In shallow lakes, wind-driven turbulence and thermally induced convection control water mixing. Together, and depending on depth, they interact with microplastic (MP) settling to determine how particles are distributed throughout the water column. To investigate these processes, two MP addition experiments were conducted in a 112 m3 aquatic mesocosm during summer using 1–5 µm microspheres. High resolution data on MP concentrations, water velocities, wind speeds, and water and air temperatures were collected. Additionally, using OpenFOAM, a three-dimensional CFD model incorporating fluid–particle interactions was configurated to quantitatively interpret the experimental data of MP transport. The results indicated that although Stokes’ settling velocity predicted MP would take up to 182 days to reach the mesocosm bottom, MP of all sizes was detected just above the bed (3 m) within only 3 days. The vertical distribution of MP, characterized using the Péclet number (Pé, settling velocity/turbulent diffusion), increased with depth but remained < 1. In the near-surface layer (< 0.25 m), approximately 10

Simulating flows between surface water and groundwater on all continents is a crucial step towards a more accurate representation of the freshwater system in large-scale hydrological and Earth System models. Reliable simulations would improve assessments of renewable groundwater resources, particularly in water-scarce regions, and water pollution as well as our understanding of how groundwater supports environmental flows. Dynamic modeling of surface water-groundwater interactions requires the computation of hydraulic head gradients. However, the accuracy of large-scale models that simulate such interactions has not yet been evaluated. We coupled the global hydrologic model WaterGAP with the gradient-based groundwater model G³M and compared it with the estimates of the well-established Central Valley Model (CVHM) and observation-based estimates of losing and gaining streams from Jasechko et al. (2021) across the USA. Compared to Jasechko et al., WaterGAP-G³M simulates the same (>90% temporal agreement) gaining and losing stream segments for only 9% of the cells with available data, while showing disagreement (<50% temporal agreement) for 56%. Compared to CVHM, WaterGAP-G³M shows a higher spatial agreement on gaining (49.8%) than on losing (4.1%) stream segments, likely due to uncertainties in representing abstractions. Surprisingly, we find that CVHM and Jasechko et al., too, strongly disagree on gaining (agreement 10.5%) or losing (17.2%) conditions. And Jasechko et al. found 54% and 77% agreement with local studies for losing and gaining conditions, respectively. This means that we face substantial uncertainty and we require more robust methods to estimate gaining and losing conditions across large scales from observations and models.

Fully-coupled solvers have proven to be suitable and computationally efficient tools for studying surface water-groundwater (SW-GW) interactions. Most existing fully-coupled codes use the two-dimensional, depth-averaged shallow water equations for surface flows. As a result, three-dimensional (3D) flow dynamics are ignored in the surface domain, including phenomena important for the SW-GW exchange such as turbulence. Computational Fluid Dynamics (CFD) solvers allow to capture 3D information on the surface turbulent flow by solving the full Navier-Stokes equations. Consequently, they are well-suited for the study of the actual exchange flows of water and solutes across the sediment-water interface. Among the available CFD software, the open-source toolbox OpenFOAM provides a flexible modelling framework to implement user-defined, fully-coupled models for the detailed investigation of SW-GW interaction processes. Based on this CFD platform, Lee et al. (2021) developed hyporheicScalarInterFoam, a fully-coupled 3D solver capable of solving the flow and transport processes in both surface and subsurface domains as well as the interactions across their interface. Despite the potential of this new code to tackle SW-GW interactions, its application to real-world hydrogeological scenarios is constrained by limitations in boundary conditions, parameter heterogeneity and key hydrodynamic and transport processes, among others, which hinder the accurate representation of the complex characteristics of natural systems. To overcome this, an updated and extended version of hyporheicScalarInterFoam, called darcyInterTransportFoam, is presented in this paper. The new fully-coupled solver enhances the applicability of the original by introducing novel simulation features. These include internal solver updates, such as the definition of heterogeneous and anisotropic subsurface fields and the simulation of heat transfer in both domains, as well as newly implemented add-ons, including pre- and post-processing utilities and additional boundary conditions. A complete description of all the new features is provided in this paper. Moreover, the utility of darcyInterTransportFoam is demonstrated in a test case, where the SW-GW flow, solute transport and heat transfer processes are simulated in a highly conductive river-aquifer system.

