Orals: Wed, 6 May, 08:30–12:30 | Room 3.29/30
Excess phosphorus inputs to surface waters are a major driver of freshwater eutrophication. Although groundwater can be an important source of phosphorus to surface waters in some settings, its contribution remains poorly quantified. Quantifying phosphorus loads to surface waters is particularly challenging due to heterogeneous groundwater flow pathways and concentrations, combined with the high reactivity of phosphorus with subsurface materials and dynamic biological uptake and release processes. This presentation will synthesize lessons learned from multi-scale studies we have conducted that aimed to quantify groundwater phosphorus inputs to streams and lakes and identify the factors that drive hot spots and hot moments of phosphorus release from groundwater to surface waters. The synthesis draws on studies spanning i) regional-scale longitudinal stream surveys using radon-222 and phosphorus measurements, ii) year-round groundwater sampling in a riparian zone, and iii) high resolution measurements of streambed phosphorus and groundwater-surface water exchange. Collectively these studies highlight the importance of accounting for dynamic processes that control phosphorus delivery to surface waters and underscore the need for caution when upscaling localized groundwater measurements that do not capture spatial variability in phosphorus concentrations.
How to cite: Robinson, C. and Roy, J.: Assessing Phosphorus Loading to Surface Waters from Groundwater: Challenges and Lessons Learnt, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8311, https://doi.org/10.5194/egusphere-egu26-8311, 2026.
Streams are integral components of the global carbon cycle, functioning not only as conduits for terrestrial carbon transport to the ocean but also as active sites of carbon transformation, storage, and greenhouse gas (GHG) evasion. Despite their importance, estimates of stream CO₂ and CH₄ emissions remain highly uncertain, particularly in headwater systems where groundwater inputs may represent a dominant yet poorly quantified source.
In this study, we quantified the role of groundwater discharge in regulating dissolved carbon dynamics and GHG evasion within an urban headwater stream (Manly Creek, Sydney, Australia). Groundwater discharge zones were identified using radon (²²²Rn) as a natural tracer, and groundwater inflows were quantified using a steady-state radon mass-balance approach. Dissolved CO₂ and CH₄ concentrations were measured in surface water and groundwater to assess groundwater-derived gas inputs and their influence on stream-atmosphere exchange.
Groundwater exhibited substantially elevated radon, CO₂, and CH₄ concentrations relative to surface water, confirming strong subsurface accumulation before discharge. A distinct mid-reach groundwater discharge zone was identified, where stream CO₂ and CH₄ concentrations were approximately five-fold and more than two-hundred-fold higher, respectively, than in adjacent surface-water-dominated reaches. Within this groundwater-influenced reach, water–air evasion fluxes ranged from 1037–1959 mmol m⁻² d⁻¹ for CO₂ and 271–511 mmol m⁻² d⁻¹ for CH₄, indicating intense, spatially focused GHG emissions associated with groundwater discharge. Radon mass-balance results showed that advective groundwater inputs overwhelmingly dominated over sediment diffusion and radioactive decay, indicating that groundwater discharge is the primary mechanism sustaining elevated dissolved gas concentrations and evasion fluxes in this reach.
By explicitly linking groundwater discharge to localized but disproportionately high stream GHG emissions, this study demonstrates how unresolved groundwater-surface water interactions can lead to systematic underestimation of inland-water emissions in bottom-up carbon budgets. Incorporating such spatially focused groundwater-driven fluxes provides a pathway toward reconciling bottom-up stream emission estimates with top-down atmospheric constraints, thereby improving assessments of inland-water contributions to climate-relevant carbon cycling.
How to cite: AhaniAmineh, Z., Andersen, M., Rutlidge, H., Glamore, W., Henderson, R., Sadat-Noori, M., and Davie, A.: Groundwater-Driven Greenhouse Gas Fluxes in a Headwater Stream, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8806, https://doi.org/10.5194/egusphere-egu26-8806, 2026.
Potentially toxic elements (PTEs), including arsenic (As), pose a risk to groundwater and surface water quality, particularly in agricultural catchments where organic matter loading can enhance PTE mobility from bedrock sources. The hyporheic zone (HZ) comprising the saturated interstices of streambed sediments forms a dynamic groundwater-surface water interface characterised by variable hydraulic gradients, geological heterogeneity and intense biogeochemical activity. Steep redox gradients, driven by hydrological exchange and organic matter inputs may exert strong control on the fate of redox-sensitive contaminants such as arsenic (As). This study is undertaken as part of the HYPOFLUX project which investigates the fate of geogenic PTEs in headwater agricultural catchments in extreme vulnerability fractured bedrock environments in Ireland. A multi-scale monitoring network comprising shallow riparian groundwater monitoring wells, clustered piezometers, field drains and nested stream sites were used to characterise spatial and temporal variability in pH, DO, ORP, DOC, DIC, alkalinity, PTEs, Fe, Mn, Si and specific ultraviolet absorbance at 254 nm (SUVA). Riparian and streambed core were collected for extraction and analysis of PTEs using pXRF. Results demonstrate that deep field drainage and permeable bedrock pathways control the interaction between groundwater and surface water at stream reach scale. Solid phase As was found to be generally very low in heavily weathered mudstone bedrock and alluvial sediments and not strongly related to Fe or organic matter content. Dissolved As was spatially variable but elevated with an average of 43.9 ± 31.3 µg L–1 in DOC-rich shallow riparian groundwater (15.1 ± 5.2 mg L–1). Streambed piezometer profiles showed pronounced declines in dissolved As from 150 cm to 20 cm depth below bed level with concomitant increase in DO and solid phase As (measured from streambed cores). These findings provide field evidence of in-situ attenuation of groundwater As in the HZ and underscore the need to explicitly incorporate HZ processes into groundwater-surface water frameworks and models in agricultural catchments.
How to cite: Palenius, E., Lilburn, S., Quishi, X., Van Acken, D., and Weatherill, J.: Hyporheic zone controls on arsenic fluxes from shallow groundwater to streams in agricultural catchments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14493, https://doi.org/10.5194/egusphere-egu26-14493, 2026.
For decades, wastewater containing significant amounts of emerging organic contaminants (EOCs) has been discharged into the aquatic environment. The introduction of EOCs into aquifers due to losing conditions of polluted rivers poses a serious risk for groundwater quality, which is particularly concerning in catchments with drinking water production facilities. Thus, understanding EOCs’ reactive transport processes in connected groundwater-surface water (GW-SW) systems are crucial for assessing their impact on water resources.
The present study investigates the GW-SW interaction at the Landgraben stream which is located within the Hessian Ried, Germany. Treated municipal and industrial wastewaters have been discharged into the Landgraben for decades, introducing a wide range of EOCs into the aquatic environment. To monitor the GW-SW interaction, we installed a complex monitoring system within and along a transect next to the Landgraben stream. The aims of our study are (i) to improve the understanding of the behaviour and fate of EOCs within the GW-SW interface and (ii) to link EOCs behaviour to the hydraulic responses of the seasonally varying stream. For this, we analysed 22 EOCs, as well as major ions, trace elements, dissolved organic carbon, rare earth elements and water stable isotopes over 16 months comprising two summers. Water samples were collected from the river, the hyporheic zone and the nearby groundwater at three different distances and at three depths at intervals ranging from biweekly to monthly.
