Surface Water Isotope Research Articles

Groundwater recharge in the oasis-desert areas of northern Tarim Basin, Northwest China

AbstractGroundwater is an important source for maintaining desert ecological processes in arid areas. With the increasing intensity of climate change and human activities, the rivers in Tarim Basin are severely dried-up. Aiming at the dried-up river, vegetation degradation and oasis maintenance in the middle and lower reaches of dried-up river basin, groundwater recharge and groundwater-surface water interaction have become hotspots, but are not well known. We examined spatial distributions and controlling factors of groundwater stable isotopes and recharge at oasis scale using data from 247 samples surveyed in the four headwaters in the northern Tarim Basin. Stable isotopes of surface water and groundwater were different from each other, and varied among sampling sites. Surface water and groundwater isotopes generally became enriched towards the east throughout the study area, while surface water isotopes showed enrichment towards the upstream direction within each catchment, mainly due to cultivated area expansion. Surface water mainly originated from precipitation, groundwater, and meltwater, while shallow groundwater derived from lateral groundwater flow, river and irrigated water infiltration, and little precipitation. The mainstream water was directly recharged by the headwaters. The results could provide a new insight into groundwater cycling in oases of dried-up river basins, which is helpful for regional groundwater management.

The Geochemistry of the Mineral Waters and Hydrogenic Sediments of the Antonovsky Hydrosulfuric Mineral Spring, Sakhalin Island

Original data on the isotope and chemical compositions of mineral springs, cold ground and surface waters, as well as hydrogenic deposits of the Antonovsky manifestation of hydrosulfuric mineral waters (the coast of the Tatar Strait, Sakhalin Island), are presented. The data on the contents of oxygen and hydrogen isotopes in ground and surface waters, as well as the volumetric activity of radon and rare-earth elements are obtained for the first time. Based on automatic monitoring of the physical parameters of the mineral water, its hydrogeological characteristics are determined and balneological properties are assessed. It is found that the mineral waters of the spring are weakly alkaline, hydrocarbonic–chloride calcium–sodium with total dissolved solids (TDS) of up to 1 g/L. According to the isotope data, the genesis of the waters is atmospheric and the circulation time is less than 60 years. The gas composition is dominated by nitrogen of atmospheric genesis (δ15N = 0.0). High contents of organic carbon (Corg = 56.6 mg/L) are explained by the processes of interaction between groundwater and organic matter in water-bearing rocks. When organic matter is decomposed in the presence of sulfates and without access to oxygen, hydrosulfuric acid is released, producing several intermediate and stable sulfur compounds in hydrogenic sediments and bacterial mats. Elemental, sulfide, and sulfate forms of sulfur have been detected in the bottom sediments and in the new formations of the mineral spring, using electron microscopy. The interaction of waters with organic matter of rocks plays a significant role in the formation of the elemental composition of hydrogenic and biogenic deposits of the Antonovsky hydrosulfuric mineral spring and their enrichment with such metals as Zn, Ge, As, Sr, Ba, U, and Th. One characteristic feature of the mineral waters is the presence of a markedly pronounced positive europium anomaly, which is typical of waters with low Eh values.

Lithium isotope signatures of weathering in the hyper-arid climate of the western Tibetan Plateau