Groundwater conflicts are increasing worldwide, particularly during droughts. While conflict dynamics have been widely studied in historically water-scarce regions, emerging hotspots in Europe remain understudied. We introduce a text-mining approach to monitor conflicts over time and apply it to Germany, where droughts have exacerbated tensions among water users. Using a corpus of over 12,000 news articles published between 2000 and 2022, we map the spatiotemporal distribution of reported conflicts and identify their drivers using a metric of reported conflict intensity combining frequency and prominence of conflict reporting. Groundwater conflicts are geographically widespread, with recurring hotspots and new conflict areas emerging during the 2018-2022 multi-year drought. While water pollution and environmental protection have diminished in importance, scarcity, agriculture, and drought have become key drivers. Reported conflicts show only moderate spatial correlations with groundwater withdrawal, recharge, and pollution, suggesting media dynamics and social context shape which conflicts become publicly visible.

Understanding the contributions of diffuse and point sources to nitrate pollution is crucial for managing river water quality. We conducted a long-term modeling study for the Rhine and Elbe basins and their 146 subbasins from 1950 to 2021 to quantify the roles of diffuse and, in particular, point sources in driving stream NO _₃ –N concentrations. In both basins, simulated results show a decline in point source contributions from 1950 to 2000, followed by a relatively stable level at around 25% in the Rhine and fluctuations around 30% levels in the Elbe. The decline in the simulated stream NO _₃ –N concentrations in both basins after 1990 was largely driven by a decrease in point sources, and stream NO _₃ –N concentrations remained high (∼2 mg l ^−1 ) during 2010–2021, even when point sources were excluded. At the subbasin level, changes in point source contributions and stream NO _₃ –N concentrations reflected the overall trends of their respective basins, although individual subbasins exhibited diverse patterns. In subbasins with high stream NO _₃ –N concentrations during 2010–2021, point source contribution accounted for around 30% (median values across subbasins), and the fractions of agricultural, urban, and industrial land cover were relatively high. These results highlight that point source management alone is not sufficient to reduce stream nitrate to a good ecological status (< 2 mg l ^−1 ), and spatial targeted management is required to achieve good ecological status at both the regional and local levels.

The extensive use of plastics has led to a widespread presence of microplastics (MPs) across various environmental compartments. Rivers and their floodplains not only play a crucial role in transporting these particles from terrestrial sources to lakes and oceans, but can also act as temporary sinks. Despite the significance of rivers as transport pathways for MPs to the ocean, our understanding of the dominant transport and retention process in river corridors is still limited. This study investigates the transport, deposition and remobilization processes of MP along a 3.5km reach of the river Rhine between Cologne and Düsseldorf, Germany. A three-dimensional hydrodynamic model with the morphological module (D-Morphology) was developed using the Delft3D FM software. Two types of microplastic particles with diameter of 0.1mm and different densities, 1030 kg/m3 Polystyrene and 1195 kg/m3 Polyvinyl Chloride (PVC) were used to assess their transport behavior under different flow scenarios. The model was calibrated against observed water levels on Manning’s roughness coefficient and subsequently validated against an independent data set. A continuous flux of microplastics at a concentration of 1μg/m³ was introduced into the hydrodynamic model at the upstream boundary.First, results indicate that advection and flow turbulence are the dominant processes governing microplastic transport. Higher discharge rates enhance microplastic transport by increasing suspended concentrations, while reducing the mass of the sedimented particles. The percentage of sedimented Polystyrene was found to be about 2.5% of total input at the end of a simulated flood event in 2021. Resuspension was found to be about 40% of the sedimented mass along the river banks and floodplain during peak flood. During the recession limb of the flood event, sedimented microplastic load increased gradually whereas suspended load decreased. Additionally, the density and size of the microplastic particles along with hydrodynamic conditions significantly influence their spatial distribution and storage within the river corridor.