Results show that the infiltration dynamics were strongly influenced by seasonal groundwater fluctuations, with influent conditions during summer and predominantly effluent conditions during winter. The second summer was characterized by a pronounced infiltration of river water into the aquifer driven by the preceding winter precipitation deficit.
The hydrochemical analysis showed a wide concentration range of EOCs in the river water samples. Of the compounds analysed, only Atenolol and Ciprofloxacin were not detected across the 29 sampling campaigns. Concentrations of the remaining 20 EOCs ranged from a few ng L⁻¹, for example Fluconazole (median = 52 ng L⁻¹), to several µg L⁻¹, such as Oxipurinol (median = 1957 ng L⁻¹). Overall, the detected EOCs cover a broad spectrum of chemical speciation and polarity, implying substantially different transport and attenuation behaviours along the SW-GW pathway. This variability is reflected in the sampled groundwater, where some compounds are no longer detected (eg., Sitagliptin, Venlafaxine), whereas others persist and even exhibit higher concentrations than those found in the stream during the sampling period (eg., Candesartan, Carbamazepine). Thus, this study highlights the significance of GW-SW interactions in the transport and attenuation of EOCs, providing insights into their differing fates within aquatic systems and how these relate to physicochemical properties.
How to cite: Hillmann, S., Muñoz-Vega, E., Richard-Cerda, J. C., and Schulz, S.: Assessing the fate of emerging organic contaminants in a coupled surface water-groundwater system, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19139, https://doi.org/10.5194/egusphere-egu26-19139, 2026.
The hyporheic zone plays a key role in controlling stream water quality by regulating the transport and transformation of nutrients and contaminants. Recently, growing attention has focused on hyporheic exchange as a driver of microplastic (MP) burial and resuspension, because of its ubiquity, persistence and their documented ecological impacts. Here, we investigate how MPs enter, become trapped, buried and released from sand-bed rivers with mobile dunes. We developed a semi-analytical solution of the flow field to delineate the MP trajectories considering the coupled effect of pumping (due to pressure variation at the water-sediment interface) and turnover (due to bedform migration). Along each exchange path, we then solved an advection–dispersion-reaction equation (ADRE) analytically. To represent progressive resistance/clogging effects, we incorporate spatially varying velocity, dispersion, retardation and first order removal coefficients.
Results show MP retention within the hyporheic zone is dictated by the interplay between stream hydro-morphology (dune geometry, migration speed and alluvial depth) and along-path retention processes. The transport formulation induces an exponential decay of MP concentration with both trajectory length and residence time, meaning that, beyond a certain point, longer subsurface travel does not necessarily equate to higher retention efficiency. Importantly, bedform migration does not simply increase or decrease MP burial uniformly but instead redistributes retention between near-surface and deeper sediment layers by enhancing shallow recirculation while intermittently disrupting long, deep pathways. The proposed framework shows how bedform dynamics influence the transport and persistence of microplastics with direct implications for ecosystem health and risk assessment at the watershed scale.
How to cite: Marzadri, A., Tonina, D., and Portillo de Arbeloa, N. K.: How hyporheic pumping and bedform migration redistribute microplastic burial in sand-bed rivers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8070, https://doi.org/10.5194/egusphere-egu26-8070, 2026.
In the arid plains of Northwest China, intensive agricultural activities and extreme climatic conditions have profoundly reshaped regional hydrological cycles. This study integrates hydrochemical analysis with multi-isotope tracing, including stable water isotopes (δ²H and δ¹⁸O) and nitrate isotopes (δ¹⁵N and δ¹⁸O), to quantify the complex interactions between surface water (SW) and groundwater (GW). Our findings demonstrate that large-scale agricultural irrigation serves as the primary physical driver enhancing the intensity and frequency of SW–GW exchanges. Isotopic signatures reveal that persistent irrigation return flows have strengthened the hydraulic connectivity between surface water bodies and shallow aquifers. This physical interaction further triggers significant biogeochemical responses. The irrigation-induced leaching of soil salts, coupled with intensive evaporation, is identified as the core factor governing groundwater salinization and quality deterioration, with Total Dissolved Solids (TDS) values ranging from 516 to 2684 mg/L. Hydrochemical modeling confirms that anthropogenic intervention, specifically cropland expansion and groundwater overexploitation, has superseded natural rock-water interactions in controlling the hydrochemical facies. To maintain water–ecology–agriculture security in arid regions, we propose that optimizing irrigation quotas and enhancing floodwater utilization are essential for sustainable groundwater management. This research provides critical insights into the mechanism of water quality evolution under the dual pressure of climate change and human interference.
How to cite: Wang, W.: Agricultural Irrigation as a Dominant Driver of Surface Water–Groundwater Interactions and Hydrochemical Evolution in Arid Northwest China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15021, https://doi.org/10.5194/egusphere-egu26-15021, 2026.
Although the interaction between groundwater (GW) and surface water (SW) has been the subject of numerous studies, few of them have focused on braided river environments. Yet these environments form essential freshwater resources and unique ecosystems. This work analyses GW-SW interactions in the Var River alluvial plain (30 km²), which is located in south-east France. The alluvial aquifer of the Var River is a crucial resource for the nearby, densely populated coastal area of the French Riviera. The objective is therefore twofold: i) to improve the conceptual understanding of GW-SW exchanges in an anthropized Mediterranean braided river, and ii) to support the operational management of the water resource. In order to achieve a multi-scale understanding of the different processes involved in GW-SW interactions, a multidisciplinary approach is being implemented in the Var alluvial plain. It incorporates methods from hydrometry, hydrogeophysics, geochemistry, aerial and satellite imaging, hydrosedimentology and numerical modelling. Here, we show results obtained by combining two methods: unmanned aerial vehicle (UAV)-based infrared thermal imagery and electrical resistivity tomography (ERT). Thermal infrared imaging of the Var riverbed was carried out in July 2025 (when piezometric levels began to decrease) using a light-weight UAV, and ERT was performed at four study sites along the river between September and November 2025 (when piezometric levels were at their lowest). The results obtained led to a new conceptualization of the connectivity and interactions between the river and the aquifer. This conceptualization is intended to be further enriched by combining more results from the multidisciplinary approach, particularly from a fully integrated modelling of surface and subsurface flows.
How to cite: Picourlat, F., Granados-Bolaños, S., Ouassanouan, Y., Ouhamdouch, S., Vidal, M., Besso, R., Lebourg, T., Billaud, F., Game, P., Targosz, J., Fantino, N., Altschuler, S., Viguier, B., and Abily, M.: Groundwater–surface water interactions in an anthropized Mediterranean braided river: insights from combined UAV-based thermal imagery and electrical resistivity tomography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13915, https://doi.org/10.5194/egusphere-egu26-13915, 2026.
