Nutrient supply, processing, and transport in fluvial ecosystems have received increasing attention over recent decades due to their ecological significance and influence on water quality. Within catchments, riparian zones are widely recognized as critical control points for nutrient exports, serving as the last buffer zone of terrestrial nutrient exports. However, understanding their influence on downstream nutrient patterns, compared with other nutrient sources, remains poorly constrained. In this talk, we examine some of the key mechanisms by which riparian zones influence stream nutrient dynamics across different spatial and temporal scales. By combining synoptic surveys across multiple catchment compartments (soils, groundwater, stream) with modelling approaches, we demonstrate the dual role of riparian zones as both major regulators for terrestrial nutrient inputs and controls of stream metabolism and associated nutrient processing. Further, we discuss the hydrological and biogeochemical significance of riparian zones, with particular emphasis on its variability across seasons, along the river continuum and among biomes. Overall, this talk aims to highlight the need for an integrated, landscape-scale perspective to advance our understanding of catchment biogeochemistry.

How to cite: Lupon, A., J. Ledesma, J. L., Poblador, S., Jativa, C., Sabater, F., Sponseller, R., Martí, E., Laudon, H., and Bernal, S.: Where land and water meet: The role of riparian zones in catchment nutrient cycling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9392, https://doi.org/10.5194/egusphere-egu26-9392, 2026.

Release of dissolved organic carbon (DOC) from soils into pore waters can occur via three distinct processes: (i) microbial decomposition of soil organic matter, producing soluble organic molecules; (ii) desorption of DOC from soil mineral surfaces, such as oxyhydroxides and clays, driven by increasing pH in pore waters and (iii) reduction of ferric iron in waterlogged soils, leading to the dissolution of amorphous Fe(III) oxyhydroxides that were previously coprecipitated with dissolved organic matter. Once released into pore waters, DOC can either be mineralised or exported to surface waters.

Riparian wetlands are important sources of organic matter and sites where all three DOC release mechanisms may occur. DOC release via microbial decomposition (process i) is typically associated with a rapidly cycling DOC pool that fuels microbial mineralisation, whereas desorption and reductive dissolution processes (ii and iii) are often linked to DOC mobilisation and export of a potentially more stable, mineral-associated pool of soil organic matter. Despite their importance, the relative contributions of these processes and their controlling factors remain incompletely understood. Improved understanding of these mechanisms may provide insight into the extent to which these processes are likely to influence the long-term carbon storage capacity of wetland soils.

Against this background, we propose and demonstrate a modelling approach to disentangle the processes and drivers of seasonal DOC mobilisation in riparian pore waters of the Kervidy-Naizin Critical Zone Observatory in western France. First, a principal component analysis was applied to weekly to biweekly measurements (period from November 2022 until May 2023 and 17 sites) of pH, nitrate, DOC, soluble reactive phosphorus, ferrous iron, five variables describing dissolved organic matter composition based on fluorescence properties, and hydrological variables. Scores of the first two principal components - interpreted as proxies for DOC desorption and reductive dissolution of coprecipitates - were extracted. Second, these component scores, together with two additional variables assumed to represent lateral DOC leaching and microbial decomposition of soil organic matter, respectively, were used as predictors in a generalized additive model (GAM) of DOC concentrations. Third, the GAM was used to quantify the relative contributions of the four processes to DOC increases.

Our analysis suggests that desorption was the dominant process responsible for DOC release during winter and spring in the studied riparian zones of the Kervidy-Naizin catchment. These results demonstrate that disentangling the processes contributing to seasonal DOC mobilisation in riparian soils - such as pH driven desorption and reductive dissolution of coprecipitates - is feasible using a combined multivariate and additive modelling approach. To further improve the quantification of individual process contributions, additional measures of DOC quality, for example derived from FT-ICR MS, are likely to be beneficial.

How to cite: Selle, B., Dupas, R., Fovet, O., Jaffrezic, A., Jeanneau, L., and Lechtenfeld, O.: A modelling approach to disentangle DOC release processes from riparian wetlands , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8247, https://doi.org/10.5194/egusphere-egu26-8247, 2026.

