HS2.3.2

The importance of riparian zones in shaping the hydrology and biogeochemistry of forest headwater catchments has been well-established in the last couple of decades. However, most studies rely on single catchment approaches or focus on a single ecoregion. Here, we present a multiple-site approach using data from four catchments located in four European ecoregions and encompassing a wide range of hydrological and biogeochemical conditions: the boreal Krycklan, the temperate Rappbode, the sub-humid Mediterranean Font del Regàs, and the semi-arid Mediterranean Fuirosos.

At long temporal scales, both climate and topography interrelate to determine the dominant source layer (DSL), i.e. the riparian zone depth stratum that contributes the most to water and solute fluxes to streams. Generally, shallower DSLs are found in wetter climates characterized by higher ratios of annual precipitation to potential evapotranspiration (e.g. boreal), but locally can occur in the flatter contributing areas defined by higher topographic wetness indexes. Riparian soils show large differences in carbon and nutrient content across ecoregions. Mediterranean riparian soils are characterized by lower organic matter content and small dissolved organic carbon (DOC) exports from deeper, mineral layers, resulting in overall lower stream DOC concentrations compared with the temperate and, especially, the boreal sites. On the other hand, the denitrification potential of the Mediterranean riparian zones, especially at the semi-arid site, is limited due to drier conditions that oxygenate the riparian soils and may even promote nitrification, resulting in higher stream nitrate (NO₃^-) concentrations compared to the boreal site. The denitrification potential of the temperate site is counterbalanced by a significantly higher nitrogen deposition compared to the other sites.

At the event scale, antecedent soil moisture conditions and associated hydrological connectivity within the catchments becomes an important factor defining solute export from riparian profiles and hysteresis patterns between riparian groundwater tables and stream discharge. Generally, the wetter conditions in the boreal site generate anticlockwise patterns between groundwater tables and discharge, indicating high catchment hydrological connectivity. Clockwise patterns are characteristic of the Mediterranean sites and imply low hydrological connectivity, except during very wet antecedent conditions. The temperate site is characterized by linear patterns and intermediate conditions. Solutes displaying relatively high concentrations in the riparian profile are generally transport limited, especially during events preceded by dry conditions, whereas source limitation occurs during events preceded by wet conditions, especially for solutes displaying relatively low concentrations.  

We highlight that multiple-site approaches can help identifying common patterns and differences in riparian hydrological and biogeochemical functions across ecoregions, as well as the factors driving these patterns and the resulting dynamics of stream water chemistry.

How to cite: Ledesma, J. L. J., Musolff, A., Grabs, T., and Bernal, S.: Riparian zone control on catchment hydrology and biogeochemistry across European ecoregions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2540, https://doi.org/10.5194/egusphere-egu22-2540, 2022.

Inland waters can be interpreted as “active pipelines” in which the complex dissolved organic carbon (DOC) dynamics occur, eventually contributing to negative net ecosystem production. Hydrological factors highly contribute to the DOC balance at the reach scale. Seasonal and event-based hydrological variability, particularly in headwater streams, affects both stream-hillslope organic matter exchanges and overall fluvial network connectivity, leading to significant space and time changes in sources and processes regulating DOC. Technological advances allow fine and continuous time scale measurements of several physicochemical parameters with optical aquatic sensors, providing great potential for a better understanding of aquatic ecosystems functioning.

This study investigates the spatial and temporal dynamics of DOC concentration in a Mediterranean headwater catchment (Turbolo River catchment, southern Italy) equipped with two multiparameter sondes providing approximately 2.5 years (May 2019 to January 2022) of continuous high-frequency measurements of several chemical-physical variables, among which DOC-related parameters (fluorescent dissolved organic matter - fDOM, streamwater temperature and turbidity) at two different outlets. One sonde was installed in a quasi-pristine sub-catchment, while the other at the catchment outlet, characterized by some anthropogenic disturbances.

The specific features of the upslope sub-catchments were considered to address the connection between seasonal and hydrologic dynamics and DOC changes. Continuous observations were supported by meteo-hydrological observations and discrete monitoring carried out in the period January-April 2021 when 59 samples were collected on-field and analysed in the laboratory to achieve reference DOC values, used for an original correction method of measured fDOM values.

