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HS2.3.2

EDI
Water quality and water age analysis to understand flow and transport processes in catchments

Water quantity and quality is typically assessed and managed at the scales of catchments and aquifers. However, flow and transport integrate a multitude of hydrological transport and biogeochemical reaction processes interacting at different temporal and spatial scales, and thus hampering the understanding of underlying cause-effect relationships.
Recent advances in high-frequency measurements, machine learning, and the use of age tracers and their modelling have enhanced process understanding of flow and transport in catchments and aquifers. Our session brings together studies approaching this challenge from different angles and with different tools:
- Data-driven interpretation of water quality time series observed at the catchment outlet
- Isotope- and model-driven evaluation of transit times and water ages in catchments including the groundwater compartment
- Linkages of water transit times, hydrochemical and ecohydrological response

Convener: Andreas Musolff | Co-conveners: Giorgia LucianettiECSECS, Andreas Hartmann, Ingo Heidbüchel, Stefanie LutzECSECS, Camille MinaudoECSECS
Presentations
| Mon, 23 May, 08:30–11:50 (CEST)
 
Room B

Mon, 23 May, 08:30–10:00

Chairpersons: Andreas Musolff, Stefanie Lutz, Andreas Hartmann

08:30–08:35
Inroduction Data-driven Water Quality Assessment

08:35–08:45
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EGU22-11739
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solicited
Adam Ward and Steven Wondzell

Catchment hydrologists have long puzzled over the question: How can catchments rapidly generate storm flows in response to storm events? That question is complex because changes in stream water chemistry during storms suggest that proportions of old (or pre-event) water and new (or event) water also change during the storm. Conceptual models viewing catchments as composed of discrete source areas generating flow at unique time scales and with unique chemical characteristics have been used to explain the observed changes in flow and water chemistry. Surprisingly, those conceptual models usually do not treat the stream channel as one of the potential source areas. Here, we propose the channel source hypothesis in which the stream itself should be considered as a potential source with the same rigor as other contributing areas. We pose this in the spirit of the scientific use of the word: a hypothesis[1] is not a proven idea but rather a provisional supposition serving as the basis for further study. We suggest that the channel should be considered as a potential source for dissolved organic carbon (DOC). Channels store substantial amounts of organic matter, and stream ecologists have long studied stream carbon cycling. From those studies we know that leaching and decomposition can generate DOC from particulate organic carbon (POC). Further, POC is stored in channel “dead-zones” - regions of low flow velocity - that can be activated as flow velocity increases, thus releasing accumulated DOC during storms. All catchments are different; there is no reason to assume that channel sources are always important, in every catchment, in every storm. Thus, the channel source hypothesis does not replace existing conceptual models. Instead, it adds another potential mechanism that may explain DOC dynamics observed in streams. The channel source hypothesis has substantial implications for catchment studies examining sources of DOC in stream water or using DOC as a tracer to determine the locations of, and proportional contributions of, different source areas for streamflow generation.


[1] Hypothesis: "a provisional supposition from which to draw conclusions that shall be in accordance with known facts and serves as a starting point for further investigation by which it may be proved or disproved and the true theory arrived at" quoted from the Oxford English Dictionary (OED), 1985.

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How to cite: Ward, A. and Wondzell, S.: The Channel Source Hypothesis: Empirical Evidence for In-Channel Sourcing of Dissolved Organic Carbon to Explain Hysteresis in a Headwater Mountain Stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11739, https://doi.org/10.5194/egusphere-egu22-11739, 2022.

08:45–08:50
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EGU22-40
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ECS
Katharina Blaurock et al.

Dissolved organic carbon (DOC) is an important link between the terrestrial and aquatic carbon cycle. With regard to climate change, it is important to quantify DOC export from catchments as accurately as possible.  The goal of our study was to investigate the influence of topography on DOC export during different seasons.

We investigated DOC export in a small, forested headwater catchment in the Bavarian Forest National Park, Germany. From April 2020 until June 2021, we measured in-stream DOC concentrations at a 15 minutes interval at three nested sub-catchments using UV-Vis spectrometry. We compared DOC export between the different seasons (winter, snowmelt, wet early summer, wet autumn) and the different locations (two upper steep sub-catchments, one lower flat catchment).

