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Brominated organic compounds such as 1,2-dibromoethane (1,2-DBA) are highly toxic groundwater contaminants. Multi-element compound-specific isotope analysis bears the potential to elucidate the biodegradation pathways of 1,2-DBA in the environment, which is crucial information to assess its fate in contaminated sites. This study investigates for the first time dual C−Br isotope fractionation during in vivo biodegradation of 1,2-DBA by two anaerobic enrichment cultures containing organohaliderespiring bacteria (i.e., either Dehalococcoides or Dehalogenimonas). Different εbulk C values (−1.8 ± 0.2 and −19.2 ± 3.5¿, respectively) were obtained, whereas their respective εbulk Br values were lower and similar to each other (−1.22 ± 0.08 and −1.2 ± 0.5¿), leading to distinctly different trends (ΛC−Br = Δδ13C/Δδ81Br ≈ εbulkC /εbulkBr ) in a dual C−Br isotope plot (1.4 ± 0.2 and 12 ± 4, respectively). These results suggest the occurrence of different underlying reaction mechanisms during enzymatic 1,2-DBA transformation, that is, concerted dihaloelimination and nucleophilic substitution (SN2-reaction). The strongly pathway-dependent ΛC−Br values illustrate the potential of this approach to elucidate the reaction mechanism of 1,2-DBA in the field and to select appropriate εbulkC values for quantification of biodegradation. The results of this study provide valuable information for future biodegradation studies of 1,2-DBA in contaminated sites.

Alpine areas play a major role in water supply in downstream valleys by releasing water during warm and dry periods. However, the hydrogeology of alpine catchments, which are particularly exposed to the effects of climate change, is currently not well understood. Increasing our knowledge of alpine hydrogeological processes is thus of considerable importance for any forward‐looking hydrological investigations in alpine areas. The objectives of this study are to quantify seasonal groundwater storage variations in a small Swiss alpine catchment and to evaluate the capabilities of time‐lapse gravimetry in the identification of zones of high groundwater storage fluctuations. Time‐lapse gravimetric measurements enable rapid localisation of zones of dynamic groundwater storage changes and help to highlight aquifers with a higher storage decrease. Temperature sensors enable measurement of the temporal trend in stream and spring drying in the post‐snowmelt period. Stable isotope measurements allow us to identify the origin of surface water exiting the catchment. The results improve our comprehension of a conceptual schema highlighting two different hydrogeological systems: (a) a shallow, rapidly depleted one fed directly by snowmelt and (b) a deeper one, with a slower recession, fed by main recharge during peak snowmelt and emerging at the lower part of the catchment below the talus and moraine of the catchment where bedrock is exposed. These dynamics confirm the high variability of storage in the talus and moraine aquifers and highlight the dominant role of Quaternary deposits and their connectivity to store water over seasonal and multi‐year time‐scales. The mechanisms explaining the importance of Quaternary deposits are the combination of moraine and talus with different permeabilities allowing the storage of sufficient quantities of water permitting continuous release during drier periods of the year.

Roads in sloping fens constitute a hydraulic barrier for surface and subsurface flow. This can lead to the drying out of downslope areas of the sloping fen as well as gully erosion. Different types of road construction have been proposed to limit the negative implications of roads on flow dynamics. However, so far, no systematic analysis of their effectiveness has been carried out. This study presents an assessment of the hydrogeological impact of three types of road structures in semi-alpine, sloping fens in Switzerland. Our analysis is based on a combination of field measurements and fully integrated, physically based modeling. In the field approach, the influence of roads was examined using tracer tests in which the area upslope of the road was sprinkled with a saline solution. The spatial distribution of electrical conductivity downslope provided a qualitative assessment of the flow paths and, thus, the implications of the road structures on subsurface flow. A quantitative albeit not site-specific assessment was carried out using fully coupled numerical models jointly simulating surface and subsurface flow processes. The different road types were implemented and their influence on flow dynamics was assessed for a wide range of slopes and different hydraulic conductivities of the soil. The models are based on homogenous soil conditions, allowing for a relative ranking of the impact of the road types. For all cases analyzed in the field and simulated using the numerical models, roads designed with an L drain (i.e., collecting water upslope and releasing it in a concentrated manner downslope) constitute the largest perturbations in terms of flow dynamics. The other road structures investigated were found to have less impact. The developed methodologies and results can be used for the planning of future road projects in sloping fens.

