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The Alps are a key water resource for central Europe, providing water for drinking, agriculture, and hydropower production. Thus, understanding runoff generation processes of Alpine streams is important for sustainable water management. It is currently unclear how much streamflow is derived from old water stored in the subsurface and how much stems from more recent precipitation that reaches the stream via near-surface quick flow processes. It is also unclear how this partitioning varies across different Alpine catchments in response to hydroclimatic forcing and catchment characteristics. Here, we use stable water isotope time series in precipitation and streamflow to quantify the young water fractions (Fyw; i.e., the fraction of water younger than approximately 2–3 months) and new water fractions (Fnew; here, the fraction of water younger than 1 month) in streamflow from 32 Alpine catchments. We contrast these measures of water age between summer and winter and between wet and dry periods and then correlate them with hydroclimatic variables and physical catchment properties. New water fractions varied between 3.5 % and 9.6 %, with values of 9.2 % in rainfall-dominated catchments, 9.6 % in hybrid catchments, and 3.5 % in snow-dominated catchments (mean across all catchments of 7.1 %). Young water fractions were approximately twice as large (reflecting their longer timescale) and ranged between 10.1 % and 17.6 %, with values of 17.6 % in rainfall-dominated catchments, 16.6 % in hybrid catchments, and 10.1 % in snow-dominated catchments (mean across all catchments of 14.3 %). New water fractions were negatively correlated with catchment size (Spearman rank correlation, rS, of −0.38), q95 baseflow (rS=-0.36), catchment elevation (rS=-0.37), total catchment relief (rS=-0.59), and the fraction of slopes steeper than 40° (rS=-0.48). Large new water fractions, implying faster transmission of precipitation to streamflow, are more prevalent in small catchments, at low elevations, with small elevation differences, and with large fractions of forest cover (rS=0.36). New water fractions averaged 3.3 % following dry antecedent conditions, compared with 9.3 % after wet antecedent conditions. Our results quantify how hydroclimatic and physical drivers shape the partitioning of old and new waters across the Alps, thus indicating which landscapes transmit recent precipitation more readily to streamflow and which landscapes tend to retain water over longer periods. Our results further illustrate how new water fractions may find relationships that remained invisible with young water fractions.

Stable isotopes (δ18O) and tritium (3H) are frequently used as tracers in environmental sciences to estimate age distributions of water. However, it has previously been argued that seasonally variable tracers, such as δ18O, generally and systematically fail to detect the tails of water age distributions and therefore substantially underestimate water ages as compared to radioactive tracers such as 3H. In this study for the Neckar River basin in central Europe and based on a >20-year record of hydrological, δ18O and 3H data, we systematically scrutinized the above postulate together with the potential role of spatial aggregation effects in exacerbating the underestimation of water ages. This was done by comparing water age distributions inferred from δ18O and 3H with a total of 21 different model implementations, including time-invariant, lumped-parameter sine-wave (SW) and convolution integral (CO) models as well as StorAge Selection (SAS)-function models (P-SAS) and integrated hydrological models in combination with SAS functions (IM-SAS). We found that, indeed, water ages inferred from δ18O with commonly used SW and CO models are with mean transit times (MTTs) of ∼ 1–2 years substantially lower than those obtained from 3H with the same models, reaching MTTs of ∼10 years. In contrast, several implementations of P-SAS and IM-SAS models not only allowed simultaneous representations of storage variations and streamflow as well as δ18O and 3H stream signals, but water ages inferred from δ18O with these models were, with MTTs of ∼ 11–17 years, also much higher and similar to those inferred from 3H, which suggested MTTs of ∼ 11–13 years. Characterized by similar parameter posterior distributions, in particular for parameters that control water age, P-SAS and IM-SAS model implementations individually constrained with δ18O or 3H observations exhibited only limited differences in the magnitudes of water ages in different parts of the models and in the temporal variability of transit time distributions (TTDs) in response to changing wetness conditions. This suggests that both tracers lead to comparable descriptions of how water is routed through the system. These findings provide evidence that allowed us to reject the hypothesis that δ18O as a tracer generally and systematically “cannot see water older than about 4 years” and that it truncates the corresponding tails in water age distributions, leading to underestimations of water ages. Instead, our results provide evidence for a broad equivalence of δ18O and 3H as age tracers for systems characterized by MTTs of at least 15–20 years. The question to which degree aggregation of spatial heterogeneity can further adversely affect estimates of water ages remains unresolved as the lumped and distributed implementations of the IM-SAS model provided inconclusive results. Overall, this study demonstrates that previously reported underestimations of water ages are most likely not a result of the use of δ18O or other seasonally variable tracers per se. Rather, these underestimations can largely be attributed to choices of model approaches and complexity not considering transient hydrological conditions next to tracer aspects. Given the additional vulnerability of time-invariant, lumped SW and CO model approaches in combination with δ18O to substantially underestimate water ages due to spatial aggregation and potentially other still unknown effects, we therefore advocate avoiding the use of this model type in combination with seasonally variable tracers if possible and instead adopting SAS-based models or time-variant formulations of CO models.

Groundwater recharge is a key component of the hydrological cycle, yet its direct measurement is complex and often difficult to achieve. An alternative is its inverse estimation through a combination of numerical models and transient observations from distributed soil water monitoring stations. However, an often neglected aspect of this approach is the effect of model predictive uncertainty on simulated water fluxes. In this study, we made use of long-term soil water content measurements at 14 locations from the Austrian soil water monitoring program to quantify and compare local potential groundwater recharge rates and their temporal variability. Observations were coupled with a Bayesian probabilistic framework to calibrate the HYDRUS-1D model and assess the effect of model predictive uncertainty on long-term simulated recharge fluxes. Estimated annual potential recharge rates ranged from 44 to 1319 mm a−1 with a relative uncertainty (95 % interquantile range/median) in the estimation of between 1 % and 39 %. Recharge rates decreased longitudinally, with high rates and lower seasonality at western sites and low rates with high seasonality and extended periods without recharge at the southeastern and eastern Austrian sites. Higher recharge rates and lower actual evapotranspiration were related to sandy soils; however, climatic factors had a stronger influence on estimated potential groundwater recharge than soil properties, underscoring the vulnerability of groundwater recharge to the effects of climate change.

Deforestation can considerably affect transpiration dynamics and magnitudes at the catchment scale and thereby alter the partitioning between drainage and evaporative water fluxes released from terrestrial hydrological systems. However, it has so far remained problematic to directly link reductions in transpiration to changes in the physical properties of the system and to quantify these changes in system properties at the catchment scale. As a consequence, it is difficult to quantify the effect of deforestation on parameters of catchment-scale hydrological models. This in turn leads to substantial uncertainties in predictions of the hydrological response after deforestation but also to a poor understanding of how deforestation affects principal descriptors of catchment-scale transport, such as travel time distributions and young water fractions. The objectives of this study in the Wüstebach experimental catchment are therefore to provide a mechanistic explanation of why changes in the partitioning of water fluxes can be observed after deforestation and how this further affects the storage and release dynamics of water. More specifically, we test the hypotheses that (1) post-deforestation changes in water storage dynamics and partitioning of water fluxes are largely a direct consequence of a reduction of the catchment-scale effective vegetation-accessible water storage capacity in the unsaturated root zone (SU, max) after deforestation and that (2) the deforestation-induced reduction of SU, max affects the shape of travel time distributions and results in shifts towards higher fractions of young water in the stream. Simultaneously modelling streamflow and stable water isotope dynamics using meaningfully adjusted model parameters both for the pre- and post-deforestation periods, respectively, a hydrological model with an integrated tracer routine based on the concept of storage-age selection functions is used to track fluxes through the system and to estimate the effects of deforestation on catchment travel time distributions and young water fractions Fyw. It was found that deforestation led to a significant increase in streamflow accompanied by corresponding reductions of evaporative fluxes. This is reflected by an increase in the runoff ratio from CR=0.55 to 0.68 in the post-deforestation period despite similar climatic conditions. This reduction of evaporative fluxes could be linked to a reduction of the catchment-scale water storage volume in the unsaturated soil (SU, max) that is within the reach of active roots and thus accessible for vegetation transpiration from ∼258 mm in the pre-deforestation period to ∼101 mm in the post-deforestation period. The hydrological model, reflecting the changes in the parameter SU, max, indicated that in the post-deforestation period stream water was characterized by slightly yet statistically not significantly higher mean fractions of young water (Fyw∼0.13) than in the pre-deforestation period (Fyw∼0.12). In spite of these limited effects on the overall Fyw, changes were found for wet periods, during which post-deforestation fractions of young water increased to values Fyw∼0.37 for individual storms. Deforestation also caused a significantly increased sensitivity of young water fractions to discharge under wet conditions from dFyw/dQ=0.25 to 0.36. Overall, this study provides quantitative evidence that deforestation resulted in changes in vegetation-accessible storage volumes SU, max and that these changes are not only responsible for changes in the partitioning between drainage and evaporation and thus the fundamental hydrological response characteristics of the Wüstebach catchment, but also for changes in catchment-scale tracer circulation dynamics. In particular for wet conditions, deforestation caused higher proportions of younger water to reach the stream, implying faster routing of stable isotopes and plausibly also solutes through the sub-surface.

