Hydrological Tracer Research Articles

Enhancing low-level tritium detection in environmental waters: assessing the Hidex ULLA liquid scintillation counter

Tritium (³H) is a vital tracer in isotope hydrology and environmental studies, necessitating accurate and precise low-level detection methods. This study evaluated the performance of the new Hidex ULLA Liquid Scintillation Counter for ultra-low-level tritium assays in environmental water samples. The optimization of instrumental parameters enabled accurate and reproducible 3H measurements. Test samples (10–132 TU) were analyzed in triplicate, including 3H-free water and NIST traceable standards. The results demonstrated excellent accuracy and reproducibility, as evidenced by Zeta scores and linearity across these tritium concentrations. The system's stability over extended measurement time periods ensured reliable performance for routine ultra-low level 3H analysis. The optimization of key parameters such as coincidence time and region of interest (ROI) achieved the highest figure of merit (FoM) by using a 25 ns coincidence time and the DigPb open mode for optimal spectral fittings. Detection limits (DL) based on ISO11929 recommendations were significantly improved but highlighted the need for sample enrichment for most low-level environmental applications, with LSC counting times preferably exceeding 1000 min. Overall, the Hidex ULLA Liquid Scintillation Counter proved to be a robust and reliable instrument suitable for ultra-low-level tritium detection in environmental water samples.

Microbial community storm dynamics signal sources of "old" stream water.

Accurate characterization of the movement of water through catchments, particularly during precipitation event response, is critical for hydrological efforts such as contaminant transport modeling or prediction of extreme flows. Abiotic hydrogeochemical tracers are commonly used to track sources and ages of surface waters but provide limited details about transit pathways or the spatial dynamics of water storage and release. Alternatively, biotic material in streams is derived from thousands of taxa originating from a variety of environments within watersheds, including groundwater, sediment, and upslope terrestrial environments, and this material can be characterized with genetic sequencing and bioinformatics. We analyzed the stable water isotopes (δ18O and δ2H) and microbiome composition (16S rRNA gene amplicon sequencing) of the Marys River of western Oregon, USA during an early season storm to describe the processes, storage, and flowpaths that shape surface water hydrology. Stable water isotopes (δ18O and δ2H) typified an event response in which stream water is composed largely of 'old' water introduced to the catchment before the storm, a common though not well understood phenomenon. In contrast, microbial biodiversity spiked during the storm, consisting of early- and late-event communities clearly distinguishable from pre-event communities. We applied concentration-discharge (cQ) analysis to individual microbial taxa and found that most Alphaproteobacteria sequences were positively correlated (i.e., were mobilized) with discharge, whereas most sequences from phyla Gammaproteobacteria and Bacteroidota were negatively correlated with discharge (i.e., were diluted). Source predictions using the prokaryote habitat preference database ProkAtlas found that freshwater-associated microbes composed a smaller fraction of the microbial community during the stream rise and a larger fraction during the recession, while soil and biofilm-associated microbes increased during the storm and remained high during recession. This suggests that the "old" water discharged during the storm was likely stored and released from, or passed through, soil- and biofilm-rich environments, demonstrating that this approach adds new, biologically derived tracer information about the hydrologic pathways active during and after this event. Overall, this study demonstrates an approach for integrating information-rich DNA into water resource investigations, incorporating tools from both hydrology and microbiology to demonstrate that microbial DNA is useful not only as an indicator of biodiversity but also functions as an innovative hydrologic tracer.

The role of fault pathways for modern and deep recharge to a semi-confined aquifer: revised analysis of modern water leakage pathways in the Davis wellfield, Memphis, Tennessee, USA

Hydrologic tracer, borehole log and geophysical data provide new insight into recharge sources and pathways to the semi-confined Memphis aquifer in the Davis wellfield, Memphis, Tennessee, U.S.A. New data and recent studies highlight the importance of faults as potential pathways for the migration of modern water into the Memphis aquifer, an important regional public, industrial, and agricultural water supply. Geochemical and environmental tracer data collected over the past 28 years, combined with hydrologic, stratigraphic, and structural data, were used to evaluate changes in water quality and different recharge pathways to the Memphis aquifer. Assessment of stratigraphic and structural relationships argue for the wellfield being bound by a fault along the bluff line (west) and another fault or faults along the eastern and southeastern margins. Modern water recharge (< 60 years old) occurs through the fault along the western boundary of the wellfield, whereas modern water and, seasonally, deep-seated (helium-bearing) fluids recharge along the fault on the southeastern margin. Each recharge source and pathway present different vulnerabilities, suggesting that the role of faults in wellfields within active tectonic regions needs to be assessed.

