HS2.3.3

The riparian zone is a hydrologically and biogeochemically active zone, characterized by mixing of stream water with groundwater and transformation of nutrients such as nitrogen. The riparian zone thus plays a key role in natural attenuation of nitrate pollution. Among the attenuation processes in riparian zones, denitrification is the only one that leads to permanent removal of nitrogen from the riparian system via the release of dinitrogen gas into the atmosphere. In contrast, other biogeochemical processes such as nitrate uptake by plants merely result in a temporary nitrogen retention within riparian zones. While hydrochemical data and endmember modelling can help assess nitrate transformation in riparian aquifers, this does not allow quantifying the extent of nitrate removal via denitrification. In this talk, I will demonstrate how nitrate isotope data can be used in combination with chloride and nitrate concentration data to quantify spatial and temporal variations in the extent of denitrification and mixing between groundwater and surface water. I will illustrate how the application of this approach to a riparian groundwater study site in Central Germany revealed that denitrification is largely exceeded by other processes that merely lead to temporary nitrate removal from the riparian groundwater. In comparable settings, a major fraction of nitrogen inputs is thus likely retained in riparian zones and may eventually be discharged into rivers. Such information is crucial to determine the effectiveness of riparian zones for removing nitrate from aquatic ecosystems, which is highly relevant for many river ecosystems at risk of eutrophication because of high nitrogen inputs from agriculture.

Reference

Lutz, S. R., Trauth, N., Musolff, A., Van Breukelen, B. M., Knöller, K., & Fleckenstein, J. H. (2020). How important is denitrification in riparian zones? Combining end-member mixing and isotope modeling to quantify nitrate removal from riparian groundwater. Water Resources Research, 56, e2019WR025528. https://doi.org/10.1029/2019WR025528

How to cite: Lutz, S., Musolff, A., van Breukelen, B., Knöller, K., and Fleckenstein, J.: Nitrate isotopes reveal the effectiveness of riparian denitrification for nitrate removal from riparian zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7211, https://doi.org/10.5194/egusphere-egu22-7211, 2022.

Coastal environments around the globe provide services and support to the population. However, they are frequently impacted by human-based activities in addition to seawater intrusion problems. These lowland groundwater resources are exposed to a significant amount and types of pollutants, the most significant of which is nitrate. Tracking and quantifying the sources of nitrate that pollute surface and groundwater systems can be challenging without the use of environmental tracers such as nitrate isotopes. This study explored how changes in regional flow paths impact the nitrogen concentrations and origin of pollution in coastal waters during high tides. Water samples were collected from surface (rivers, lagoon and Atlantic shore) and groundwater (wells and boreholes) systems along the east coast of Côte d'Ivoire during the boreal summer (October). Water samples were analysed for major ions, dissolved nitrogen concentrations, coliforms presence/amount, as well as dual nitrate isotopes (δ15N-NO3- and δ18O-NO3-). Bayesian isotope mixing models were conducted to estimate the contributions of potential main sources (wastewater, seawater, atmospheric deposition, and agrochemicals). In some areas, nitrate inputs were found likely coming from wastewater sources. Nitrate concentration in groundwater was high at several sites. Some groundwater samples (n = 7) exceeded the WHO drinking water limit of nitrate concentrations of 50 mg/l, and most groundwater samples had high levels of total coliforms (>500 cfu/100ml). However, great isotope variation found in both surface and subsurface water samples suggested a spatial differential impact and origin of nitrate pollution, which is in agreement with the modelling. These water resources are the primary source of water for 53% of the local population and an alternative domestic water source for 97%, which highlights the importance of determining the main pollution sources for sustainable development. We expected that groundwater may have been better protected from nitrate pollution than lagoon surface water during wet periods, but this might have not been the case.

How to cite: Osemwegie, I., Soto, D., Stumpp, C., Coulibaly, J. K., and Reichert, B.: Tracing nitrate pollution within the Agneby subcatchment, SE Côte d'Ivoire (West Africa), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13405, https://doi.org/10.5194/egusphere-egu22-13405, 2022.