Multielement isotope fractionation studies to assess pollutant transformation are well-established for point-source pollution but are only emerging for diffuse pollution by micropollutants like pesticides. Specifically, chlorine isotope fractionation is hardly explored but promising, because many pesticides contain only few chlorine atoms so that "undiluted" position-specific Cl isotope effects can be expected in compound-average data. This study explored combined Cl, N, and C isotope fractionation to sensitively detect biotic and abiotic transformation of the widespread herbicides and groundwater contaminants acetochlor, metolachlor, and atrazine. For chloroacetanilides, abiotic hydrolysis pathways studied under acidic, neutral, and alkaline conditions as well as biodegradation in two soils resulted in pronounced Cl isotope fractionation (εCl from -5.0 ± 2.3 to -6.5 ± 0.7‰). The characteristic dual C-Cl isotope fractionation patterns (ΛC-Cl from 0.39 ± 0.15 to 0.67 ± 0.08) reveal that Cl isotope analysis provides a robust indicator of chloroacetanilide degradation. For atrazine, distinct ΛC-Cl values were observed for abiotic hydrolysis (7.4 ± 1.9) compared to previous reports for biotic hydrolysis and oxidative dealkylation (1.7 ± 0.9 and 0.6 ± 0.1, respectively). The 3D isotope approach allowed differentiating transformations that would not be distinguishable based on C and N isotope data alone. This first data set on Cl isotope fractionation in chloroacetanilides, together with new data in atrazine degradation, highlights the potential of using compound-specific chlorine isotope analysis for studying in situ pesticide degradation.

Zusammenfassung: Bei der Untersuchung von belasteten Standorten stellt sich häufig die Frage, ob unterschiedliche Schadstoffquellen zu einer Grundwasserbelastung beitragen. Chlorierte Kohlenwasserstoffe (CKWs) von verschiedenen Produzenten können unterschiedliche Kohlenstoff- und Chlor-Isotopensignaturen (C-Cl-Isotopensignaturen) aufweisen, was eine Differenzierung von CKW-Quellen ermöglicht. Dazu muss aber die Variabilität der Isotopensignatur bekannt sein. Da die ursprünglich verwendeten Lösungsmittel meist nicht mehr zugänglich sind, werden in dieser Studie die Perchlorethen (PCE) C-Cl-Isotopensignaturen an belasteten Standorten in der Schweiz untersucht. Zehn Standorte wurden ausgewählt, die verschiedene Branchen und Landesregionen abdecken. Eine Variabilität der C-Cl-Isotopensignaturen zwischen einigen Standorten bestätigt die Anwendbarkeit dieser Methode zur Quellendifferenzierung. Gewisse Standorte zeigen jedoch ähnliche Isotopensignaturen. Deswegen ist der Erfolg dieser Identifikationsmethode standortspezifisch. Außerdem ist die Variabilität geringer als publizierte Isotopensignaturen von nordamerikanischen Herstellern. Es hat sich außerdem bestätigt, dass der biologische Abbau von PCE durch reduktive Dechlorierung bei der Identifikation der Kontaminationsquellen berücksichtigt werden muss.

Pesticide metabolites are frequently detected in groundwater at concentrations often exceeding those of their parent pesticides. A well-known example is the metabolites of chlorothalonil, a non-systematic, broad spectrum fungicide. Some of the chlorothalonil metabolites occur frequently and at elevated concentrations in groundwater, which is why the use of chlorothalonil was recently banned in the European Union. To estimate the long-term evolution of the concentration of the chlorothalonil metabolites in groundwater after this ban, it is important to know if metabolite residues in soil and unsaturated zone can affect the concentrations in groundwater. We developed and validated a method for the determination of 5 chlorothalonil metabolites in soil (R471811, R417888, SYN507900, SYN548580 and R611968), including those which are frequently detected in groundwater. The developed protocols, based on a solid phase extraction approach (for R471811, R417888, SYN507900, SYN548580) or a QuEChERS approach (for R611968) followed by UHPLC-MS/MS analysis, provided excellent sensitivity (LOQ of 0.5 µg/kg for all metabolites), precision (RSD

Until now, the magnitude of isotopologue fractionation of organic compounds due to aqueous-phase diffusion has been quantified only experimentally. This study aims to determine the extent of aqueou...