The Characteristics and Application of Deuterium and Oxygen Isotopes to Karst Groundwater, Southwest China

Stable isotopes in natural water bodies serve as hydrologic tracers, with a history of extensive application in karst groundwater research. The present paper is a systematic review of previous research results, with the objective of sorting out and summarizing the stable isotopic characteristics of karst groundwater in southwest China, which is the most typical karst region worldwide. Comprehensive analysis reveals that the deuterium and oxygen isotope values of most karst groundwater are characterized temporally as ‘enriched in the rainy season and depleted in the dry season’, which is the opposite of the pattern of precipitation. While the spatial distribution feature is basically consistent with the spatial variation pattern of precipitation, which is characterized as ‘depleted from the coast to the interior’. Additionally, the main applications of stable isotopes to karst groundwater are discussed, including karst groundwater recharge source and recharge elevation identification, research on karst groundwater in the hydrological cycle, the hydrological process of karst groundwater, and karst groundwater contamination tracking. Finally, we looked forward to future research on karst groundwater based on deuterium and oxygen isotopes. It is our hope that this review may provide insight into the study of karst groundwater.

Effect of channel bed sediment on the transport behaviour of superparamagnetic silica encapsulated DNA microparticles in open channel injection experiments

AbstractRecently, superparamagnetic silica encapsulated DNA microparticles (SiDNAFe) were designed and in various experiments used as a hydrological tracer. We investigated the effect of bed characteristics on the transport behaviour and especially the mass loss of SiDNAFe in open channel injection experiments. Hereto, a series of laboratory injection experiments were conducted with four channel bed conditions (no sediment, fine river sediment, coarse sand, and goethite‐coated coarse sand) and two water qualities (tap water and Meuse water). Breakthrough curves (BTCs) were analysed and modelled. Mass loss of SiDNAFe was accounted for as a first‐order decay process included in a 1‐D advection and dispersion model with transient storage (OTIS). SiDNAFe BTCs could be adequately described by advection and dispersion with or without a first‐order decay process. SiDNAFe mass recoveries exhibited a wide range, varying from 50% to 120% from sediment‐free conditions to coarse (coated) sediment. In 6 out of 8 cases, SiDNAFe mass recovery was complete. Retention of SiDNAFe was 1–2 orders of magnitude greater than gravitational settling rates, as determined in Tang et al. (Hydrological Processes, e14801, 2023). We reason this was due to grain‐scale hyporheic flows and coupled water‐sediment‐particle interactions. The dispersive behaviour of SiDNAFe generally mimicked that of NaCl tracer. We concluded that SiDNAFe can be used in tracing experiments. However, water quality and sediment characteristics may affect the fate of SiDNAFe in river environments. SiDNAFe is a promising tool for particulate multi‐tracing in large rivers.

Metal(loid)s as Hydrologic Tracers for Particle Transport via Fluvial Processes from the Yellow Pine Mine and Mill in an Arid Environment of Southern Nevada, USA

ABSTRACT The aim of this study was to provide a broader view of the transport and mobilization of metal(loid)s in sediments from the Yellow Pine Mill site in Goodsprings, Nevada. This mine was principally a lead ore operation located in the township of Goodspring, Nevada. From the two wash systems originating from processed tailing piles of the mill the contaminant migration was evaluated. Concentrations of metal(loid)s in sediments were up to 10 times higher in the south wash than in the north. Maximum levels of As and Pb in surficial south wash sediments were 103 and 1351 mg kg−1, respectively. Similarly, the enrichment factors of Cd, Cu, and Pb were far lower in north wash tailing than those obtained for south wash. Principal component analysis (PCA) data from the south wash shows PC2 explained 13.98% of the total variance and loadings on Zn (0.982), Cu (0.875), Pb (0.778), and Cd (0.690), which can be considered anthropogenic component associated with mine tailing. Evaluation of the transport mechanism of sediments indicates that metal(loid)s in sediments were found to have been derived from anthropogenic activities and migrated at least 1050 m beyond source materials in the south wash. Sediment transport from source areas to the wider environment during storm events.