Interactions between surface flow dynamics and stream sinuosity drive significant hyporheic exchange and concomitant thermal advection within streambed sediments, processes critical to ecological and biogeochemical functioning. This study extends the laboratory meandering pool-riffle stream model established by Huang & Chui (2022) employing combined heat tracer experiments and coupled numerical simulations to investigate hyporheic zone thermal responses to in-stream temperature fluctuations. Numerical models, validated against laboratory data, elucidate the spatiotemporal complexity of the thermal regime within the HZ. Spatially, downwelling zones exhibit greater responsiveness to in-stream diel temperature variations relative to upwelling zones. Higher discharge amplifies the influence volume while preserving temporal response patterns. Temporally, hyporheic temperatures closely track in-stream fluctuations. Sinuosity substantially modulates the thermal regime by expanding regions experiencing pronounced temperature variations and extending heat residence times, thereby enhancing downstream thermal stability and enlarging zones conducive to biogeochemical activity. The intra-meander floodplain exhibits intensified thermal fluctuations, functioning as potential biogeochemical transformation hotspots due to heightened hydraulic connectivity. Collectively, stream sinuosity and discharge critically govern the heterogeneity of hyporheic thermal environments, with significant implications for ecological processes and stream restoration strategies.
How to cite: Huang, P., Zhu, S., and Chui, T. F. M.: Thermal Regime in the Hyporheic Zone of Pool-Riffle Streams, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5682, https://doi.org/10.5194/egusphere-egu26-5682, 2026.
Lake Kinneret, Israel’s primary surface freshwater reservoir, faces ongoing salinization driven by a complex interplay of geological structures and deep saline groundwater sources. While the western and northern onshore and offshore saline spring systems thought to be the major lake salinization sources, 70% of the salts entering the lake come from disperse seepeges from the shores around the lake. The southeastern “Haon” shoreline is a potential location for major salinization of the lake due to high groundwater salinity below the lake (~25,000 mg/L of total dissolved solids) and higher hydraulic head compared to the lake level. There is a need to research potential actions to try and mitigate the salt discharge into the lake. This study investigates the use of pumping wells for extraction of the underlying brine to mitigate saline lacustrine groundwater discharge. We used continuous monitoring of two shallow wells with data from two other deeper wells at Haon beach, to construct a 2D cross-sectional stratigraphy for building a flow and solute transport numerical model. The model was developed using the FEFLOW code and was calibrated to the data from the monitoring wells. Building on this calibrated framework, simulation tests were conducted using an active pumping well to quantify subsurface dynamics and salt fluxes under stress. Results show that pumping the brine below the lake lowers the groundwater hydraulic head and potentially mitigate salt discharge. Furthermore, these results provide critical data regarding the feasibility of the recent plan to use the extracted saline groundwater in Lake Kinneret nearshore aquifers for desalination purposes, offering a scientific basis for water management strategies aimed at mitigating lake salinization.
How to cite: Meidan, T., Stein, S., and Zohar, I.: The effect of shallow saline groundwater pumping on the salt discharge into Lake Kinneret (Sea of Galilee) and its salinization potential, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20565, https://doi.org/10.5194/egusphere-egu26-20565, 2026.
Surface Water–Groundwater Interactions and Flow Variability in the Everglades: Implications for Freshwater Delivery to Florida Bay
Assefa Melesse, Ethiopia Zeleke, Alemayehu Shanko, Tania, Islam, Rene Price
Department of Earth and Environment, Institute of Environment
Florida International University, Miami, USA
Abstract
The dynamic exchange between surface water and groundwater is critical for impacting the functions of the Everglades ecosystem. The spatiotemporal variability of freshwater exchanges related to freshwater delivery to Florida Bay is not adequately characterized or understood. This study focuses on the results of a comprehensive data analysis to examine the flow variability and surface–groundwater interactions, with a particular focus on freshwater delivery through Taylor sloughs to Florida Bay. Field observations and historical long-term monitoring data were used in the analysis to estimate the spatiotemporal patterns and trends of surface–groundwater interactions under varying climatic and water management conditions. Statistical approaches were applied to identify dominant controls on flow variability and exchange processes. The spatial heterogeneity of interactions was significant, indicating that freshwater exchange rates vary across different hydrogeologic settings in relation to seasonal water variations and rainfall volumes. This analysis reveals how water delivery through Taylor slough to Florida Bay is highly inter-annual and seasonal mainly driven by climatic as well as water management decisions. Given the sensitivity of the salinity regime of Florida Bay to freshwater inflow, the findings of this analysis have important implications for ecosystem restoration. This analysis provides insights into the coupled surface‒subsurface hydrological processes that govern water movement in this unique wetland system, contributing to the improved understanding necessary for effective water management and ecosystem restoration in the greater Everglades landscape.
Keywords: Everglades, Surface–groundwater interactions, Florida Bay, Taylor slough, Flow variability
How to cite: Melesse, A., Zeleke, E., Shanko, A., Islam, T., and Price, R.: Surface Water–Groundwater Interactions and Flow Variability in the Everglades: Implications for Freshwater Delivery to Florida Bay, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7674, https://doi.org/10.5194/egusphere-egu26-7674, 2026.
Lacustrine groundwater discharge (LGD) and its associated nitrogen (N) and phosphorus (P) inputs are increasingly recognized as the critical drivers of lake eutrophication. However, the intermonthly variability in LGD and its influence on lake nutrient dynamics remain poorly understood. In this study, high-frequency monitoring and hydrochemical analyses were conducted over a full hydrological year to investigate LGD-related nutrient fluxes and their effects in a typical oxbow lake in the central Yangtze Basin. Water level data and 222Rn tracing revealed a seasonal LGD pattern characterized by an increase from summer to winter, followed by a decline from winter to spring, with LGD rates ranging from 35.36 to 51.71 mm·d-1. This pattern was regulated by monthly net precipitation, which controlled the lake level fluctuations and LGD rates. The corresponding N and P loads varied synchronously with LGD and showed seasonal synchrony with lake N and P concentrations. Moreover, variations in the N/P ratio carried by LGD regulate the lake water N/P ratio, thereby influencing its relationship with the dynamic changes in chlorophyll-a. From a global perspective, in closed lakes, LGD is typically governed by climatic factors such as precipitation and evaporation, thereby serving as a key process regulating the lake’s trophic status. This study provides the first evidence that groundwater-driven nutrient loading influences lake nutrient status on an intermonthly scale offering new insights and management strategies for eutrophication control in shallow, closed lake systems worldwide.
How to cite: Sun, X.: Seasonal dynamics of closed lakes nutrient status controlled by lacustrine groundwater discharge, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6479, https://doi.org/10.5194/egusphere-egu26-6479, 2026.
In wetland ecosystems, lacustrine groundwater discharge enriched in nitrogen and phosphorus concentration is an important component of lake nutrient budgets. However, in addition to groundwater fluxes, the transport of particulate matter accompanying groundwater discharge, especially the mobile fine particles, was largely neglected in existing frameworks. In this study, we identified typical particle mobilization associated with groundwater discharge at shoreline springs in an oxbow lake. Continuous monitoring of spring flow velocity and discharge pressure, combined with principal component analysis, reveals two distinct particle-mobilization regimes. A pressure-controlled mode dominates in settings with thick surficial clay layers, whereas a velocity-controlled mode prevails where the surficial clay cover is thin. The results show that the total phosphorus load carried by particles can exceed that of dissolved groundwater by two to three orders of magnitude, although a substantial fraction consists of coarse particles and relatively inert HCl-P. Importantly, fine particles (silt and clay) transport large amounts of reactive phosphorus (OP + NaOH-P), yielding fluxes comparable to or even several times higher than dissolved groundwater phosphorus under both strong and weak discharge sites. Coupled Sr isotope and rare earth element tracing indicates that these fine particles originate from pressure-induced detachment of the top clay aquitard and velocity-driven erosion of clay lenses within the aquifer, respectively. Physical simulations indicate that preferential loss of coarse particles during mobilization leads to significant enrichment of reactive P in the discharged fine fraction relative to its source. These findings indicate that solid-phase transport driven by groundwater discharge constitutes a previously overlooked but potentially important component of phosphorus cycling in wetland ecosystems. Since particulate inputs do not experience dilution in the same way as dissolved groundwater fluxes, fine-particle transport may represent a hidden and underestimated P source even in low-permeability zones traditionally considered to have weak groundwater influence on lake water quality.