Results concern both the seasonal variability of background values and hydrological regulation of DOC export. Specifically, applying a Principal Component Analysis multivariate approach to the background values, seasonal clusters emerged with a clear temporal trajectory, highlighting similarities and differences among DOC and other measured parameters. Furthermore, DOC concentrations were positively correlated with discharge and even more with antecedent precipitation, reflecting the flushing effect of intense and prolonged precipitation. In both sites, the accumulated export of DOC for discharge values below the flow equalled or exceeded for 10% of the time (Q10) was lower than 35% of the total. Also, some differences in DOC concentration emerged between the two sites, with increased values with high flows in the catchment more affected by disturbances.

High-frequency monitoring proved to be a valuable tool to explain DOC dynamics at multiple time scales with a quantitative approach, highlighting the climate control and the hydrological regulation on DOC production and export.

How to cite: Senatore, A., Corrente, G. A., Argento, E. L., Micieli, M., Mendicino, G., Beneduci, A., and Botter, G.: Understanding climate control and hydrological regulation of dissolved organic carbon concentration in a Mediterranean headwater catchment through long-term, high-frequency monitoring, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7307, https://doi.org/10.5194/egusphere-egu22-7307, 2022.

The links between nitrate-nitrogen (N) leaching from agricultural fields and N measured in streams can in general terms be divided in two pathways: groundwater and a more surface-near transport (e.g. tile drains).  The former with a typical slower hydrological response than the latter.  Therefore, catchments with quick hydrologic pathways respond also quickly on programme of measures for N. On the other hand, catchments having bot high N-attenuation or longer N time lags makes it complicated for managers and policy makers as the response of the implemented programme of measures might both be dampened and delayed. As River Basin Management Plans (RBMPs) under the Water Framework Directive (WFD) runs in 6 years periods – such time lags might end up as an overdosing of measures. Therefore, attenuation and time lags needs to be mapped as they have major effects on the expected effects of RBMPs and its legacy for water quality. The aim of this study is to improve our understanding of N lags mapped based on 30 years of data from 160 Danish stream monitoring stations.

A national wide screening for trends in annual flow-weighted total nitrogen (TN) concentrations at 163 river monitoring stations shows in most cases a downward trend (average: 30% ± 17%) during the last 30 years 1990-2019). The N-surplus has been reduced (farm gate: -44%; field:  -45%) during the same period. Before 1990, the N-surplus in agriculture was increasing and started at first levelling off in the mid 1980ies. Diffuse N-sources and mostly agriculture contributed the most to TN in streams (93% ±8%) during the period 1990-2019). The reduction in the diffuse N loadings are paralleling the development of the N surplus for most Danish streams. However, in certain parts of Denmark several river monitoring stations shows a much different response, which in some cases is no response at all. Such a pattern can only be explained by N-flows in the catchments to be delayed in groundwater aquifers. Using long term data for national N-surplus a simple lag-time analysis shows that the time lags for N are long for 21 catchments (up to 20 years), medium long for N in 62 catchments and with nearly no delay for N in 80 catchments (Fig. 1). Moreover, all the stream stations experiencing long time lags are situated in the chalk and partly karstic landscapes of Denmark from the Danien period. The catchments having long delays for N shows in most cases also a very low attenuation of N in groundwater as measured N-concentrations are substantially higher than found in the streams having nearly no time lags. Therefore, we conclude that incorporation of biogeochemical and hydrologic time lag principles into water quality regulations will be necessary for providing managers and regulators with realistic expectations when implementing new policies for N.

Figure 1: Map of Denmark showing catchments with short (< 20% older than 10 years), medium (20-40 % older than 10 years) and long (> 40% older than 10 years) for nitrogen in groundwater.

How to cite: Windolf, J., Tornbjerg, H., Blicher-Mathiesen, G., and Kronvang, B.: Assessment of agricultural nitrogen pressures and legacies in Denmar, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3390, https://doi.org/10.5194/egusphere-egu22-3390, 2022.

One challenge for modern agricultural management systems is to reduce their detrimental effects on water quality of water bodies. With this in mind, monitoring was carried out on behalf of the State Agency for Environment, Nature Conservation and Geology Mecklenburg-Vorpommern in the drainage outlets of 19 arable fields distributed throughout the state. In long-term measurement campaigns, runoff and substance concentration were determined at the drainage outlets. As expected with intensive arable land use, the nitrogen concentrations of most samples were far above the current quality standards and target values for surface waters according to the Surface Water Ordinance. Phosphorus concentrations were also generally very high. In parallel to the measurements, extensive data on agricultural management were collected and then correlated, together with pedological and meteorological data, with the temporal dynamics and spatial patterns of substance concentrations in drainage runoff. After selection and processing, 1037 data sets with 19 measured variables were available for the analyses.