Experimental watersheds have a long tradition as research sites in hydrology and have been used since the late nineteenth and early twentieth centuries. The University of Natural Resources and Life Sciences Vienna (BOKU) recently extended its experimental research forest site “Rosalia” with an area of 950 ha towards the creation of a full ecological-hydrological experimental watershed. The overall objective is to implement a multi-scale, multi-disciplinary observation system that facilitates the study of water, energy and solute transport processes in the soil–plant–atmosphere continuum. This article describes the characteristics of the site and the monitoring network and its instrumentation that has been installed since 2015, as well as the datasets. The network includes four discharge gauging stations and seven rain gauges along with observations of air and water temperature, relative humidity, and electrical conductivity. In four profiles, soil water content and temperature are recorded at different depths. In addition, since 2018, nitrate, TOC and turbidity have been monitored at one gauging station. In 2019, a programme to collect isotopic data in precipitation and discharge was initiated. All data collected since 2015, including, in total, 56 high-resolution time series (with 10 min sampling intervals), are provided to the scientific community on a publicly accessible repository. The datasets are available at https://doi.org/10.5281/zenodo.3997140 (Fürst et al., 2020).

The time precipitation needs to travel through a catchment to its outlet is an important descriptor of a catchment's susceptibility to pollutant contamination, nutrient loss, and hydrological functioning. The fast component of total water flow can be estimated by the fraction of young water (Fyw), which is the percentage of streamflow younger than 3 months. Fyw is calculated by comparing the amplitudes of sine waves fitted to seasonal precipitation and streamflow tracer signals. This is usually done for the complete tracer time series available, neglecting annual differences in the amplitudes of longer time series. Considering inter-annual amplitude differences, we employed a moving time window of 1 year in weekly time steps over a 4.5-year δ18O tracer time series to calculate 189 Fyw estimates and their uncertainty. They were then tested against the following null hypotheses: (1) at least 90 % of Fyw results do not deviate more than ±0.04 (4 %) from the mean of all Fyw results, indicating long-term invariance. Larger deviations would indicate changes in the relative contribution of different flow paths; (2) for any 4-week window, Fyw does not change more than ±0.04, indicating short-term invariance. Larger deviations would indicate a high sensitivity of Fyw to a 1-week to 4-week shift in the start of a 1-year sampling campaign; (3) the Fyw results of 1-year sampling campaigns started in a given calendar month do not change more than ±0.04, indicating seasonal invariance. In our study, all three null hypotheses were rejected. Thus, the Fyw results were time-variable, showed variability in the chosen sampling time, and had no pronounced seasonality. We furthermore found evidence that the 2015 European heat wave and including two winters into a 1-year sampling campaign increased the uncertainty of Fyw. Based on an increase in Fyw uncertainty when the mean adjusted R2 was below 0.2, we recommend further investigations into the dependence of Fyw and its uncertainty to goodness-of-fit measures. Furthermore, while investigated individual meteorological factors did not sufficiently explain variations of Fyw, the runoff coefficient showed a moderate negative correlation of r=-0.50 with Fyw. The results of this study suggest that care must be taken when comparing Fyw of catchments that were based on different calculation periods and that the influence of extreme events and snow must be considered.

For more than two decades, research groups in hydrology, ecology, soil science, and biogeochemistry have performed cryogenic water extractions (CWEs) for the analysis of δ2H and δ18O of soil water. Recent studies have shown that extraction conditions (time, temperature, and vacuum) along with physicochemical soil properties may affect extracted soil water isotope composition. Here we present results from the first worldwide round robin laboratory intercomparison. We test the null hypothesis that, with identical soils, standards, extraction protocols, and isotope analyses, cryogenic extractions across all laboratories are identical. Two standard soils with different physicochemical characteristics along with deionized (DI) reference water of known isotopic composition were shipped to 16 participating laboratories. Participants oven-dried and rewetted the soils to 8 and 20 % gravimetric water content (WC), using the deionized reference water. One batch of soil samples was extracted via predefined extraction conditions (time, temperature, and vacuum) identical to all laboratories; the second batch was extracted via conditions considered routine in the respective laboratory. All extracted water samples were analyzed for δ18O and δ2H by the lead laboratory (Global Institute for Water Security, GIWS, Saskatoon, Canada) using both a laser and an isotope ratio mass spectrometer (OA-ICOS and IRMS, respectively). We rejected the null hypothesis. Our results showed large differences in retrieved isotopic signatures among participating laboratories linked to soil type and soil water content with mean differences compared to the reference water ranging from +18.1 to −108.4 ‰ for δ2H and +11.8 to −14.9 ‰ for δ18O across all laboratories. In addition, differences were observed between OA-ICOS and IRMS isotope data. These were related to spectral interferences during OA-ICOS analysis that are especially problematic for the clayey loam soils used. While the types of cryogenic extraction lab construction varied from manifold systems to single chambers, no clear trends between system construction, applied extraction conditions, and extraction results were found. Rather, observed differences in the isotope data were influenced by interactions between multiple factors (soil type and properties, soil water content, system setup, extraction efficiency, extraction system leaks, and each lab's internal accuracy). Our results question the usefulness of cryogenic extraction as a standard for water extraction since results are not comparable across laboratories. This suggests that defining any sort of standard extraction procedure applicable across laboratories is challenging. Laboratories might have to establish calibration functions for their specific extraction system for each natural soil type, individually.

Lagoons are important ecosystems occupying large coastal areas worldwide. Lagoons contain various mixtures of marine and freshwater sources which are highly dynamic in time. However, it often remains a challenge to identify and quantify dynamic changes of water sources, particularly in heterogeneous lagoon systems like the Köycegiz–Dalyan lagoon (KDL), which is located at the south-west of Turkey on the Mediterranean Sea coast. The objective of this study was to quantify different contributions of potential water sources i.e. surface water, groundwater and seawater in the lagoon and how these water sources changed over time and space. In the wet- and dry-season stable isotopes of water, chloride concentration (Cl-) and salinity were measured in two depths in the lagoon and surrounding water bodies (sea, lake, groundwater). Different components of water sources were quantified with a three component endmember mixing analysis. Differences in Cl- and stable isotopes over time indicated the dynamic behaviour of the system. Generally, none of the groundwater samples was impacted by water of the Mediterranean Sea. During the wet season, most of the lagoon water (> 95%) was influenced by freshwater and vertically well mixed. During the dry season, high Cl- in the deeper sampling locations indicated a high contribution of marine water throughout the entire lagoon system due to saltwater intrusion. However, a distinct layering in the lagoon was obvious from low Cl- and depleted isotope contents close to the surface supporting freshwater inflow into the system even during the dry season. Besides temporal dynamics also spatial heterogeneities were identified. Changes in water sources were most evident in the main lagoon channel compared to more isolate lagoon lakes, which were influenced by marine water even in the wet season, and compared to side branches indicating slower turnover times. We found that environmental tracers helped to quantify highly dynamic and heterogeneous contributions of different water sources in the Köycegiz–Dalyan lagoon.