How to cite: Qiu, W.: Groundwater-driven particle mobilization reveals an underestimated pathway of phosphorus input to lakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16400, https://doi.org/10.5194/egusphere-egu26-16400, 2026.
Seepage across a streambed is typically expressed by Darcy's law, q = Kc · ΔH / wc, where Kc and wc are the hydraulic conductivity and thickness of the bed, respectively, and ΔH is the hydraulic head gradient. When the stream is “disconnected” – i.e., an unsaturated zone exists beneath it – the suction at the lower bed interface no longer follows the groundwater head. Capturing this effect requires solving a nonlinear equation that incorporates the unsaturated hydraulic conductivity (UHC) relationship of the underlying aquifer (most commonly described by van Genuchten–Mualem or Brooks‑Corey–Burdine parametrizations). Because of the added computational burden, many modelling packages (e.g., MODFLOW) simplify the problem by assuming zero suction, which leads to systematic underestimation of seepage.
In this study we analyse the governing nonlinear equation using a generic UHC function that exhibits a power‑law behaviour in the dry limit. First, we examine the zero‑stage (h = 0) case and demonstrate that three distinct asymptotic solutions arise, depending on the relative magnitude of the dimensionless groups (Ka/Kc)1+1/b and B1/b wcKa/hgKc, where Ka is the aquifer hydraulic conductivity. Here b and B are shape parameters that depend on the chosen parameterisation (for example, for Van Genuchten–Mualem, b = (5n − 1)/2 and B = (1 − 1/n)2) and hg is the associated scale parameter. From these asymptotes we identify two clogging regimes. For hard clogging, seepage becomes independent of aquifer properties; in this regime the MODFLOW simplification is exact. For soft clogging, zero-stage seepage converges to q0 = Ka · (wcKa/hgKc)-b/(1+b).
We then derive an analytical approximation that smoothly bridges the asymptotic limits. Validation against the exact numerical solution shows that the new expression provides a rapid yet more accurate alternative to the conventional MODFLOW formulation. Finally, we argue that a positive stage simply adds a linear term to the flux, yielding q ≈ q0 + wc/Kc · h. Based on this insight, we propose a novel method for assessing surface water - groundwater disconnection.
How to cite: Paccolat, J.: Analytical description of seepage from disconnected surface water: improved approximate solution and insights for disconnection assessment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3724, https://doi.org/10.5194/egusphere-egu26-3724, 2026.
Filamentous benthic algae grow ubiquitously on the sediment-water interface (SWI) in streams, rivers, and lakes. They support fluvial food webs, engage in nutrient cycling, attenuate flow structures, and affect bed stability, thereby modifying the aquatic habitat. In this contribution, we are going to present some preliminary results of the direct numerical simulation (DNS) of turbulent open-channel flow over a layer of spherical sediment particles covered with benthic algae canopy in the transitionally rough regime. The flow motion is governed by Navier-Stokes equations and solved with a standard fractional-step method. The filamentous benthic algae are modeled as elastic rods and solved by an efficient and physically accurate finite difference scheme. Both the rods and particles are fully resolved and coupled with the flow using the direct-forcing immersed boundary technique. The flow structures and turbulence statistics in both the roughness sublayer and logarithmic region will be thoroughly analyzed and compared to previous studies on rough wall open-channel flow in order to investigate how benthic algae affect the flow field. The results presented here may provide physical insights and implications into sediment incipient motion and mass/momentum transfer across the sediment-water interface in natural rivers where benthic algae play dominant role and will serve as a first step for better understanding and modeling of the dynamics in hyporheic/benthic zone and entire river ecosystems.
How to cite: Zeng, X., Huang, L., and Fang, H.: Direct numerical simulation of open-channel flow over a rough wall covered with benthic algae, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22990, https://doi.org/10.5194/egusphere-egu26-22990, 2026.
Quantifying solute transport in biofilm-colonized porous media is essential for predicting microbially mediated processes in the hyporheic zone, especially in the presence of anoxic niches. However, our understanding of how pore-scale biofilm structure governs local transport remains limited, posing a significant challenge in developing accurate large-scale reactive transport models. The inherent structural heterogeneity of biofilms makes direct characterization of internal mass transfer technically difficult and complicates the interpretation of experimental observations.
In this study, we combine a high-resolution microfluidic experiment with pore-scale reactive transport modelling to investigate dissolved oxygen transport in biofilm-colonized pore spaces. Transparent planar optical sensors integrated into the microfluidic platform enabled non-invasive imaging of oxygen concentration fields at high spatial resolution, providing direct insight into intra-biofilm transport behaviour. To interpret these observations, we employed a pore-scale micro-continuum modelling framework in which the biofilm is represented as a fluid-filled microporous medium characterized by microscale transport properties, including permeability and effective diffusivity. The numerical model was calibrated against experimental datasets using an optimization-based approach, allowing for the simultaneous estimation of the permeability of the biofilm, effective diffusivity, and metabolic kinetic parameters. Results show strong agreement between simulated and measured oxygen distributions, supporting the suitability of the modelling framework to resolve complex mass transfer mechanisms at the fluid–biofilm interface. Analysis of the inferred parameters suggests that diffusive transport dominates within the biofilm matrix, while biofilm permeability is found to be relatively low. Furthermore, the resolved oxygen consumption rates were found to be significantly lower than those observed in batch reactors, highlighting the role of pore-scale environmental limitations on metabolic activity. This work establishes a robust framework for further exploring the relationship between biofilm morphology and reactive transport, providing a basis for more accurate upscaling in complex porous environments.
How to cite: Dawi, M., Ceriotti, G., Dell’Oca, A., Porta, G., and Siena, M.: Reactive Transport in Pore-Scale Biofilms: Model-Based Interpretation of a Microfluidic Experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7203, https://doi.org/10.5194/egusphere-egu26-7203, 2026.
The vertical extent of exchange between surface flow and sediment pore water is a key control on ecological functioning in fluvial systems. Across the diverse disciplines engaged in hyporheic-zone research, however, this exchange depth is characterized using different criteria and definitions. From a hydrodynamic and transport perspective, this raises the question of how far interface-driven scalar transport mechanisms penetrate into a porous sediment bed beneath turbulent open-channel flow.
Using pore-resolved Direct Numerical Simulation (DNS), we investigate scalar transport near the sediment--water interface within the framework of effective diffusivity. The porous medium is represented by a random sphere packing overlain by a turbulent open-channel flow characterized by a friction Reynolds number of Reτ = 180 and a permeability Reynolds number of ReK = 1.8. Scalar transport is modeled by solving the advection-diffusion equation for a passive scalar at Schmidt number Sc = 1, subject to a prescribed vertical concentration gradient driving bed-normal transport.