Dissolved organic matter (DOM) plays a key role in weakly-buffered surface water ecosystems by influencing pH, metal speciation, and the bioavailability of trace elements through complexation reactions. Despite a substantial decrease in acid deposition over recent decades, many Swedish streams and lakes remain relatively acidic, largely due to the natural acidity associated with DOM.  A robust understanding of the acid–base properties of DOM is therefore essential for accurately assessing natural water acidity and determining the continued need for liming as a mitigation measure. In this study, we analyzed national monitoring data comprising more than 42,000 samples collected between 1990 and 2024 from 136 streams across Sweden to quantify long-term changes in DOM acidity. Organic matter charge density at pH 5.6 (OMCD_5.6) was used as an indicator of the dissociation behaviour of dissolved organic acids. While spatial variation in median OMCD_5.6 among sites was relatively small, compared to variation within individual stations, approximately two-thirds of the sites exhibited significant temporal trends, predominantly reflecting a decline in organic matter charge density over time. Lower charge density implies reduced dissociation of organic acids and, at constant DOM concentrations, a diminished influence of DOM on stream water pH. We further examine how catchment characteristics, such as atmospheric deposition, land-use, and water chemistry, and their long-term changes, relate to observed trends in OMCD_5.6. Our findings challenge the common assumption that changes in DOM dissociation properties can be neglected in surface water chemistry assessments and highlight the need to explicitly consider shifts in DOM acid–base properties.  

How to cite: Lackner, A., Vogt, R. D., Sjöstedt, C., Gustafsson, J.-P., and Bishop, K.: Dynamics of the Dissociation of Dissolved Organic Acids across Swedish Streams over 35 Years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9743, https://doi.org/10.5194/egusphere-egu26-9743, 2026.

Understanding the patterns and mechanisms of DOC export from catchments is key to interpreting and quantifying land-to-ocean lateral carbon transport process. The DOC export is highly dynamic and storm-driven, especially in mountainous headwater catchments. Yet traditional low-frequency monitoring and modelling often misses rapid DOC changes and fail to identify potential shifts on DOC export pattern and process, limiting a deeper understanding of short-term export mechanisms. We have conducted high-frequency measurements combined with process-based modeling in a mountainous flash flood headwater catchment in China, to reveal how hydrological processes control DOC mobilization and export under varying storm conditions. Based on high-frequency measurements and hysteresis analysis, we have found a three-phase concentration-discharge (C-Q) relationship of DOC and a shift on DOC export pattern from transport-limited to source-limited across extreme storms. The studied catchment streamflow and DOC dynamics were successful reproduced by the process-based INCA-C model at hourly steps, further supporting the quantification of different flow pathways’ contributions on DOC output. The results showed that more DOC were exported by subsurface flows from shallow organic soil with greater peaks and shorter time-to-peaks at higher storm intensities. DOC is primarily sourced from subsurface runoff from the mineral layer (73 %–77 %) during moderate events, whereas it is primarily sourced from subsurface runoff from the organic layer (61 %–79 %) during extreme events. The two contrasting contributions suggest that hydrological pathway controls and DOC dynamic patterns can shift owing to runoff generation influenced by storm intensity. Our research revealed mechanisms on shifted DOC export regimes at a typical flash flood catchment with increasing storm intensity and changing flow-path contributions. The findings highlight high-frequency measurements and modellings for insights into hydrological controls on DOC export process.

How to cite: Wu, Y., Cheng, L., Su, H., Fu, C., and Qin, S.: Dissolved organic carbon export mechanism at a typical flash flood catchment: insights from high-frequency measurements and modellings, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8828, https://doi.org/10.5194/egusphere-egu26-8828, 2026.

High-resolution, water quality observations reveal complex instream transformations and mixing of solutes dependent on the source and the prevailing flow conditions, with both concentration and dilution effects apparent. Here we explore river confluence behaviour using a combination of an Acoustic Doppler Current Profiler (ADCP) and multiparameter water quality sonde to resolve solute and particulate behaviours of tributary mixing. We hypothesise that confluence-scale functional heterogeneity is spatially persistent, but that hydrological forcing systematically alters the predominance of source- and sink-type behaviours spatially across the confluence. To test this, we implemented an innovative quasi-synoptic field campaign at the Kennet-Thames River confluence (Reading, UK). Using an uncrewed moving-boat platform (ArcBoat), we collected simultaneous, high-resolution data on velocity, nutrients (NH₄⁺, NO₃⁻), fluorescent dissolved organic matter (fDOM), and turbidity on three days. Each time, the reach was subdivided into 14 fixed spatial zones, allowing reproducible analysis across the three hydrologically distinct campaigns: higher winter flow (12 Feb 2025), a rainfall event pulse (26 Feb 2025), and low spring baseflow (14 Mar 2025). 