Groundwater flow system and microbial dynamics of groundwater in a headwater catchment

Besides surface water systems, groundwater also provides a habitat for prokaryotes. The prokaryotes inhabiting groundwater are affected by the environment that they grow in. Owing to the environmental specificity, some prokaryotic community constituents might thus provide information on local subsurface growth conditions as well as advective transport processes in case of rapid dislocation. In this study, we investigated the spatiotemporal distribution of prokaryotic communities in groundwater and spring water to examine the relationship between the groundwater flow system and prokaryotic dynamics. We sampled and monitored the water in a mountainous forested catchment in Kawamata Town, Fukushima Prefecture, Japan. Seven piezometers and a spring located across a ridge, hillslope, and valley were monitored. Prokaryotic information, such as density estimated by total direct counting, community constituents and diversity using the next-generation sequencing methods, physicochemical parameters, general water quality, and groundwater mean residence time using sulfur hexafluoride (SF6) were analyzed. Our results showed that the mean residence time of groundwater and spring water ranged from less than 1 year to 30 years. The prokaryotic density ranged from 5.37 × 103cells/mL to 2.01 × 106cells/mL (n = 60). As the mean residence time increased, the density of prokaryotes in the shallow subsurface also increased, whereas that of the prokaryotes in the deep subsurface decreased. The contribution of deep groundwater to the examined groundwater is inferred using the groundwater residence time and prokaryotic information, including density and specific constituents. The stable water isotopes did not suggest the deep groundwater contribution to the groundwater at the ridge in the high-rainfall period; however, the SiO2 concentration indicated that. Elucidating the prokaryotic information along with existing hydrological tracers may enable a better understanding of groundwater flow processes.

Enhancing urban runoff modelling using water stable isotopes and ages in complex catchments

AbstractIncreased urbanization, coupled with the projected impacts of climatic change, mandates further evaluation of the impact of urban development on water flow paths to guide sustainable land‐use planning. Though the general urbanization impacts of increased storm runoff peaks and reduced baseflows are well known; how the complex, non‐stationary interaction of the dominant water fluxes within dynamic urban water stores sustain streamflow regimes over longer periods of time is less well quantified. In particular, there is a challenge in how hydrological modelling should integrate the juxtaposition of rapid and slower flow pathways of the urban ‘karst’ landscape and different approaches need evaluation. In this context, we utilized hydrological and water stable isotope datasets within a modelling framework that combined the commonly used Hydrologic Engineering Center Hydrological Modelling System (HEC‐HMS) urban runoff model along with a simple hydrological tracer module and transit time modelling to evaluate the spatial and temporal variation of water flow paths and ages within a heavily urbanized 217 km2 catchment in Berlin, Germany. Deeper groundwater was the primary flow component in the upper reaches of the catchment within fewer urbanized regions, while the addition of wastewater effluent in the mid‐reaches of the catchment was the dominant water supply to sustain baseflow in the lower main stem stream, with additional direct storm runoff and shallow subsurface contributions in the more urbanized lower reaches. Water ages from each modelling approach mirrored flow contributions and water age mixing potential in subsurface storage; with older average water and lower young water contributions in less urbanized sub‐catchments and younger average water and higher young water contributions in more urbanized regions. The results from the first step towards more integrated tracer‐aided hydrologic modelling tools for similar peri‐urban catchments, given the potential limitations of simpler model frameworks. The results have broader implications for assessing the uncertainty in evaluating urban impacts on hydrological function under environmental change.

Predicting spatial distribution of stable isotopes in precipitation by classical geostatistical- and machine learning methods

Stable isotopes of precipitation are important natural tracers in hydrology, ecology, and forensics. The spatially explicit predictions of oxygen and hydrogen isotopes in precipitation are obtained through different interpolation techniques. In the present study we aim to examine the performance of various interpolation techniques when predicting the spatial distribution of precipitation stable isotopes. The efficiency of combined geostatistical tools (i.e. regression kriging; RK) and various machine learning methods (including regression enhanced random forest methods: MRRF, RERF) are compared in interpolating the spatial variability of precipitation stable oxygen isotope values from two different sampling networks in Europe. To assess the performance of the models, mean squared error (MSE), nonparametric Kling Gupta efficiency (KGE), absolute differences and relative mean absolute error metrics were employed. It was found that the combination of the different regression techniques with Random Forest can produce estimations with comparable accuracy in terms of descending order of overall average MSE, MRRF: 2.61, RK: 2.77, RERF: 2.99, RF: 3.08. The best performing combined random forest model variant (MRRF) outperformed regression kriging in terms of a hybrid error metric (KGE) by 7.5%. Sequential random rarefying the station networks showed that machine-learning methods are more capable of maintaining high prediction accuracy even with fewer input data. This can be a great advantage when a suitable method is needed to predict the stable isotope composition of precipitation for large spatial domains where the spatial density of data stations shows large differences.