In the geosciences, inverse problems, wherein observations corresponding to model outputs are known and model parameters are unknown, are commonplace. Many of these problems involve coupled physical, chemical, and other processes that can be modelled using forward finite-element models. Here, we present a novel interface, COMPEST, that connects the parameter estimation and uncertainty analysis package, PEST, with the finite-element modelling package, COMSOL Multiphysics. To demonstrate some of the capabilities of this approach, we also develop and present a novel model for the degradation and transport of chlorohydrocarbons in low-permeability units. This model integrates isotopic fractionation arising from degradation and diffusion. Three implementations of this model with increasing complexity are used to demonstrate the functionality of the developed interface. This linkage provides a means for parameter estimation, uncertainty analysis, and singular value decomposition to gain insight into the behaviour, identifiability, and interdependence of the various parameters in the model. COMPEST is written so as to be suited to a wide range of scientific and engineering applications and thus can be used to link any COMSOL model with PEST. This enables the use of advanced inverse modelling techniques previously unavailable to COMSOL users.

Hydrologists and hydrogeologists both study the flux and storage of water with the numerous interactions and feedback mechanisms of surface water and groundwater. Traditionally however, focus, models and scales of the studies differ. In this commentary, situations are illustrated where boundary conditions that each discipline assumes, preserves and actively uses, can and have to be overcome. These situations occur when the domain of one discipline cannot be separated from the other one because of existing interaction and feedback mechanisms at the boundaries. Highlighted are especially these boundary conditions, where closer collaboration between catchment hydrologists and hydrogeologists would be most useful. Often such collaborations would be relatively straight-forward and rather requiring an increased awareness than novel methods.

For the interpretation of stable isotope ratio trends in saturated geochemical systems, the magnitude of aqueous phase diffusion-induced isotope fractionation needs to be known. This study reviews how five diffusion models (Fick, Maxwell-Stefan, Einstein, Langevin, Mode-Coupling Theory Analysis (MCTA) of diffusion) predict isotope fractionation due to aqueous phase diffusion and compares them with experimental results. The reviewed diffusion models were not consistent regarding the prediction of the mass (m) dependency of the aqueous phase diffusion coefficient (D). The predictions range from a square root power law (D ∝ m−0.5) to an opposite mass dependency of D (D ∝ mβ). Experimental studies exhibited consistently a weak power law mass dependency of the diffusion coefficient (D ∝ m−β with β

Global methane (CH4) emissions are becoming increasingly important due to the contribution of CH4 to global warming. Leaking oil and gas wells can lead to subsurface CH4 gas migration (GM), which can cause both aquifer contamination and atmospheric emissions. Despite the need to identify and quantify GM at oil and gas well pads, effective and reliable monitoring techniques are lacking. In this field study, we used CH4 and carbon dioxide (CO2) efflux measurements together with soil gas stable carbon isotopic signatures to identify the occurrence and to characterize the spatio-temporal migration of fugitive gas across 17 selected well pads in Northeastern British Columbia, Canada. At 13 of these sites, operators had previously reported the occurrence of GM; however, subsequent inspections based on visual, olfactory or auditory evidence only identified GM at two of these sites. Using the soil gas efflux method, evidence for GM was found at 15 of the 17 well pads with CH4 and CO2 effluxes ranging from 0.017 to 180 μmol m−2 s−1(0.024 to 250 g CH4 m−2 d−1) and 0.50 to 32 μmol m−2 s−1 (1.9 to 122 g CO2 m−2 d−1), respectively. Stable carbon isotopic composition was assessed at 10 of the 17 well pads with 9 well pads showing evidence of GM. The isotopic values indicated that CH4 in soil gas was from the same origin as CH4 in the surface casing vent flow gas. There was no correlation between CH4 effluxes and the distance from the well head; an equal portion of elevated effluxes were detected >10 m from the well head as were detected

The effects of contaminant sources removal in 2005 (i.e. barrels, tank, pit and wastewater pipe sources) on carbon tetrachloride (CT) and chloroform (CF) concentration in groundwater were assessed at several areas of a fractured multi-contaminant aquifer (Odena, Spain) over a long-term period (2010–2014). Changes in redox conditions, in these chlorinated methanes (CMs) concentration and in their carbon isotopic compositions (δ13C) were monitored in multilevel wells. δ13C values from these wells were compared to those obtained from sources (barrels, tank and pit before their removal, 2002–2005) and to commercial solvents values in literature. Additionally, CMs natural attenuation processes were identified by C-Cl isotope slopes (Λ). Analyses revealed the downstream migration of the pollutant focus and an efficient removal of DNAPLs in the pit source's influence area. However, the removal of the contaminated soil from former tank and wastewater pipe was incomplete as leaching from unsaturated zone was proved, evidencing these areas are still active sources. Nevertheless, significant CMs degradation was detected close to all sources and Λ values pointed to different reactions. For CT in the tank area, Λ value fitted with hydrogenolysis pathway although other possible reduction processes were also uncovered. Near the wastewater pipe area, CT thiolytic reduction combined with hydrogenolysis was derived. The highest CT degradation extent accounted for these areas was 72 ± 11% and 84 ± 6%, respectively. For CF, the Λ value in the pit source's area was consistent with oxidation and/or with transport of CF affected by alkaline hydrolysis from upstream interception trenches. In contrast, isotope data evidenced CF reduction in the tank and wastewater pipe influence areas, although the observed Λ slightly deviates from the reference values, likely due to the continuous leaching of CF degraded in the non-saturated zone by a mechanism different from reduction.