The present study explores the signature of stable isotopes (δ¹⁸O and δD) and major inorganic solute ion concentrations of Satluj river water and its tributaries to gain insight into the dominant hydrogeochemical process that controls the water chemistry in the Satluj River Basin (SRB), Himalayas, India. In isotopic and geochemical terms, the surface water of SRB is poorly characterized for its whole length; their potential variability has yet to be widely used as an aid in hydrological research. The δ¹⁸O values in the SRB range from – 14.50‰ to – 7.35‰ and δD from – 100.30‰ to − 44.70‰, showing general enrichment from Khaab to downstream Harike barrage. The upper reaches of Satluj River and its major upstream tributaries like Spiti and other small streams contributing to the Satluj are relatively depleted in δ¹⁸O and δD, possibly indicative of precipitation originating at higher elevations and/ or recharge from snow/glacial meltwater. While lower reaches are relatively enriched in δ¹⁸O and δD. The local water line for the SRB was found to be δD = 8.12 × δ¹⁸O + 18.89. The higher slope and higher intercept as compared to GMWL indicate a system recharged by snow/glacier meltwater and recycled moisture derived from continental sources in addition to monsoonal climates. In addition, a higher intercept indicates that the moisture source of precipitation (snow/rainfall) in this region originates from the Western Disturbance (WD). The Deuterium excess (d-excess) in the SRB varies between 12.50‰ and 25.81‰ with an average of 17.40‰, which is mostly higher than the long-term average for the Indian summer monsoon (~ 8‰). The higher d-excess value is because of the contribution of moisture from Mid-latitude westerlies. A significant negative correlation of Satluj river water δ¹⁸O with elevation was observed with a vertical lapse rate of 0.15‰/100 m. Geochemical analysis showed that the solute concentrations show spatial heterogeneity with decreasing elevation in the SRB. This is related to the complex lithologic compositions and different water sources from different elevations that contribute to the Satluj river. This study also established a relationship between total cation abundance (∑Cat*, corrected for cyclic components) and δ¹⁸O in waters of the Satluj mainstream. The variation in δ¹⁸O and ∑Cat* along the course of the Satluj is brought about by independent processes, the intensity of chemical weathering in the catchments and associated sub-catchments (tributaries), and altitude effect. ∑Cat* is higher at higher altitudes because of intense weathering, and δ¹⁸O is more depleted because the source waters are from depleted snow/glacial melt and cloud that is depleted in ¹⁸O because of previous rainouts during its ascent. The relationship between total cation abundance (∑Cat*) and δ¹⁸O suggests that ∑Cat* would increase by a factor of 0.93 for every 1‰ increase in δ¹⁸O.

How to cite: Jahan, A., Dar, T., Khan, U., Rai, N., and Kumar, S.: Investigation of Stable isotope systematics and geochemical signatures of surface water from the Satluj River Basin (SRB), Himalayas, India., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-353, https://doi.org/10.5194/egusphere-egu22-353, 2022.

Re-use of treated wastewater is receiving increasing attention as method to reduce water stress resulting from population growth, socio-economic development and climate change. In 2018, the European Commission issued a policy strategy entailing minimum water quality requirements for water re-use for agriculture and aquifer recharge. However, the environmental impact of such solution is yet to be determined.

The VUB in collaboration with private and public sector partners set up a field experiment in Kinrooi (Belgium) in which the effects of re-using treated domestic wastewater for sub-irrigation of an agricultural field are monitored. This is an interdisciplinary project which includes analyses of the effects on water quality and quantity in the subsurface saturated and unsaturated zone and nearby surface water, the effects on crops as well as research on the public perception.

Within this project, one of the aims is to create an advection-dispersion groundwater transport model to investigate how the chemical composition of the shallow groundwater would change after the treated domestic wastewater is applied through sub-irrigation. Observation data of tracers of the re-used water in the groundwater are needed to calibrate the transport model. Therefore, it is critical to choose a suitable tracer, allowing to unambiguously tell apart the effluent and groundwater end members. Literature suggests the use of chemical properties such as stable isotopes and Cl/Br ratios to use as wastewater tracers. Stable isotopes of hydrogen and oxygen are investigated, but the focus is currently on the use of Cl/Br ratios, which shows promising results. The use of this ratio as tracer is based on the close to ideal conservative behaviour of bromide and chloride ions in water caused by their small size and hydrophilic characteristics. This implies that physical processes such as dilution and evaporation happening in the environment influence the absolute concentrations of the ions but leaves their ratio constant. At the moment, 21 monitoring wells are installed on the field of which 9 monitoring wells have been sampled for data on Cl/Br tracers.