Event-based vision sensors produce asynchronous event streams with high temporal resolution based on changes in the visual scene. The properties of these sensors allow for accurate and fast calculation of optical flow as events are generated. Existing solutions for calculating optical flow from event data either fail to capture the true direction of motion due to the aperture problem, do not use the high temporal resolution of the sensor, or are too computationally expensive to be run in real time on embedded platforms. In this research, we first present a faster version of our previous algorithm, ARMS (Aperture Robust Multi-Scale flow). The new optimized software version (fARMS) significantly improves throughput on a traditional CPU. Further, we present hARMS, a hardware realization of the fARMS algorithm allowing for real-time computation of true flow on low-power, embedded platforms. The proposed hARMS architecture targets hybrid system-on-chip devices and was designed to maximize configurability and throughput. The hardware architecture and fARMS algorithm were developed with asynchronous neuromorphic processing in mind, abandoning the common use of an event frame and instead operating using only a small history of relevant events, allowing latency to scale independently of the sensor resolution. This change in processing paradigm improved the estimation of flow directions by up to 73% compared to the existing method and yielded a demonstrated hARMS throughput of up to 1.21 Mevent/s on the benchmark configuration selected. This throughput enables real-time performance and makes it the fastest known realization of aperture-robust, event-based optical flow to date.

The occurrence and persistence of microplastics (MPs) in natural environments are of increasing concern. Along with this, the transport of MPs in sediments has been investigated mainly focusing on the effect of plastic size and shape, media size effect, and solution chemistry. Yet, the influence of particle density is only partially understood. Therefore, column experiments on the transport of variably buoyant MPs in saturated natural sediments and glass beads were conducted, and transport parameters were quantified using a two-site kinetic transport model with a depth-dependent blocking function (the amount of retained MPs does not decrease at a constant rate with increasing depth, the majority of MPs were retained near the column inlet). Neutral, sinking, and buoyant MPs within the same size range were selected, with stable water isotope applied as conservative tracer to explore water and MP movement in the tested sediments. The results showed that 95.5 ± 1.4% of sinking MPs remained in columns packed with gravel, followed by buoyant and neutral MPs, thus indicating that particle density does affect MP mobility. Similar recovered amounts of MPs were found in columns packed with glass beads, indicating that tested sediment types do not affect the deposition behavior of MPs. The breakthrough curves of MPs were accurately described by the selected model. However, the simulated retention profiles overestimated the observed MP amount in layers closest to the column inlet. The coupled experimental and modeled results suggest an enhanced retention of sinking MPs, while neutrally and buoyant MPs exhibit a higher mobility in comparison. Thus, neutral or buoyant MPs can potentially pose a higher contamination risk to subsurface porous media environments compared to sinking MPs. Discrepancies between observed and simulated retention profiles indicate that future model development is needed for advancing the MP deposition as affected by particle density., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Microplastics (MPs) present in terrestrial environments show potential leaching risk to deeper soil layers and aquifer systems, which threaten soil health and drinking water supply. However, little is known about the environmental fate of MPs in natural sediments. To examine the MPs transport mechanisms in natural sediments, column experiments were conducted using different natural sediments and MPs (10-150 µm) with conservative tracer. Particle breakthrough curves (BTCs) and retention profiles (RPs) were numerically interpreted in HYDRUS-1D using three different models to identify the most plausible deposition mechanism of MPs. Results show that the retention efficiency for a given particle size increased with decreasing grain size, and RPs exacerbated their hyper-exponential shape in finer sediments. Furthermore, the amounts of MPs present in the effluent increased to over 85 % as MPs size decreased to 10-20 µm in both gravel and coarse sand columns, while all larger MPs (125-150 µm) were retained in the coarse sand column. The modeling results suggested that the blocking mechanism becomes more important with increasing particle sizes. In particular, the attachment-detachment without blocking was the most suited parameterization to interpret the movement of small MPs, while a depth-dependent blocking approach was necessary to adequately describe the fate of larger particles., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Groundwater is a vital source of freshwater, serving ecological, environmental, and societal needs. In regions with springs as a predominant source, such as the Northern Apennines (Italy), resilience of these springs to climate-induced recharge changes is crucial for water supply and ecosystem preservation. In this study, Nadìa Spring in the Northern Apennines is examined through an unprecedented array of multidisciplinary analyses to understand its resilience and unique characteristics. The Nadìa Spring's exceptional response, characterized by a sustained base flow even in the face of drought, is attributed to a combination of factors including a substantial groundwater reservoir, a complex network of faults/fractures, slope instabilities, and karst dissolution. The investigation reveals a dual porosity system in the aquifer, consisting of fast-flow conduits and a diffuse fracture network. While fast-flow conduits contribute to rapid responses during high-flow conditions, the diffuse system becomes predominant during low-flow periods. This dual porosity structure helps the spring maintain a consistent base flow in the face of climate-induced recharge fluctuations. The study shows that Nadìa Spring exhibits remarkable resilience to year-to-year variations in recharge, as evidenced by stable minimum discharge values. While the spring has undergone a decline in discharge over the past century due to long-term climate change, it is becoming more resilient over interdecadal timescales due to transition to a diffuse drainage system that mitigates the impact of reduced recharge. The availability of a century-long spring discharge monitoring was a crucial piece of information for understanding the spring's discharge response and drawing conclusions about its long-term resilience to recharge fluctuations. Continuing long-term monitoring and research in the future will be essential to validate and expand upon these findings in the context of changing climatic conditions. This research serves as a model for assessing strategic groundwater discharge points in geological settings similar to the Northern Apennines., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

A new conceptual model of the hydrogeological systems in Cameroon's Douala Coastal Sedimentary Basin (DCSB) was constructed. The model is based upon the basin's known geology, plus data from recent field campaigns that allowed the collection of rainwater and groundwater samples for analyses of stable isotopes (δ 2 H, δ 18 O, δ 13 C), radiogenic isotopes ( 3 H, 14 C), and water chemistry. Aquifer characteristics that were thereby deciphered include recharge, isotopic distributions, residence times, and mixing processes. Rainfall samples (mean δ 18 O = -2.0 ‰; mean δ 2 H = -6.80 ‰; weighted mean = -2.4 ‰ δ 18 O, -9.85 ‰ δ 2 H) scatter along two distinct lines, thus indicating that local rainfall events undergo processes during convective events, variability in humidity, amount effects, and seasonal variations. Stable isotope values of river water samples are close to the weighted mean of local precipitation, with some downstream enrichment. The Quaternary/Mio-Pliocene superficial aquifer system (depth < 70 m) and the intermediate Oligocene/Upper Eocene aquifer system (depth: 70 to 200 m) exhibit evidence of similar fractionation processes through an enrichment gradient of δ-values. The enrichment is more pronounced at the top of the superficial aquifer, which is very exposed to direct rainfall water infiltration, evaporation, and amount effects. The depth profiles of δ-values coupled to water chemistry and tritium contents, evidence leakage between (i) the superficial system's Quaternary alluvium sands and Mio-Pliocene sands; and (ii) the superficial and intermediate systems. Thus, the aquifers that contain modern, post nuclear groundwater are characterized by flow exchanges and direct recharge from rainfall events. In contrast, the Upper Eocene system has depleted δ-values and lower bicarbonate contents, suggesting not only that this system was recharged by rapid infiltration (with limited effect of evaporation), but that this recharge occurred during a cooler time in the past. The residence times (computed from 14 C dates) indicate uncorrected ages ranging from hundreds to thousands of years., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Recently, δ 17 O and its excess (Δ' 17 O) have become increasingly significant "triple-oxygen-isotope" indicators of distinctive hydrological processes in hydrology and climatology. This situation mirrors the research regarding δ 18 O and δ 2 H in the 1960s towards a solid theoretical base and a surge in application examples and field studies worldwide. Currently, systematic global measurements for δ 17 O in precipitation are still lacking. As a result, attempts have been made to define a Global δ 17 O/δ 18 O Meteoric Water Line (GMWL), often by using regional or local datasets of varying systematicity. Different definitions of the global reference slope (λ ref ) for determining Δ' 17 O values have been proposed, by ongoing debate around a proposed consensus value of 0.528. This study used worldwide samples archived in the IAEA Global Network of Isotopes in Precipitation (GNIP) to (a) derive a δ 17 O/δ 18 O GMWL based on four-year monthly records from 66 GNIP stations, (b) formulate local δ 17 O/δ 18 O meteoric water lines (LMWL) for these stations' areas, and (c) evaluate regional and seasonal variations of Δ' 17 O in precipitation. The GMWL for δ 17 O/δ 18 O was determined to be δ' 17 O = 0.5280 ± 0.0002 δ' 18 O + 0.0153 ± 0.0013, in keeping with the consensus value. Furthermore, our results suggested that using a line-conditioned 17 O-excess is a viable alternative over the global λ ref in the context of regional hydrology and paleoclimatology interpretations; however, without challenging the global λ ref as such., (© 2023. The Author(s).)