These data were first subjected to a principal component analysis. The first seven principal components were each assigned to specific effects. The values of the principal components were interpreted as quantitative measures of the strength of the expression of these effects in the individual water samples. These were then placed in relation to extensive meteorological, hydrological, soil and management data. Classical correlation analyses revealed a bewildering variety of significant effects. Using random forest models, however, it was possible to map a large part of the observed spatial and temporal variance with just a few explanatory variables in each case.

The temporal dynamics of the nutrient concentrations in the outlet of the drains were mainly determined by hydrological conditions and weather. In contrast, direct short-term effects of individual arable farming measures on nutrient dynamics in the drainages could not be identified. Instead, clear indications of long-term effects of agriculture were found. In particular, the nitrogen and phosphorus balances of the areas played a decisive role. Soil recovery from long-term fertilisation thus does not seem to be achievable either through minor changes in agricultural management or in the short term.

Furthermore, it was shown that the many years of intensive use of the land had a massive impact not only on the nitrogen and phosphorus contents, but also on almost all other substances studied. Nitrogen and phosphorous data alone can only provide limited information on the source and development of soil eutrophication. This is still given too little attention in studies on the substance balance of agriculturally used land.

How to cite: Steidl, J., Lischeid, G., Engelke, C., and Koch, F.: Evaluation of a state-wide drainage monitoring in Mecklenburg-Vorpommern using artificial intelligence methods, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1712, https://doi.org/10.5194/egusphere-egu22-1712, 2022.

The classification of metabolic regimes in aquatic ecosystems is based on gross primary production (GPP) and ecosystem respiration (ER). A recent advancement in sensor technology and modeling capabilities has enabled the metabolic regimes classification to be applied also to stream ecosystems. Current information about stream metabolism exists mostly for temperate climate while data in semi-arid and urban environments remain scarce. In this study, we used long-term high-frequency measurements of water quality parameters in the Yarkon Stream (Israel) to study the dynamics of water quality and metabolic regimes. The Yarkon is a lowland, urban, Mediterranean stream in which about 75% of the discharge consist of tertiary level treated wastewater. A multi-sensor monitoring station was installed in 2019 and includes sensors for measuring turbidity, carbon dioxide, electrical conductivity, nitrate, dissolved oxygen (DO), water level, temperature, and pH. In addition, photosynthetically active radiation (PAR) is measured near the stream. Real-time data can be accessed in the following link: https://tinyurl.com/Yarkon-public-view. Results show a very strong impact of seasonal floods on water quality, however, the stream returns back to pre-flood conditions relatively quickly due to the weak link between the stream and the subsurface. First floods of the winter also exhibit a strong hypoxic response due to the flush of contaminants that are accumulated during the long dry summer. Only weak seasonal patterns were observed in water quality as a result of the dominant fraction of treated wastewater relative to the freshwater in the stream, and the relatively low in-stream nutrient transformations. Preliminary results also show that a clear diurnal cycle in oxygen concentrations is not clearly visible throughout the year. GPP exhibits low values throughout the year, with slightly higher values during spring and summer. The average annual net ecosystem production (NEP) is negative because ER is much higher than GPP throughout the year. The long-term data from the Yarkon Stream provides an insight into the dynamics in water quality and stream metabolism of an urban Mediterranean stream ecosystem and allows to compare the dynamic behavior to streams from temperate climates.

How to cite: adar, Z., Yogev, N., and Arnon, S.: Water quality and metabolism dynamics in a lowland urban Mediterranean stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-341, https://doi.org/10.5194/egusphere-egu22-341, 2022.