In general, the results indicate that finding a suitable tracer is not straightforward because chemical and isotopic compositions of the groundwater and treated wastewater are often similar. Therefore, the research continues to focus on improving the analytical methods used to analyse the currently used tracers (e.g. Cl/Br ratio and stable isotopes) and on the selection of other tracers such as anthropogenic organic compounds (e.g. pharmaceuticals and artificial sweeteners) to quantify the influence of the effluent end member and to enhance modelling performance.

How to cite: Speijer, L., Vandeputte, D., Zawadzki, M., Su, Y., Luo, M., Gao, Y., Elskens, M., Verhoest, P., Bauwens, J., Coussement, T., Elsen, F., Raes, B., Eisenreich, S., and Huysmans, M.: Re-use of treated wastewater for irrigation and groundwater recharge: environmental impact assessment based on tracer method at the experimental site in Kinrooi, Belgium, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6080, https://doi.org/10.5194/egusphere-egu22-6080, 2022.

Concentration-discharge relationships have been extensively utilized to infer the routing of water and pathways of (bio)geochemical reactions in the Critical Zone. To date, relatively little complementary development of stable isotope ratio - discharge relationships has been made, despite the fact that these tracers are commonly used to disentangle weathering reactions in the fluids draining from headwater systems. A process-based understanding of the extent to which fluid flow rates, antecedent hydrological conditions, and water age distributions impact these isotopic signatures in stream or river exports would present a pivotal advancement in the quantitative analysis of Critical Zone structure and function. Here, we explore how these factors regulate variations in stable lithium isotope ratios (expressed as δ⁷Li) of streamflow and the underlying water-rock interactions recorded by this signal during periods of hydrological transience. We present novel data sets collected during two distinct flooding events in Sapine Creek, a small, granitic catchment located on the southern flank of the Mont-Lozère, France and part of a Critical Zone Observatory within the French OZCAR Research Infrastructure. The data from this site are used to parametrize and validate an isotope-enabled, multicomponent reactive transport model capable of running transient simulations designed to mimic two sampling conditions at Sapine: (1) a storm preceded by a low-flow period and (2) a storm event during the wet season. The models are initiated from an unweathered granite subject to steady state uplift and infiltration. From this point, two synthetic storm events are simulated. Simulation of a storm event ending a period of dry conditions results in a net increase in streamflow lithium isotope ratios due to enhanced secondary mineral formation promoted by relatively long water residence times. In contrast, when the model is used to simulate a succession of relatively larger (~2 times greater in magnitude) storms during a wet season, a net decrease in δ⁷Li is observed. These lower isotope ratios are a consequence of attenuated secondary mineral formation. These preliminary results and trends demonstrate the influence of antecedent hydrological conditions and storm intensity on the magnitude and duration over which stream δ⁷Li is perturbed and, hence, the seasonal dependence of this signal as a record of transient behavior.

How to cite: Golla, J., Bouchez, J., Didon-Lescot, J.-F., Domergue, J.-M., Grard, N., Ayral, P.-A., Josselin, D., and Druhan, J.: Stream lithium isotope ratios record antecedent and transient hydrological conditions in catchment weathering exports, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8917, https://doi.org/10.5194/egusphere-egu22-8917, 2022.

Export of carbon from terrestrial catchments remains a poorly understood flux within the global carbon budget. In order to better understand the release and in-stream transport of dissolved organic carbon (DOC) in a small forested headwater catchment within the Bavarian Forest National Park (Germany), we characterized the stream-groundwater exchange along the headwater stream. We divided a 3000 m long stretch of the headwater stream into three topographically delineated sections that consisted of a steep upstream section (length 620m, elevations 888-967 m a.s.l.), a transition section (length 770 m, elevations 805-855 m a.s.l.) and a flat and wide downstream section (length 1330 m, elevations 770-805 m a.s.l., outlet of headwater catchment). Using sequential tracer injections of known masses of sodium chloride, we determined stream discharge and lateral exchange fluxes between stream and the surrounding riparian zone in the three stream sections and evaluated the effects of the resulting hydrologic turnover on stream water composition. We also compared the calculated lateral exchange fluxes with previously measured longitudinal profiles of Radon activities that can be used to locate groundwater inflow points. Discharge increased over the investigated 3000 m stretch in downstream direction, as expected, although the transition section did not show any change in discharge. The analysis of recovered tracer masses revealed that in the steep upstream section, the exchange fluxes consisted mainly of inflow of water into the stream. The transition section was characterized by an absence of exchange fluxes. In the downstream section, however, large inflows were offset by slightly lower outflows, resulting in a very pronounced exchange between stream water and riparian zone groundwater in this flat valley bottom. The spatial pattern of exchange fluxes was supported by the longitudinal Radon profiles, which pointed to a marked groundwater inflow in the downstream section. Our results suggest that the dominant source area of in-stream DOC at the outlet of our headwater catchment is the flat and wide valley bottom in the downstream section as most of the DOC released into the stream along the steep upstream section will be removed from the stream during the passage through the downstream section.