Our results show that DOC export varied strongly between seasons. Whereas DOC export was only 2.3  – 2.7 kg/day/km2 on average in all sub-catchments during winter, it increased to 15.6 – 28.8 kg/day/km2 during the rainy early summer. Snowmelt also contributed to DOC export with 14.9 – 18.7 kg/day/km2 on average and was therefore almost as important as precipitation events in early summer and autumn. During winter and snowmelt, all sub-catchments contributed proportionally to total DOC export compared to their area. However, during the rainy seasons, the upper sub-catchments gained in relative importance leading to a disproportional contribution to total DOC export.

Our high-resolution data allowed us to obtain detailed quantities of DOC export over a long period covering different hydrological seasons. These numbers can help us to better understand the importance of DOC export during different seasons. Moreover, our results show that DOC export can vary strongly between small sub-catchments due to the importance of different hydrological processes. This finding is especially relevant as the number of drought periods and extreme rain events will increase and therefore not only influence the distribution of DOC export during the year but also have an impact on the contribution of different sub-catchments.

How to cite: Blaurock, K., Gilfedder, B. S., Fleckenstein, J. H., Peiffer, S., and Hopp, L.: High-resolution DOC measurements indicate seasonal differences of the contribution of sub-catchments to DOC export , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-40, https://doi.org/10.5194/egusphere-egu22-40, 2022.

08:50–08:55
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EGU22-2540
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ECS
José L. J. Ledesma et al.

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.

08:55–09:00
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EGU22-7307
Alfonso Senatore et al.

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.

09:00–09:05
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EGU22-962
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ECS
Qian Zhang et al.

Anthropogenic nutrient inputs have led to nutrient enrichment in many waterbodies worldwide, including Chesapeake Bay (USA). River water quality integrates the spatial and temporal changes of watersheds and forms the foundation for disentangling the effects of anthropogenic inputs. We demonstrate with the Chesapeake Bay Non-Tidal Monitoring Network that machine learning approaches – i.e., hierarchical clustering and random forest – can be combined to better understand the regional patterns and drivers of total nitrogen (TN) trends in large monitoring networks. Cluster analysis revealed regional patterns of short-term TN trends (2007-2018) and categorized the stations into three distinct clusters, namely, V-shape (n = 23), monotonic decline (n = 35), and monotonic increase (n = 26). Random forest models were developed to predict the clusters using watershed characteristics and major N sources, providing information on regional drivers of TN trends. Results show encouraging evidence that improved agricultural nutrient management has contributed to water-quality improvement. Moreover, water-quality improvements are more likely in watersheds underlain by carbonate rocks, reflecting the relatively quick groundwater transport. By contrast, water-quality improvements are less likely in Coastal Plain watersheds, reflecting the effect of legacy N in groundwater. Notably, TN trends are degrading in forested watersheds, suggesting new and/or remobilized sources that may compromise management efforts. Finally, the developed random forest models were used to predict TN trend clusters for the entire Chesapeake watershed at the scale of river segments (n = 979), providing fine-level information that can facilitate targeted watershed management, especially in unmonitored areas. More broadly, this combined use of clustering and classification approaches can be applied to other monitoring networks to address similar questions.

 

How to cite: Zhang, Q., Bostic, J., and Sabo, R.: Regional patterns and drivers of total nitrogen trends in the Chesapeake Bay watershed: Insights from machine learning approaches and management implications, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-962, https://doi.org/10.5194/egusphere-egu22-962, 2022.

09:05–09:10
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EGU22-2340
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ECS
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Highlight
Alexander Wachholz et al.