To obtain statistically representative descriptions of the strongly three-dimensional flow and transport fields, double averaging, in time and over horizontal planes, is employed. Within this framework, the effective diffusivity model relates the plane-averaged scalar concentration to the vertical scalar flux, enabling a quantitative decomposition into turbulent transport, dispersive transport, and molecular diffusion. Revisiting the theory of horizontal averaging, we discuss the implications of different formulations of effective diffusivity and show that seemingly minor differences become significant in regions of rapidly varying porosity, such as the sediment--water interface.
In a systematic pre-study, we assess the influence of grid resolution, sampling duration, and sediment-bed depth on the resulting transport statistics. Based on this analysis, simulations are conducted at a constant flow-depth-to-sphere-diameter ratio of hf / D = 3, combined with three sediment-bed depths corresponding to hb / D ∈ [2, 5, 9]. To isolate interface-induced transport processes, simulations with overlying turbulent flow are compared to reference cases of scalar transport in the porous medium without free flow.
This comparison enables a clear distinction between transport mechanisms intrinsic to the porous medium and those induced by the sediment--water interface. The effective diffusivity associated with interface-driven transport decays exponentially with increasing depth below the sediment surface. Turbulent scalar transport is confined to the uppermost sediment layer, penetrating only to depths of approximately z / D ≅ 1 - 2. In contrast, dispersive transport induced by pressure fluctuations at the sediment crest dominates scalar exchange within the sediment bed. Beyond z / D ≅ 5, dispersive transport becomes negligible and scalar transport is governed predominantly by molecular diffusion, indicating the onset of a Darcy-type transport regime. Within the effective diffusivity framework, cases with hb / D = 5 and 9 show indistinguishable behavior, whereas the shallow bed case hb / D = 2 exhibits a pronounced attenuation of dispersive transport.
These results provide a quantitative, transport-based definition of the effective depth of interface-driven hyporheic exchange. The exponential decay of effective diffusivity, isolated from background porous-medium transport, offers a promising basis for improving reduced-order models of scalar transport in the hyporheic zone.
How to cite: Ott, P., Manhart, M., v. Wenczowski, S., and Sakai, Y.: Quantifying the Extent of the Hyporheic Zone Using Pore-Resolved DNS of Turbulent Flow over a Random Sphere Packing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21043, https://doi.org/10.5194/egusphere-egu26-21043, 2026.
This study presents a theoretically sound operational framework for calibrating large-scale, high-fidelity integrated surface water-groundwater models to improve their reliability for water resources management. The approach combines ParFlow-CLM simulations of three-dimensional variably saturated flow with local sensitivity analysis and Gaussian Process Regression surrogates to enable efficient multi-stage calibration against water table depth and river discharge observations. The framework is applied to the entire system associated with the Po River District (87,000 km2) in northern Italy, resulting in the first robustly calibrated high-fidelity model at this spatial scale. Calibrated model parameters include hydraulic conductivities of the main subsurface geomaterials and Manning roughness coefficients of major rivers in the area. Our results show that clay hydraulic conductivity is a primary driver for groundwater table dynamics, while channel roughness dominates river discharge. Overall, the proposed strategy provides a robust computational framework for scenario analysis and sustainable water management under climate and anthropogenic pressures.
How to cite: Guadagnini, A., Sandoval, L., Condon, L., and Riva, M.: Modular approach to calibration of supra-regional scale integrated surface-groundwater models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19134, https://doi.org/10.5194/egusphere-egu26-19134, 2026.
Generating hydropower alters the streamflow dynamics of rivers, especially in the river reaches downstream of hydroelectric plants. This causes hydropeaking, i.e. sudden and frequent fluctuations in river discharge. If a river is hydraulically connected with the adjacent aquifer, these fluctuations can be observed in the groundwater, depending on the distance from the river. In this study, we used this interaction to estimate the hydraulic diffusivity of the aquifer and the streambed resistance of two rivers located in the Adige valley (Northern Italy) by applying an analytical solution. We compared our model results with the observations of several piezometers located near the Noce and Adige rivers. While seasonal streamflow variability is significantly reduced and short term (sub-daily and weekly) fluctuations are increased in the Noce river, hydropeaking is less pronounced in the Adige. We compared the optimal model results of six different time windows, including low flow, medium and high flow events. Our results allowed for the estimation of the hydraulic diffusivity and revealed high spatial and temporal variability in the streambed resistance. The use of an analytical solution enables a rapid estimation of theses parameters, which can assist in calibrating numerical groundwater models. However, we highlight the importance of satisfying the necessary conditions required to apply this analytical approach; we demonstrate that while these conditions may be met under normal flow, they are not necessarily maintained under extreme conditions (i.e., floods and droughts).
How to cite: Höbenreich, K., Chiogna, G., and Basilio-Hazas, M.: Estimating Hydraulic Diffusivity and Streambed Resistance Using Surface Water-Groundwater Interactions under Hydropeaking Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19635, https://doi.org/10.5194/egusphere-egu26-19635, 2026.
Posters on site: Wed, 6 May, 14:00–15:45 | Hall A
Trace organic compounds (TrOCs) in water bodies worldwide are a major concern. In addition to reducing the loads and improving the understanding of the ecotoxicological effects of TrOC cocktails, it is important to gain a better understanding of the pathways and fate of this large group of compounds in the environment. The lowland River Erpe in Berlin/Brandenburg, Germany, which receives treated wastewater from an urban wastewater treatment plant, is an excellent site for such research. The exceptionally high TrOC concentrations in the River Erpe enable reliable process studies with minimal analytical effort, as prior enrichment steps aren’t required. The river system also offers a variety of reaches that differ in terms of hydrology and streambed morphology, enabling different types of investigation. Over the past 15 years, more than 100 researchers have conducted several large-scale and numerous smaller studies on the River Erpe. Topics have included the role of hyporheic zones in the self-purification capacity of streams with respect to TrOCs; seasonal changes in instream processes; interactions between easily degradable organic matter and TrOC attenuation; the importance of identifying flow paths to understand biogeochemical processes; the effects of management actions, such as the removal of macrophytes, on the fate of TrOCs; the effects of losing conditions on TrOC input to aquifers and bank filtration systems; the effects of discharging treated effluents from a large, new industrial site on the composition of the river water; and identifying microbial key players associated with TrOC removal. Current research topics include bioremediation, the impact of migrating bedforms on TrOC fate, as well as the seasonal development of loads. Research highlights and future directions are presented.
How to cite: Lewandowski, J., Neumann, J., Posselt, M., Reith, C. J., Schaper, J. L., Villa Arroyave, M. A., Arnon, S., Putschew, A., and Spahr, S.: Key findings from 15 years of research on attenuation of trace organic compounds in the River Erpe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10319, https://doi.org/10.5194/egusphere-egu26-10319, 2026.