How to cite: Hopp, L., Kehr, J., and Blaurock, K.: Elucidating source areas of in-stream DOC by characterizing stream-groundwater exchange in a low mountain forested headwater catchment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9628, https://doi.org/10.5194/egusphere-egu22-9628, 2022.

In the peripheral aquifer of the Salar de Atacama, recharge is a key term of the water balance. This recharge is produced under arid conditions in the sub-basins surrounding the Salar and is dominated by medium salinity water due the intense evapo-concentration conditions. To solve the uncertainty in closing the water balance in the Salar de Atacama basin, this study aims to characterize the isotopic composition of precipitation, groundwater and surface water and to identify the recharge area. The results show that winter precipitation is more depleted in heavy isotopes, δ²H and δ¹⁸O, than summer precipitation. Surface water is evaporated and it has the same isotopic footprint as groundwater in each sub-basin, indicating that surface water runoff is a main recharge component. The meteoric source of surface and underground water in the basins of the Altiplano-Puna Plateau is isotopically lighter than the other waters found in the Salar de Atacama basin, although there is no significant transfer of isotopically lighter water to the peripheral aquifer of the Salar de Atacama from areas significantly outside the hydrographic basin.

How to cite: Valdivielso, S., Vázquez-Suñe, E., Herrera, C., and Custodio, E.: Identification of the recharge zone using stable isotopes in the Salar de Atacama, Chile, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9097, https://doi.org/10.5194/egusphere-egu22-9097, 2022.

Dissolved organic matter DOM corresponds to a complex mixture of molecules and macromolecules playing a major role in terrestrial and aquatic biogeochemical process. DOM heterogeneous composition allows it to have innumerable interactions with organic and inorganic compounds, affecting both their bioavailability and mobility in the environment. At the watershed scale, there are many sources of DOM towards the river such as diffuse inputs by leaching of different types of soils (e.g. forest, meadows, crops, urban impermeable areas...) or urban point inputs (storm overflows, WWTP discharges, stormwater discharges....). However, the influence of of all these sources on the composition of the aquatic DOM remains poorly understood to this day.

In this context, the main objective of this study was to build a methodology to detect the different sources of DOM in the rivers. Using a detailed characterization of DOM, the aim was to identify physicochemical markers and construct source-specific fingerprints. For this purpose, approximately 150 samples were collected from natural and anthropogenic sources of DOM (forest, agricultural, wastewater and urban runoff). All samples were analyzed using an innovative approach based on the use of a wide range of analytical techniques: dissolved organic carbon measurement, optical properties (UV-Visible, 3D fluorescence, size exclusion chromatography coupled with UV-fluorescence detection) and molecular composition (high-resolution mass spectrometry coupled with liquid chromatography). A large amount of data has been generated, and processed by classical (Anova, TukeyHSD) and multivariate (PCA, MFA, DFA) statistical approaches.

The results obtained allowed highlighting optical and molecular markers relevant for the identification of the selected sources. These markers inform on specific characteristics of DOM, such as the size of the molecules, the aromaticity content, the degree of humification, polarity and reactivity. In addition, complementarities and redundancies between optical and molecular characterization techniques is investigated. This research also contributes to select relevant markers for geochemical tracing models.

How to cite: Boukra, A., Masson, M., Brosse, C., Richard, L., Sourzac, M., Parlanti, E., and Miège, C.: Multi-analytical approach to investigate sources of dissolved organic matter in a peri-urban watershed, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11790, https://doi.org/10.5194/egusphere-egu22-11790, 2022.