We studied the origin of elements of an undisturbed stream basin during the dry season as derived by atmospheric inputs and lithological processes. Α mass balance model was applied taking into account atmospheric (rain and vapor) inputs and their origin from marine aerosol and dust, as well as the contribution of rock mineral weathering and dissolution of soluble salts. The model results were enhanced using element enrichment factors, element ratios and water stable isotopes. Weathering and dissolution of bedrock and soil minerals contributed the main element portions, besides sodium and sulfate that chiefly derived from wet deposition. Vapor was shown to contribute water to inland waters of the basin. However, rain was the main source of elements compared to vapor, with marine aerosol being the only atmospheric chloride source, contributing also over 60 % of atmospheric sodium and magnesium. Silicate derived from mineral weathering (mainly plagioclase and amorphous silica), while soluble salt dissolution contributed the main portions of the rest of major elements. In headwater springs and streams, element concentrations were more affected by atmospheric inputs and silicate mineral weathering was more intense, contrary to lowland waters that were more affected by soluble salt dissolution. Effective self-purification processes were mirrored in low nutrient levels, despite the significant inputs from wet deposition, with rain being more important contributor than vapor for the majority of nutrient species. Relatively high nitrate concentrations in headwaters were attributed to increased mineralization and nitrification, while the downstream nitrate diminishing was due to prevailing denitrification processes. The ultimate goal of this study is to contribute in establishing stream elements' reference conditions using mass balance modeling approaches., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

In coastal salinized groundwater systems, contamination from various nitrate (NO 3 ) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO 3 sources and identify potential NO 3 transformtation processes. Effective field-based NO 3 studies in coastal areas are needed to improve the understanding of NO 3 contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO 3 contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δ 18 O H2O , δ 2 H H2O , δ 15 N NO3 , δ 18 O NO3 , and δ 11 B) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO 3 sources and their contributions, and (ii) to describe the potential NO 3 transformation processes. The measured NO 3 concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO 3 indicates that manure, soil organic matter, and sewage are the potential sources of NO 3 , while δ 11 B values constrain the NO 3 contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO 3 dominates over other likely sources. Nitrate derived from manure in the study area is attributed to organic fertilizers used to promote crop growth, and livestock that deposit manure directly on the ground surface. Evidence for ongoing denitrification in groundwaters of the study area is supported by an enrichment in both 15 N and 18 O in the remaining NO 3 , although isotopic mass balances between the measured and the theoretical δ 18 O NO3 values also suggest the occurrence of nitrification. The simultaneous occurrence of these biogeochemical processes with heterogeneous distribution across the study area reflect the complexity of interactions within the investigated coastal aquifer. The multiple isotopic tracer approach used here can identify the effect of multiple NO 3 anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

As an alternative to activated carbon, biochar is a promising, environmentally friendly sorbent that can be used to remove organic groundwater pollutants, such as chlorinated ethenes (CEs). Stable isotope fractionation in biofilters is used to quantify pollutant degradation and to distinguish degradation from pollutant sorption on e.g. biochar. However, the sorption of CEs on biochar, and the potential abiotic fractionation processes remain to be tested. The sorption process of CEs and ethene on activated carbon and biochar was investigated with regard to the isotope effects for the differentiation from microbial degradation processes. Results from physical and chemical characterization of biochar indicated that biochar feedstock and pyrolysis conditions determined sorption performance depending on the surface chemistry and the pore size distribution of the coarse sorbent particles. The sorption capacity of the activated carbon was significantly higher with highly chlorinated ethenes, but similar to the biochars with low chlorination. Apparent carbon isotope fractionation factors (ε) of +0.1 to -4.4 ‰ were found above measurement uncertainties of GC/IRMS. The extent of isotope enrichment of the 13 C bearing isotopologues in the residual aqueous phase (ε < 0) was characteristic for individual pairs of pollutant and sorbent material and could be related to pore-filling processes limited by the micropore size distribution of sorbent materials and the chemical properties of sorbed pollutants. Especially the large isotope fractionation during the sorption of ethene led to the assumption that diffusion processes within the pore matrix of the sorbent particles contributed to the observed isotope effects, but should still be considered a property of sorption. Concluding on the results indicated that sorption processes can have a significant contribution to carbon isotope fractionation in CEs and ethene. These should not be neglected in the evaluation of biofilters for groundwater purification, in which CEs are simultaneously degraded by microbes., Competing Interests: The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Andrea Watzinger reports financial support was provided by Kommunalkredit Public Consulting GmbH. Andrea Watzinger reports equipment, drugs, or supplies was provided by Donau Carbon GmbH., (© 2023 The Authors.)

δ 17 O and Δ' 17 O are emerging tracers increasingly used in isotope hydrology, climatology, and biochemistry. Differentiating small relative abundance changes in the rare 17 O isotope from the strong covariance with 18 O imposes ultra-high precision requirements for this isotope analysis. Measurements of δ 17 O by Cavity Ringdown Spectroscopy (CRDS) are attractive due to the ease of sample preparation, automated throughput, and avoidance of chemical conversions needed for isotope-ratio mass spectrometry. However, the CRDS approach requires trade-offs in measurement precision and uncertainty. In this protocol document, we present the following:•New analytical procedures and a software tool for conducting δ 17 O and Δ' 17 O measurements by CRDS.•Outline a robust uncertainty framework for Δ' 17 O determinations.•Description of a CRDS performance framework for optimizing throughput, instrumental stability, and Δ' 17 O measurement precision and accuracy., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2023 International Atomic Energy Agency. Published by Elsevier B.V.)

Rivers are the "tip of the iceberg", with the underlying groundwater being the unseen freshwater majority. Microbial community composition and the dynamics of shallow groundwater ecosystems are thus crucial, due to their potential impact on ecosystem processes and functioning. In early summer and late autumn, samples of river water from 14 stations and groundwater from 45 wells were analyzed along a 300 km transect of the Mur River valley, from the Austrian alps to the flats at the Slovenian border. The active and total prokaryotic communities were characterized using high-throughput gene amplicon sequencing. Key physico-chemical parameters and stress indicators were recorded. The dataset was used to challenge ecological concepts and assembly processes in shallow aquifers. The groundwater microbiome is analyzed regarding its composition, change with land use, and difference to the river. Community composition and species turnover differed significantly. At high altitudes, dispersal limitation was the main driver of groundwater community assembly, whereas in the lowland, homogeneous selection explained the larger share. Land use was a key determinant of the groundwater microbiome composition. The alpine region was more diverse and richer in prokaryotic taxa, with some early diverging archaeal lineages being highly abundant. This dataset shows a longitudinal change in prokaryotic communities that is dependent on regional differences affected by geomorphology and land use.