We evaluated solute behaviour using a Reactivity Index (RI) based on conservative mixing and integrated it with a Hydrodynamic Index (H) to classify each observation into process-informed categories (e.g., Reactive, Retentive, Low-energy depletion, Attenuating, and Conservative). Extending the same logic to turbidity yielded complementary particulate-transport classes (Local Input, Advective Input, Sediment Deposition, Advective Dilution and Conservative mixing).

Zone-wise analysis revealed exceptionally strong and persistent spatial structuring of functional classifications across campaigns (Kruskal–Wallis ε² = 0.28–0.81, p < 0.0001 for solute RIs). Across the fortnightly transition from event to baseflow conditions, the Kennet-influenced pathway exhibited a coherent regime shift in both dissolved and particulate classifications: during the rainfall snapshot, NH₄⁺ enrichment and turbidity input classes dominated, whereas under baseflow the same corridor shifted toward attenuation-depletion and dilution-deposition dominance.

These results demonstrate that confluence function is organised into distinct functional zones. Hydrological forcing does not erase these zones but alters the predominant process, driving downstream branch-wide (Kennet-influenced, middle mixing corridor, and Thames-influenced branches) switches from source to sink dominated regimes. Because confluences integrate signals from contrasting tributary sub-catchments, this approach provides a transferable workflow for translating high-resolution synoptic patterns into process-based diagnostics that complement fixed-station monitoring and help locate source- versus sink-dominant behaviour within river catchments.

How to cite: Rameshwaran, P., Pesso, C., Wade, A. J., and Everard, N.: From Pattern to Process at a River Confluence: A Process-based Reactivity-Hydrodynamic Framework from High-resolution Synoptic Monitoring, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13450, https://doi.org/10.5194/egusphere-egu26-13450, 2026.

River water quality shapes both ecosystem health and human well-being. However, rapid fluctuations in a river's water quality can emerge between sampling intervals and escape detection. These brief events are often linked to intense rainfall or post-drought flushing. Consequently, low-frequency grab sampling can result in an incomplete representation of water quality in a river. By contrast, high-frequency monitoring at hourly or finer intervals can reveal these previously hidden water quality dynamics. What remains unresolved is how frequently we must sample to detect short-lived water-quality events without exceeding realistic monitoring effort. To address this, we systematically examine the effect of sampling frequency on accurately capturing riverine water-quality dynamics, with particular focus on extreme values. We use a novel, large-sample, Germany-wide dataset of multi-year hourly river water quality records from 72 catchments, covering more than 70 per cent of Germany's land area. We focus on eight primary parameters, including conductivity, dissolved oxygen, ammonia, nitrate, pH, phosphate, turbidity, and water temperature. Each time series is sub-sampled along a continuous range of interval lengths (hourly to annually). We analyse different measurement objectives through skewness, water quality duration curves, and weighted regression on time, discharge and season (WRTDS). For each interval, we compute skewness to diagnose extreme values' behaviour, as well as annual duration curves of water quality. WRTDS was applied to determine whether a relatively simple model can overcome sampling-interval-induced inaccuracy. Our results show that low-frequency intervals of one week or longer are consistently associated with a considerable loss of information, most substantially for dissolved oxygen, pH and conductivity. This loss was pronounced for extreme values, while the mean and median were less affected. WRTDS did not substantially combat the information loss associated with increasing sampling intervals. Estimates of skewness and coefficient of variation worsened, while median values showed only minor improvements. We conclude that sampling frequency must align with the monitoring objective and be explicitly incorporated into the interpretation of the results. Coarse sampling can approximate central tendencies, but not extremes or variability. These findings underscore the need for tailored sampling strategies to optimise water quality monitoring and ensure that critical fluctuations are not overlooked.