Hydrogen and oxygen isotope values of precipitation are critically important quantities for applications in Earth, environmental, and biological sciences. However, direct measurements are not available at every location and time, and existing precipitation isotope models are often not sufficiently accurate for examining features such as long-term trends or interannual variability. This can limit applications that seek to use these values to identify the source history of water or to understand the hydrological or meteorological processes that determine these values. We developed a framework using gradient boosted regression tree-based machine learning, which we used to implement a procedure for calculating isotope time series at monthly resolution using available climate and location data. Here we present two new updates to our model, Piso.AI, one of which applies the original approach to new climate predictor data to extend the time series to the 1950-2020 time interval, and the second of which uses a restricted set of predictors to allow time series to be generated that span the range from 1901-2020 with slightly reduced accuracy compared to the original model. Both new products can be applied over most of Europe, and were trained on the historic archive of precipitation isotope data available from the Global Network of Isotopes in Precipitation. These model products facilitate simple, user-friendly predictions of precipitation isotope time series that can be generated on demand and are accurate enough to be used for exploration of interannual and long-term variability in both hydrogen and oxygen isotopic systems. These predictions provide important isotope input variables for ecological and hydrological applications, as well as powerful targets for paleoclimate proxy calibration, and they can serve as resources for probing historic patterns in the isotopic composition of precipitation with a high level of meteorological accuracy. Predictions from our modelling framework are available at https://isotope.bot.unibas.ch/PisoAI/.

How to cite: Nelson, D. B., Basler, D., and Kahmen, A.: An updated model for generating historic precipitation isotope time series from machine learning applied in Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8287, https://doi.org/10.5194/egusphere-egu22-8287, 2022.

We present preliminary results of the monitoring of oxygen and hydrogen isotopes in the water cycle of Georgia. The objective of the monitoring is to create maps of isotopic composition of precipitation over Georgia, estimate seasonal amplitudes in the isotopic composition of precipitation, and relationships with the surface and ground waters. Monthly cumulative precipitation samples have been collected since 2013 at 17 sites over entire Georgia at elevations 14 m a. s. l to 2220 m a. s. l. Local meteoric water lines (LMWLs) indicated differences between western, central, and eastern Georgia. Although the LMWLs slopes were mostly not substantially different, only the highest stations (Bakuriani and Gudauri) exhibited slopes that were slightly greater than 8.  The lowest LMWL slope (7.4) was found for stations Chaladidi and Sabueti located in western and central Georgia. LMWL derived for Gudauri (the Greater Caucasus) had a significantly greater intercept (19.9) than at all other sites. Isotopic gradients in delta 18O and delta 2H calculated between stations Tbilisi (430 m a.s.l.) and Gudauri (2220 m a.s.l.) were -0.26 per mil and -1.8 per mil per 100 m, respectively. Monthly samples were collected in several major rivers (Rioni, Mtkvari, Alazani, Iori). Isotopic composition of the greatest Georgian river Mtkvari was best correlated with precipitation in the central Lesser Caucasus (Bakuriani and Sabueti) and southern slopes of the eastern Greater Caucasus (Lagodekhi). One-fourth of groundwater samples were collected in boreholes between mid-July and mid-August and almost all samples from the second half of September (i.e. in the periods without groundwater replenishment) contained evaporated water. Slopes of the evaporation lines were 5.4 and 4.8, respectively.

How to cite: Melikadze, G., Holko, L., Todadze, M., Tchankvetadze, A., Gventsadze, A., Gaphrindashvili, M., Chitanava, R., and Chikadze, T.: Mapping stable isotopes of oxygen and hydrogen in precipitation, surface waters and groundwaters of Georgia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6106, https://doi.org/10.5194/egusphere-egu22-6106, 2022.

The determination of aquifer recharge zones is necessary for optimal and sustainable management of water resources. Stable isotopes (δ¹⁸O and δ²H) are an effective tool to better understand the relationship between precipitation and groundwater. However, there are areas in the world such as northern Chile, where there is a lot of available isotopic information on groundwater but very heterogeneous isotopic information on precipitation. This study contributes to a better understanding of the spatial and meteorological variables that control the isotopic composition in precipitation in Northern Chile and to estimate these meteorological and stable isotopes in precipitation. Results show that in summer, the significant features for temperature, relative humidity and precipitation are altitude-latitude, latitude and altitude-latitude respectively. The stable isotopes of precipitation are controlled by temperature, altitude, latitude, longitude, and precipitation. The monthly estimation models of temperature, relative humidity and precipitation and three isotopic models (summer, winter and annual) are created based on the controlling features.