Determination of Uranium Concentrations and 234U/238U Isotopic Ratios in Plants and the Groundwater Used in Their Cultivation in an Area with High Background Radiation

The weathering of rocks and soil causes uranium to collect in groundwater. In this article, we attempted to assess the activity concentrations of uranium and 234U/238U isotopic ratios in various groundwater samples and plants obtained from the Ha’il region, Saudi Arabia. The results showed that the concentrations of 238U ranged between 0.08 ± 0.04 and 1.11 ± 0.06 Bq L−1, with an average of 0.67 ± 0.05 Bq L−1. Activity concentrations of 234U ranged between 0.12 ± 0.01 and 2.2 ± 0.8 Bq L−1, with an average of 1.26 ± 0.11 Bq L−1. The international permissible limit of uranium in groundwater is 0.372 Bq L−1. The 234U/238U activity ratios in the collected groundwater samples ranged between 1.5 ± 0.09 and 2.49 ± 1.27 Bq L−1, with an average of 1.95 ± 0.52 Bq L−1. In fruit-bearing trees, the 234U/238U activity ratios were 1.57 ± 0.84, 2.58 ± 1.01, and 2.69 ± 0.95 Bq L−1 for lemon, fig, and narenj. In fruit-bearing shrubs, the 234U/238U activity ratios were 2.37± 0.69 and 2.68 ± 0.69 Bq L−1 for green pepper and eggplant. Our goal is to increase the understanding of the factors that govern uranium’s activity and isotopic composition to better understand its prospective utilization as a tracer in groundwater chemistry and hydrology, as well as the possibility of exploiting this area for irrigation. In this investigation area, the groundwater and plants were unfit for human consumption.

A Virological Perspective on the Use of Bacteriophages as Hydrological Tracers

Hydrological tracers, commonly used for characterizing water flow paths and sources, do not often meet all the requirements of an ideal tracer. Trans-disciplinary approaches are advocated as the way forward to enlarge the number of tracers available for investigating hydrological processes. Since the 19th century, hydrological tracers have been increasingly used, particularly in underground areas. The tracer toolbox at hand includes a large variety of options, including fluorescent dyes, isotopes, salts or bacteriophages, with each tracer offering specific qualities and complementarities. While their potential for hydrological studies has been studied in karstic environments since the 1960s, bacteriophages remain insufficiently understood. According to the selection methodology used in this review, more than thirty experiments have been listed, involving in total around seventeen different bacteriophages. These have facilitated the investigation of groundwater, surface water (i.e., river, lake and marine water), wetland and wastewater hydrological processes. The tracing experiments have also highlighted the possible interaction between bacteriophages and the surrounding environments. Bacteriophages have successfully helped researchers to understand the water flow within watersheds. Certain advantages, such as the sensitivity of detection, the ease of producing high concentrations of bacteriophages to be injected, their specificity for a host and their non-pathogenicity for human and animal cells, make bacteriophages appreciable tracer candidates for tracing experiments. However, the adsorption process or environmental factors such as temperature, pH and UV light considerably impact the fate of bacteriophages, thereby leading to an attenuation of the phage signal. Considering both the flaws and the qualities of bacteriophages, their use as hydrological tracers requires new insight and further discussions regarding experimental tracing conditions.

The extent to which soil hydraulics can explain ecohydrological separation

Field measurements of hydrologic tracers indicate varying magnitudes of geochemical separation between subsurface pore waters. The potential for conventional soil physics alone to explain isotopic differences between preferential flow and tightly-bound water remains unclear. Here, we explore physical drivers of isotopic separations using 650 different model configurations of soil, climate, and mobile/immobile soil-water domain characteristics, without confounding fractionation or plant uptake effects. We find simulations with coarser soils and less precipitation led to reduced separation between pore spaces and drainage. Amplified separations are found with larger immobile domains and, to a lesser extent, higher mobile-immobile transfer rates. Nonetheless, isotopic separations remained small (<4‰ for δ2H) across simulations, indicating that contrasting transport dynamics generate limited geochemical differences. Therefore, conventional soil physics alone are unlikely to explain large ecohydrological separations observed elsewhere, and further efforts aimed at reducing methodological artifacts, refining understanding of fractionation processes, and investigating new physiochemical mechanisms are needed.