How to cite: Hettler, W., Stahl, K., Ebeling, P., Fohrer, N., Kiesel, J., and Winter, C.: Using large-sample high-frequency records to optimise water quality sampling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-445, https://doi.org/10.5194/egusphere-egu26-445, 2026.

Quantifying solute and particulate export from river catchments is particularly challenging when concentration measurements are sparse, irregular, or limited to low‑frequency monitoring schemes. In such conditions, traditional regression‑based approaches used to interpret concentration–discharge (C–Q) relationships often fail to capture the full range of hydrological and hydrochemical variability. Here we present a bootstrap‑based methodology designed to robustly infer export mechanisms and associated uncertainties using minimal observational data.

The approach combines daily discharge series (Q) with available concentration measurements (C), explicitly separating rising and falling limbs of the hydrograph to account for hysteresis effects and asymmetric mobilization dynamics. In each bootstrap iteration (N≈1000), entire (Q, C) pairs are resampled and the C–Q relationship is fitted. For each parameter set, cumulative load duration curves (Mp%, p = 1–99) are computed, enabling the derivation of confidence intervals for both regression parameters and export‑pattern metrics.

The method yields a full empirical distribution of Mp%, allowing process‑based inference even in catchments with very limited measurements. Results demonstrate that bootstrap‑derived parameter distributions reliably distinguish dilution vs. mobilization patterns, identify “hot moments” of export, and quantify hysteresis strength. Percentile‑based confidence intervals effectively communicate uncertainty without assuming parametric error structures, making the framework well-suited for diverse catchment types and monitoring strategies.

This work shows that bootstrap resampling provides a powerful, model‑agnostic tool for moving “from pattern to process” in data‑scarce environments. The methodology enables more defensible interpretation of C–Q behaviours, supports targeted design of water‑quality monitoring networks, and provides transferable insights for ungauged or poorly sampled catchments.

How to cite: Szalińska, W., Ciupak, M., and Tokarczyk, T.: From Sparse Measurements to Robust Inference: A Bootstrap Framework for Solute and Particulate Export Mechanisms Assessment in River Catchments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20890, https://doi.org/10.5194/egusphere-egu26-20890, 2026.

To test hypotheses about catchment processes inferred from hydrological and hydrochemical patterns observed at the outlet, measurements are needed at a high temporal frequency from multiple water sources distributed in space. Here we present the Water Analysis Trailer for Environmental Research (WATER), a trailer-based mobile sampling platform capable of autonomously measuring stable water isotopes, nitrate, electrical conductivity, pH and temperature for 72 samples per day, collected from up to 11 sources. As a proof of concept, the WATER was deployed to the Schwingbach Environmental Observatory (1.03 km²) in Hesse, Germany, where six water sources were analysed (2 × stream water, 3 × groundwater, 1 × precipitation) for a period of six months. The multi-source, high-frequency data offered new insights into catchment functioning that had not been revealed by previous, lower-resolution sampling campaigns. For example, rapid vertical movement of incoming precipitation into the soil and a strong linkage between shallow sub-surface flow paths and the stream became apparent during events. In addition, streamflow generation and water quality at the catchment outlet showed likely signs of influence from nearby water sources and arable farming practices. Simulating the reduced sampling frequency associated with connecting additional sources to the WATER indicated that key features of the collected data would likely be preserved if sampling occurred over a period of several months. Overall, the WATER provides a mobile and scalable approach for moving from pattern-based inference to process understanding through multi-source, high-temporal-frequency measurements.

How to cite: Neill, A., Windhorst, D., Kraft, P., Sahraei, A., and Breuer, L.: The Water Analysis Trailer for Environmental Research (WATER): Proof-of-concept and process insights from multi-source, high-frequency measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17655, https://doi.org/10.5194/egusphere-egu26-17655, 2026.