How to cite: Vázque-Suñe, E., Valdivielso, S., Hassanzadeh, A., Custodio, E., and Criollo, R.: Factors that control the isotopic composition of precipitation in northern Chile, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9346, https://doi.org/10.5194/egusphere-egu22-9346, 2022.

The impact of climate change had caused that precipitation has significantly decreased in Georgia which caused significant decrease of surface water flows and depletion of groundwater amounts in natural springs. In the same time, the waters recharged in karstic aquifers, occurring on the southern slopes of the Greater Caucasus Mountains, can be considered as alternative groundwater resources for the communities in lowlands and the adjacent foothills. Here, about half of the renewable groundwater resources in artesian basins and confined groundwater systems in Georgia can be considered as belonging to the above mentioned water-bearing horizon. In order to assess water resources, the pathways between the recharge zones along the Caucasus and aquifers need to be addressed and risks of groundwater contamination along these pathways need to be evaluated.

On the territory of West Georgia, hydrogeological and hydrogeochemical surveys were performed in order to define the main hydrogeological features of the region. In the frame of this research, more than one hundred water sources (springs, wells, boreholes, rivers) were sampled during 2019. Physical parameters (pH, O₂, EC, temperature) were measured on site during sampling. Water samples were collected for chemical (major ions) and isotope analysis (¹⁸O and ²H). Karstic areas of West Georgia were covered by mapping. Isotopic composition of water in the study area evolves according to a line parallel with the global meteoric water line. Available isotopic data indicate several groups of groundwater types. Some of them very probably represent older waters, with substantially long mean residence time. Samples with pronounced isotope composition variability indicate the evolution of groundwater isotopic composition from the recharge area in the mountains through river valleys to the exfiltration areas. Deuterium excess shows higher values, typical for mountain precipitation and snow in mountain ranges. The conjunctive use of isotopic approaches demonstrates a high potential for future water resources studies in Georgia.

How to cite: Vepkhvadze, S., Malik, P., Melikadze, G., Todadze, M., Ghlonti, L., and Tchikadze, T.: Assessment karstic water origin along South slope of Grate Caucasus Mountain range, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4127, https://doi.org/10.5194/egusphere-egu22-4127, 2022.

High Alpine catchments are precious water-resources since they act as natural storage reservoirs, storing water in the snow cover and in the subsurface and thereby providing water during the dry seasons. Thus, a deeper knowledge of the hydrological functioning of these systems is necessary, in particular to make climate change projections. The role of seasonality is crucial in these catchments that generally exhibit a snow-dominated hydro-climatic regime.

Here we use high-frequency observations of stable isotopes of water to identify the seasonal origin of streamwater in a high-elevation Alpine catchment located in the Valle d’Aosta Region, Italy. We quantify the relative contribution of winter and summer precipitation reaching the stream through the Seasonal Origin Index (SOI_Q), calculated using the δ¹⁸O values and the volumes of precipitation and streamflow. Highly negative SOI_Q values are obtained suggesting that streamwater is mainly composed of winter precipitation. Conversely, the Seasonal Origin Index for evapotranspiration (SOI_ET), which can be directly inferred from SOI_Q, returns a positive value reflecting that plants preferentially take up water deriving from summer precipitation.

These findings allow us to develop a conceptual model of this Alpine system. This conceptual model suggests:

• a deep infiltration component, mainly composed by snowmelt water, reaching the stream through a preferential flow.
• a shallow infiltration component, predominantly represented by summer rainfall, that dominates the shallow soils and that is used by plants.

Therefore, we presume a seasonal compartmentalisation of water in this high-elevation catchment.

Nevertheless, a previous study in Switzerland revealed SOI_Q ≈ 0 for the Allenbach and Dischmabach snow-dominated catchments, indicating that similar fractions of summer and winter precipitation become streamflow. This different result achieved in systems with an apparently similar functioning highlights the need for a deep insight into the flow paths governing high-elevation catchments and it opens the way for new challenges to understand the hydrological processes hidden behind this difference.