The tradition of mining in the Western Harz Mountains ranges from the Middle Ages to the 20th century. To evaluate the water-rock interactions as well as the environmental impact of trace metals, the mine water of three mining districts – Rammelsberg, St. Andreasberg and Clausthal-Zellerfeld – was investigated. The water samples were also analysed for the main ions. During the sampling campaigns pH-values and specific electrical conductivites (SEC) of the mine waters were measured. Compared to the most regions of Northern Germany, high precipitation rates (>850 mm/a) and a realatively low mineralisation of surface waters are found in the Upper Harz Mountains. The logging of temperature and SEC of spring and mine water over several months clearly showed that water mineralisation decreases with increasing precipitation rates (Bozau et al., 2017). The Devonian SEDEX deposit Rammelsberg is characterised by layer-bound ores and is famous for its non-ferrous metals. Acidification due to the oxidation of metal sulphides is rare. During the sampling campaigns only one sample of the Rammelsberg mine displayed a pH-value <3 (pH = 2.8). The majority of the measured pH-values ranges from 6.8 to 7.9. SEC values from 540 up to 2680 µS/cm were measured in the Rammelsberg mine. The adit "Ernst-August-Stollen" has a lenght of 40 km and is the biggest drainage tunnel of the mining district Clausthal-Zellerfeld. The portal of the adit is located in the triassic rocks of the Harz foreland, where SEC values from 980 to 1173 µS/cm and pH-values between 7.3 and 8.2 were measured. The SEC of the adit "Ernst-August-Stollen" ranges from 629 (inflow of the Kneseberg manhole) to 4710 µS/cm at the outflow of an oil separator into the main chanel (Bothe-Fiekert et al., 2021). The ICP-MS data of unfiltered and filtered (0.45 µm and 0.2 µm) water samples of this adit show that most of the iron (> 95 %) is transported as particulate fraction in the mine water. The mines of "St. Andreasberg" are situated on the highest elevations and receives the highest precipitation amount compared to the other two mining districts. The mine water of this area shows the lowest SEC values which range from 113 to 193 µS/cm and pH-values between 6.9 and 7.6. Due to the increased occurrence of Fe-Ni-Co arsenides in the St. Andreasberg gangue ore deposit and the association with antimony minerals, increased arsenic and antimony concentrations were observed in some water samples of this mining district. Although there are different and changing trace element concentrations in the single mines and adits, significant hydrochemical characteristics for the three mining districts are observed. Threshold limit values for drinking water were seldom exceeded during the investigations.

References:

Bothe-Fiekert, M., Bozau, E., Ließmann, W., 2021. Hydrogeochemical characteristics of an old mine adit in the Harz Mountains (Germany). Goldschmidt Virtual 2021.

Link: https://2021.goldschmidt.info/goldschmidt/2021/meetingapp.cgi/Paper/3614

Bozau, E., Licha, T., Ließmann, W., 2017. Hydrogeochemical characteristics of mine water in the Harz Mountains, Germany. Chemie der Erde 77 (2017), 614-624.

How to cite: Kolling, M., Bozau, E., and Ließmann, W.: Water quality and trace metal concentrations of mine water in the Western Harz Mountains, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2497, https://doi.org/10.5194/egusphere-egu22-2497, 2022.

Vegetation exhibits critical feedback with runoff generation. Trees show distinct water uptake patterns relying on soil water from different depths and groundwater with a mixture of water sources that is commonly very different from runoff in terms of age distribution and isotopic composition. Here we present a multi-method and multi-model approach to study the spatio-temporal patterns of sap flow and age distribution of tree water uptake and streamflow in a headwater catchment (mixed forest, 43 ha). For this, we monitored sap flow spatially distributed at >30 trees over two years, sampled ²H and ¹⁸O bi-weekly and spatially distributed in xylem for two years and in streamflow sub-daily for four years. This was supplemented by tritium sampling in streamflow over two years for different flow stages. We used statistical modeling to determine spatio-temporal patterns of transpiration and isotopic composition of xylem water and their drivers. We used a multi-model approach to derive catchment travel times through (i) composite Storage Selection (SAS) functions, (ii) conceptual hydrological modeling, and (iii) coupled land surface-subsurface modeling (ParflowCLM) combined with particle tracking. Statistical data analysis revealed that tree species, tree diameter, and topographic wetness index at the tree location were the main driver of spatial variability of sap flow, while soil water was the main source of xylem water with little groundwater influence. The travel time modeling showed a strong seasonal and event-based variability of travel times and allowed to include information on vegetation behavior with different complexity. Last, our detailed sampling of vegetation offers a blueprint for a sampling strategy of isotopes in xylem water for travel time studies. Our data revealed that approximately 20 sampled trees are sufficient to capture the mean isotopic value of xylem water of a species at our study site, while we needed around 100 samples to detect landscape influence on the xylem isotopes needed for considering spatial patterns in the travel time analysis. Our results underline the feedbacks between vegetation and runoff generation and show their relevance for better simulating catchment travel times.