Hyporheic exchange between surface water and groundwater governs the biogeochemical cycling of groundwater in riparian zones and plays a crucial role in the spatiotemporal dynamics of watershed aquatic environments. The spatiotemporal dynamics of oxygen during this exchange determine the redox zonation of groundwater, thereby controlling the activity of reactions such as aerobic respiration, nitrification, and denitrification. However, most current studies assume that oxygen-rich river water is the sole source of oxygen in riparian aquifers, overlooking the significant oxygen supply process via atmospheric diffusion and dissolution into the unsaturated zone. Therefore, this study developed a numerical model of gas–liquid two-phase flow and reactive solute transport in the phreatic aquifer of a riparian zone to investigate nitrogen migration and transformation processes under the influence of atmospheric oxygen diffusion and dissolution. By comparing the classical model with an oxygen diffusion model under varying hydraulic conductivities, river stage fluctuation amplitudes, and rainfall infiltration rates, we found that: (1) Oxygen diffusion from the atmosphere increases dissolved oxygen (DO) concentrations in the aquifer by over 200%, leading to a 220% increase in nitrification rates and a 40% increase in denitrification rates; (2) The influence of oxygen diffusion on nitrogen cycling in riparian zones is positively correlated with hydraulic conductivity. oxygen is more readily supplied under high-permeability conditions, and then accelerating nitrogen cycling reactions; (3) Although oxygen-rich rainfall infiltration provides a direct DO input, it weakens the dissolution–diffusion process of oxygen. Under the “competitive supply” of these two processes, the DO flux into the riparian zone is positively correlated with infiltration rate. These results highlight the critical role of atmospheric oxygen diffusion in shaping subsurface redox conditions and nitrogen dynamics, underscoring the need to incorporate unsaturated zone processes into future riparian biogeochemical models.
How to cite: Li, Y., Ouyang, D., Tong, M., Wen, Z., and Zhu, Q.: The impact of atmospheric oxygen supply on the nitrogen cycle during hyporheic exchange in the river bank aquifer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8535, https://doi.org/10.5194/egusphere-egu26-8535, 2026.
Recent research on braided rivers in New Zealand has revealed how groundwater is recharged from braided rivers, and alluvial rivers in general. A defining feature of alluvial rivers is the development of a thin (2-5m thick) high permeability braidplain aquifer/ bed reservoir. This feature forms through the process of sediment mobilisation during flood events. Field observations in braided rivers reveal that water is freely exchanged between river channels and the bed reservoir, which accommodates parafluvial flow. The river channels and associated reservoir together constitute a river system (Wilson et al. 2024). Exchange between the river system and regional aquifer is controlled by the water level in the bed reservoir rather than river stage. This has important implications for how groundwater-surface water exchange occurs in alluvial systems.
Firstly, flow exchange between channels and the bed reservoir is preferentially lateral via the banks rather than vertically via the bed. In isotropic sediments, Darcy’s Law predicts that flow exchange will occur vertically through the river bed, and that flux will vary with river stage. However, alluvial sediments are strongly anisotropic, with lateral hydraulic conductivities magnitudes greater than the vertical. Groundwater temperature observations show that specific discharge adjacent to the bed reservoir is much greater than that beneath the bed reservoir. The flux beneath the river system is stable, with variations in flux only occurring along the river margins.
Secondly, exchange between the river system and adjacent aquifer is primarily controlled by transmissivity rather than hydraulic gradient. It is commonly considered that groundwater recharge is controlled by the head difference between the river system and adjacent regional aquifer. Under this scenario, we would expect river losses to increase when groundwater levels in the regional aquifer become very low due to an increased hydraulic gradient. However, in the Wairau system we observe the opposite, and that river flows are sustained during periods of low groundwater level. Because the river bed reservoir is very thin, changes in hydraulic gradient are small compared to changes in reservoir saturation. When groundwater levels are low, the transmissivity along the bed reservoir margin is lower, resulting in lower exchange rates.
Wilson, S. R., Hoyle, J., Measures, R., di Ciacca, A., Morgan, L. K., Banks, E. W., Robb, L., & Wöhling, T. (2024). Conceptualising surface water–groundwater exchange in braided river systems. Hydrology and Earth System Sciences, 28(12), 2721–2743. https://doi.org/10.5194/hess-28-2721-2024
How to cite: Wilson, S. and Wöhling, T.: Groundwater-surface water exchange processes in alluvial rivers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15170, https://doi.org/10.5194/egusphere-egu26-15170, 2026.
Riparian zones are dynamic eco-hydrological interfaces regulating groundwater-surface water (GW-SW) exchange by controlling vertical and lateral fluxes of water, heat, sediments, and biogeochemical constituents along river corridors. Most Earth Observation (EO) studies, however, treat them as static land-cover units rather than functional exchange domains, limiting insights into subsurface connectivity and hydrological processes. To address this gap, this study develops a process oriented EO framework to delineate functional riparian zones based on hydrological connectivity and climatic sensitivity. Multi-decadal EO datasets were integrated with hydro-climatic indicators of spectral vegetation indices, inundation frequency, surface moisture proxies, and land surface temperature metrics to distinguish permanent and seasonal riparian interfaces and were interpreted as proxies for GW-SW exchange intensity, residence time, and flow directionality. The framework was applied across climatically heterogeneous river corridors spanning multiple Köppen-Geiger climate classes across India, representing distinct precipitation-temperature regimes. The results indicate that Permanent riparian interfaces occupy only 18-27% of the geomorphic floodplain area but account for >55% of persistent surface-subsurface connectivity. Contrastingly, seasonal riparian zones expand by up to 2.6 times during monsoon or high-precipitation periods. This further highlights the climate-driven activation of transient GW-SW pathways. Humid climatic regions exhibit stable vegetation persistence and low thermal variability and are indicative of sustained gaining conditions and shallow groundwater tables. Semi-arid reaches show high seasonal variability, episodic losing conditions, and rapid contraction of active interfaces. Climatic transition zones display the highest temporal instability from bidirectional GW-SW fluxes governed by threshold-controlled switching between hydrological states. Moreover, trend and non-parametric breakpoint analysis of extreme climate indices indicate regime shifts in 32-41% of seasonal riparian interfaces across the varying climatic zones across India after the early 2000s. Further, rainfall dominated basins show the strongest response due to weak hydrological memory and event-driven processes. Thus, Riparian zones emerge as transient control volumes regulating GW-SW coupling under changing climatic forcing. This approach advances riparian analysis from spatial mapping to functional characterisation and supports scalable, process-based riparian management focused on buffering capacity, resilience, and subsurface connectivity.
How to cite: Roy, S., Singh, S., and Goyal, M. K.: Earth Observation Based Functional Characterization of Riparian Interfaces across Climatic Zones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8955, https://doi.org/10.5194/egusphere-egu26-8955, 2026.