Deterioration of groundwater quality due to nitrate loss from intensive agricultural systems can only be mitigated if methods for in-situ monitoring of nitrate leaching under active farmers’ fields are available. In this study, three methods were used in parallel to evaluate their spatial and temporal differences, namely ion-exchange resin-based Self-Integrating Accumulators (SIA), soil coring for extraction of mineral N (Nmin) from 0 to 90 cm in Mid-October (pre-winter) and Mid-February (post-winter), and Suction Cups (SCs) complemented by a HYDRUS 1D model. The monitoring, conducted from 2017 to 2020 in the Gäu Valley in the Swiss Central Plateau, covered four agricultural fields. The crop rotations included grass-clover leys, canola, silage maize and winter cereals. The monthly resolution of SC samples allowed identifying a seasonal pattern, with a nitrate concentration build-up during autumn and peaks in winter, caused by elevated water percolation to deeper soil layers in this period. Using simulated water percolation values, SC concentrations were converted into fluxes. SCs sampled 30% less N-losses on average compared to SIA, which collect also the wide macropore and preferential flows. The difference between Nmin content in autumn and spring was greater than nitrate leaching measured with either SIA or SCs. This observation indicates that autumn Nmin was depleted not only by leaching but also by plant and microbial N uptake and gaseous losses. The positive correlation between autumn Nmin content and leaching fluxes determined by either SCs or SIA suggests autumn Nmin as a useful relative but not absolute indicator for nitrate leaching. In conclusion, all three monitoring techniques are suited to indicate N leaching but represent different transport and cycling processes and vary in spatio-temporal resolution. The choice of monitoring method mainly depends (1) on the project’s goals and financial budget and (2) on the soil conditions. Long-term data, and especially the combination of methods, increase process understanding and generate knowledge beyond a pure methodological comparison. Supplementary Information The online version contains supplementary material available at 10.1007/s10661-021-09605-x.

Climate change affects both water resources and agricultural production. With rising temperatures and decreasing summer precipitation, it is expected that agricultural production will be increasingly limited by drought. Where surface- or groundwater resources are available for irrigation, an increase in water withdrawals for irrigation is to be expected. Therefore, quantitative approaches are required to anticipate and manage the expected conflicts related to increased water abstraction for irrigation. This project aims to investigate how agricultural production, water demand for irrigation, runoff and groundwater dynamics are affected by future climate change and how climate change impacts combined with changes in agricultural water use affect groundwater dynamics. To answer these research questions, a comprehensive, loosely coupled model approach was developed, combining models from three disciplines: an agricultural plant growth model, a hydrological model and a hydrogeological model. The model coupling was implemented and tested for an agricultural area located in Switzerland in which groundwater plays a significant role in providing irrigation water. Our suggested modelling approach can be easily adapted to other areas. The model results show that yield changes are driven by drought limitations and rising temperatures. However, an increase in yield may be realized with an increase in irrigation. Simulation results show that the water requirement for irrigation without climate protection (RCP8.5) could increase by 40% by the end of the century with an unchanged growing season and by up to 80% with varietal adaptations. With climate change mitigation (RCP2.6) the increase in water demand for irrigation would be limited to 7%. The increase in irrigation (+12 mm) and the summer decrease in recharge rates (~20 mm/month) with decreasing summer precipitation causes a lowering of groundwater levels (40 mm) in the area in the late summer and autumn. This impact may be accentuated by an intensification of irrigation and reduced by extensification.