This study focuses on coastal aquifers subject to uncontrolled land use development by investigating the combined effects of seawater intrusion and nitrate contamination. The research is undertaken in a Mediterranean coastal agricultural area (Plain of the El-Nil River, Algeria), where water resources are heavily impacted by anthropogenic activities. A multi-tracer approach, integrating hydrogeochemical and isotopic tracers (δ 2 H H2O , δ 18 O H2O , δ 15 N NO3 and δ 18 O NO3 ), is combined with a hydrochemical facies evolution diagram, and a Bayesian isotope mixing model (MixSIAR) to assess seawater contamination with its inland intrusion, and distinguish the nitrate sources and their apportionment. Results show that seawater intrusion is circumscribed to the sector neighboring the Mediterranean Sea, with two influencing functions including classic inland intrusion through the aquifer, and upstream seawater impact through the river mouth connected to the Mediterranean Sea. Groundwater and surface water samples reveal nitrate concentrations above the natural baseline threshold, suggesting anthropogenic influence. Results from nitrate isotopic composition, NO 3 and Cl concentrations, and the MixSIAR model show that nitrate concentrations chiefly originate from sewage and manure sources. Nitrate derived from the sewage is related to wastewater discharge, whereas nitrate derived from the manure is attributed to an excessive use of animal manure to fertilise agricultural areas. The dual negative impact of seawater intrusion and nitrate contamination degrades water quality over a large proportion of the study area. The outcomes of this study are expected to contribute to effective and sustainable water resources management in the Mediterranean coastal area. Furthermore, this study may improve scientists' ability to predict the combined effect of various anthropogenic stressors on coastal environments and help decision-makers elsewhere to prepare suitable environmental strategies for other regions currently undergoing an early stage of water resources deterioration., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Implementing sustainable groundwater resources management in coastal areas is challenging due to the negative impacts of anthropogenic stressors and various interactions between groundwater and surface water. This study focuses on nitrate contamination and transport via groundwater-surface water exchange in a Mediterranean coastal area (Guerbes-Senhadja region, Algeria) that is heavily affected by anthropogenic activities. A multi-tracer approach, integrating hydrogeochemical and isotopic tracers (δ 2 H H2O , δ 18 O H2O , 3 H, δ 15 N NO3 and δ 18 O NO3 ), is combined with a Bayesian isotope mixing model (MixSIAR) to (i) elucidate the nitrate sources and their apportionments in water systems, and (ii) describe potential interactions between groundwater and surface water. Results from nitrate isotopic composition and the MixSIAR model show that nitrate concentrations mainly originate from sewage and manure sources. Nitrate derived from the sewage is attributed to urban and rural wastewater discharge, whereas nitrate derived from the manure is related to animal manure used to fertilise agricultural areas. High apportionments of nitrate-based atmospheric precipitation are identified in groundwater and surface water; a finding that is specific to this study. The multi-origin stresses combined with evidence of interactions between surface water and groundwater contribute to negatively impacting large parts of the study coastal area. The outcomes of this study are expected to contribute to sustainable management of coastal ecosystems by drawing more attention towards groundwater use and protection. Furthermore, this study may improve scientists' ability to predict the behavior of anthropogenically impacted coastal ecosystems and help decision-makers elsewhere to prepare suitable environmental strategies for other coastal ecosystems currently undergoing an early stage of groundwater resources deterioration., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In Germany, river monitoring for tritium started in the early 1970s. Today this monitoring network consists of 50 stations and includes stable isotopes. The stable isotope time series to the end of 2021 are at least four years and for some stations up to 30 years long. Daily river water samples were collected during an extraordinary dry season from October 2018 until end of January 2019 from six selected stations of the Rhine and five stations of the Elbe basin. The most dominating stable isotope effects in river water are the seasonal and altitude effects, but also a continental effect is visible. The isotopes indicate snow and ice melt contributions in the Rhine and Danube during the summer months and a consecutive dilution of these signals by mixing with tributary rivers. Close to the coasts in northern Germany, stable isotope patterns reflect influence of seawater and tides. Daily patterns during the dry season 2018/2019 surprisingly do not exhibit extreme changes but rather trends of enhanced groundwater contribution. Long-term continual data across scales are important for comparing and identifying hydrological processes in German river basins of different size and mean catchment altitudes, and highlight the benefits of a co-organized national network.

We present field data on the effects of heavy rainfall after drought on the mobility of glyphosate and redox conditions in a clayey floodplain soil. By applying glyphosate together with deuterated water as conservative tracer in combination with time resolved in situ redox potential measurements, the spatial and temporal patterns of water infiltration and pesticide transport as well as the concomitant changes of the redox conditions were revealed. Our findings demonstrate that shrinkage cracks in dry soils can serve as effective transport paths for atmospheric oxygen, water and glyphosate. The rain intensity of a typical summer storm event (approx. 25 mm within one hour) was sufficient to translocate deuterated water and glyphosate to the subsoil (50 cm) within 2 hours. Soil wetting induced partial closure of the shrinkage cracks and stimulated microbial activity resulting in pronounced dynamics of in situ soil redox conditions. Redox potentials in 40 to 50 cm depth dropped permanently to strongly reducing conditions within hours to days but fluctuated between reducing and oxidizing conditions in 10 to 30 cm depth. Our findings highlight the close link between the presence of macropores (shrinkage cracks), heavy rainfall after drought, redox dynamics and pesticide translocation to the subsoil and thus call for further studies addressing the effects of dynamic redox conditions as a limiting factor for glyphosate degradation.

Riverbank filtration is an established and quantitatively important approach to mine high-quality raw water for drinking water production. Bacterial fecal indicators are routinely used to monitor hygienic raw water quality, however, their applicability in viral contamination has been questioned repeatedly. Additionally, there are concerns that the increasing frequency and intensity of meteorological and hydrological events, i.e., heavy precipitation and droughts leading to high and low river levels, may impair riverbank filtration performance. In this study, we explored the removal of adenovirus compared with several commonly used bacterial and viral water quality indicators during different river levels. In a seasonal study, water from the Rhine River, a series of groundwater monitoring wells, and a production well were regularly collected and analyzed for adenovirus, coliphages, E. coli, C. perfringens, coliform bacteria, the total number of prokaryotic cells (TCC), and the number of virus-like particles (TVPC) using molecular and cultivation-based assays. Additionally, basic physico-chemical parameters, including temperature, pH, dissolved organic carbon, and nutrients, were measured. The highest log 10 reduction during the >72 m of riverbank filtration from the river channel to the production well was observed for coliforms (>3.7 log 10 ), followed by E. coli (>3.4 log 10 ), somatic coliphages (>3.1 log 10 ), C. perfringens (>2.5 log 10 ), and F+ coliphages (>2.1 log 10 ) at high river levels. Adenovirus decreased by 1.6-3.1 log units in the first monitoring well (>32 m) and was not detected in further distant wells. The highest removal efficiency of adenovirus and most other viral and bacterial fecal indicators was achieved during high river levels, which were characterized by increased numbers of pathogens and indicators. During low river levels, coliforms and C. perfringens were occasionally present in raw water at the production well. Adenovirus, quantified via droplet digital PCR, correlated with E. coli, somatic coliphages, TCC, TVPC, pH, and DOC at high river levels. At low river levels, adenoviruses correlated with coliforms, TVPC, pH, and water travel time. We conclude that although standard fecal indicators are insufficient for assessing hygienic raw water quality, a combination of E. coli, coliforms and somatic coliphages can assess riverbank filtration performance in adenovirus removal. Furthermore, effects of extreme hydrological events should be studied on an event-to-event basis at high spatial and temporal resolutions. Finally, there is an urgent need for a lower limit of detection for pathogenic viruses in natural waters. Preconcentration of viral particles from larger water volumes (>100 L) constitutes a promising strategy., (Copyright © 2021. Published by Elsevier Ltd.)