Wetting Pattern of Cow Urine Patch in an Andisol Assessed through Bromide Concentration Distribution: A Pilot Study

Cow urine is a rich source of mobile nutrients such as nitrate (NO3−) and potassium (K+). The aim of this experiment was to evaluate the wetting pattern distribution through soil profile of cow urine patch in an andisol. Two field experiments across two consecutive years were carried out to compare cow urine patches in relation to initial wetting pattern and volume of soil affected. Bromide (Br−) has successfully been used as an inert hydrologic tracer to indicate the movement of NO3− and K+ in soil–water systems. The distribution of Br− (used as a urine tracer) on the soil surface and down the profile was irregular in all the patches. Cow urine patches covered a surface area of 0.27 and 0.35 m2, respectively, and penetrated to a depth of 70 cm. The rapid downward movement of urine occurred through macropore flow but even so, between 27% and 40% of the applied Br− was detected in the 0–5 cm soil layer. Br− showed concentrations greater than 1500 mg kg−1 and up to 3000 mg kg−1, and as the concentration of Br− decreases, the frequency and depth of affected layers increases. Despite the differences in moisture and in the distribution of the Br− concentration in both years, the concentration frequency of 500 to 1500 mg kg−1 represented around 37% of the affected volume of soil (bulb of urine) in both years. Up to 40% of the bulb represented N equivalent rates between 187 and 975 kg N ha−1. These values can potentially be emitted in gases such as NH3, N2O, and N2. It is suggested that the presence of N in the volume of affected soil could vary due to the moisture content of the soil, and that in andisols of southern Chile under permanent grasslands there are a large number of macropores that would induce preferential flows.

Pathways and Estimate of Aquifer Recharge in a Flood Basalt Terrain; A Review from the South Fork Palouse River Basin (Columbia River Plateau, USA)

Aquifer recharge is one of the most important hydrologic parameters for understanding available groundwater volumes and making sustainable the use of natural water by minimizing groundwater mining. In this framework, we reviewed and evaluated the efficacy of multiple methods to determine recharge in a flood basalt terrain that is restrictive to infiltration and percolation. In the South Fork of the Columbia River Plateau, recent research involving hydrologic tracers and groundwater modeling has revealed a snowmelt-dominated system. Here, recharge is occurring along the intersection of mountain-front alluvial systems and the extensive Miocene flood basalt layers that form a fractured basalt and interbedded sediment aquifer system. The most recent groundwater flow model of the basin was based on a large physio-chemical dataset acquired in laterally and vertically distinctive locations that refined the understanding of the intersection of the margin alluvium and the spatially variable basalt flows that filled the basin. Modelled effective recharge of 25 and 105 mm/year appears appropriate for the basin’s plain and the mountain front, respectively. These values refine previous efforts on quantifying aquifer recharge based on Darcy’s law, one-dimensional infiltration, zero-flux plane, chloride, storage, and mass-balance methods. Overall, the combination of isotopic hydrochemical data acquired in three dimensions and flow modelling efforts were needed to simultaneously determine groundwater dynamics, recharge pathways, and appropriate model parameter values in a primarily basalt terrain. This holistic approach to understanding recharge has assisted in conceptualizing the aquifer for resource managers that have struggled to understand aquifer dynamics and sustainable withdrawals.

Preferential interactions of surface-bound engineered single stranded DNA with highly aromatic natural organic matter: Mechanistic insights and implications for optimizing practical aquatic applications