Diffuse agricultural pollution is a major driver of nutrient enrichment in European surface waters, and its impacts are expected to intensify as climate change alters the timing, magnitude, and intensity of precipitation events. The Po River basin, like many nutrient hotspots worldwide, exhibits a chronic excess of nitrogen (N) driven by the preferential export of highly mobile nitrate. Extreme precipitation events, however, can also mobilize less mobile nutrient pools through erosion-driven transport. Phosphorus (P), which preferentially accumulates in the solid phase due to its strong affinity for soil particles, is therefore mainly exported during high-flow conditions, leading to marked, event-driven shifts in nutrient stoichiometry. Yet, how these processes vary across basins with contrasting nutrient surpluses, land management, and physical settings remains poorly understood.

In this study, we analyzed N and P export dynamics in two agricultural basins of the Po River watershed that experienced similar climatic anomalies but differ strongly in their biogeochemical and physical characteristics. The Chiese basin is characterized by high livestock density and a marked nutrient surplus, whereas the Volano basin exhibits lower livestock pressure and overall nutrient deficit. The two basins further contrast in soil permeability and topography, providing a natural experiment to investigate control mechanisms on nutrient export.

Monthly samplings carried out at the basin outlets throughout 2024-2025 were combined with high-frequency autosampler measurements (every 3 hours) during hydrological extremes. Dissolved and particulate phosphorus, including its bioavailable fraction and particulate nitrogen, together with dissolved inorganic nitrogen species (NO₃^-, NO₂^-, NH₄⁺), were quantified and linked to continuous discharge records.

Both basins displayed elevated nutrient concentrations, with contrasting partitioning between dissolved and particulate forms linked to differences in hydrological functioning. Hydrological extremes exerted divergent controls on nutrient behaviour, resulting in dilution or strong mobilization responses. Event-specific concentration-discharge relationships varied with seasonality and rainfall intensity. Overall, extreme events induced variable but generally P-dominated shifts in nutrient stoichiometry, with short-lived pulses accounting for a substantial fraction of annual export and temporarily altering N:P ratios.

How to cite: Magri, M., Severini, E., Soana, E., Gervasio, M. P., Vincenzi, F., Castaldelli, G., and Bartoli, M.: Extreme precipitation controls P export mechanisms and N:P stoichiometry in contrasting Po River agricultural basins, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12167, https://doi.org/10.5194/egusphere-egu26-12167, 2026.

Oil palm plantations have become a major land-use in parts of the wet tropics over the past 40 years. There has been much concern about its hydrological, erosional, water quality and nitrous emissions (and hence climate change) consequences.  These impacts, however, may vary considerably with terrain  (hilly terrain with bench-terracing being very different from low- and    moderate-slope terrain without bench-tarracing), details of land management practices, and  over the c. 25 years life-cycle of oil palm plantations – but remain largely unassessed.  With regard to water quality, this paper presents evidence from catchment studies in hilly terrain since 2012 in the headwaters of the Brantian and Kalabakan river basins in Sabah (Malaysian Borneo), with a  particular focus on results from a bench-terraced, mature oil palm catchment (3.27 km²).  Studies there formed part of Stability of Altered Forest Ecosystems (SAFE) Project, within which comparisons were drawn between the oil palm (OP) catchment, a near-primary (VJR Virgin Jungle Reserve) catchment, and six multiple-logged catchments. All catchments were instrumented from 2011 with sensors recording 15-minute data on conductivity, turbidity, water temperature and water depth (and hence discharge). This was supplemented by (a) a programme of monthly spot sampling for water chemistry, (b) opportunistic storm event sampling of water chemistry since 2014, (c) a regional survey of baseflow water chemistry of oil palm catchments in 2014; and (d) exploratory application in June 2025 of a multi-isotope approach (using δ²H–H₂O, δ¹⁸O–H₂O, δ¹⁵N–NO₃⁻, and δ¹⁸O–NO₃⁻) in exploring water chemistry (particularly nitrate values) of the oil palm and VJR catchments. The regional survey highlights significant but relatively modest increases in nitrate, sulphate and chloride levels at baseflow that also vary in magnitude between catchments.   For the OP catchment, (1) nitrate levels at baseflow differ little from values in forested catchments, but levels of chloride and sulphate are much elevated; (2) both the conductivity records and storm-event sampling, however, indicate the importance of flushing of fertilizer-derived nitrates and sulphates during some (but not all) storm events.  Reasons for the regional survey and OP catchment results are explored, particularly the influence of (1) the enhanced stormflow and reduced baseflow of hilly oil palm terrain with its bench-terracing and high track density and (2) the varying degree of efficiency of fertilizer uptake linked to different application techniques and frequency strategies. Results of the multi-isotope (nitrogen, oxygen and hydrogen) isotope approach of 2025 highlight differences in isotope values (a) downstream within the OP catchment, (b) with changes in discharge before and after a rainstorm, (c) between the OP and VJR catchments and (d) between the OP catchment and published results from other studies in low slope terrain in Peninsular Malaysia.  Links with nitrate source processes are explored.  Possible ways in which impacts on pollution might be reduced are presented and discussed, including how to avoid possible conflicts with strategies to reduce erosion, storm runoff (and downstream flooding) and nitrous atmospheric emissions from oil palm terrain.    