River systems have undergone a massive transformation since the Anthropocene. The natural properties of river systems have been drastically altered and reshaped, limiting the use of management frameworks, their scientific knowledge base and their ability to provide adequate solutions for current problems and those of the future, such as climate change, biodiversity crisis and increased demands for water resources. To address these challenges, a socioecologically driven research agenda for river systems that complements current approaches is needed and proposed. The implementation of the concepts of social metabolism and the colonisation of natural systems into existing concepts can provide a new basis to analyse the coevolutionary coupling of social systems with ecological and hydrological (i.e., 'socio-ecohydrological') systems within rivers. To operationalize this research agenda, we highlight four initial core topics defined as research clusters (RCs) to address specific system properties in an integrative manner. The colonisation of natural systems by social systems is seen as a significant driver of the transformation processes in river systems. These transformation processes are influenced by connectivity (RC 1), which primarily addresses biophysical aspects and governance (RC 2), which focuses on the changes in social systems. The metabolism (RC 3) and vulnerability (RC 4) of the social and natural systems are significant aspects of the coupling of social systems and ecohydrological systems with investments, energy, resources, services and associated risks and impacts. This socio-ecohydrological research agenda complements other recent approaches, such as 'socio-ecological', 'socio-hydrological' or 'socio-geomorphological' systems, by focusing on the coupling of social systems with natural systems in rivers and thus, by viewing the socioeconomic features of river systems as being just as important as their natural characteristics. The proposed research agenda builds on interdisciplinarity and transdisciplinarity and requires the implementation of such programmes into the education of a new generation of river system scientists, managers and engineers who are aware of the transformation processes and the coupling between systems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Understanding microbial community dynamics in the alpine cryosphere is an important step toward assessing climate change impacts on these fragile ecosystems and meltwater-fed environments downstream. In this study, we analyzed microbial community composition, variation in community alpha and beta diversity, and the number of prokaryotic cells and virus-like particles (VLP) in seasonal snowpack from two consecutive years at three high altitude mountain summits along a longitudinal transect across the European Alps. Numbers of prokaryotic cells and VLP both ranged around 10 4 and 10 5 per mL of snow meltwater on average, with variation generally within one order of magnitude between sites and years. VLP-to-prokaryotic cell ratios spanned two orders of magnitude, with median values close to 1, and little variation between sites and years in the majority of cases. Estimates of microbial community alpha diversity inferred from Hill numbers revealed low contributions of common and abundant microbial taxa to the total taxon richness, and thus low community evenness. Similar to prokaryotic cell and VLP numbers, differences in alpha diversity between years and sites were generally relatively modest. In contrast, community composition displayed strong variation between sites and especially between years. Analyses of taxonomic and phylogenetic community composition showed that differences between sites within years were mainly characterized by changes in abundances of microbial taxa from similar phylogenetic clades, whereas shifts between years were due to significant phylogenetic turnover. Our findings on the spatiotemporal dynamics and magnitude of variation of microbial abundances, community diversity, and composition in surface snow may help define baseline levels to assess future impacts of climate change on the alpine cryosphere., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Fillinger, Hürkamp, Stumpp, Weber, Forster, Hausmann, Schultz and Griebler.)

Groundwater pollution threatens human and ecosystem health in many regions around the globe. Fast flow to the groundwater through focused recharge is known to transmit short-lived pollutants into carbonate aquifers, endangering the quality of groundwaters where one quarter of the world's population lives. However, the large-scale impact of such focused recharge on groundwater quality remains poorly understood. Here, we apply a continental-scale model to quantify the risk of groundwater contamination by degradable pollutants through focused recharge in the carbonate rock regions of Europe, North Africa, and the Middle East. We show that focused recharge is the primary reason for widespread rapid transport of contaminants to the groundwater. Where it occurs, the concentration of pollutants in groundwater recharge that have not yet degraded increases from <1% to around 20 to 50% of their concentrations during infiltration. Assuming realistic application rates, our simulations show that degradable pollutants like glyphosate can exceed their permissible concentrations by 3 to 19 times when reaching the groundwater. Our results are supported by independent estimates of young water fractions at 78 carbonate rock springs over Europe and a dataset of observed glyphosate concentrations in the groundwater. They imply that in times of continuing and increasing industrial and agricultural productivity, focused recharge may result in an underestimated and widespread risk to usable groundwater volumes., Competing Interests: The authors declare no competing interest.

The modeling of thick vadose zones is particularly challenging because of difficulties in collecting a variety of measured sediment properties, which are required for parameterizing the model. Some models rely on synthetic data, whereas others are simplified by running as homogeneous sediment domains and relying on a single set of sediment properties. Few studies have simulated flow processes through a thick vadose zone using real and comprehensive data sets comprising multiple measurements. Here, we develop a flow model for a 7-m-thick vadose zone. This model, combining the numerical codes CTRAN/W with SEEP/W, includes the measured sediment hydraulic properties of the investigated vadose zone and incorporates the actual climate and subsurface conditions of the study site (precipitations, water-table elevations, and stable isotope data). The model is calibrated by fitting the simulated and measured vertical profiles of water content. Our flow model calculates a transit time of 1 year for the travel of water through the 7-m vadose zone; this estimate matches stable isotope-based results obtained previously for this site. A homogeneous sediment domain flow model, which considers only a single set of sediment properties, produces a transit time that is approximately half the duration of that of the heterogeneous flow model. This difference highlights the importance of assuming heterogeneous material within models of thick vadose zones and testifies to the advantage gained when using real sediment hydraulic properties to parametrize a flow model., (© 2020, National Ground Water Association.)

Pharmaceutically active compounds have increasingly been detected in groundwater worldwide. Despite constituting a risk for human health and ecosystems, their fate in the environment has still not been exhaustively investigated. This study characterizes the transport behavior of five selected pharmaceutically active compounds (antipyrine, atenolol, caffeine, carbamazepine and sulfamethoxazole) in two sediments (coarse quartz sand and sandy loam) using column experiments with long-term injection of spiked groundwater. Transport parameters were estimated using an analytical reactive transport model. When five selected compounds were injected simultaneously, transport behavior of antipyrine, carbamazepine and the antibiotic sulfamethoxazole were similar to the conservative tracer in both sediments and under varying redox conditions. Atenolol and caffeine were retarded significantly stronger in the sandy loam sediment than in the coarse quartz sand. Biodegradation of caffeine was observed in both sediments after an adaption period and depended on dissolved oxygen. The identification of biodegradation processes was supported by monitoring of intracellular adenosine triphosphate (ATP itc ) as a measure for microbial activity. ATP itc was present in varying concentrations in all sediments and was highest when biodegradation of pharmaceuticals, especially caffeine, was observed. When only caffeine and sulfamethoxazole were injected simultaneously, sulfamethoxazole was degraded while caffeine degradation was reduced. The latter seemed to be influenced by low concentrations in dissolved oxygen rather than the presence of the antibiotic sulfamethoxazole. Results of these experiments emphasize the impact on pharmaceutical sorption and (bio)degradation of sediment type and redox conditions, as well as available time for microbial adaption and the combination of pharmaceuticals that are released together into groundwater., (Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.)