Engineered short-chain single stranded DNA (ssDNA) are emerging materials with various environmental applications, such as aptasensor, selective adsorbent, and hydrological tracer. However, the lack of fundamental understanding on the interactions of such materials with natural organic matter (NOM) hinders the improvement of their application performance in terms of sensitivity, selectivity, and stability. In this study, we investigated the interactions of ssDNA (four strands with systematically varied length and sequence) with two humic acids (Suwannee River humic acid (SRHA) and Aldrich humic acid (AHA)) and two humic-like NOM present in local aquatic matrices (ROM in river water and WOM in wastewater). Detailed, molecular level interaction mechanisms were obtained by probing the colloidal stability of the ssDNA-coated gold nanoparticles, coupled with product characterization using a suite of microscopic and spectroscopic techniques. Our study revealed that π-π interactions and divalent cation bridging were the major mechanisms for ssDNA-NOM interactions. ssDNA preferentially interacted with NOM with high aromaticity (AHA > SRHA/WOM/ROM). With divalent cations present (especially Ca2+), even a small amount of AHA could completely shield ssDNA, whereas the extent of shielding by SRHA/WOM/ROM depended on the relative content of ssDNA and NOM and whether bridges formed. The extent of shielding of ssDNA by NOM provides a potential answer to the reported conflicting effects of natural water matrices on the performance of DNA-based sensors. Taken together, our findings provide insights into the transformations of engineered ssDNA under environmentally relevant conditions as well as implications for their performance optimization in practical aquatic applications (e.g., from DNA design to pretreatment strategy).

Variability in observed stable water isotopes in snowpack across a mountainous watershed in Colorado

AbstractIsotopic information from 81 snowpits was collected over a 5‐year period in a large, Colorado watershed. Data spans gradients in elevation, aspect, vegetation, and seasonal climate. They are combined with overlapping campaigns for water isotopes in precipitation and snowmelt, and a land‐surface model for detailed estimates of snowfall and climate at sample locations. Snowfall isotopic inputs, describe the majority of δ18O snowpack variability. Aspect is a secondary control, with slightly more enriched conditions on east and north facing slopes. This is attributed to preservation of seasonally enriched snowfall and vapour loss in the early winter. Sublimation, expressed by decreases in snowpack d‐excess in comparison to snowfall contributions, increases at low elevation and when seasonal temperature and solar radiation are high. At peak snow accumulation, post‐depositional fractionation appears to occur in the top 25 ± 14% of the snowpack due to melt‐freeze redistribution of lighter isotopes deeper into the snowpack and vapour loss to the atmosphere during intermittent periods of low relative humidity and high windspeed. Relative depth of fractionation increases when winter daytime temperatures are high and winter precipitation is low. Once isothermal, snowpack isotopic homogenization and enrichment was observed with initial snowmelt isotopically depleted in comparison to snowpack and enriching over time. The rate of δ18O increase (d‐excess decrease) in snowmelt was 0.02‰ per day per 100‐m elevation loss. Isotopic data suggests elevation dictates snowpack and snowmelt evolution by controlling early snow persistence (or absence), isotopic lapse rates in precipitation and the ratio of energy to snow availability. Hydrologic tracer studies using stable water isotopes in basins of large topographic relief will require adjustment for these elevational controls to properly constrain stream water sourcing from snowmelt.

Neodymium Isotopic Composition and Rare Earth Element Concentration Variations in the Coral and Solomon Seas

Significant progress has been made in the last decade on the understanding of the role of the Coral and Solomon Seas as major suppliers of waters and chemical elements to the equatorial Pacific. Yet, the location, depth, and processes of chemical enrichment of these waters remain poorly constrained. Neodymium (Nd) isotopic compositions (εNd) and rare earth element concentrations (REE) are powerful tracers of land-ocean chemical exchanges. Combined, they can greatly refine the characterization of these exchanges. Here we report profiles of εNd at 21 stations located in the Coral and Solomon Seas as part of the GEOTRACES GP-12 cruise that complement the rare earth element concentration (REE) profiles of Pham (Chemical Geology, 2019, 524 (May), 11–36). Waters exiting the Solomon Sea are generally slightly more radiogenic than the incoming ones, suggesting inputs of radiogenic material along their pathways across the Solomon Sea. This radiogenic material is brought to the surface waters via natural processes (rivers, volcanic dusts) and likely local mining activities. Noticeable εNd increases are also observed in subsurface and intermediate layers. All these processes indicate the occurrence of local Boundary Exchange (BE) processes, which are estimated to occur within a few days. Coupling hydrological and chemical tracers allows highlighting the land-ocean interactions affecting some water layers and quantifying the exchanged fluxes of Nd. Modifications of the Nd concentration and isotopic composition in the lower thermocline layer require an external flux of 7.9 ± 2.0 t(Nd)/yr only partly balanced by a scavenging flux of 1.8 ± 2.3 t(Nd)/yr, leading to a net influx of 6.1 ± 1.7 t(Nd)/yr. Regarding the Upper Circumpolar Deep Water, a total net flux of 105 ± 50 t(Nd)/yr is estimated, the external flux is relatively high (86 ± 31 t(Nd)/yr while the scavenging flux remains. These results refine the role of the Solomon Sea as a supplier of continental chemical elements to the Pacific equatorial waters.