How to cite: Klaus, J., Chun, K. P., Fabiani, G., Fresne, M., Hrachowitz, M., McGuire, K., Moussa, A., Penna, D., Pfister, L., Rodriguez, N., Schoppach, R., Sulis, M., and Zehe, E.: A multi-method and multi-model approach for predicting spatio-temporal patterns of sap flow, xylem isotopic composition, and water ages in the critical zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1738, https://doi.org/10.5194/egusphere-egu22-1738, 2022.

Transpiration is an important component of the catchment water balance, and tree water uptake can significantly influence discharge dynamic and travel times. Tree usually use water of a different age than the water that generates discharge. Most hydrological models that track water age rely on discharge and stream water isotopes data for calibration. This calibrated age-tracking model enables to simulate time-variant discharge travel times. However, transpiration and related water isotopes data are rarely integrated. We currently lack understanding on the age distribution of transpiration and how discharge travel times relate to this age distribution. In this context, the objectives of this work are to 1) analyse spatio-temporal patterns of xylem water isotopes, 2) determine transpiration age distribution and investigate its temporal dynamic over two growing seasons and 3) evaluate how discharge travel times respond to changes in the transpiration age distribution. We determined the spatial controls on xylem water isotopes (δ¹⁸O and δ²H) and simulated travel times in discharge and transpiration in a forested catchment in Luxembourg. We used a lumped, process-based catchment model for water fluxes and tracer transport based on storage-age selection functions. Using discharge, transpiration, stream and xylem water δ²H measurements, the model was calibrated over a period of three years (October 2017-September 2020) by means of a Monte Carlo optimization and a multi-objectives approach. Preliminary results indicate that tree species and diameter are the main drivers of xylem water isotopes variation. Vegetation controls are especially dominant during a drier growing season while landscape variables (hillslope position, topographical position index, flow accumulation) appear to also control xylem water δ²H in wetter conditions. Investigations of transpiration and discharge age distributions will help to better understand how tree water uptake influences discharge travel times over the growing seasons. The spatial analysis of xylem water isotopes further provides a foundation for investigating the spatial variability of the transpiration age distribution. This study will also profit in assessing the value of transpiration and associated isotopes data for discharge travel times simulations and improving our current model representations of hydrological processes in forested catchments.

How to cite: Fresne, M., Chun, K. P., Fabiani, G., Hrachowitz, M., McGuire, K., Moussa, A., Schoppach, R., and Klaus, J.: Integrating transpiration and xylem water stable isotopes in a process-based model to determine transpiration and discharge age distributions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3046, https://doi.org/10.5194/egusphere-egu22-3046, 2022.

Quantifying the transfer of organic carbon from the terrestrial to the riverine ecosystems is  of crucial importance to fully appreciate the carbon cycle at the catchment, regional and global scales. Specifically, the entity of dissolved organic carbon (DOC) fluctuations in streamflow is of particular interest also for their impact on the nutrient cycles and on water quality, with implications also for drinking water treatment. In this study, we propose a framework for modelling the flux of DOC from hillslopes to stream and river networks which couples a transport model based on water travel time distributions with the reactivity continuum theory to model DOC degradation. We test the model by applying it to the Plynlimon catchments (UK) exploiting both weekly and high-frequency (7-hour interval) time-series. Besides DOC concentration data, we use information about chloride to get an independent estimate of water travel times using the framework of StorAge Selection functions. The composition and the degradation of DOC along the flowpaths is described assuming a continuous spectrum of quality which initially follows a gamma distribution. Results show that, chiefly for high-frequency measurements, the model is able to reproduce reasonably well both chloride and DOC streamflow concentrations and to capture the complex hysteretic relation between DOC concentration and discharge. The distribution of the age of the water comprised in the streamflow proves thus a key variable to predict the quantity but also the quality of the DOC exported from soils, and the effect of hydrologic variability on this process. Starting from the proposed framework and the results obtained, we discuss how future developments could help in shedding light on the complex relations among carbon, water cycle and the metabolic balance of riverine ecosystems.

How to cite: Grandi, G. and Bertuzzo, E.: Catchment dissolved organic carbon transport: a modeling approach combining water travel times and reactivity continuum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5100, https://doi.org/10.5194/egusphere-egu22-5100, 2022.