How to cite: Gentile, A., Bevilacqua, I., Canone, D., Ceperley, N., Gisolo, D., N'Sassila, M., Previati, M., Zuecco, G., Schaefli, B., and Ferraris, S.: Seasonal compartmentalisation of water in a grassland at 2600 m a.s.l., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9190, https://doi.org/10.5194/egusphere-egu22-9190, 2022.

During 2017-2018, the City of Cape Town, South Africa faced an unprecedented drought crisis with the six main water storages supplying Cape Town falling to a combined capacity of just under 20%. Throughout the crisis, groundwater was considered the most important alternative urban water supply source but also the most vulnerable to contamination through accidental return flows from the municipal network, private residences and agricultural industries. This project aimed to constrain the stable isotope chemistry of the water supply network in the Stellenbosch municipality and monitor the augmentation of groundwater into the network using stable isotopes. Long-term monitoring points have been established at 35 tap water sites, 20 private wells as well as at the 3 supply reservoirs that feed the municipal network. Rainwater (4 locations) and local surface water (6 locations) were also monitored over the one-year sampling period in 2021. Preliminary data show distinct isotopic signals associated with each supply reservoir as well as in the local groundwater. Rainfall is predominantly received in the winter season (May-Aug) and typically has more negative isotope delta values. Typical residence times in storage dams and reservoirs appear to be between 2-3 weeks in the winter and 3-4 weeks in the summer, according to stable isotope hydrograph separations. Domestic water supply is consolidated at 2 water treatment facilities in Stellenbosch, where isotope values of all 3 supply reservoirs mix. The amounts of water received from each reservoir changes throughout the year according to dam levels, this change is evident in the stable isotope values at the tap water sample locations. The data also shows significant return flow into the alluvial aquifer system during warmer months when private stakeholders’ water consumption is at its highest. Groundwater is expected to supplement this urban supply network in Q1-2 of 2022 and will likely disrupt the current distribution of stable isotopes in the network, providing further insight into the potential return flow into the local groundwater system. For longer term monitoring, tap water locations that receive the same supply have been identified and single locations (8 in total) have been selected to monitor through 2022 to optimise the monitoring network. Similarly, only 4 rainfall collection sites will continue monitoring. Interested stakeholders and policy makers include municipal supply managers as well as local farmers and industry that can use this data to develop water management strategies and identify areas where leaks or overuse is likely. Hydrograph separations would be more accurate with longer term monitoring rainfall, reservoir and tap water by identifying trends in storage residence time, mixing and release schedules throughout the supply systems. Although the data indicates that there are return flows into local groundwater, these results could not distinguish the mechanisms by which water is entering the system. Further monitoring in targeted areas is needed to constrain if return flows originate from leaks, irrigation or other urban recharge processes.

How to cite: van Rooyen, J., Meyer, C., Ortega, L., and Miller, J.: Stable Isotope Techniques for the Evaluation of Water Sources for Domestic Supply in Stellenbosch, South Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11177, https://doi.org/10.5194/egusphere-egu22-11177, 2022.

Key words: Mass balance filtering, baseflow, BFI, time-resolution, electrical conductivity, permeability

Baseflow plays an important role in sustaining freshwaters quality and quantity at the global scale. Quantitative estimates of baseflow are necessary but no direct observations are available to the purpose, thus hydrograph filtering appears as a valid solution to quantify the baseflow process. The mass balance filter (MBF), based on electrical conductivity (EC) observations, is one of the most objective filtering techniques.

The aim of the present study is to analyze the impact of data time resolution in the assessment of the long-term scale baseflow index (BFI, the ratio between baseflow and total flow) by hydrograph filtering. The MBF method was used to estimate the long-term BFI of 64 catchments across Continental United States ranging from about 5 to 50000 Kmq, from arid to continental climate conditions and accounting for 5 to 10 years of continuous observations. Streamflow and EC data were collected from the United States Geological Survey (USGS) National Water Information System website at the 15 minutes time resolution and aggregated at the daily scale. BFI₁₅, the BFI computed at the 15 min time resolution, was compared with BFI₂₄, that is with the BFI computed at the daily scale. The difference among the two indices was investigated in relation to catchment climate and physiographic characteristics.