The amount of dissolved organic carbon (DOC) in surface waters is an important water quality indicator. High levels of DOC in surface waters cause negative impacts on the aquatic ecosystem (e.g., via reducing light penetration and increasing water temperature) and increase the water treatment cost for drinking water supply. DOC mobilization and export from catchments into streams are hydrologically controlled and strongly affected by catchment-specific characteristics (such as topography, soils, and land cover type) and climatic factors. In this study, we developed a simplified process-based model that explicitly includes the hillslope, riparian, and groundwater compartments with the hydro-biogeochemical concept mainly based on the mesoscale Hydrologic Model (mHM) and INCA-Carbon models. The proposed model also allows dynamic carbon input from litterfall and root breakdown. We hypothesize that such a model is needed to understand the role of different catchment compartments and land cover and climate change on instream DOC export. We applied the proposed model for instream DOC simulation in four temperate forest and agriculture catchments located in the Harz Mountains, Germany. Here, we calibrated the model for the period which includes drought years (2018-2019) and the subsequent forest dieback (starting from 2018). The models showed satisfactory results in terms of instream DOC concentrations. Here, we will further evaluate if the model provides the right results for the right reasons by analyzing the physical soundness of the internal carbon export dynamics among different model compartments from our calibrated model. Such evaluation is important when further applying this modeling concept to other areas under similar circumstances.

Interactions between groundwater (GW) and surface water (SW) have been a focus of hydrologic research for some time. Seminal early work by Toth (1963) and later 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. Finally an outlook will be given on future requirements for adequate monitoring and modeling of coupled GW-SW ecosystems.

Numerical modeling is an efficient tool for quantifying transport and sedimentation patterns of microplastic (MP) particles in lentic systems. To evaluate these patterns based on a specific research area we set up a three-dimensional hydrodynamic and transport model for a reservoir in Germany.We partition the computational domain with an unstructured mesh to optimally capture the geometry of the reservoir and to adapt the mesh resolution. Thereby, shallow areas and those with steep bathymetry gradients are represented at a particularly high resolution. In vertical direction, we use a combination of z- and sigma-layers. To quantify the effects of the grid on the model results, we perform a sensitivity analysis for different horizontal and vertical mesh resolutions.For the hydrodynamic simulations we use the Delft3D Flexible Mesh Suite (Delft3D FM). We calibrate and validate the hydrodynamic model utilizing monthly measured vertical temperature profiles for two different years. For simulating the MP transport, we rely on the sediments and morphology module of Delft3D FM. This module is based on a Eulerian approach which allows us to efficiently simulate large concentrations of MP particles.

Interactions between groundwater (GW) and surface water (SW) play a pivotal role in influencing water quantity, quality, and associated biogeochemical and ecological processes in stream networks. Understanding the spatial pattern of gaining and losing rivers is crucial for managing water resources at catchment scale. Each method to identify losing and gaining rivers, from point to reach to catchment scale, has distinct advantages and limitations. These limitations can potentially be mitigated by combining different approaches.In this study, we combined local information from hydraulic head differences between GW and SW with the regional information derived from topography-driven groundwater flow to robustly identify and characterize the spatial pattern of gaining and losing rivers in two study areas located at Central Germany –the Bode catchment and Free State of Thuringia. Central Germany has faced a drought period in the last five years, which has impacted groundwater levels. To evaluate local head differences, we compared the measured averaged groundwater levels (GWLs) and estimated surface levels (SWLs). The GWL data were obtained from 49 and 826 groundwater monitoring wells within a 1500 m distance from rivers in the Bode catchment and Thuringia, respectively. We developed a method for estimating SWLs across river networks by correcting a coarse DEM (25 m) based on the river bed elevations and river water depths recorded at gauging stations and river network topology. Uncertainties of SWL were also estimated and considered in the classification of gaining and losing rivers. Topography-driven discharge (gaining rivers) and recharge (losing rivers) areas are derived from groundwater upward and downward flow directions according to a 3D spectral solution.The analysis of head differences reveals a widespread occurrence of losing rivers. However, when combining the losing and gaining classifications from topographical-driven groundwater flow with the classifications from head differences, the fraction of river segments having the same classification from both methods is relatively low (around 50% in both study areas). Many river segments showed contradictory classifications from the two methods, with a notable observation being that rivers have losing classifications from head differences but gaining classifications from topographic analyses. Specifically, 41% of river segments in Thuringia and 7 out of 9 (78%) in the Bode catchment fall into this category. This mismatch typically occurred in urban and mining areas, indicating anthropogenically lowered GWLs.By combining local and regional scale methods, our study contributes to a more robust representation of patterns of gaining and losing rivers. Our analysis reveals the prominence of losing rivers despite the topographical classification of a gaining river highlights the anthropogenic impacts on local groundwater levels.