Transit time distributions (TTDs) of water moving through a catchment can be estimated using water isotope data. However, different isotopes are characterized by different information contents, which may affect the estimation of TTDs. Stable isotopes, such as ²H and ¹⁸O can provide insights into the part of TTDs that describes water ages of up to a few years. However, they are “blind” to ages older than that. Radioactive isotopes, such as tritium (³H), on the other hand, have been shown to describe old water, and thus the tails of TTDs much better. Direct comparisons of the different information contents and the resulting differences in catchment TTDs estimated from stable and radioactive isotopes are rare, mostly due to very limited data availability. The objectives of this study are therefore to quantify the differences in TTDs together with their temporal variability and sensitivity to climatic variability in multiple components of the hydrological system estimated from both tritium and stable isotopes using a distributed wise-process based model in the Neckar river basin in Germany. More specifically, we test the hypotheses that (1) stable isotope- and tritium-based estimates of TTDs exhibit significant differences for both young and old water ages, (2) they are characterized by distinct sensitivities to climatic variability, and that (3) combined use of stable isotopes and tritium results in more robust estimates of TTDs. The analysis is carried out based on long term hydrological (1958-2016) and isotope data (1990-2016), using a distributed hydrological model coupled with StorAge Selection (SAS) functions, which is simultaneously calibrated (and evaluated) with respect to multiple variables and hydrological signatures including, amongst others, streamflow, tritium, and stable isotope data.

How to cite: Wang, S., Hrachowitz, M., Schoups, G., and Stumpp, C.: Temporal variability of transit time distributions in response to climatic variability: do stable water isotopes and tritium tell the same tale?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7347, https://doi.org/10.5194/egusphere-egu22-7347, 2022.

In the last decade a large and various set of groundwater modelling tools were developed and provide wide range of applications. In contrast, the number of tools for field studies remains limited. One of the tools are Environmental Tracers: substances of natural or anthropogenic origin which enter the groundwater system and were traced on their way to the discharge zone. Methods to interpret the tracer concentrations and how groundwater dynamics could be derived were published also in textbooks by now. One parameter to describe groundwater dynamics is residence time, age or flow velocity. All these terms are interchangeable.

Here, I will present an overview on applied Environmental Tracers to determine groundwater ages and assess their current limitation. Specially, I will make some predictions on the applicability for the near future. This includes some estimates about the concentration changes for the input of the tracers for younger groundwater and the consequences for reliability of the derived ages.

Also, I give an overview on the state of the measurement capacities and the possible development. It will be estimated how the analytical errors affect the ages. These constraints must be recognized for planning of field studies for groundwater age determination.

How to cite: Sültenfuß, J.: Suitability of Environmental Tracers as groundwater dating tools in the next decade, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7839, https://doi.org/10.5194/egusphere-egu22-7839, 2022.

The distribution of groundwater ages in aquifers is a key indicator for flow processes, solute transport and biogeochemical reactions. A lagged rejuvenation of groundwater ages has been observed at a 0.47 km² subcatchment of the Krycklan catchment in 2017¹. Chlorofluorocarbons (CFCs) were measured in 9 wells at different depths located close to the stream and revealed an overall representative age stratification for the subcatchment. Immediately below the water table at 1-2 meters depth, groundwater was already 30 years old. This lag in rejuvenation was successfully modeled on the assumption that it was caused by seepage flow of groundwater in the subsurface discharge zone that evolves along the interface between two soil types with different hydraulic permeability. The comparison of the observed groundwater age stratification with a simple analytical approximation shows that the lag in rejuvenation is an indicator for the extent of the subsurface discharge zone and the vertical gradient for the overall aquifer recharge.

To test this hypothesis a second sampling campaign in 2021 was performed. CFCs were measured in 49 sampling locations at different depths and distances to the stream within the subcatchment and neighboring subcatchment.  CFC-based groundwater ages show the extent of the subsurface discharge zone and reveal groundwater flow patterns. This study provides further information on the hydrological connectivity of groundwater in the hydrological cycle. 

References:

¹Kolbe, T, Marçais, J, de Dreuzy, J-R, Labasque, T, Bishop, K. Lagged rejuvenation of groundwater indicates internal flow structures and hydrological connectivity. Hydrological Processes. 2020; 34: 2176– 2189. https://doi.org/10.1002/hyp.13753

How to cite: Kolbe, T., Marçais, J., Vergnaud, V., Yvard, B., and Bishop, K.: Story continued: The Lagged Rejuvenation Phenomenon at the Krycklan catchment - 49 new samples reveal groundwater flow patterns and hydrological connectivity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-605, https://doi.org/10.5194/egusphere-egu22-605, 2022.