How to cite: Walsh, R., Mazilamani, L., Ogiya, M., Nainar, A., Annammala, K., Nurhidayu, S., Reynolds, G., and Ewers, R.: Terrain and land management practice influences on water quality in oil palm compared with forested catchments in Sabah, Malaysian Borneo, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20763, https://doi.org/10.5194/egusphere-egu26-20763, 2026.

Urban expansion is altering hydrological processes and water quality dynamics in many catchments. At a regional scale, catchments are often only partially urbanized, with land cover combining both natural and developed areas. A common configuration is a predominantly rural upper part, where forests, agricultural land, and natural landscapes prevail, which transitions to increasingly urbanized areas downstream. This land cover arrangement creates a rural–urban gradient along the stream network, providing a natural framework to assess how land cover influences hydrochemical dynamics.

Here, we investigate the influence of this land-cover gradient on streamwater chemistry in two parallel catchments in the Lausanne area (Switzerland). Five stream gauges monitor nested sub-catchments (5–22 km²) spanning urban land cover fractions from 5% to 40%. Stream gauges continuously measure streamflow and water quality parameters (electrical conductivity, temperature, turbidity, and fDOM), complemented by weekly and event-based streamwater sampling for major ions and trace metals.

Results from the first year of measurements confirm that urbanization strongly alters hydrochemical dynamics. We find that individual solutes respond differently along the land-cover gradient, reflecting contrasting dominant sources (geogenic, agricultural, and urban). This results in distinct downstream patterns in their mean concentrations. Despite these differences, most solutes exhibit increasingly dilution-dominated concentration–discharge (C–Q) relationships in the more urbanized downstream sub-catchments. This behavior is consistent with relatively constant solute inputs (from point sources or spatially diffuse sources) that become rapidly diluted during high-flow conditions by low-solute runoff generated from impervious surfaces. Together, these observations provide new insights into how urbanization influences the storage and release of water and solutes. The approaches developed and insights gained will support a more holistic understanding of water and solute dynamics across diverse catchment types, as urban areas represent an ever-growing proportion of landscapes worldwide.

How to cite: Miazza, R. and Benettin, P.: Assessing changes in streamwater chemistry along a rural–urban gradient, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6542, https://doi.org/10.5194/egusphere-egu26-6542, 2026.