The large dataset was divided into two groups, a poorly-drained group and a well-drained group on the base of the catchment permeability assessed by the GLobal HYdrogeology MaPS (GLHYMPS) project. The first group corresponds to BFI values < 0.5, the second to BFI values > 0.5. Overall the BFI₁₅ was found to be larger than BFI₂₄, with an average difference of about 7%, probably caused by the fact that at finer time scale a wider spectra of streamflow processes and components can be detected, more evidently during the peak flow conditions. The average difference drops to 1% in the case of the well-drained hydrological systems, which appear likely the less impacted by the monitoring time resolution and increase on average to 11% (with maximum values approaching 50%) in the case of the poorly drained systems.

How to cite: Khasanova, L., Longobardi, A., Khasanov, I., and Elizaryev, A.: Impact of data time resolution in long term baseflow index assessment from mass balance filtering, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8681, https://doi.org/10.5194/egusphere-egu22-8681, 2022.

Alpine regions are becoming more sensitive to climate change and to understand the hydrogeological processes that follow extreme climatic events (flooding, drought, heavy precipitation and fast snow melt), the hydrologic conditions and geologic realities need to be understood. Our research project “Understanding of Extreme Climatological Impacts from Hydrogeological 4D Modelling” (EXTRIG; funded by the Austrian Academy of Sciences) contributes to this challenge by applying an innovative interdisciplinary approach in an Austrian alpine research area (Sibratsgfäll, Vorarlberg) with a catchment of 5 km² situated at an altitude between 800 and 1.400 m based on a cooperation between hydrogeologists, meteorologists, social scientists and the local population.

This poster study emphasizes on preliminary results of applied hydrogeological methods conducted between 2019 and 2021 to understand the local water cycle.

• To determine surface discharge, radar monitoring stations was installed at each of the four main streams, flowing into the main river beneath the village. Calibration of the results was conducted with several salt dilution measurements. Additional salt dilution measurements helped to estimate diffuse surface and groundwater discharge into the main river.
• Precipitation was measured in Sibratsgfäll (906m) and compared with precipitation measurements of two nearby weather stations and with past precipitation measurements of the area since 1990.
• Evapotranspiration was calculated as ET0 with the Hargreaves method in two different approaches, one using the air temperature reconstructed from surrounding weather stations, the other using temperature measurements from the monitoring station in Sibratsgfäll.

The resulting data was used to calculate the amount of water infiltrating into the ground via water balance calculation. The yearly precipitation from December 2019 including November 2020 sums up to 2.600 mm/a and approx. 50% discharges via the main streams. Evapotranspiration can be estimated to be 22 to 32% with a large uncertainty leaving 18 to 28% of precipitating water to diffuse discharge and infiltration into the groundwater. Estimating that surface-near discharge is at least 10%, between 8 and 18% of precipitating water may infiltrate into the ground. Diffuse surface near discharge has shown to be higher after snow melt and in summer, but almost absent during colder periods. Furthermore, a complex network of shallow agricultural drainages may only partially dewater to streams but also contribute to surface-near discharge.

The monitoring of the stable ²H/¹⁸O-isotopes in a meteorological station (906m) and of Flysch-springs close to the mountain ridge of the recharge area allow to differentiate the recharge altitude. The vertical unsaturated infiltration in silt/sand dominated glaciolacustrine sediments were estimated by seasonal variation of ²H/¹⁸O-isotopes in soil-water to be 1m/year approximately. Precipitation in the Flysch dominated area at higher altitudes is transported slope-parallel in the upper part of the glacial sediments. The Mean Residence Time (MRT) of the shallow groundwater (<40m) estimated by a combination of isotopes ²H/¹⁸O, ³H/³He, ¹³C/¹⁴C and tracer gases (CFC, SF₆) indicate ages between some months and 4 years. Deeper (>40m) artesian wells in the western part are dominated by MRT older than 30 years.

How to cite: Holzschuster, R., Elster, D., Kralik, M., and Stumpp, C.: Alpine water cycle unraveled by hydrogeological measurements, isotopes (18O/2H, 3H, 3He, 14C) and tracer gas analyses (CFC-11,-12,-113, SF6), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11417, https://doi.org/10.5194/egusphere-egu22-11417, 2022.