This study of Lithium (Li) isotopes in surface waters and sediments in the catchment of Lake Bangong and the Upper Indus on the western Tibetan Plateau aims to identify processes that control Li isotope variations during weathering under a cold and hyper-arid climate. Additionally, Li isotope ratios in the Yarlung Tsangpo – Brahmaputra River were investigated. The lake and river sediments of Lake Bangong catchment display remarkable low δ7Li values between −4.7‰ and −0.6‰ relative to L-SVEC. Li isotopes in river bed sediments correlate with weathering intensity tracers such as the chemical index of alteration (CIA), K/(Na + K), or Na/Ti, and δ7Li values decrease continuously within the sediment cascade. These observations cannot be explained by mixing of different lithologies but reflects increasing intensity of weathering. The hyper-arid climate on the western plateau results in considerably long sediment residence times, which allows for overcoming the limitation of water availability on chemical weathering reactions. Samples from the Lake Bangong basins display low δ7Li values between +8.1‰ and +11.1‰. The major inflows have dissolved δ7Li values of +6.1‰ and +8.9‰. High Li/Na ratios in the stream waters indicate some contribution of hydrothermal Li. However, low δ7Li values in surface waters result from impeded silicate weathering processes in the thin soils. The samples from Indus headwaters and Yarlung Tsangpo provide evidence for low δ7Li all over the western and southern Tibetan Plateau. Using data of the Bangong Co, the Indus headwaters, and Yarlung Tsangpo, as well as published data from the northeastern plateau, we explore what controls Li isotope variation across the Tibetan plateau. Mass balance calculations suggest that similar proportions of dissolved Li and particulate Li are exported by river water and sediments on the western plateau. In contrast, high δ7Li values around +17‰ of the dissolved load in rivers on the northeastern Tibetan Plateau reflect a particulate Li export flux that is about five times higher than the export flux of dissolved Li. There is no first-order control by silicate weathering rates. The δ7Li differences largely follow the precipitation gradient across the Tibetan Plateau, which results in high net-incorporation of Li into clays on the northeastern but limits soil formation on the western Tibetan Plateau and, therefore limited, processing of lithium in the weathering zone.

Craig–Gordon model validation using stable isotope ratios in water vapor over the Southern Ocean

Abstract. The stable oxygen and hydrogen isotopic composition of water vapor over a water body is governed by the isotopic composition of surface water and ambient vapor, exchange and mixing processes at the water–air interface, and the local meteorological conditions. These parameters form inputs to the Craig–Gordon models, used for predicting the isotopic composition of vapor produced from the surface water due to the evaporation process. In this study we present water vapor, surface water isotope ratios and meteorological parameters across latitudinal transects in the Southern Ocean (27.38–69.34 and 21.98–66.8∘ S) during two austral summers. The performance of Traditional Craig–Gordon (TCG) (Craig and Gordon, 1965) and the Unified Craig–Gordon (UCG) (Gonfiantini et al., 2018) models is evaluated to predict the isotopic composition of evaporated water vapor flux in the diverse oceanic settings. The models are run for the molecular diffusivity ratios suggested by Merlivat (1978), Cappa et al. (2003) and Pfahl and Wernli (2009), referred to as MJ, CD and PW, respectively, and different turbulent indices (x), i.e., fractional contribution of molecular vs. turbulent diffusion. It is found that the UCGx=0.8MJ, UCGx=0.6CD, TCGx=0.6MJ and TCGx=0.7CD models predicted the isotopic composition that best matches with the observations. The relative contribution from locally generated and advected moisture is calculated at the water vapor sampling points, along the latitudinal transects, assigning the representative end-member isotopic compositions, and by solving the two-component mixing model. The results suggest a varying contribution of the advected westerly component, with an increasing trend up to 65∘ S. Beyond 65∘ S, the proportion of Antarctic moisture was found to be prominent and increasing linearly towards the coast.

Urban water systems under climate stress: An isotopic perspective from Berlin, Germany