Understanding pollution dynamics in tropical coastal rivers and groundwater is critical due to the complex landscape interface between terrestrial, mangrove and marine processes, especially in the lower Bengal Delta of Bangladesh. We are conducting hydrogeochemical statistics by combining in-situ and ex-situ hydrochemical data at high and low tide to estimate pollution dynamics in 90 km long river networks of three major rivers along with 30 groundwater wells in the catchments. We used the Hydrochemical Facies Evolution Diagram (HFED) to assess seawater intrusion and freshening stages with dominant ions (cation and anion) visualised through Piper Diagram. Results showed that river water in both high and low tide and groundwater chemistry is dominated by Na⁺ and Cl^- during the dry season, with all river water samples (100%) plotted in the seawater intrusion zone (Na-Cl).  In contrast, 50% of groundwater chemistry is dominated by Na⁺ and Cl^- and plotted in the seawater intrusion zone (Na-Cl), whereas 23.33% is dominated by Ca²⁺ + Na⁺ and HCO₃^- and plotted in the mixed zone, 23.33% is dominated by Ca²⁺ + Mg²⁺ and HCO₃^-, and plotted in the temporary hardness zone, and lastly 3.34% is dominated by Na⁺ + K⁺ and HCO₃^- + CO₃^2- and plotted in the alkali carbonate zone. Furthermore, Ca²⁺, Na⁺, and HCO₃^- ions indicated that groundwater chemistry is significantly influenced by rock weathering processes, which is also evident in the Gibbs diagram. Regression analysis illustrated significant positive relationship (r²) between Na⁺ and Cl^- in river (r² = 0.99) and groundwater (r² = 0.93). Further, box and whisker plots illustrated variation in ions’ concentration in three rivers, where 2^nd river has higher variation compared to the 1^st river (upstream) and 3^rd river (downstream). These variations align with diverse land use patterns along 2^nd riverbank including mega coastal city, industries, food processing facilities, agriculture, and aquaculture. The degree of pollution including nutrient parameters such as NO₃^- indicated high pollution levels in river catchments which ranged from 95.6% to 99%. Downstream groundwater samples showed higher pollution levels (89.3%) compared to upstream groundwater (3.4%). Possible reasons for increasing water pollution include variations in freshwater flow associated with precipitation and temperature patterns. Additionally, diverse land use patterns from upstream to downstream in the river catchments have significant impact on water pollution levels and are considered as an important anthropogenic pollution source. The research offers a novel approach for providing in-depth pollution characterization through ion concentration analysis, which aids regional-scale water quality management in the lower Bengal Delta of Bangladesh.

How to cite: Akter, S. and Gassmann, M.: Pollution dynamics in the lower Bengal Delta of Bangladesh, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3393, https://doi.org/10.5194/egusphere-egu26-3393, 2026.

River water quality, governed by complex natural and anthropogenic drivers and further intensified by global climate change, poses growing threat to aquatic ecosystems and human water use. To systematically disentangle drivers in controlling river water quality, an ecosystem-oriented framework that bridged and included diverse catchment attributes  (e.g., climate, topography, lithology, land cover, and human influence) was proposed and tested against monthly data from 149 catchments in the Yellow River Basin (YRB). The framework addressing the vegetation and soil mediated mechanisms achieved satisfactory performance and reiterated the dominant role of human influence as primary source controlling water quality. Legacy effects of topographical and lithological controls, which shaped present-day soil, vegetation, and geomorphological characteristics by regulating long-term energy and material fluxes, led to great positive effect of aspect and a negative effect of pb (basic plutonics rocks) proportion on concentrations of water quality metrics represented by organic pollution indicators (BOD5, COD, and permanganate index) and nitrogen and phosphorus loads (ammonia nitrogen, total nitrogen, and total phosphorus). Within the ecosystem-oriented framework, vegetation and soil exerted opposite mediating effects on climate-water quality relationships, partially offsetting each other. Moreover, direct effects of warming-wetting climate positively influenced the concentrations of organic pollution indicators, while negatively affected nitrogen and phosphorus loads. The proposed framework clarifies the relative roles of diverse catchment attributes and offers a transferable basis for anticipating future water quality trajectories under ongoing climate change, supporting tailored river water quality management.

How to cite: Wang, F., Du, J., and Wang, Y.: An ecosystem-oriented framework reveals coupled climate and human controls on river water quality, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20925, https://doi.org/10.5194/egusphere-egu26-20925, 2026.

End-member mixing models are widely used in catchment geochemistry to interpret the observed stream chemistry as a linear combination of underlying end-members, and infer lithological source contributions, carbon fluxes or weathering rates. In pursuit of their conceptual simplicity, these models rely on a set of assumptions including conservative behavior of tracers, exactly identified (a-priori) number of sources with known and constant tracer signatures. While the conservativity of certain tracers is well-established, the accurate identification of geochemical end-member signatures from field investigations remains a challenge, contributing to uncertainties in inferred mixing fractions and subsequent interpretations. However, another layer of uncertainty, pertaining to the validity of the mixing model is often overlooked in this process. 