Isotopic fractionation of groundwater contaminants can occur due to degradation, diffusion and sorption. Of these, only degradation has been extensively explored, yet diffusive isotopic fractionation (DIF) and sorptive isotopic fractionation (SIF) can have significant effects on the isotopic enrichment of groundwater contaminants. Understanding how to mathematically describe and model these processes is vital to the correct interpretation of compound-specific isotope analysis (CSIA) data in the field. Here, models for these physical fractionation processes are developed and described, including the definition of a sorption enrichment factor. These models are then implemented numerically using inverse and finite-element methods to investigate two scenarios, diffusion-sorption and diffusion-sorption-advection, that have been measured in the laboratory. Concentration, δ37Cl, and δ2H data from cis-dichloroethene (cDCE) and trichloroethene (TCE) are used as inputs to the models. Unknown transport parameters including diffusive fractionation exponents are determined from an inverse modelling approach. DIF is shown to have a stronger influence on chlorine isotopologues than on hydrogen isotopologues. For both cDCE and TCE, the sorption enrichment factor of chlorine is found to be negative while that of hydrogen is positive. The presented approach and results provide novel tools and insight into DIF and SIF and underline that these processes should be taken into account when using CSIA to assess contaminant fate.

Understanding the mixing between surface water and groundwater as well as groundwater travel times in vulnerable aquifers is crucial to sustaining a safe water supply. Age dating tracers used to infer apparent travel times typically refer to the entire groundwater sample. A groundwater sample, however, consists of a mixture of waters with a distribution of travel times. Age dating tracers only reflect the proportion of the water that is under the dating range of the used tracer, thus their interpretation is typically biased. Additionally, end‐member mixing models are subject to various sources of uncertainties, which are typically neglected. In this study, we introduce a new framework that untangles groundwater mixing ratios and travel times using a novel combination of in‐situ noble gas analyses. We applied this approach during a groundwater pumping test carried out in a pre‐alpine Swiss valley. First, we calculated transient mixing ratios between recently infiltrated river water and regional groundwater present in a wellfield, using helium‐4 concentrations combined with a Bayesian end‐member mixing model. Having identified the groundwater fraction of recently infiltrated river water (Frw) consequently allowed us to infer the travel times from the river to the wellfield, estimated based on radon‐222 activities of Frw. Furthermore, we compared tracer‐based estimates of Frw with results from a calibrated numerical model. We demonstrate (i) that partitioning of major water sources enables a meaningful interpretation of an age dating tracer of the water fraction of interest and (ii) that the streambed has a major control on the estimated travel times., Water Resources Research, 57 (4), ISSN:0043-1397, ISSN:1944-7973

Krypton-85 and other radioactive noble gases are widely used for groundwater dating purposes. 85Kr analysis require large volumes of water to reach the analytical requirements. Conventionally, this water is pumped to the surface to be degassed with a gas extraction system. The large pumping rate may disturb the natural flow field and requires substantial field logistics. Hence, we propose a new in-situ degassing method, in which membrane contactors are used to degas the groundwater directly in the well and gas is collected at the surface. This way, field work is facilitated, groundwater system disturbance is minimized, and the gas sample is collected at a specific depth. We demonstrate the tightness of the system regarding atmospheric air contamination for a collection times of 24 h, which is sufficient for both low-level counting and laser-based counting methods for 85Kr. The minimal borehole diameter is 7.5 cm for the prototype presented in this research but can easily be reduced to smaller diameters. In a case study, we compare the results obtained with the new passive method with those from a conventional packer setup sampling. Additionally, 3H/3He samples were collected for both sampling regimes and the dating results were compared with those from 85Kr. A good agreement between tracer ages is demonstrated and the age stratigraphy is consistent with the expected age distribution for a porous unconfined aquifer. In addition, our study emphasizes the differences between the age information sampled with various methods. In conclusion, we demonstrate that the new in situ quasi-passive method provides a more representative age stratigraphy with depth in most cases.

Over the last decade, activated carbon amendments have successfully been applied to retain chlorinated ethene subsurface contamination. The concept of this remediation technology is that activated carbon and bioamendments are injected into aquifer systems to enhance biodegradation. While the scientific basis of the technology is established, there is a need for methods to characterise and quantify the biodegradation at field scale. In this study, an integrated approach was applied to assess in situ biodegradation after the establishment of a cross sectional treatment zone in a TCE plume. The amendments were liquid activated carbon, hydrogen release donors and a Dehalococcoides containing culture. The integrated approach included spatial and temporal evaluations on flow and transport, redox conditions, contaminant concentrations, biomarker abundance and compound-specific stable isotopes. This is the first study applying isotopic and microbial techniques to assess field scale biodegradation enhanced by liquid activated carbon and bioamendments. The injection enhanced biodegradation from TCE to primarily cis-DCE. The Dehalococcoides abundances facilitated characterisation of critical zones with insufficient degradation and possible explanations. A conceptual model of isotopic data together with distribution and transport information improved process understanding; the degradation of TCE was insufficient to counteract the contaminant input by inflow into the treatment zone and desorption from the sediment. The integrated approach could be used to document and characterise the in situ degradation, and the isotopic and microbial data provided process understanding that could not have been gathered from conventional monitoring tools. However, quantification of degradation through isotope data was restricted for TCE due to isotope masking effects. The combination of various monitoring tools, applied frequently at high-resolution, with system understanding, was essential for the assessment of biodegradation in the complex, non-stationary system. Furthermore, the investigations revealed prospects for future research, which should focus on monitoring contaminant fate and microbial distribution on the sediment and the activated carbon.