AbstractLarge urban areas are typically characterized by a mosaic of different land uses, with contrasting mixes of impermeable and permeable surfaces that alter “green” and “blue” water flux partitioning. Understanding water partitioning in such heterogeneous environments is challenging but crucial for maintaining a sustainable water management during future challenges of increasing urbanization and climate warming. Stable isotopes in water have outstanding potential to trace the partitioning of rainfall along different flow paths and identify surface water sources. While isotope studies are an established method in many experimental catchments, surprisingly few studies have been conducted in urban environments. Here, we performed synoptic sampling of isotopes in precipitation, surface water and groundwater across the complex city landscape of Berlin, Germany, for a large ‐scale overview of the spatio‐temporal dynamics of urban water cycling. By integrating stable isotopes of water with other hydrogeochemical tracers we were able to identify contributions of groundwater, surface runoff during storm events and effluent discharge on streams with variable degrees of urbanization. We could also assess the influence of summer evaporation on the larger Spree and Havel rivers and local wetlands during the exceptionally warm and dry summers of 2018 and 2019. Our results demonstrate that using stable isotopes and hydrogeochemical data in urban areas has great potential to improve our understanding of water partitioning in complex, anthropogenically‐affected landscapes. This can help to address research priorities needed to tackle future challenges in cities, including the deterioration of water quality and increasing water scarcity driven by climate warming, by improving the understanding of time‐variant rainfall‐runoff behaviour of urban streams, incorporating field data into ecohydrological models, and better quantifying urban evapotranspiration and groundwater recharge.

Comment on “On the calculation of lake metabolic rates: Diel O2 and 18/16O technique” by Peeters et al. [Water Res. 165 2019, 114990

Quantifying metabolic rates in lakes and other aquatic ecosystems is a complex task, as methods are continually evolving and are not currently standardized. Recently, Peeters et al. presented a valuable simulated dataset that advances the field by comparing the strengths and limitations of individual and combined metabolic techniques. The authors conclude that calculating metabolic rates from point sampling and mass balancing of surface water oxygen concentration and isotope composition is flawed, because the technique does not capture sub-daily patterns of metabolic variability, which they argue invalidates past applications and interpretations. These conclusions are inconsistent with how the method has been used, and are based on a biased construction of scenarios and interpretation of model results, especially because their parameterization of the stable isotopic model employs input values that appear unrepresentative of most lake conditions. Here, we establish that 1) empirical evidence supports the isotopic approach’s suitability to approximate daily or longer metabolic patterns in most lakes. 2) The authors’ own simulations show agreement between metabolic estimates from point isotopic measurements and average metabolic rates under most scenarios. 3) The authors’ invalidation of isotopic measurements are based on the most extreme model deviations observed in simulated hypereutrophic environments. While we welcome a critical evaluation of the isotopic approach, we argue that isotopic model uncertainty needs to be placed within an appropriate context. We emphasize that isotopic sampling and steady state metabolic modelling has a key role to play in constraining metabolic patterns in the global lake landscape, but that the research questions addressed with the method need to be commensurate with the limitations and uncertainties of the approach.

A Lagrangian View of Trace Elements and Isotopes in the North Pacific

AbstractOcean time‐series sites are influenced by both temporal variability, as in situ conditions change, as well as spatial variability, as water masses move across the fixed observation point. To remove the effect of spatial variability, this study made sub‐daily Lagrangian observations of trace elements and isotopes (Al, Sc, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, 232Th, and 230Th) in surface water over a 12‐day period (July–August 2015) in the North Pacific near the Hawaii Ocean Time‐series Station ALOHA. Additionally, a vertical profile in the upper 250 m was analyzed. This dataset is intercalibrated with GEOTRACES standards and provides a consistent baseline for trace element studies in the oligotrophic North Pacific. No diel changes in trace elements could be resolved, although day‐to‐day variations were resolved for some elements (Fe, Cu, and Zn), which may be related to organic matter cycling or ligand availability. Pb concentrations remained relatively constant during 1997–2015, presenting a change from previous decreases. Nutrient to trace element stoichiometric ratios were compared to those observed in phytoplankton as an indication of the extent of biological trace element utilization in this ecosystem, providing a basis for future ecological trace element studies.