The Coxilha das Lombas Aquifer System is located in the coastal plain of the Pelotas Basin, southern Brazil. Associated with Pleistocene eolian deposits, it is considered the most productive aquifer system in the Porto Alegre Metropolitan Region, capital of the state of Rio Grande do Sul. Despite its strategic importance and the pressure related to its use, the system still lacks a more comprehensive understanding of its groundwater flow and hydrochemical characteristics. This aquifer system constitutes a groundwater recharge area extending approximately 70 km in length (NE–SW direction) and about 10 km in width. Its topographic highs coincide with a regional groundwater divide, characterized by divergent flow paths. According to several authors, groundwater discharge contributes to lagoons, wetlands, and watercourses that drain toward the Lagoa dos Patos to the southeast—the world’s largest choked coastal lagoon—and toward the Gravataí River basin to the northwest. In this context, a hydrochemical study of the Coxilha das Lombas Aquifer System was carried out, integrating pre-existing data with new field data obtained from the collection and analysis of 85 groundwater samples along the aquifer extent, encompassing different depths and well types. The analyzed parameters included pH, electrical conductivity, total dissolved solids, and the major anions and cations. The results indicate predominantly sodium–chloride hydrochemical facies, with a median pH of 5.3, characterizing acidic waters, likely of meteoric origin. Low values of total dissolved solids, electrical conductivity, and alkalinity were observed (TDS = 36 mg/L, EC = 46 μS/cm, and alkalinity = 4.83 mg/L), reflecting the low degree of mineralization of the aquifer waters, probably related to a short groundwater residence time. Additionally, 21 groundwater and surface water samples were collected for oxygen and hydrogen isotope analyses. These results will be evaluated using precipitation data from the Global Network of Isotopes in Precipitation (GNIP-POA) station in Porto Alegre, operated by the Geological Survey of Brazil, and are expected to provide insights into water origin and groundwater contribution to wetlands and surface watercourses in the region through groundwater-surface water mixing relationships.
This work is part of the project “Geological evolution and hydrostratigraphy of Coxilha das Lombas, northwestern coastal plain of Rio Grande do Sul” (no. 407572/2023-6), supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
How to cite: Collischonn, L., Ronchetti, F., Correa da Camara Rosa, M. L., and Kirchheim, R. E.: Hydrochemical and isotopic evidence of groundwater contribution to wetlands in a coastal plain aquifer, southern Brazil, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15049, https://doi.org/10.5194/egusphere-egu26-15049, 2026.
Small weirs are ubiquitous in agricultural watersheds, yet how their repeated seasonal operation reshapes groundwater–surface water interactions (GSI) remains poorly understood. This study explores the hydraulic, thermal, and geochemical responses to cyclic opening and closure of a small intake weir throughout the monitoring period along a 2.5-km reach of the Hyogyo Stream, South Korea. High-frequency monitoring of water level and electrical conductivity, together with multi-depth subsurface temperature observations, was conducted at three locations upstream of the weir within the study reach under both open and closed conditions. These continuous observations were complemented by four discrete δ¹⁸O–δD isotope surveys. Water level, electrical conductivity, isotope values, and subsurface temperature all captured individual weir-opening and -closure events. These event-scale responses included water-level rise, channel inundation, and transient flooding of streambanks and were accompanied by electrical-conductivity and isotope shifts indicative of enhanced surface water–subsurface water mixing. During the initial closure, diurnal thermal signals penetrated to depths of 3.5–4.0 m, indicating an expanded hyporheic exchange zone. Unlike other parameters, thermal signals at depth (3.5–4.0 m) progressively converged to a stable temperature with repeated weir opening and closure, implying contraction and reorganization of the active exchange zone despite comparable hydraulic forcing. These cumulative thermal responses may induce long-term restructuring of the hyporheic thermal regime. Our findings highlight the need to consider small weirs in integrated water management, particularly by emphasizing long-term thermal monitoring in agricultural watersheds.
Acknowledgement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2022R1A2C1006696). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT)(No.RS-2022-NR070842). This work was supported by a grant from the National Institute of Environmental Research (NIER), funded by the Ministry of Environment (ME) of the Republic of Korea (NIER-2025-04-02-051).
How to cite: Seok, H.-Y., Baek, J.-Y., Kaown, D., Lee, S.-S., and Lee, K.-K.: Small Weirs Reshape the Hyporheic Zone: Cumulative Thermal Responses across Seasonal operation cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2295, https://doi.org/10.5194/egusphere-egu26-2295, 2026.
Interactions between groundwater (GW) and surface water (SW) have been a focus of hydrologic research for some time. Seminal early work by J. Toth (1963) and later T. Winter (1999) had shown the existence of nested GW flow systems and stressed that surface water bodies are integral parts of these flow systems. Despite this early, integral perspective, a simpler perception of GW and SW as two distinct compartments, which interact via some often loosely defined transfer mechanisms, still prevails. This perception can be found in many hydrologic models, but can be misleading, as it implies the existence to two clearly separable compartments, while in fact GW and SW are part of a hydrologic continuum (as a part of the terrestrial hydrologic cycle), in which water dynamically transitions back and forth between surface water bodies (rivers, lakes, wetlands) and shallow aquifers. For example, shallow riparian groundwater may become stream water in one moment and return back to the alluvial aquifer in the next with implications for water and solute exchange and biogeochemical turnover. While simplified conceptualizations of the GW-SW hydrologic continuum may be acceptable for the simulation of catchment streamflow response, they usually fall short, when trying to represent fluxes and dynamics of nutrients and other solutes, which are typically controlled by hydrological and biogeochemical processes in the transition zone between GW and SW. I argue that in our quest to understand coupled hydrological and biogeochemical processes and GW dependent ecosystems at the catchment and landscape scales, we needed to revisit the perception of GW and SW as a hydrologic continuum. I will use the example of dissolved organic carbon (DOC) export from a headwater catchment to stress this point and illustrate how rich field data and an integral numerical model can help to refine and improve a simplified conceptual model for catchment-scale DOC export.
Toth, J. (1963) A Theoretical Analysis of Groundwater Flow in Small Drainage Basins, Journal of Geophysical Research, 68(16)
Winter, T. (1999) Relation of streams, lakes, and wetlands to groundwater flow systems, Hydrogeology Journal, 7:28-45
How to cite: Fleckenstein, J.: Groundwater – surface water interactions revisited, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19520, https://doi.org/10.5194/egusphere-egu26-19520, 2026.
Growing environmental challenges are forcing hydrogeologists to create
increasingly accurate models that faithfully reflect the complex
groundwater-surface water interactions in the hydrological cycle.
However, the construction of complex, monolithic models is often faced
barriers such as lack of detailed input data and time-consuming
calculations. An alternative is to couple dedicated domain models, which
allow existing resources to be leveraged and increase the accuracy of
simulations.
This paper presents a custom approach to coupling the SWAT+ catchment
model (representing surface and soil processes) with the MODFLOW 6
groundwater model. The integration was performed using Python scripts,
which ensured flexibility in data exchange. The study utilizes the SWAT
model developed in QSWAT and a hydrogeological model which was migrated
from the older MODFLOW 96 engine to the latest version of MODFLOW 6 with
using the DIS grid.
The study area is an agricultural catchment of approximately 250 km² in
south-central Poland. The key methodological challenge addressed in this
paper is the issue of spatial scaling, i.e. upscaling and mapping the
irregular hydrological response units (HRUs) from the SWAT model to the
regular computational grid of the MODFLOW model. The algorithm developed
in Python enables bidirectional exchange of fluxes (groundwater
recharge, river flow), which represents the dynamic interaction between
surface water and saturated zone. Preliminary results indicate that the
presented approach improves consistency of the water balance at the
interface.
Acknowledgements. The work was carried out as part of WATERLINE project
(2020/02/Y/ST10/00065), under the CHISTERA IV programme of the EU
Horizon 2020 (grant no. 857925) funded by National Science Centre,
Poland and a partially by AGH University of Krakow, Faculty of Geology,
Geophysics and Environmental Protection (grant no. 16.16.140.315).