Snowfall is the dominant form of precipitation in high mountainous areas and its driving melt water has an indispensable role in the hydrological cycle and groundwater recharge, particularly in karstic landscapes with high infiltration capacity. Monitoring snow cover area (SCA) and its melting process is essential for the investigation of climatic variables, hydrology, hydrogeology, and water resource management. Prodigious advances of satellite imaginary technology in the past decades made it possible to monitor spatiotemporal distribution of snow and its melting process. In this research, SCA was investigated using cloud-free images of Landsat-8 from December 2014 to June 2016 and Sentinel-2 from November 2015 to June 2016 at Lasem area (north of Iran) by normalized difference snow index. Simultaneously, the discharges of the main karstic springs were monitored over May 2015 to June 2016. The catchment subdivided into three sub-zones based on the hydrogeological characteristics and snow melting time. Fractional SCA time series within each subdomain used to develop snow melting curve in each subzone. Comparison of melting peaks between the 2014–2015 and 2015–2016 water years shows that melting shifted in average 20 days later in 2016 at north-facing subdomains. North-facing slopes show quite fast transmitting time (20–35 days) of the peak snowmelt to the springs, while the south-facing springs are more silent to the recharge pulses (70–80 days), indicating a higher degree of karstification in north-facing domains. More concentrated snowmelt in 2016 led to increasing peak flow by an average of 15% in the springs fed by north-facing domains.

Knowing how stream discharge in an ungauged catchment reacts to dry spells is a major challenge for managing water resources. The role of geology on these dynamics is poorly understood. For the Swiss Molasse basin, we therefore explored how the geology influences the groundwater contribution to stream flow during low-flow conditions. Using existing data from geological reports and maps as well as from deep boreholes, we constructed a basin-wide overview of the hydrogeological quality of the bedrock and investigated five catchments in 3D. We found that catchments with the most permeable sedimentary bedrock are least sensitive to low flows (marine sandstone, K = 10&minus, 4 to 10&minus, 5 m/s, Peff = 5&ndash, 10%). In contrast, if bedrock K is low (K &lt, 10&minus, 6 m/s), the presence of a productive Quaternary volume becomes decisive for groundwater contribution to stream flow. Limitations exist due to a restricted database for K and Peff values of the Molasse and limited information on continuation of lithologies with depth. This emphasizes the need for more hydrogeologically relevant data for the future management of water resources. Our results highlighting what lithotypes favor groundwater contribution to stream flow are valid also in other regions for the assessment of a catchment&rsquo, s sensitivity to low flows.

Desphenylchloridazon (DPC), the main metabolite of the herbicide chloridazon (CLZ), is more water soluble and persistent than CLZ and frequently detected in water bodies. When assessing DPC transformation in the environment, results can be nonconclusive if based on concentration analysis alone because estimates may be confounded by simultaneous DPC formation from CLZ. This study investigated the fate of DPC by combining concentration-based methods with compound-specific C and N stable isotope analysis (CSIA). Additionally, DPC formation and transformation processes were experimentally deconvolved in a dedicated lysimeter study considering three scenarios. First, surface application of DPC enabled studying its degradation in the absence of CLZ. Here, CSIA provided evidence of two distinct DPC transformation processes: one shows significant changes only in C-13/C-12, whereas the other involves changes in both C-13/C-12 and N-15/N-14 isotope ratios. Second, surface application of CLZ mimicked a realistic field scenario, showing that during DPC formation, C-13/C-12 ratios of DPC were depleted in C-13 relative to CLZ, while N-15/N-14 ratios remained constant. Finally, CLZ depth injection simulated preferential flow and demonstrated the importance of the topsoil for retaining DPC. The combination of the lysimeter study with CSIA enabled insights into DPC transformation in the field that are superior to those of studies of concentration trends.