Long-term monitoring shows evidence of persistent changes in the magnitude and timing of the seasonal pattern and C-Q relations of nitrate concentrations in rivers, with possibly grave effects on aquatic ecosystems. Seasonal patterns of riverine nutrient concentrations are driven by a complex interplay of inputs, transport, and in-stream processing. Over multi-decadal periods, each of these factors may change due to socio-economic factors such as consumption patterns, governance regimes, or technological control measures. Here we test the hypothesis that observed multi-decadal changes in stream nitrate seasonality could be explained by changes in the relative importance of catchment nutrient sources over time. We analyze 66 years of shifting nitrate seasonality in a large, central-European river (Elbe) during a period of significant socio-political changes (1954 to 2019), with correspondingly significant changes in the sources of anthropogenic nitrate emissions. We show that the in-stream nitrate seasonality of the River Elbe changed significantly from a weak seasonal pattern (chemostatic) with peak concentrations during summer in the 1950s to a strong seasonal pattern (chemodynamic) with peak concentrations during winter in the 1990s. We link these shifts to a succession of technical/ political developments which influence the contribution of point and diffuse sources over time. Such shifts in seasonal concentration patterns can significantly impact the macronutrient (carbon, nitrogen, phosphorus) ratios in rivers, which in turn highly affect the health of aquatic ecosystems.

How to cite: Wachholz, A., Jawitz, J. W., Büttner, O., Jomaa, S., Merz, R., Yang, S., and Borchardt, D.: Drivers of multi-decadal in-stream nitrate regime shifts in a large European catchment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2340, https://doi.org/10.5194/egusphere-egu22-2340, 2022.

09:10–09:15
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EGU22-2392
Rémi Dupas et al.

Landscape organized (or structured) heterogeneity is often assumed to influence hydrological and biogeochemical patterns across space and time. In this study, we quantified landscape organized heterogeneity with two indices describing the spatial configuration of nitrogen sources or sinks regarding 1) their hydrological distance to the nearest stream (i.e. upslope/downslope heterogeneity: in the lateral dimension) and 2) their hydrological distance to the outlet in the river network (i.e. upstream/downstream heterogeneity: in the longitudinal dimension). The nitrogen sources considered are agricultural fields, defined from interpretation of satellite images, and the sinks are riparian wetland, defined from a topoclimatic index. Using public nitrate concentration and discharge data from 180 catchments in western France (5-150km²), we tested whether landscape organized heterogeneity influenced riverine nitrate concentration and dynamics. The metrics computed to characterize nitrate concentration and dynamics were the flow-weighted concentration (FWNO3), the slope of the log(C)-log(Q) relationship (slope b) and the ratio of the coefficients of variation of concentration and discharge (CVratio). Results showed a high positive correlation between slope b and the CVratio, but no correlation between the later and FWNO3. 43% of the catchment exhibited a positive b slope, indicating maximum nitrate during the winter high flow period and 17% exhibited a negative b slope, indicating maximum nitrate during the summer/fall low flow period; the remaining 40% exhibited a near-zero slope. Landscape organized heterogeneity was larger in the lateral dimension for both nitrogen source and sinks than in the longitudinal dimension. In the lateral dimension, nitrogen sources were primarily located upslope and nitrate sinks downslope. In the longitudinal dimension, no general trend was observed for nitrogen sources and nitrate sinks were rather located upstream. Heterogeneity in the lateral dimension was highly variable among catchments for the smaller catchments and less variable for the larger ones. Heterogeneity in the longitudinal dimension did not exhibit a visible relationship with catchment size. No relationship was found between indices of landscape heterogeneity and FWNO3, arguably because other primary factors (such as the nitrogen surplus or runoff) control most of the regional variability in FWNO3. We found non-linear relationships between our indices of nitrogen sink organization and the b-slope or the CVratio, both in the lateral and longitudinal dimensions. The catchments with a negative b-slope (maximum nitrate during low-flow season) had their wetlands located more upstream and/or more upslope than the average. The relationship with nitrogen sources were opposite by construction (agricultural fields are often located outside wetland areas) but less clear. Further work is ongoing to explore the influence of landscape spatial organization on phosphorus concentration and dynamics.

How to cite: Dupas, R., Casquin, A., Durand, P., and Viaud, V.: The influence of landscape organized heterogeneity on riverine nitrate dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2392, https://doi.org/10.5194/egusphere-egu22-2392, 2022.