Karst hydrosystems are complex systems but often undergo high anthropogenic pressure on their water resources in Mediterranean area since it is shared by many actors. The addition of thermal and/or marine components in these systems makes the interpretation of classical methods more difficult. The thermal karst aquifer of Thau Basin (South of France) illustrates well the complexity of such underpressured karst hydrosystems. In the Balaruc-les-Bains area, groundwater results from the convergence and mixing of (i) young and cold karst water, (ii) old, hot and mineralized thermal water and (ii) marine water (Thau Lagoon and/or seawater). In the framework of the Dem'Eaux Thau CPER/FEDER project (2017-2022), we propose to combine tracers of water-rock interaction processes (Sr, Li) and residence time tracers (⁴He, ¹⁴C, ³⁶Cl) to better understand the origin and mean residence times of flow that takes place in the system, with a focus on the thermal water. In particular, the originality of this work was to constraint geological and hydrogeological informations using natural tracers and to calibrate He dating using ¹⁴C ages.

The combination of Si geothermometer and isotopic Sr signature (⁸⁷Sr/⁸⁶Sr) indicates that the thermal reservoir is located in the Jurrassic carbonate formation until 2000m depth. In addition Li concentrations show the existence of deep flows from granitic bed-rock. These new geochemical results allowed to better constraint the location of the thermal reservoir on the geological 3D map of the area and points out the major role of a local fault. However, there is significant uncertainty on the porosities of this reservoir impacting He age dating method. We used Carbon-14 dating in a deep karst well to constrain ⁴He ages and therefore determine the reservoir porosity. In parallel, the identification and quantification of the thermal flux by Li concentrations, allowed us to correct the ⁴He concentrations, and to propose a residence time of the thermal water of several thousands of years (10 000 to 50 000 years) which were consistent with the ³⁶Cl results. Thermal water subsequently feed a shallow reservoir (100 – 300 m) through local fractures and mix with variable proportions of recent karst flows. 

How to cite: Ranchoux, C., Ladouche, B., De Montety, V., Seidel, J.-L., and Batiot-Guilhe, C.: Combining residence time and isotopic tracers to better understand  groundwater reservoir and flows in a karst thermal aquifer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-768, https://doi.org/10.5194/egusphere-egu22-768, 2022.

Bottom-up catchment scale distributed hydrological transport models based on physical process descriptions improve our understanding of the hillslope scale in a physically consistent way. They allow for characterization of the flow domain as a multi-continuum, and are amenable to account for biogeochemical processes. However, these models are computationally expensive, and the spatial heterogeneity of the forcings and the boundary conditions are hard to render, which leads to ill posed inverse problems that adversely affect their predictive skill and convenience in real world applications (Hrachowitz et al., 2016).

On the other hand, flexible lumped or semi-distributed modelling approaches based on interplay of conceptual stores have proven their worth in reproducing hydrological responses despite their simplicity. The physical basis of these models is however not well understood – they differentiate between the timescales of hydrological response (governed by wave celerity arising from pressure diffusion) and water quality response (governed by flow velocity and solute dispersion) using passive mixing volumes with zero hydraulic pressure. Being depth based, they’re not scalable either. Conceptual models are thus mostly suitable for inverse modelling to compare relatable conceptual parameters in similar catchments.

In this study, we attempted to bridge the gap between these 2 different ideologies for groundwater flow and transport by building a semi-distributed grid-based model which discretizes a catchment based on its hydrodynamic dispersivity. The flow part was based on the concept of Mean Action Times along hillslopes (Simpson et al., 2013) and the transport part was based on solving the pore-scale advection dispersion equation by discretizing the domain as a series of well-mixed reactors to mimic the optimal behavior between the extremes of complete segregation and maximum mixedness for a given catchment. We verified the model with FEFLOW for a synthetic homogenous unconfined aquifer for unsteady flow and in the process established mathematical relationships between physical and conceptual parameters for groundwater flow and solute transport. We then applied the same framework to Kerrien, an agricultural and groundwater dominated headwater catchment located in the French Critical Zone Observatory of Brittany and gained insights on the sensitivity of different parameters on solute breakthrough behavior and transit time.

Ref:

Hrachowitz, M., Benettin, P., Van Breukelen, B. M., Fovet, O., Howden, N. J., Ruiz, L., ... & Wade, A. J. (2016). Transit times—The link between hydrology and water quality at the catchment scale. Wiley Interdisciplinary Reviews: Water, 3(5), 629-657.