Atrazine is a frequently detected groundwater contaminant. It can be microbially degraded by oxidative dealkylation or by hydrolytic dechlorination. Compound-specific isotope analysis is a powerful tool to assess its transformation. In previous work, carbon and nitrogen isotope effects were found to reflect these different transformation pathways. However, chlorine isotope fractionation could be a particularly sensitive indicator of natural transformation since chlorine isotope effects are fully represented in the molecular average while carbon and nitrogen isotope effects are diluted by non-reacting atoms. Therefore, this study explored chlorine isotope effects during atrazine hydrolysis with Arthrobacter aurescens TC1 and oxidative dealkylation with Rhodococcus sp. NI86/21. Dual element isotope slopes of chlorine vs. carbon isotope fractionation (?ArthroCl/C = 1.7 +/- 0.9 vs. ?RhodoCl/C = 0.6 +/- 0.1) and chlorine vs. nitrogen isotope fractionation (?ArthroCl/N = -1.2 +/- 0.7 vs. ?RhodoCl/N = 0.4 +/- 0.2) provided reliable indicators of different pathways. Observed chlorine isotope effects in oxidative dealkylation (epsilon(Cl) = -4.3 +/- 1.8 parts per thousand) were surprisingly large, whereas in hydrolysis (epsilon(Cl) = -1.4 +/- 0.6 parts per thousand) they were small, indicating that C-Cl bond cleavage was not the rate-determining step. This demonstrates the importance of constraining expected isotope effects of new elements before using the approach in the field. Overall, the triple element isotope information brought forward here enables a more reliable identification of atrazine sources and degradation pathways.

Relating stream dynamics to catchment properties is essential to anticipate the influence of changing environmental conditions and to predict flows of ungauged rivers. Although the importance of subsurface processes in catchment hydrology is widely acknowledged, geological characteristics are rarely explicitly included in studies assessing physiographic controls on catchment dynamics. In this investigation of 22 catchments of the Swiss Plateau and Prealpes, we use a simple linear regression approach to analyze the relationship between streamflow indicators and various geological and hydrogeological properties of the bedrock and quaternary deposits, along with meteorological, soil, land use and topographical characteristics. We use long-term discharge percentiles, as well as dimensionless flow duration curves (FDC, standardized by long-term mean discharge) that allow to evaluate the catchment response to climate forcing. While climate conditions dominate the high to medium discharge percentiles (Q5–Q50), the capacity of the catchments to buffer the meteorological forcing can only be attributed to geological characteristics. The sandstone proportion in the catchments explains 54% of the variance of both extremities of the dimensionless FDC (Q5/Qmean and Q95/Qmean) and productive quaternary deposits are responsible of 55% resp. 58% of the variance of the two ratios. Examining the hydrogeological characteristics of both bedrock and quaternary lithologies considerably improves the understanding of catchment dynamics.

Stable isotopes oxygen-18 (δ18O) and deuterium (δD) and the hydrochemistry of the main springs of well-developed karst aquifers in Lar watershed in the Haraz basin, northern Iran, were investigated. Water samples were collected in the recession period for analysis of major ions and the stable isotopes. Predominant hydrochemical types of main karst springs samples were Ca2+ – Mg2+ – $$ {\mathrm{HCO}}_3^{-} $$ type. The hydrochemical composition of the karst springs is dominated by Ca2+, Mg2+, $$ {\mathrm{HCO}}_3^{-} $$ and $$ {\mathrm{SO}}_4^{2-} $$ ions. The hydrochemical results indicate that dissolution of carbonate rocks (limestone and dolomite) governs the major ion concentrations, with minor effects of silicate weathering, ion exchange and precipitation effects on concentrations of Na+, Cl−, K+, and $$ {\mathrm{SO}}_4^{2-} $$ . In the northern parts of the study area, hydrothermal fluid affects the chemistry of the draining springs. The isotopic content ranges from −43.8 to −52.9‰ and from −7.09 to −8.97‰ for δD and δ18O, respectively. The karst spring samples lie above the local meteoric water line, similar to the δ18O and δD signature of the snowpack samples, and have small spatial and temporal variability. The similarity of the isotopic composition of the springs to the snowpack suggests that the dominant recharge is by snowmelt water.