ГЕОХИМИЯ МИНЕРАЛЬНЫХ ВОД И ГИДРОГЕННЫХ ОТЛОЖЕНИЙ АНТОНОВСКОГО СЕРОВОДОРОДНОГО МИНЕРАЛЬНОГО ИСТОЧНИКА (О. САХАЛИН)

Original data on the isotopic and chemical composition of mineral waters, cold ground and surface waters, as well as hydrogenic deposits of the Antonovsky manifestation of hydrogen sulfide mineral waters (the coast of the Tatar Strait, Sakhalin Island) are provided in the article. For the first time, data on the content of oxygen and hydrogen isotopes in groundwaters and surface waters, the volumetric activity of radon and the rare-earth elements as well were obtained. Based on automatic monitoring of the physical parameters of the waters, their hydrogeological characteristics were determined and balneological properties were assessed. It has been determined that the mineral waters of the spring are weakly alkaline, hydrocarbonate-chloride calcium-sodium, with TDS of up to 1 g/l. According to isotope data, the genesis of waters is atmospheric, and the circulation time is less than 60 years. The gas composition is dominated by nitrogen of atmospheric genesis (δ15N = 0.0). High contents of organic carbon (Corg. = 56.6 mg/l) are explained by the processes of interaction of groundwaters with organic matter of water-bearing rocks. When organic matter decomposes in the presence of sulfates and without oxygen, hydrogen sulfide is released, giving rise to a number of intermediate and stable sulfur compounds in hydrogenic sediments and bacterial mats. Elemental, sulfide and sulfate forms of sulfur have been determined in the bottom sediments as well as new formations of the mineral spring, using electron microscopy. The interaction of waters with organic matter of rocks plays a significant role in the formation of the elemental composition of hydrogenic and biogenic deposits of the Antonovsky hydrogen sulphide mineral spring and their enrichment with such metals as Zn, Ge, As, Sr, Ba, U, and Th as well. A characteristic feature of mineral waters is the presence of a pronounced positive europium anomaly, which is typical for waters with low Eh values.

Stable Isotope Reveals Tap Water Source under Different Water Supply Modes in the Eastern Margin of the Qinghai–Tibet Plateau

Based on 1260 tap water samples gathered monthly and 136 surface water samples collected seasonally in the eastern margin of the Qinghai–Tibet Plateau, the local tap water line, the basic spatiotemporal characteristics of tap water isotopes, and their indication for water source under different water supply modes were discussed, linking the local tap water supply and water source information. A new tap water isotopes data set based on dense sampling sites was established, which was reliable for the analysis of tap water isotope features, tap water supply management, and tap water sources. The main conclusions are: (1) The local tap water lines in Gannan and Longnan are δ2H = (7.06 ± 0.17) δ18O + (3.24 ± 1.75) (r2 = 0.81, p < 0.01) and δ2H = (5.66 ± 0.09) δ18O + (−8.12 ± 0.82) (r2 = 0.82, p < 0.01), respectively. (2) The annual mean δ2H and δ18O in tap water show an increasing trend from southwest to northeast. The seasonal differences of δ2H and δ18O in tap water in Gannan and Longnan are small. (3) The correlation of tap water isotopes with those in main source water is high, while that of isotopes in tap water with those in non-water source is low. Under the central water supply mode by local tap water company, tap water isotopes in Gannan where groundwater is the direct water source show weak connection with those in surface water and precipitation, and those in tap water in Longnan with surface water as main source water reveal good connection with isotopes in surface water. Under mixed water supply modes, tap water isotopes indicate that surface water is the main tap water source in Gannan and Longnan with multiple water sources.

Using Water Stable Isotopes for Identifying Groundwater Recharge Sources of the Unconfined Alluvial Zagreb Aquifer (Croatia)