09:15–09:20
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EGU22-11933
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ECS
Felipe Saavedra et al.

Nitrate contamination of rivers from agricultural sources, is a challenging problem for water quality management. The relationship between solute concentrations and streamflow rates (C-Q) observed at catchment outlets provide useful information on hydrological functioning and biogeochemical transformations at catchment scale. Nevertheless, nitrate C-Q relationships (linear regression in log space) often exhibit a considerable scatter. 

 

During runoff events, different nitrate transport paths within the catchment might be activated, generating a variety of responses in nitrate concentration. We hypothesize that the differences in characteristics of runoff events, such as different portions of rainfall and snowmelt contributions or antecedent wetness states, can explain some observed scatter in long-term C-Q relationships of nitrate.

 

To investigate this hypothesis, we analyzed low frequency nitrate data from 184 German catchments during different types of runoff events and quantified the deviations of the C-Q relationship for different event types. First, we computed the long-term C-Q relationships for each study catchment. Then, we attributed each nitrate grab sample to the corresponding runoff event type or non-event conditions, based on the nature of the inducing event and the antecedent wetness states of the catchments. Finally, we quantified the deviations from the long-term C-Q relationship.

 

We found pronounced deviations of different event types from the long-term C-Q relationships in most of the study catchments. During snow-impacted events, deviations are normally positive, indicating higher nitrate concentrations than the long-term C-Q relationships. On the other hand, deviations of rainfall events during dry antecedent conditions are mostly negative. Moreover, for rainfall events during wet antecedent conditions, we do not find persistent deviations from long-term C-Q patterns. Pronounced differences in event runoff coefficients among different event types indicate that contrasting levels of hydrological connectivity are a key control of C-Q deviations among different event types.

How to cite: Saavedra, F., Musolff, A., Von Freyberg, J., Merz, R., Basso, S., and Tarasova, L.: Can runoff event types explain some scatter in nitrate C-Q relationships?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11933, https://doi.org/10.5194/egusphere-egu22-11933, 2022.

09:20–09:25
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EGU22-3390
Jørgen Windolf et al.

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.

09:25–09:30
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EGU22-8223
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ECS
Razi Sheikholeslami and Jim Hall

Livestock are known to be one of the leading sources of nitrogen and other organic pollutant contents in surface waters. In response to growing demand for animal-source foods, livestock production has shifted into more high-input systems, accompanied by increased fertilizer and animal manure application rates to produce feed, which has resulted in large losses of nitrogen. The effects of nitrogen contamination of water bodies on human and ecosystem health are not limited and localized but have large-scale implications and are rapidly becoming widespread. However, current approaches to estimate the changes in stream water quality often fail to explicitly account for livestock activities across large spatial scales. To overcome this challenge, we adopted a data-driven approach and developed a spatio-temporal water quality model. Our model is based on a popular supervised machine learning technique, known as random forests, that can efficiently handle large, heterogeneous geo-environmental datasets. The model was trained using several site-level measurements and a large set of gridded environmental covariates to predict monthly nitrogen concentrations across the world. We then performed variable importance analysis on the proposed model to identify influential drivers of nitrogen variability at global scale. Our results confirmed the prominent role of livestock population and nitrogen fertilizer use in pollution of the river systems. Finally, we quantified how much livestock has contributed to nitrogen pollution in 115 major river basins of the world. We found that during 1992-2010 the average increase in nitrogen concentrations due to livestock was about 15% globally. Importantly, model results also indicate that at some large basins livestock population is responsible for more than 50% of raise in the levels of nitrogen. These regions point to the global livestock ‘‘hot spots’’ where high nitrogen loading to waterways may be expected. The results and insights gained in this study can have important implications for better management of livestock faming systems and pollution control policies.

How to cite: Sheikholeslami, R. and Hall, J.: The role of livestock in nitrogen pollution of large river basins: A machine learning-based assessment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8223, https://doi.org/10.5194/egusphere-egu22-8223, 2022.

09:30–09:35
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EGU22-1712
Jörg Steidl et al.

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.

09:35–09:40
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EGU22-341
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ECS
zafrir adar et al.

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.