Dense non-aqueous phase liquids (DNAPLs) can easily penetrate into karst aquifers, may accumulate in the sediment-filled depressions of conduits, and constitute a long-term source of groundwater contamination. In contrast to porous media, DNAPL dissolution in karst settings has not received much attention. This study investigated the dissolution of sediment-entrapped DNAPLs using a laboratory analogue model corresponding to a syphon structure partly filled by sediments and using numerical simulations. The mass transfer rate increased with an increasing conduit Reynolds number (Re) and increasing conduit angle, as both of these factors led to a more intense hyporheic flow in the sediments. In addition, the mass transfer rate increased linearly with the average flow velocity in the sediment and was higher than expected for an infinite porous medium considered as a reference case. The enhanced mass transfer rate can be explained by the curved streamlines associated with hyporheic flow, which transport solutes away from the DNAPL interface. This study demonstrates that hyporheic flow through karst sediment is a possible mechanism for the mobilization of DNAPL components and might explain the persistent occurrence of such contaminants in karst springs.

This modeling study aims to investigate how reactive processes in aquitards impact plume persistence in adjacent aquifers. For that purpose the migration of a trichloroethene (TCE) plume in an aquifer originating from dense non-aqueous phase liquid (DNAPL) source dissolution and back-diffusion from an underlying reactive aquitard was simulated in a 2D-numerical model. Two aquitard degradation scenarios were modeled considering one-step degradation from TCE to cis-dichloroethene (cDCE): a uniform (constant degradation with aquitard depth) and a non-uniform scenario (decreasing degradation with aquitard depth) and were compared with a no-degradation scenario. In the no-degradation scenario, a long-term TCE tailing above the Maximum Contaminant Level (MCL) caused by back-diffusion after source removal was observed. In contrast, in the aquitard degradation scenarios, TCE back-diffusion periods were shorter, whereby the extent of back-diffusion reduction depended on the aquitard degradation depth and the rate. For high degradation rates (half-life: 30-80days), an aquitard degradation depth greater than 65cm prevented TCE plume persistence after source removal but generated a long-term tailing above the MCL for the produced cDCE. For slow degradation rates (half-life:200days), TCE was only partially degraded after source removal, independent of the aquitard degradation depth, leading to a long-term dual contamination of the aquifer by cDCE and TCE. A sudden enrichment of

Groundwater quality in many regions with intense agriculture has deteriorated due to the leaching of nitrate and other agricultural pollutants. Modified agricultural practices can reduce the input of nitrate to groundwater bodies, but it is crucial to determine the time span over which these measures become effective at reducing nitrate levels in pumping wells. Such estimates can be obtained from hydrogeological modeling or lumped-parameter models (LPM) in combination with environmental tracer data. Two challenges in such tracer-based estimates are (i) accounting for the different modes of transport in the unsaturated zone (USZ), and (ii) assessing uncertainties. Here we extend a recently published Bayesian inference scheme for simple LPMs to include an explicit USZ model and apply it to the Dunnerngau aquifer, Switzerland. Compared to a previous estimate of travel times in the aquifer based on a 2D hydrogeological model, our approach provides a more accurate assessment of the dynamics of nitrate concentrations in the aquifer. We find that including tracer measurements (3H/3He, 85Kr, 39Ar, 4He) reduces uncertainty in nitrate predictions if nitrate time series at wells are not available or short, but does not necessarily lead to better predictions if long nitrate time series are available. Additionally, the combination of tracer data with nitrate time series allows for a separation of the travel times in the unsaturated and saturated zone.

To provide a sound understanding of the sources, pathways and residence times of groundwater water in alluvial river-aquifer systems, a combined multi-tracer and modelling experiment was carried out in an important alluvial drinking water wellfield in Switzerland. 222Rn, 3H/3He, atmospheric noble gases and the novel 37Ar-method were used to quantify residence times and mixing ratios of water from different sources. With a half-life of 35.1 days, 37Ar allowed to successfully close a critical observational time gap between 222Rn and 3H/3He for residence times of weeks to months. Covering the entire range of residence times of groundwater in alluvial systems revealed that, to quantify the fractions of water from different sources in such systems, atmospheric noble gases and Helium isotopes are tracers suited for end-member mixing analysis. A comparison between the tracer-based mixing ratios and mixing ratios simulated with a fully-integrated, physically-based flow model showed that models, which are only calibrated against hydraulic heads, cannot reliably reproduce mixing ratios or residence times of alluvial river-aquifer systems. However, the tracer-based mixing ratios allowed the identification of an appropriate flow model parameterization. Consequently, for alluvial systems we recommend the combination of multi-tracer studies that cover all relevant residence times with fully-coupled, physically-based flow modelling to better characterize the complex interactions of river-aquifer systems.