The main purpose of this study was to understand the interactions between precipitation, surface water, and groundwater in the Zagreb aquifer system using water stable isotopes. The Zagreb aquifer is of the unconfined type and strongly hydraulically connected to the Sava River. As the groundwater is the main source of drinking water for one million inhabitants, it is essential to investigate each detail of the recharge processes of the aquifer to ensure adequate protection of the groundwater. Measuring the content of water stable isotopes in surface waters and groundwater enabled the creation of two- and three-component mixing models based on the isotopic mass balance for the purpose of the quantification of each recharge component. The mixing models gave ambiguous results. Observation wells equally distant from the Sava River did not have the same recharge component ratio. This indicated that there were more factors (in addition to the distance from the river) that were affecting groundwater recharge, and the properties of the unsaturated zone and surface cover data were therefore also taken into consideration. The thickness of the unsaturated zone and the characteristics of different soil types were identified as important factors in the recharge of the Zagreb aquifer. The areas with high thickness of the unsaturated zone and well-permeable soil had a very similar recharge component ratio to the areas with small thickness of the unsaturated zone but low-permeable soil.

Iron Isotope Fractionation during Bio- and Photodegradation of Organoferric Colloids in Boreal Humic Waters.

Biodegradation and photolysis of dissolved organic matter (DOM) in boreal high-latitude waters are the two main factors controlling not only the aquatic fluxes and residence time of carbon but also metal nutrients associated with DOM such as Fe. The DOM is usually present in the form of organic and organomineral colloids, which also account for the majority of dissolved Fe. Here, we use the stable Fe isotope approach to unravel the processes controlling Fe behavior during bio- and photodegradation of colloids in boreal Fe- and DOM-rich humic waters (a stream and a fen). The adsorption of Fe colloids onto heterotrophic bacteria Pseudomonas aureofaciens produced enrichment in +0.4‰ (δ57Fe) in the heavier isotopes of the cell surface relative to the remaining solution. In contrast, long-term assimilation of Fe by live cells yielded preferential incorporation of lighter isotopes into the cells (-0.7‰ relative to aqueous solution). The sunlight-induced oxidation of Fe(II) in fen water led to the removal of heavier Fe isotopes (+1.5 to +2.5‰) from solution, consistent with Fe(III) hydroxide precipitation from Fe(II)-bearing solution. Altogether, bio- and photodegradation of organoferric colloids, occurring within a few days of exposure time, can produce several per mil isotopic excursions in shallow lentic and lothic inland waters of high-latitude boreal regions. Considerable daily scale variations of Fe isotopic composition should therefore be taken into account during the interpretation of the riverine flux of Fe isotopes to the ocean or tracing weathering processes using Fe isotopes in surface waters at high latitudes.

Stable Isotope Ratios in Tap Water of a Riverside City in a Semi-Arid Climate: An Application to Water Source Determination

Stable isotopes (e.g., δ2H and δ18O) in tap water are important tools to understand the local climate or environment background, water sources and the state of regional water supply. Based on 242 tap water samples, 35 precipitation samples and 24 surface water samples gathered in the urban area of Lanzhou, the basic spatiotemporal characteristics of isotopes in tap water, their connection with isotopes in other water bodies and change during the process from raw water to tap water are discussed in detail, combining the information of local tap water supply and water source. It can provide reliable help for understanding the isotope characteristics of local tap water, regional water supply management and determination of tap water source of in a small area. Except for the establishment of a new data set of isotopes in tap water with complete time series and uniform spatial distribution of sampling sites, other results show that: (1) The Local Tap Water Line (LTWL) of Lanzhou is δ2H = (6.03 ± 0.57) δ18O + (−8.63 ± 5.44) (r2 = 0.41, p < 0.01). (2) For seasonal variations, δ2H and δ18O in tap water both are higher in autumn and lower in spring. The diurnal and daily variations of isotopes in tap water are not large. As for spatial variations, the monthly mean values of δ2H and δ18O in tap water at each sampling site show little difference. The isotopes in tap water collected from one single sampling site can be considered as a representative for isotopes in tap water in the area with a single tap water source. (3) Isotopes in tap water show weak connection with precipitation isotopes, but exhibit good connection (consistent seasonal variation, similar numerical range, small numerical difference and high correlation) with isotopes in surface water, which is the direct water source. Isotopes in water change little from raw water to tap water. Isotopic composition of tap water in Lanzhou can be used as a representative of isotopes in surface water.