Simpson, M. J., Jazaei, F., & Clement, T. P. (2013). How long does it take for aquifer recharge or aquifer discharge processes to reach steady state? Journal of Hydrology, 501, 241-248.

How to cite: Bhaduri, B., Muddu, S., and Ruiz, L.: An attempt to bridge the gap between physical and conceptual hydrological models used for transit time determination, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2119, https://doi.org/10.5194/egusphere-egu22-2119, 2022.

During rainfall, water infiltrates the soil, and percolates through the unsaturated zone until it reaches the water table. Groundwater then flows through the aquifer, and eventually emerges into streams to feed surface runoff. We reproduce this process in a  two-dimensional laboratory aquifer recharged by artificial rainfall (Fig. 1). As rainwater infiltrates, it forms a body of groundwater which can exit the aquifer only through one of its sides. The outlet is located high above the base of the aquifer, and drives the flow upwards. The resulting vertical flow component violates the Dupuit-Boussinesq approximation. In this configuration, the velocity potential that drives the flow obeys the Laplace equation, the solution of which crucially depends on the boundary conditions. Noting that the water table barely deviates from the horizontal, we linearize the boundary condition at the free surface, and solve the flow equations in steady state. We derive an expression for the velocity potential, which accounts for the shape of the experimental streamlines and for the propagation rate of tracers through the aquifer  (Fig. 1). This theory allows us to calculate the travel times of tracers through the experimental aquifer, which are in agreement with the observations. The travel time distribution has an exponential tail, with a characteristic time that depends on the aspect ratio of the aquifer. This distribution depends essentially on the geometry of the groundwater flow, and is weakly sensitive to the hydrodynamic dispersion that occurs at the pore scale.

Figure 1 : Streamlines in a laboratory aquifer recharged by artificial rainfall. Flow is from left to right. The streamlines converge towards the aquifer outlet, in the upper right corner of the picture.

How to cite: Lajeunesse, E., Devauchelle, O., Jules, V., Guérin, A., Jaupart, C., and Lagrée, P.-Y.: Flow and residence time in a laboratory aquifer recharged by rainfall, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3439, https://doi.org/10.5194/egusphere-egu22-3439, 2022.

The residence time of groundwater is an essential parameter for water resource management. In the presented study, environmental tracers (Rn-222, S-35, H-3, δ¹⁸O) and hydrogeochemical groundwater components are used for assessing groundwater mean residence times in a near surface aquifer.

At a barrage station at the River Moselle, four groundwater monitoring wells and two surface water spots were sampled at a 4-week interval over an 18-month period. At each sampling event, isotopes as well as other hydrogeochemical parameters (e.g. water level, water temperature, oxygen, pH value and electrical conductivity) were measured and evaluated.

All tracers showed different concentrations and signatures in ground- and surface water samples. As expected, Radon showed high concentrations in groundwater (up to 25 Bq/L) and low concentrations (about 0.2 Bq/L) in surface water. The tritium content of groundwater (13 Bq/L; ~110 TU) was similar to the long-term average concentration measured in surface water (~14 Bq/L); these comparatively high concentrations are way above the natural background concentration of about 1 Bq/L and result from the release of tritium from the French nuclear power plant Cattenom (situated about 250 km upstream of the sampling site). S-35, produced in the atmosphere and entering the hydrological cycle via precipitation, could be determined only once (January 2021) due to technical obstacles. The S-35 concentration measured in surface water (0.035 Bq/L) was about 4 times higher than the concentration in groundwater (0.0093 Bq/L). Finally, the median δ¹⁸O signature in surface water (-8.13 ‰) was similar to the signature found in groundwater (-7.78 ‰).

The selected isotopes and water parameters indicate that (i) the aquifer is predominantly recharged by surface water and (ii) the groundwater mean residence times varies between 5 and 6 months based on S-35 and δ¹⁸O.

Hence, it can be concluded that the selected isotopes are suitable as tracers for estimating groundwater mean residence times. However, further studies are needed, especially to minimize the time gap between the established tracers Radon (useful for up to 40 days) and tritium (useful from about one year). The novel tracer S-35 seems promising, but long-term data series of S-35 in surface and precipitation water are still missing to establish the necessary input functions.

How to cite: Schmidt, A., Engel, M., Mischel, S., and Radny, D.: Estimation of groundwater mean residence times in a near-surface aquifer using different natural tracers (Radon-222, Sulfur-35, Tritium, and δ18O), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8196, https://doi.org/10.5194/egusphere-egu22-8196, 2022.