This study investigates the multiple contamination sources of a coastal Mediterranean aquifer in northeastern Algeria that is bordered by two rivers and neighboring densely populated areas. Hydrogeochemical and isotopic groundwater characterization is carried out, including the analyses of major elements, water stable isotopes δ 2 H-H 2 O and δ 18 O-H 2 O, and stable isotopes of nitrate δ 15 N-NO 3 and δ 18 O-NO 3 , and then integrated into the history of land use over the study area. Groundwater nitrate concentrations ranging from 1.6 to 235 mg/L with a median value of 69 mg/L are evidence of the degradation of groundwater quality induced by anthropogenic sources. The combined of δ 15 N-NO 3 and δ 18 O-NO 3 ratios showed that nitrate in groundwater is attributable to (i) the uncontrolled development of inadequate private sanitation systems over the study area, and (ii) the unsafe application of animal manure to fertilize crops. Very active saltwater intrusion is confirmed by several hydrogeochemical indicators. Interestingly, the intrusion mechanism appears to be more complex than a direct intrusion from the Mediterranean Sea. During the high-water period, saltwater intrusion may also originate from the two rivers bordering the aquifer, via upstream migration of seawater through the river mouths. The heavier ratios in δ 2 H-H 2 O and δ 18 O-H 2 O of surface water collected from the rivers suggest that water from the Mediterranean Sea is mixing with water in the rivers. Multi-source contamination not only contributes to complex chemical reactions within the aquifer, but also contributes, via the cumulative effect of the various sources, to affecting large parts of the study area. The present study may serve as a warning to the effect that historical land-use practices may exert seriously deleterious impacts on groundwater quality and greatly limit conditions for the sustainable management of Mediterranean coastal areas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Green Roofs (GRs) have proven to be a sustainable solution to stormwater management in urban areas. To boost their adoption at the large scale, there is a need to develop numerical models, which are accurate, computationally cheap, and as complex as needed to reproduce the hydrological behavior of GRs. Alternative conceptual and mechanistic approaches have been proposed and tested, however the most appropriate level of model complexity for GRs' analysis is still unknown. To cover this scientific gap, we provide a Bayesian comprehensive perspective of GR hydrological modeling, which includes a statistically rigorous Bayesian comparison of one conceptual and multiple Richards-based mechanistic GR models, and a probabilistic assessment of the information content of different observations. The analysis of the marginal likelihoods reveals that the conceptual and the unimodal van Genuchten - Mualem models are the most appropriate parameterizations, and that further layers of model complexity are not fully supported by the measurements. In addition to that, the estimated Kullback-Leibler divergences suggest that the measured volumetric water content outperforms the measured subsurface outflow and tracer concentrations in terms of informativeness, leading to the lowest model predictive uncertainty for the simulation of water fluxes. The findings of this study represent a first step to clarify the role of model complexity in GRs' analysis, and open new perspective on GRs' model-based experimental design., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Terrestrial mud volcanoes are unique structures driven by tectonic pressure and fluids from the deep subsurface. These structures are mainly found in active tectonic zones, such as the area near the Los Bajos Fault in Trinidad. Here we report a chemical and microbiological characterization of three mud volcanoes, which included analyses of multiple liquid and solid samples from the mud volcanoes. Our study confirms previous suggestions that at least some of the mud volcano fluids are a mixture of deeper salt-rich water and surficial/precipitation water. No apparent water quality differences were found between sampling sites north and south of a major geological fault line. Microbiological analyses revealed diverse communities, both aerobic and anaerobic, including sulfate reducers, methanogens, carbon dioxide fixing and denitrifying bacteria. Several identified species were halophilic and likely derived from the deeper salt-rich subsurface water, while we also cultivated pathogenic species from the Vibrionaceae, Enterobacteriaceae, Shewanellaceae, and Clostridiaceae. These microorganisms were likely introduced into the mud volcano fluids both from surface water or shallow ground-water, and perhaps to a more minor degree by rain water. The identified pathogens are a major health concern that needs to be addressed., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Despite the increasing public concern about the frequent occurrence of pharmaceuticals in the water bodies, their transport and fate are not yet well known; in particular in groundwater. In this laboratory study, batch experiments were conducted to investigate the sorption behaviour of selected pharmaceuticals. The choice of compounds was motivated by their chemical properties as well as by their occurrence at the relevant field sites. It included: antipyrine, atenolol, caffeine, carbamazepine, ciprofloxacin, diclofenac, ketoprofen, ofloxacin, and sulfamethoxazole. Sorption behaviour has already been investigated for some of these compounds (e.g. carbamazepine), but for the others (e.g. antipyrine, ketoprofen), extensive studies are missing so far. For the experiments, artificial and actual aquifer materials from complementary field sites were selected: technical coarse quartz sand and sediments from alluvial Vistrenque Aquifer, France (sandy loam), and fluvio-deltaic Baix Fluvià Aquifer, Spain (sandy clay loam, clay, and medium sand). In these field sites occurrence of pharmaceuticals in groundwater was previously stated, and the presented laboratory experiments were complementary to the field investigations. Five concentration steps for determining the sorption isotherms were investigated. Correlation analysis showed dependencies of K-coefficients of individual compounds and sediment properties; however, no clear, universal patterns for all compounds were found. Batch experiments suggest that sorption behaviour was governed by compound-specific properties rather than by sediment properties. These results contribute to the understanding sorption behaviour of pharmaceuticals in heterogeneous sediments, although some inconsistencies were revealed between laboratory scale results and field scale observations., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Fate and transport of solutes in heterogeneous porous media is largely affected by diffusive mass exchange between mobile and immobile water zones. Since it is difficult to directly measure and determine the effect in the aquifers, multi-tracer experiments in combination with mathematical modeling are used to obtain quantitative information about unknown system parameters such as the effective mobile and immobile porosity, and the diffusive mass exchange between mobile and immobile water zones. The Single Fissure Dispersion Model (SFDM) describing nonreactive transport of solutes in saturated dual-porosity media, has been employed as a modeling approach to explain dual-porosity experiments in the field and laboratory (column experiments). SFDM optimization with conventional methods of minimization was immensely difficult due to its complex analytical form. Thus, previous studies used a trial and error procedure to fit it to the experimental observations. In this study, a rigorous optimization technique based on the newly developed scatter search method is presented that automatically minimizes the SFDM to find the optimal values of the hydrogeologically related parameters. The new program (OptSFDM) is accompanied with an easy-to-use graphical user interface (GUI) that is flexible and fully integrated. The program usability is showcased by a few, previously presented experimental case studies, and compared against the currently available, trial-and-error based, command-line executable, SFDM code., (Copyright © 2019 Elsevier B.V. All rights reserved.)

In 2014 reclaimed water (RW) started to be used for agricultural irrigation of 200 ha in Maneadero Valley, Baja California, Mexico. The RW had relatively high electrical conductivity (4-5 dS m -1 ), and K + , Mg 2+ , PO 4 3- and Cl - concentrations relative to irrigation guidelines. However, no accumulation of these elements, nor total nitrogen, was observed in the studied soils. The main effect of RW irrigation was the downcore leaching of Cl - and Na + below 120 cm, causing an increase in electrical conductivity in the lower soil horizons. Al, Fe and Zn showed a twofold to eightfold accumulation in the top soil layer after 2 years. Moderate to severe chemical clogging of dripping systems and potential biological clogging are likely to occur. Mexican national guidelines for the reuse of RW do not take into account specific variables that could change the quality of agricultural soil or lead to clogging of drip irrigation systems.

Hydrated, magnesium-rich minerals and subglacial brines exist on the martian surface, so the habitability of high-Mg 2+ environments on Earth has extraterrestrial (as well as terrestrial) implications. Here, we report the discovery of a MgCl 2 -dominated (4.72 M) brine lake on the floor of the Mediterranean Ridge that underlies a 3500-m water column, and name it Lake Hephaestus. Stable isotope analyses indicated that the Hephaestus brine is derived from interactions between ancient bishofite-enriched evaporites and subsurface fluids. Analyses of sediment pore waters indicated that the Hephaestus depression had contained the MgCl 2 brine for a remarkably short period; only 700 years. Lake Hephaestus is, therefore, the youngest among currently known submarine athalassohaline brine lakes on Earth. Due to its biologically hostile properties (low water-activity and extreme chaotropicity), the Hephaestus brine is devoid of life. By contrast, the seawater-Hephaestus brine interface has been shown to act as refuge for extremely halophilic and magnesium-adapted stratified communities of microbes, even at MgCl 2 concentrations that approach the water-activity limit for life (0.653).

In recent years, constructed wetland systems have become into focus as means of cost-efficient organic contaminant management. Wetland systems provide a highly reactive environment in which several removal pathways of organic chemicals may be present at the same time; however, specific elimination processes and hydraulic conditions are usually separately investigated and thus not fully understood. The flow system in a three dimensional pilot-scale horizontal subsurface constructed wetland was investigated applying a multi-tracer test combined with a mathematical model to evaluate the flow and transport processes. The results indicate the existence of a multiple flow system with two distinct flow paths through the gravel bed and a preferential flow at the bottom transporting 68% of tracer mass resulting from the inflow design of the model wetland system. There the removal of main contaminant chlorobenzene was up to 52% based on different calculation approaches. Determined retention times in the range of 22d to 32.5d the wetland has a heterogeneous flow pattern. Differences between simulated and measured tracer concentrations in the upper sediment indicate diffusion dominated processes due to stagnant water zones. The tracer study combining experimental evaluation with mathematical modeling demonstrated the complexity of flow and transport processes in the constructed wetlands which need to be taken into account during interpretation of the determining attenuation processes., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Hydrochemical solute concentrations in the shallow subsurface can be spatially highly variable within small scales, particularly at interfaces. However, most monitoring systems fail to capture these small scale variations. Within this study, we developed a high resolution multilevel well (HR-MLW) with which we monitored water across the interface of the unsaturated and saturated zone with a vertical resolution of 0.05-0.5 m. We installed three of these 4 m deep HR-MLWs in the riparian zone of a third-order river and analyzed for hydrochemical parameters and stable water isotopes. The results showed three distinct vertical zones (unsaturated zone, upper saturated zone, lower saturated zone) within the alluvial aquifer. A 2 m thick layer influenced by river water (upper saturated zone) was not captured by existing monitoring wells with higher screen length. Hydrochemical data (isotopes, total ions) were consistent in all HR-MLWs and showed similar variation over time emphasizing the reliability of the installed monitoring system. Further, the depths zones were also reflected in the NO 3 -N concentrations; with high spatial variabilities between the three wells. The zonation was constant over time, with seasonal variability in the upper saturated zone due to the influence of river water. This study highlights the use of high resolution monitoring for identifying the spatial and temporal variability of hydrochemical parameters present in many aquifer systems. Possible applications range from riparian zones, agricultural field sites to contaminated site studies, wherever an improved understanding of biogeochemical turnover processes is necessary.