Towards a global interpretation of dual nitrate isotopes in surface waters

Modern anthropogenic activities have significantly increased nitrate (NO3−) concentrations in surface waters. Stable isotopes (δ15N and δ18O) in NO3− offer a tool to deconvolute some of the human-made changes in the nitrogen cycle. They are often graphically illustrated on a template designed to identify different sources of NO3− and denitrification. In the two decades since this template was developed, δ15N- and δ18O-NO3− have been measured in a variety of ecosystems and through the nitrogen cycle. However, its interpretation is often fuzzy or complex. This default is no longer helpful because it does not describe surface water ecosystems well and biases researchers towards denitrification as the NO3− removal pathway, even in well oxygenated systems where denitrification is likely to have little to no influence on the nitrogen cycle. We propose a different scheme to encourage a better understanding of the nitrogen cycle and interpretation of NO3− isotopes. We use a mechanistic understanding of NO3− formation to place bounds on the oxygen isotope axis and provide a means to adjust for different environmental water isotope values, so data from multiple sites and times of year can be appropriately compared. We demonstrate that any interpretation of our example datasets (Canada, Kenya, United Kingdom) show clear evidence of denitrification or a mixture of NO3− sources simply because many data points fall outside of arbitrary boxes which cannot be supported once the range of potential δ18O-NO3− values has been considered.

Recharge and dynamics of a karst groundwater system in Kakheti (Eastern Georgia)

Abstract Monitoring temporal variations of 18O and 2H isotopes in precipitation, groundwater and surface water was performed in the region of Kakheti (East Georgia). Data were collected from three meteorological stations at altitudes between 400 - 1,100 m a.s.l., from two shallow and one deep hydrogeological boreholes, and from two surface water monitoring stations (Alazani River and Patmasuri karstic stream). 18O values in precipitation show an annual variation between -22 ‰ and +1 ‰ and a distinct altitude effect. A clear correlation exists between the seasonal isotope composition of precipitation, shallow groundwater and surface water. A five-fold amplitude dampening and a delay of 10-15 days was observed. The data show that precipitation in the Caucasus Mountains to the North infiltrates into the Upper Jurassic - Lower Cretaceous karstic aquifer and travels to the Alazani valley towards south-east. The isotopic signature of winter precipitation is reflected in stream water as well as in shallow groundwater isotope data of groundwater in a 2,000-m-deep hydrogeological borehole at Heretiskari show a distinctly different character with δ18O ranging between -2.8 ‰ to -2.2 ‰ and a deuterium excess of -25 ‰.

Spatial and temporal variability of stable isotopes (δ18O and δ2H) in surface waters of arid, mountainous Central Asia

AbstractCharacterization of spatial and temporal variability of stable isotopes (δ18O and δ2H) of surface waters is essential to interpret hydrological processes and establish modern isotope–elevation gradients across mountainous terrains. Here, we present stable isotope data for river waters across Kyrgyzstan. River water isotopes exhibit substantial spatial heterogeneity among different watersheds in Kyrgyzstan. Higher river water isotope values were found mainly in the Issyk‐Kul Lake watershed, whereas waters in the Son‐Kul Lake watershed display lower values. Results show a close δ18O–δ2H relation between river water and the local meteoric water line, implying that river water experiences little evaporative enrichment. River water from the high‐elevation regions (e.g., Naryn and Son‐Kul Lake watershed) had the most negative isotope values, implying that river water is dominated by snowmelt. Higher deuterium excess (average d = 13.9‰) in river water probably represents the isotopic signature of combined contributions from direct precipitation and glacier melt in stream discharge across Kyrgyzstan. A significant relationship between river water δ18O and elevation was observed with a vertical lapse rate of 0.13‰/100 m. These findings provide crucial information about hydrological processes across Kyrgyzstan and contribute to a better understanding of the paleoclimate/elevation reconstruction of this region.