Stable isotope variations of dew under three different climates

As a supplementary or the only water source in dry regions, dew plays a critical role in the survival of organisms. The new hydrological tracer 17O-excess, with almost sole dependence on relative humidity, provides a new way to distinguish the evaporation processes and reconstruct the paleoclimate. Up to now, there is no published daily dew isotope record on δ2H, δ18O, δ17O, d-excess, and 17O-excess. Here, we collected daily dew between July 2014 and April 2018 from three distinct climatic regions (i.e., Gobabeb in the central Namib Desert with desert climate, Nice in France with Mediterranean climate, and Indianapolis in the central United States with humid continental climate). The δ2H, δ18O, and δ17O of dew were simultaneously analyzed using a Triple Water Vapor Isotope Analyzer based on Off-Axis Integrated Cavity Output Spectroscopy technique, and then d-excess and 17O-excess were calculated. This report presents daily dew isotope dataset under three climatic regions. It is useful for researchers to use it as a reference when studying global dew dynamics and dew formation mechanisms.

Late Summer Water Sources in Rivers and Lakes of the Upper Yana River Basin, Northern Eurasia, Inferred from Hydrological Tracer Data

Major ions, stable isotopes, and trace elements, including rare earth elements (REEs), are used as natural tracers in the qualitative assessment of potential water sources in lakes and rivers of the upper Yana River basin, between Verkhoyansk and Chersky Ranges, during the late summer period. Three distinct regions were sampled, and a dominant water source in each region was qualitatively inferred from water chemistry data. The REE distribution pattern was found to be highly regional and controlled by pH and carbonate contents. Mountain headwater stream at the Verkhoyansk Range north slope, the Dulgalakh River, shows an input from a mixture of shallow groundwater and icing meltwater, with a depleted isotopic signature (δ18O below –21‰), d-excess (dex = δ2H − 8·δ18O) above 18, enrichment in Mg and Sr, and depletion in heavy REEs. The Derbeke Depression lakes and streams are fed by rainfall having ultra-low total dissolved solids (TDS) content, below 25 mg/L, and a convex-up REE pattern. In a medium mountainous river at the Chersky Range flank, the Dogdo River, leaching through fissured Jurassic carbonates is a dominant runoff pathway. Riverine water is heavily depleted in light REEs, but enriched in Mo, Rb, Sb, W and U. In the Dulgalakh River water, high positive Sm and Gd anomalies were observed, attributed either to local geology (greenshists), historical mining legacy, or contemporary winter road operations.

Transport characteristics of DNA-tagged silica colloids as a colloidal tracer in saturated sand columns; role of solution chemistry, flow velocity, and sand grain size

In recent years, DNA-tagged silica colloids have been used as an environmental tracer. A major advantage of this technique is that the DNA-coding provides an unlimited number of unique tracers without a background concentration. However, little is known about the effects of physio-chemical subsurface properties on the transport behavior of DNA-tagged silica tracers. We are the first to explore the deposition kinetics of this new DNA-tagged silica tracer for different pore water chemistries, flow rates, and sand grain size distributions in a series of saturated sand column experiments in order to predict environmental conditions for which the DNA-tagged silica tracer can best be employed. Our results indicated that the transport of DNA-tagged silica tracer can be well described by first order kinetic attachment and detachment. Because of massive re-entrainment under transient chemistry conditions, we inferred that attachment was primarily in the secondary energy minimum. Based on calculated sticking efficiencies of the DNA-tagged silica tracer to the sand grains, we concluded that a large fraction of the DNA-tagged silica tracer colliding with the sand grain surface did also stick to that surface, when the ionic strength of the system was higher. The experimental results revealed the sensitivity of DNA-tagged silica tracer to both physical and chemical factors. This reduces its applicability as a conservative hydrological tracer for studying subsurface flow paths. Based on our experiments, the DNA-tagged silica tracer is best applicable for studying flow routes and travel times in coarse grained aquifers, with a relatively high flow rate. DNA-tagged silica tracers may also be applied for simulating the transport of engineered or biological colloidal pollution, such as microplastics and pathogens.