09:40–09:45
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EGU22-2497
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ECS
Mario Kolling et al.

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.

09:45–09:50
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EGU22-8455
|
ECS
Nicolai Brekenfeld et al.

Stream water chemistry at catchment outlets is commonly used to infer the flowpaths of water through the catchment and to quantify the relative contributions of various flowpaths. High-frequency and multi-elemental timeseries could shed light on the dynamic activation/deactivation and the changing relative contributions of different flowpaths during storm events or diel cycles in summer. Here, we present multi-year, high-frequency (< 60 minutes) timeseries of the major cations and anions from the outlet of three small (0.8 – 40 km²) french catchments with contrasting land-use (forest, field crops and mixed farming-cropping productions). Instead of analysing the concentration dynamics of individual elements, we use elemental ratios in order to identify the contrasting temporal variations of different elements during storm events. We try to link the dynamics of the elemental ratios to specific flowpaths, constrained by the processes likely to modify the ratios. Then, we compare the inferred flowpath contributions with our perceptual understandings of the three catchments. These findings contribute to our understanding of dynamic flowpath activation in catchments and the value of high-frequency, multi-elemental stream concentration timeseries.

How to cite: Brekenfeld, N., Fovet, O., Blanchouin, A., Cordier, L., Cotel, S., Faucheux, M., Floury, P., Fourtet, C., Hamon, Y., Henine, H., Petitjean, P., Pierson-Wickman, A.-C., Pierret, M.-C., and Gaillardet, J.: High-frequency, multi-elemental stream concentration timeseries as a tool for catchment flowpath identification, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8455, https://doi.org/10.5194/egusphere-egu22-8455, 2022.

09:50–09:55
|
EGU22-12985
|
Highlight
Claire Oswald et al.

In watersheds impacted by urban growth and road salt usage, increasing stream chloride (Cl-) concentrations are well-documented. Peaks in stream Cl- concentrations that exceed chronic and/or acute water quality guidelines are typical in the winter salting season when Cl- (from Cl--based de-icers) is flushed from the landscape but are not easily measured with grab samples. In some cases, chronic Cl- conditions persist into the summer growing season due to a build-up of Cl- in the subsurface. Estimating the proportion of Cl- loads transported in the salting and non-salting seasons is of interest for tracking the relative role of subsurface Cl- pools to the annual load, as well as the influence of runoff events on loads across the two periods. In this study, we made use of a 6-year record of high-frequency stream Cl- concentrations from an urbanizing watershed in southern Ontario, Canada. High-frequency measurements revealed that the acute and chronic water quality guidelines for Cl- were exceeded for 7 and 97 % of the study period, respectively. Salting season Cl- loads were 2 to 5 times higher than in the non-salting season, but surprisingly, inter-event periods contributed 21 to 56 % of the annual load across years. The results of this study illustrate the utility of high-frequency sensors for identifying water quality extremes that negatively impact aquatic ecosystems, identifying Cl- transport pathways, and tracking the build-up of legacy Cl- in the subsurface.

How to cite: Oswald, C., Ross, C., Moslenko, L., and Wellen, C.: High-frequency observations reveal acute chloride pulses and chloride legacy effects in an urbanizing watershed impacted by road salting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12985, https://doi.org/10.5194/egusphere-egu22-12985, 2022.

09:55–10:00
Discussion

Mon, 23 May, 10:20–11:50

Chairperson: Andreas Hartmann

10:20–10:25
Introduction to Water Ages in Ecohydrology

10:25–10:30
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EGU22-5852
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ECS
|
Highlight
Stefan Seeger and Markus Weiler

At the catchment scale, incoming precipitation is typically partitioned into runoff and evapotranspiration. Most transit time modeling approaches focus on the runoff part of the water balance and are consequently evaluated with respect to water ages in streamflow. Nevertheless, most modeling approaches also include a more or less detailed representation of evapotranspiration. In order to evaluate whether a modeling approach appropriately captures evapotranspiration and its water ages, it is important to understand what processes are required to be considered in the models. While evaporation from the soil and canopy surfaces is relatively easy to observe, transpiration involves root water uptake from a range of depths below the soil surface and water transport within plants. In addition, experimental data to evaluate transit time models are sampled in different organs and locations of plants.