Contribution of recycled moisture to precipitation in the monsoon marginal zone: Estimate based on stable isotope data

The hydrogen and oxygen isotope composition of precipitation and water vapor are useful tracers in the water cycle. The water balance of inland river basins located in the monsoon marginal zone are difficult to determine, because little is known about the contribution of various vapor involved. Based on the three-component isotopic mixing model, the monthly precipitation, surface water and plant water isotope data of the seven sampling stations from April to October 2017 were used to assess the vapor contribution to precipitation in the Shiyang River basin.The average contribution of plant transpiration, land surface evaporation and advected vapor to precipitation accounted for 11%, 6% and 83% in the Mountain regions, for 21%, 7% and 72% in the Oasis regions, and for 10%, 5% and 85% in the Desert regions, respectively. The contribution of plant transpiration and surface evaporation vapor was similar in time variation, and the maximum value appeared in July. The average plant transpiration vapor contribution at each station was always greater than the surface evaporation vapor. Different landscape and special underlying surfaces are important factors influencing the difference in water vapor contribution. The average surface evaporation water vapor contribution of the Xiying (M1) station is the largest in the study area, with a contribution fraction of 9%. The contribution of Wuwei oasis recycled moisture can reach 30%, the plant transpiration vapor accounted for 73%.

Temperature-dependent ocean–atmosphere equilibration of carbon isotopes in surface and intermediate waters over the deglaciation

High resolution and precise records of the carbon isotopic composition of atmospheric carbon dioxide from ice cores are now providing a valuable constraint on the origin of the rise in CO2 over the last deglaciation. Previous studies using isotopic measurements on the tests of planktonic foraminifera from marine sediments had shown a deglacial minimum in the carbon isotopic composition of seawater carbon in the surface and thermocline waters of the Southern Hemisphere and the tropics, a feature also seen in the atmospheric record. Here we show that this deglacial minimum also can be clearly seen in carbon isotope measurements on planktonic foraminifera from sediment cores in the North Atlantic. There are also new high-resolution and precise records of the carbon isotopic composition of intermediate and mode waters generated using benthic foraminifera which show a history of change that is strikingly similar to that in the atmosphere. Similarities in these new highly-resolved deglacial records suggest that air–sea exchange can effectively globalize the isotopic signature of a release of carbon from the deep ocean. This limits the utility of carbon isotopes for constraining the precise location and mechanism of carbon release, indicating that more direct nutrient tracers are needed. However, given that the equilibrium partitioning of carbon isotopes between the atmosphere and ocean is temperature dependent, the difference between the atmosphere and upper ocean values could potentially provide an independent estimate of sea surface temperature change.

Application of the pore water stable isotope method and hydrogeological approaches to characterise a wetland system

