as a “reciprocal solution” consisting of independent cationic and anionic substitutions, namely (NaAl)⁵⁺?(CaSi)⁵⁺and¹⁸O?¹⁶O. The results show that the logarithm of the oxygen isotope fractionation factor, α, between plagioclase and a coexisting phase varies linearly with the anorthite content of plagioclase. The proportionality constant is given by the oxygen isotope fractionation factor between albite and anorthite, and has been derived from the experimental data of two groups of workers, O'Neil and Taylor [2] and Matsuhisa et al. [3], on the isotopic fractionation between each plagioclase end-member and aqueous solutions. It is found that O'Neil and Taylor's data on isotopic exchange of plagioclase end-members with only 2–3 M chloride solution, rather than with both pure water and the chloride solution, lead essentially to zero intercept of the ln α(Ab-An) vs. 1/T² relation, in accord with Bottinga and Javoy's [10] conclusion about the oxygen isotope fractionation between two anhydrous silicates at T<500°C.     

Groundwater Origin in Qanats,Chemo-Isotopic,and Hydrogeological Evidence

A systematic study of the chemo-isotopic characteristics and origin of the groundwater was carried out at six major qanats in the hyper-arid Gonabad area, eastern Iran. These qanats as a sustainable groundwater extraction technology have a long history, supporting human life for more than a thousand years in this region. The Gonabad qanats are characterized by outlet electrical conductivity (EC) values of 750 to 3900 µS/cm and HCO₃-Na-Mg and Cl-Na water types. The Gonabad meteoric water line (G_nMWL) was drawn at the local scale as δ²H = 6.32×δ¹⁸O + 8.35 (with R² = 0.90). It has a lower slope and intercept than the global meteoric water line due to different water vapor sources and isotope kinetic fractionation effects during precipitation in this arid region. The altitude effects on isotopic content of precipitation data were derived as δ¹⁸O = (−0.0031 × H_(m.a.s.l))−1.3). The δ²H and δ¹⁸O isotopes signatures demonstrate a meteoric origin of the groundwater of these qanats. The shift of the qanat's water samples from the local meteoric water line (LMWL) in a dry period with higher temperatures is most probably due to evaporation during the infiltration process and water movement in qanat gallery. Based on the isotopic results and mass balance calculations, the qanats are locally recharged from an area between 2000 to 2400 m.a.s.l of nearby carbonate formations and coarse alluvial sediments. The dissolution of evaporate interlayers in Neogene deposits deteriorates the groundwater quality, especially in Baidokht qanat.