We will present the results of studies dealing with the transit times of root water uptake (between precipitation and water uptake) and with transit times internally in trees (between water uptake at the root tips and transpiration at the leaves) in temperate forests. A novel in-situ measurement technique enabled us to measure stable water isotopes in the soil and within tree stem xylem with unprecedented temporal resolution and thereby enabled us to refine our understanding of plant water uptake and tree internal water transport. Based on our experimental observations, we developed a new approach to model water transport and hence water transit time internally in trees.

How to cite: Seeger, S. and Weiler, M.: Water ages at the soil root interface and beyond, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5852, https://doi.org/10.5194/egusphere-egu22-5852, 2022.

10:30–10:35
Discussion on Seeger & Weiler

10:35–10:40
|
EGU22-1738
Julian Klaus et al.

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.

10:40–10:45
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EGU22-3046
|
ECS
Maëlle Fresne et al.

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.

10:45–10:50
|
EGU22-4898
|
ECS
Arianna Borriero et al.

Water transit time distributions (TTDs) are important descriptors of hydrological functioning and solute mobilization in catchments. The use of transport models based on StorAge Selection (SAS) functions is promising for characterizing non-stationary TTDs. Model parameters are typically calibrated using tracer concentration in inflow (e.g., precipitation) and outflow (e.g., streamflow) in order to obtain suitable values of SAS function parameters and, thereby, simulate TTDs at catchment-scale. However, due to uncertainties in tracer data and equifinality problems in SAS modelling, modeled TTDs can be subject to considerable uncertainty. Therefore, we need alternative and independent methods that can help constrain model parameters. An example is the young water fraction (Fyw), which quantifies the proportion of catchment outflow younger than approximately 2–3 months. Our work attempts to explore the robustness of Fyw in constraining SAS model parameter values and, in turn, reducing predictive uncertainty of TTDs in multiple contrasting sub-catchments in the Central European Bode River Basin. We simulated TTDs using sparse (i.e., monthly) stable water isotope data (δ¹⁸O) in streamflow for calibration in an experimental SAS modelling framework. In a subsequent step, we directly compared the model estimates of long-term average (marginal) TTDs with Fyw derived from the seasonal cycles of δ¹⁸O measured in precipitation and streamflow. Our results showcase if and to what extent Fyw is a valuable additional constraint to infer SAS parametrizations as well as improve TTD predictions and the characterization of water age selection dynamics, and identify potentials and gaps in isotope-based TTD models. Our results also show how the effectiveness of Fyw in reducing the predictive uncertainty of TTDs may depend on the water use by plants and land use change across physiographically different sub-catchments. Overall, as the relevance of Fyw in TTD modeling is not yet well established, our aim is to investigate whether additional indicators such as Fyw are useful for TTD modeling and thus allow improving the description of flow and transport in catchment areas, especially in situations where a high-resolution tracer data are lacking.

How to cite: Borriero, A., Kumar, R., Nguyen, T., Fleckenstein, J., and Lutz, S.: Evaluating the added value of young water fractions for determining water transit times in diverse catchments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4898, https://doi.org/10.5194/egusphere-egu22-4898, 2022.

10:50–10:55
|
EGU22-5100
|
ECS
Giulia Grandi and Enrico Bertuzzo

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.

10:55–11:00
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EGU22-7347
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ECS
Siyuan Wang et al.

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.

11:00–11:05
Introduction to estimation of groundwater ages

11:05–11:15
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EGU22-7839
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solicited
|
Highlight
Jürgen Sültenfuß

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.

11:15–11:20
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EGU22-605
|
ECS
Tamara Kolbe et al.

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.

11:20–11:25
|
EGU22-768
|
ECS
Coralie Ranchoux et al.

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.

11:25–11:30
|
EGU22-2119
|
ECS
Baibaswata Bhaduri et al.

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: Water3(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 Hydrology501, 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.

11:30–11:35
|
EGU22-3439
Eric Lajeunesse et al.