Here, we use high-frequency stream chemistry and discharge datasets from multiple small and mesoscale catchments to demonstrate the existence of large inaccuracies in inferred mixing fractions, irrespective of the perfect identification of (supposed) end-members. We employ a geometrical approach to visualize observed stream chemistry with reference to an ideal mixing space spanned by a given set of tracers. We then relate deviations from this reference space to hydrology (using discharge as a proxy) to argue that such deviations cannot be explained by the existence of another static unidentified end-member, but only through either discharge-dependent shifts in the source signatures themselves, or secondary geochemical processes correlated with discharge. This is evidenced by strong synchronization between the mixing model residuals and discharge time series, highlighting the role of discharge as a confounding factor in geochemical mixing analyses.

Preliminary results indicate that the variance unaccounted for by the mixing model due to the violation of its assumptions is comparable to, or even exceeds, the magnitude of the inferred mixing contributions. This raises questions on treating mixing as the primary control of observed variability in stream chemistry. Furthermore, we also demonstrate how data-driven inference of the number/nature of geochemical end-members using Principal Component Analysis, a frequently adopted practice, could be heavily biased due to this uncertainty. 

Overall, our results dispute the common practice of using mixing models to infer catchment geochemical processes, and call for exercising caution in downstream analyses such as calculating weathering and solute fluxes. We suggest that establishing the existence of a tenable mixing space must be a prerequisite for any subsequent geochemical interpretation.

How to cite: Mallik, A. P. and Gaillardet, J.: Discharge as a confounding control on geochemical mixing inferences, and implications on catchment-scale weathering flux estimates, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12695, https://doi.org/10.5194/egusphere-egu26-12695, 2026.

In recent years, extreme weather events such as intense rainfall and prolonged droughts are occurring with increasing frequency all over Central Europe. To deal with the hydrological consequences an improved understanding of storage and flow dynamics within hydrological catchments might be essential. Hence, the objective of this study is to develop and test a parsimonious work-routine to identify dominant streamflow components and dynamic catchment storages in complex hydrogeological settings.

Based on a two-year field campaign at public streamflow gauging stations in western Germany, we investigated water quality dynamics in two nested catchments with long-term hydrological records. We collected daily water samples and analyzed dissolved organic carbon, nitrate, electrical conductivity, silica, and stable water isotopes. Using these data, we evaluated a hydrographical filter algorithm and a subsequent linear storage detection method, focusing on their hydro-chemical interpretability. We then applied the resulting workflow to published datasets from several nested catchments in Switzerland and the United States. The dynamic hydrographical filter algorithm, DelayedFlowIndex (DFI), is derived from the classical Baseflow Index (IH-UK). It produces Characteristic Delay Curves (CDCs) that describe average catchment drainage behavior with filter widths from 0 to 60 days after a streamflow increase. We improved an existing workflow that divides CDCs into several linearly draining subsets. The new automated routine determines the catchment-specific number of sub-storages. Testing this approach on more than 100 catchments showed that both CDCs and the number of sub-storages can be linked to distinct morphological catchment characteristics. To increase confidence in the hydrological interpretability of hydrographically derived streamflow components, we compared stream hydro-chemical information from our field sites and from published datasets with the resulting flow components. We also examined their roles in streamflow composition throughout the year. At our test sites, rapid flow components showed elevated DOC concentrations. Intermediate components displayed pronounced nitrate peaks. Delayed components had increased silica concentrations, while highly delayed components were associated with higher electrical conductivity. In the downstream sub-catchment, these hydro-chemical signals were additionally shaped by seasonal effects. Water samples collected during a 45-day drought in spring 2025 at both gauges provided valuable information for validating the hydro-chemical signatures of very slow storage components, which are rarely observable.

The streamflow components derived from the DFI show consistent correlations with distinct solute signatures, demonstrating that they are hydro-chemically meaningful. Consequently, DFI analysis combined with the automated storage-delineation algorithm provides a robust, streamflow-based method for identifying catchment-specific flow components. This approach offers valuable insight into storage depletion processes and groundwater dynamics, particularly under extreme low-flow conditions in (mid-)mountain regions.

How to cite: Frietsch, S. and Schuetz, T.: Are hydro-graphically assessed streamflow components hydro-chemically meaningful?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-998, https://doi.org/10.5194/egusphere-egu26-998, 2026.