Microbial communities are the driving force behind the degradation of contaminants like aromatic hydrocarbons in groundwater ecosystems. However, little is known about the response of native microbial communities to contamination in pristine environments as well as their potential to recover from a contamination event. Here, we used an indoor aquifer mesocosm filled with sandy quaternary calciferous sediment that was continuously fed with pristine groundwater to study the response, resistance and resilience of microbial communities to toluene contamination over a period of almost two years, comprising 132days of toluene exposure followed by nearly 600days of recovery. We observed an unexpectedly high intrinsic potential for toluene degradation, starting within the first two weeks after the first exposure. The contamination led to a shift from oxic to anoxic, primarily nitrate-reducing conditions as well as marked cell growth inside the contaminant plume. Depth-resolved community fingerprinting revealed a low resistance of the native microbial community to the perturbation induced by the exposure to toluene. Distinct populations that were dominated by a small number of operational taxonomic units (OTUs) rapidly emerged inside the plume and at the plume fringes, partially replacing the original community. During the recovery period physico-chemical conditions were restored to the pristine state within about 35days, whereas the recovery of the biological parameters was much slower and the community composition inside the former plume area had not recovered to the original state by the end of the experiment. These results demonstrate the low resilience of sediment-associated groundwater microbial communities to organic pollution and underline that recovery of groundwater ecosystems cannot be assessed solely by physico-chemical parameters., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Contaminants like nitroaromatic compounds can be degraded in the subsurface to similar or even more toxic metabolites. Degradation or transformation rates are dependent on physical, chemical and biological properties which can be different in sedimentological layers or other heterogeneous structures of aquifers. Sediments with low hydraulic conductivity can even consist of immobile water. These regions are only accessible by diffusion. Most modelling approaches accounting for immobile water regions focused on the mathematical description of the transport and decay of the parent compound. The objective of this study was to develop an analytical model to quantify the transport and formation of a metabolite in dual-porosity media describing the exchange between mobile and immobile water regions based on the metabolite's diffusion coefficient. Column experiments with a well-defined immobile water region were performed under anoxic conditions at three different water flow velocities. The model compound 4-Cl-nitrobenzene was reduced to 4-Cl-aniline (4-Cl-An) by surface-bound Fe (II) species within the immobile water region. Transport and formation of the metabolite were quantified with a modified solution of the single fissure dispersion model assuming additionally for the region with immobile water first-order metabolite production, irreversible sorption and an instantaneous equilibrium sorption. The number of unknown fitting parameters was reduced to two (sorption rate and retardation factor) by stepwise parameter estimation using tracer and parent compound data. Experimental results of the metabolite for each water flow velocity were successfully described with a first-order production term (λ prod  = 1.51 ± 0.08 h -1 ), retardation factor (R im  = 2.94 ± 0.45) and first-order irreversible sorption rate (K im  = 0.39 ± 0.16 h -1 ) within the immobile water region. Model results supported that 4-Cl-An was formed within the immobile water region. 4-Cl-An sorbed instantaneously onto the clay matrix while a fraction was irreversibly sorbed. Experimental results and the provided analytical solution help to improve the understanding about reactive transport and the formation of metabolites in dual-porosity media.

Nitroaromatic compounds (NACs) are reduced by structural or surface bound Fe (II) species under anaerobic conditions in the subsurface. This reaction preferentially occurs on clay minerals which are mainly present in areas with low hydraulic conductivity containing nearly immobile water. Diffusion is the dominating transport process in these zones. Due to the complexity in such heterogeneous systems, the mathematical prediction of reactive solute transport taking into account diffusive mass exchange into immobile water regions still remains challenging. Therefore, the influence of immobile water regions on the fate of 4-Cl-Nitrobenzene (4-Cl-Nb) was quantified in dual-porosity column experiments at three different mean transit times under saturated anaerobic conditions in the presence of soluble Fe (II). A multi-tracer approach and a Single Fissure Dispersion Model (SFDM) were used to estimate input parameter to further model the transport of 4-Cl-Nb. Reactive solute transport of 4-Cl-Nb was quantified considering instantaneous sorption on to the clay matrix and a reduction within the immobile water region following first-order kinetics. The experimental results indicated that sorption onto the clay matrix enhanced the mass exchange of 4-Cl-Nb into immobile water region compared to nonreactive solutes. At the same time the abiotic reduction of 4-Cl-Nb limited the process of back diffusion to mobile water regions. Fitted retardation factors (Rim=4.62±0.68) and decay rates (k=1.51±0.08h(-1)) were independent on tested flow velocities. Findings of this study can advance the understanding on the fate of NACs in the subsurface which is essential for prediction of reactive solute transport at field scale., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Recently, laser-based water stable isotope spectrometers have become popular as they enable previously impossible approaches of environmental observations. Consequently, they have been subjected to increasingly heterogeneous atmospheric conditions. However, there is still a severe lack of data on the impact of nonstandardized gas matrices on analyzer performances. Against this background, we investigated the influence of changing proportions of N2, O2, and CO2 in the carrier gas on the isotope measurements of a typical laser-based water stable isotope analyzer (Picarro L2120-i). We combined environmentally relevant mixtures of N2, O2, and CO2 with referenced, flash-evaporated water and found that isotope readings of the same water were altered by up to +14.57‰ for δ(18)O and -35.9‰ for δ(2)H. All tested relationships between carrier gas changes and respective isotope readings were strongly linearly correlated (R(2) > 0.99). Furthermore, an analyzer-measured variable allowed for reliable postcorrection of the biased isotope readings, which we additionally tested on field data. Our findings are of importance for environmental data obtained by analyzers based on the same technology. They are relevant for assays where inconsistent gas matrices or a mismatch in this regard between unknown and reference analyses cannot be excluded, which is in particular common when investigating the soil-vegetation-atmosphere continuum.

Compound-specific isotope analysis (CSIA) receives increasing interest for its ability to detect natural degradation of pesticides and pharmaceuticals. Despite recent laboratory studies, CSIA investigations of such micropollutants in the environment are still rare. To explore the certainty of information obtainable by CSIA in a near-environmental setting, a pulse of the pesticide bentazone, the pesticide metabolite 2,6-dichlorobenzamide (BAM), and the pharmaceuticals diclofenac and ibuprofen was released into a mesoscale aquifer with quasi-two-dimensional flow. Concentration breakthrough curves (BTC) of BAM and ibuprofen demonstrated neither degradation nor sorption. Bentazone was transformed but did not sorb significantly, whereas diclofenac showed both degradation and sorption. Carbon and nitrogen CSIA could be accomplished in similar concentrations as for "traditional" priority pollutants (low μg/L range), however, at the cost of uncertainties (0.4-0.5‰ (carbon), 1‰ (nitrogen)). Nonetheless, invariant carbon and nitrogen isotope values confirmed that BAM was neither degraded nor sorbed, while significant enrichment of (13)C and in particular (15)N corroborated transformation of diclofenac and bentazone. Retardation of diclofenac was reflected in additional (15)N sorption isotope effects, whereas isotope fractionation of transverse dispersion could not be identified. These results provide a benchmark on the performance of CSIA to monitor the reactivity of micropollutants in aquifers and may guide future efforts to accomplish CSIA at even lower concentrations (ng/L range).