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.

11:35–11:40
|
EGU22-4932
Christian Moeck et al.

3H and tritiogenic 3He concentrations and their interpretation as  3H/3He apparent water ages have been proven to offer crucial insights on groundwater flow and transport processes. However, the analysis is expensive as well as labor‐ and time‐intensive. Recent developments of portable field‐operated gas equilibrium membrane inlet mass spectrometer (GE‐MIMS) systems provide however, a unique opportunity to measure relatively fast dissolved gas concentrations, such as 4He, in groundwater systems with a high resolution at relatively low costs but they are not capable of providing an apparent age. However, 4He accumulation rates are often obtained from 3H/3He ages and it has been shown that non-atmospheric 4He concentrations determined in the laboratory (e.g., by static (noble gas) mass spectrometry) and by field-based (GE-MIMS) methods closely agree. This agreement allowed to quantify the local (radiogenic) 4He accumulation, e.g., we were able to establishing an inter‐relationship between 3H/3He apparent groundwater ages and the non-atmospheric 4He excess (e.g., calibrating the 4He excess in terms of residence time).

We demonstrate that the 4He excess concentrations derived from the GE‐MIMS system serve as adequate proxy for the experimentally demanding laboratory based analyses. The combined use of 3H/3He lab‐ based ages and calibrated  4He ages opens new opportunities for site characterization due to the measurements facilitated by the GE‐MIMS.

For our urban and contaminated study site, we combine groundwater ages with hydrochemical data, water isotopes (δ18O and δ2H), and perchloroethylene (PCE) concentrations (1) to identify spatial inter‐aquifer mixing between artificially infiltrated surface water and groundwater originating from regional flow paths and (2) to explain the spatial differences in PCE contamination. Moreover, for some wells, we identify fault‐induced aquifer connectivity as a preferential flow path for the transport of older groundwater, leading to elevated PCE concentrations.

How to cite: Moeck, C., Popp, A., Brennwald, M., Kipfer, R., and Schirmer, M.: Combined Use Of 3H/3He Apparent Age And On‐Site Helium Analysis To Identify Groundwater Flow Dynamics And Transport Of PCE, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4932, https://doi.org/10.5194/egusphere-egu22-4932, 2022.

11:40–11:45
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EGU22-8196
Axel Schmidt et al.

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.

11:45–11:50
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EGU22-12903
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ECS
Elena Petrova et al.

Quantification of the travel time distribution and transport parameters in fractured aquifers is crucial for understanding contaminant transport in fractured systems. Although knowledge about the travel time distribution is a helpful tool to assess the solute transport, it can’t be measured directly. Travel time is typically back-computed from different tracers in groundwater by applying well established analytical methods. However, in fractured aquifers diffusive exchange with the rock matrix, intersection of streamtubes and associated mixing, as well as other processes can cause deviation of the estimated travel time from the mean advective travel time. Direct numerical modelling of the tracer’s reactive behavior with the travel time as one of the calibrated parameters can lead to non-uniqueness of the result. These non-unique solutions typically lead to a high level of parametric uncertainty especially on catchment scale. In this work, we address the reduction of uncertainty in mean travel time, shape parameter of travel time distribution, fracture aperture, and porosity by means of multiobjective optimization enhanced by surrogate modelling. For pre-selection of potentially plausible model runs Gaussian Processes Emulation (GPE) was applied within four-parametric space. We use the GPE with multitracer conditioning for pre-selection of plausible parameter combinations. Posterior distributions were employed to estimate the mean groundwater travel times at sampling locations, to distinguish between different rock facies of captured streamlines, and to get an estimate of fracture apertures. We confirm the hypothesis that using tritium and helium isotopes together with radiogenic helium measurements helps to achieve a unimodal posterior distribution and reduces uncertainty significantly.

How to cite: Petrova, E., Osenbrück, K., Mayer, K. U., Finkel, M., and Grathwohl, P.: Travel Time Uncertainty Reduction by Multiobjective Optimisation of Isotope Age Tracer Models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12903, https://doi.org/10.5194/egusphere-egu22-12903, 2022.