Abstract. Three naturally intact wetland systems (swamps) were characterised based on sediment cores, analysis of surface water, swamp groundwater, regional groundwater and pore water stable isotopes. These swamps are classified as temperate highland peat swamps on sandstone (THPSS) and in Australia they are listed as threatened endangered ecological communities under state and federal legislation. This study applies the stable isotope direct vapour equilibration method in a wetland, aiming at quantification of the contributions of evaporation, rainfall and groundwater to swamp water balance. This technique potentially enables understanding of the depth of evaporative losses and the relative importance of groundwater flow within the swamp environment without the need for intrusive piezometer installation at multiple locations and depths. Additional advantages of the stable isotope direct vapour equilibration technique include detailed spatial and vertical depth profiles of δ18O and δ2H, with good accuracy comparable to other physical and chemical extraction methods. Depletion of δ18O and δ2H in pore water with increasing depth (to around 40–60 cm depth) was observed in two swamps but remained uniform with depth in the third swamp. Within the upper surficial zone, the measurements respond to seasonal trends and are subject to evaporation in the capillary zone. Below this depth the pore water δ18O and δ2H signature approaches that of regional groundwater, indicating lateral groundwater contribution. Significant differences were found in stable pore water isotope samples collected after the dry weather period compared to wet periods where recharge of depleted rainfall (with low δ18O and δ2H values) was apparent. The organic-rich soil in the upper 40 to 60 cm retains significant saturation following precipitation events and maintains moisture necessary for ecosystem functioning. An important finding for wetland and ecosystem response to changing swamp groundwater conditions (and potential ground movement) is that basal sands are observed to underlay these swamps, allowing relatively rapid drainage at the base of the swamp and lateral groundwater contribution. Based on the novel stable isotope direct vapour equilibration analysis of swamp sediment, our study identified the following important processes: rapid infiltration of rainfall to the water table with longer retention of moisture in the upper 40–60 cm and lateral groundwater flow contribution at the base. This study also found that evaporation estimated using the stable isotope direct vapour equilibration method is more realistic compared to reference evapotranspiration (ET). Importantly, if swamp discharge data were available in combination with pore water isotope profiles, an appropriate transpiration rate could be determined for these swamps. Based on the results, the groundwater contribution to the swamp is a significant and perhaps dominant component of the water balance. Our methods could complement other monitoring studies and numerical water balance models to improve prediction of the hydrological response of the swamp to changes in water conditions due to natural or anthropogenic influences.

Cumulative effects of cascade dams on river water cycle: Evidence from hydrogen and oxygen isotopes

Cascade dams are known to influence the river water cycle, but their cumulative effects (CEs) are still not well understood. Water hydrogen (H) and oxygen (O) isotopes are hypothesized to be used to characterize the CEs. To test this hypothesis, we investigated water δD, δ18O, and related environmental factors in cascade reservoirs on the Wujiang River, Southwest China. The δD and δ18O ranged from −64.2‰ to −45.4‰ and from −9.7‰ to −6.8‰, respectively, and showed obvious temporal and spatial variations. Water temperature is an important factor influencing these variations. After damming, an increase of water retention time caused the enrichment of heavy H-O isotopes in reservoir surface water, and thermal stratification induced a decrease of δD and δ18O with depth. Due to bottom discharge, released water showed more negative δD and δ18O than reservoir surface water, and these δD and δ18O differences were controlled by water retention time and mean water depth of the reservoir. Overall, the CEs of cascade dams caused δD and δ18O to display a jagged increase from upstream to downstream in the impounded Wujiang River. Therefore H-O isotopes can be used to estimate the CEs of cascade dams. As cascade dams can modify H-O isotope signatures, caution should be exercised when using H and O isotopes to trace the source of the impounded river water.

Isotopes, Microbes, and Turbidity: A Multi‐Tracer Approach to Understanding Recharge Dynamics and Groundwater Contamination in a Basaltic Island Aquifer

AbstractWells designated as groundwater under the direct influence (GUDI) of surface water have caused an ongoing boil‐water advisory afflicting the island of Tutuila, American Samoa for almost a decade. Regulatory testing at these wells found turbidity and indicator bacteria spikes correlated with heavy rainfall events. However, the mechanism of this contamination has, until now, remained unknown. Surface water may reach wells through improperly sealed well casings, or through the aquifer matrix itself. In this study, three independent surface water tracers, turbidity, indicator bacteria, and water isotopes were used to assess recharge timing and determine contamination mechanisms. Results from each method were reasonably consistent, revealing average GUDI well breakthrough times of 37 ± 21 h for turbidity, 18 to 63 h for bacteria, and 1 to 5 days for water isotopes. These times match well with estimated subsurface flow rates through highly permeable aquifer materials. In contrast, where one well casing was found to be compromised, turbidity breakthrough was observed at 3 to 4 h. These results support local management decisions and show repairing or replacing wells will likely result in continued GUDI contamination. Additionally, differences in observed rainfall response for each tracer provide insight into the recharge dynamics and subsurface flow characteristics of this and other highly conductive young‐basaltic aquifers.