How to cite: Nikiel, M. and Żurek, A. J.: Coupling SWAT+ and MODFLOW 6 using Python: A flexibleapproach to representing groundwater-surface water interactions in anagricultural catchment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22367, https://doi.org/10.5194/egusphere-egu26-22367, 2026.
The hyporheic zone is a key interface where surface water and groundwater interact, triggering a wide range of biogeochemical reactions. This interaction is governed by mixing processes whose dynamics remain poorly understood in hyporheic environments. Here we investigate hyporheic mixing using numerical simulations of Darcy-scale flow and solute transport beneath river bedforms in heterogeneous porous media. We consider a range of conditions that vary solute advection and dispersion, groundwater upwelling intensity, and the ratio between the heterogeneity correlation length and bedform size. Mixing is quantified through the scalar dissipation rate and by assessing the emergence of ergodic behavior in hyporheic transport. Finally, we interpret the resulting mixing dynamics within a lamellar framework, in which the stretching and elongation of material elements traveling through the hyporheic zone control the efficiency of mixing.
How to cite: Fistolera, D. and Dell'Oca, A.: Solute mixing in the hyporheic zone: impact of dispersion, upwelling and heterogeneity., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7416, https://doi.org/10.5194/egusphere-egu26-7416, 2026.
Floodplains are highly dynamic systems and can accumulate large quantities of microplastics (MPs), yet the mechanisms controlling their vertical redistribution after deposition remain poorly constrained. We investigate MP infiltration and transport in undisturbed Rhine floodplain soils using intact 0–20 cm cores subjected to a controlled flooding scenario. A particle mix of polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) (20–75 µm), cryomilled and pre-incubated in Rhine water to allow biofilm formation, was applied to the soil surface prior to flooding. Particles were labelled with Rhodamine-B and metal oxides to enable complementary optical and elemental tracing. Water flow and tracer transport were monitored using D₂O breakthrough curves and continuous gravimetric measurements.
After freezing, soil columns were sectioned into 2 cm layers, and MPs were quantified by fluorescence microscopy, µ-XRF, and AI-assisted particle recognition. Results indicate rapid MP infiltration and vertical transport within the soil. MP breakthrough was observed in all columns, although breakthrough timing and concentrations varied among replicates. Vertical transport was strongly governed by spatial heterogeneity and preferential flow paths, particularly biogenic macropores, whereas saturated hydraulic conductivity alone did not reliably predict MP movement. These findings highlight the dominant role of soil structural controls in floodplain MP transport and challenge the use of bulk hydraulic parameters for predicting MP redistribution during flooding events.
How to cite: Echstenkämper, L., Rolf, M., Khaleel, R., Laermanns, H., Pohl, F., and Bogner, C.: Soil structure strongly controls vertical microplastic transport in floodplain soils, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18620, https://doi.org/10.5194/egusphere-egu26-18620, 2026.
The transient storage model (TSM) has been widely used to interpret solute transport in river corridors. Its key components are a mobile domain, representing the main channel, and an immobile domain, accounting for solute storage within the hyporheic zone, where biogeochemical transformations are known to occur. In this study, we consider a reactive immobile compartment characterized by a linear reaction whose rate fluctuates over time. We systematically explore combinations of characteristic storage times, reaction-rate fluctuation periods, and the relative importance of advection and dispersion in the mobile domain to identify the regimes in which temporal fluctuations exert a significant control on solute dynamics. Furthermore, we derive a semi-analytical expression for the effective reactivity of the river corridor, highlighting deviations from the classical scenario assuming a time-invariant reaction rate in the immobile domain.
How to cite: Dell Oca, A., Aquino, T., and Roche, K.: Transient storage model: the impact of temporally varying reaction rate, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17313, https://doi.org/10.5194/egusphere-egu26-17313, 2026.
Posters virtual: Tue, 5 May, 14:00–18:00 | vPoster spot A
EGU26-16155 | ECS | Posters virtual | VPS8
Microbial Heterogeneity Outweighs Sediment Variability in Regulating Hyporheic Nitrogen RemovalTue, 05 May, 14:36–14:39 (CEST) vPoster spot A
The hyporheic zone serves as a critical hotspot for nitrogen attenuation, driven by flow in streambed sediments, biogeochemical reactions, and enhanced microbial activity. It has, however, yet to be determined how the interaction of heterogeneity in sedimentary physical (e.g., permeability) and chemical (e.g., organic matter content) properties influences nitrogen cycling in complex hyporheic environments. Here we developed numerical models coupling porous flow, reactive transport, and microbial dynamics for realistic heterogeneous streambed scenarios. Simulations reveal that small-scale spatial variations in sediments physical and chemical properties exert negligible effects on nitrogen removal, whereas the spatial heterogeneity in functional microbial biomass dominates nitrogen removal dynamics. This is caused by biofilm-induced bioclogging that drastically reduces hyporheic exchange, thereby weakening the role of sedimentary heterogeneity. This study represents the first quantitative assessment of how sedimentary and microbial spatial heterogeneities jointly regulate nitrogen removal in hyporheic systems, offering critical insights for predictive modeling of bedform interfaces.
How to cite: Xian, Y., Xiao, Z., Wen, Z., and Krause, S.: Microbial Heterogeneity Outweighs Sediment Variability in Regulating Hyporheic Nitrogen Removal, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16155, https://doi.org/10.5194/egusphere-egu26-16155, 2026.
EGU26-16236 | ECS | Posters virtual | VPS8
Flood-Driven Groundwater Recharge for IndiaTue, 05 May, 15:00–15:03 (CEST) vPoster spot A
Rapid groundwater depletion, driven by intensive pumping and growing climate variability, poses a critical threat to water security across India. Concurrently, climate change is intensifying the frequency and magnitude of flood events, generating episodic but potentially significant opportunities for natural aquifer replenishment. However, the contribution of floods to groundwater in India remains poorly quantified. In this study, we systematically quantify flood‐driven groundwater recharge across the major river basins of India. Using the integrated, physically based ParFlow-CLM hydrological model, we evaluate three fundamental attributes of flood recharge: (i) the contribution of flood runoff to total groundwater recharge, (ii) the temporal lag between flood peaks and aquifer response, and (iii) the persistence of flood‐induced recharge signals following an event. These metrics are evaluated across diverse hydrogeological settings to identify where floodwaters are most effectively captured and retained within aquifers. Our results show strong spatial contrasts in flood recharge efficiency. The highly permeable alluvial aquifers of the Indus, Ganga and Brahmaputra basins exhibit the highest flood-to-recharge contribution and the longest persistence, indicating a strong capacity to capture and retain floodwater. In contrast, less permeable and fractured hard-rock aquifers in large parts of central and southern India show weaker and shorter-lived recharge responses to floods. By explicitly linking flood dynamics to subsurface hydrologic response, this study provides a framework for identifying priority regions for flood‐based groundwater management. The results demonstrate how increasing flood extremes under climate change can be strategically harnessed to enhance the resilience of India’s groundwater resources.
How to cite: Roy, R. and Mishra, V.: Flood-Driven Groundwater Recharge for India , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16236, https://doi.org/10.5194/egusphere-egu26-16236, 2026.