Nitrate pollution in streams, although attempts have been made to combat it, remains a persistent problem, especially in highly anthropogenically impacted landscapes such as Western Europe. Nitrate concentrations and discharge typically vary with the seasons, as does the vulnerability of water bodies to high nitrate inputs. However, the degree of variability and seasonal timing vary in space and time while nitrate inputs in catchments have undergone drastic long-term changes. The changing N sources and distribution in the catchments and their variable hydrological activation suggest that different nitrate seasonality has emerged across catchments over the decades. In this study, we hypothesize that nitrate concentrations respond faster to changes in input during the high-flow season than during the low-flow season, as shallow sources are typically activated during high flow and are the first to be affected by changes in management. To test this hypothesis, we propose a hysteresis approach of long-term nitrate seasonality during low- and high-flow seasons, which we applied in 290 catchments in Germany and France with nitrate and discharge time series of 20 or more years. Our results show that in the majority of catchments, nitrate and discharge vary synchronously with peaks in winter. Deviating average nitrate-discharge typologies could be linked to topography and hydroclimatic seasonality as well as to the regionally characteristic source heterogeneity and lithology in northwestern France. Contrary to our hypothesis, we found both types of trajectories with preceding high-flow and low-flow nitrate concentrations were equally present. We could exemplarily show high-flow concentrations responded first in an agricultural catchment and low-flow concentrations reacted first in a more point source intense catchment. However, across the large number of catchments, consistency was not observed suggesting higher complexity of interacting processes. In a further step, we plan to investigate the long-term trajectories of phosphorus to account for the ratios of the major nutrients affecting the resulting impact of land-stream transfer processes on eutrophication.References: Ebeling, P., Dupas, R., Abbott, B., Kumar, R., Ehrhardt, S., Fleckenstein, J. H., & Musolff, A. (2021). Long-term nitrate trajectories vary by season in Western European catchments. Global Biogeochemical Cycles, 35, e2021GB007050. https://doi.org/10.1029/2021GB00705

Microplastic (MP) pollution in the aquatic environment has become a problem of growing concern due to potential adverse effects on aquatic organisms and ecosystems. While MP transport and fate in marine systems has been researched to quite some extent relatively little is known about the transport mechanisms of MP particles in terrestrial surface waters and in saturated porous media like in groundwater or the hyporheic zone (HZ). We investigated the transport and fate of small (1, 3 and 10 μm diameter) polystyrene MP particles in a rippled, sandy stream bed (D50 = 1.04 mm) using CFD simulations calibrated to a set of flume experiments. A novel detection system for fluorescent MP particles (Boos et al. 2021) was used to track and quantify particle movement in the turbulent open water and in the hyporheic sediments in the laboratory flume following a pulse injection of MP particles into the surface water compartment. A new, integrated CFD simulation scheme within the OpenFOAM suite of CFD solvers was implemented for the flume system for a seamless simulation of water flow and particle transport in the open water and in the hyporheic sediments (Dichgans et al. 2023). Additionally we simulated the transport and fate of a range of “virtual” particles in the open water for different channel geometries using a Lagrangian approach. Simulations show that 1 μm MP particles are transported through the HZ like a solute, following the typical hyporheic flow cells below the bedforms. Transport and particle progression through the HZ could be adequately described with an advection-dispersion equation. Larger 10 µm MP particles instead showed retarded transport through the HZ, while retardation increased with travel distance in the sediments. Our results indicate that advective pumping across the streambed interface can transport very small MP particles through the HZ, while larger particles are increasingly retained. Distinct flow structures in the open water are found to be decisive for the fate of MP particles in the river channel. References: Dichgans, F., Boos, J.P., Ahmadi, P., Frei, S., Fleckenstein, J.H. (2023), Integrated numerical modeling to quantify transport and fate of microplastics in the hyporheic zone, Water Research, 243, https://doi.org/10.1016/j.watres.2023.120349 Boos, J.-P., Gilfedder, B. S., & Frei, S. (2021), Tracking microplastics across the streambed interface: Using laserinduced-fluorescence to quantitatively analyze microplastic transport in an experimental flume. Water Resources Research, 57, e2021WR031064.https://doi.org/10.1029/2021WR031064 Boos, J.-P., Dichgans, F., Fleckenstein, J.H., Gilfedder, B. S., Frei, S. (2024) Assessing the Behavior of Microplastics in Fluvial Systems: Infiltration and Retention Dynamics in Streambed Sediments. Water Resources Research, accepted

Microplastic (MP) pollution has garnered global attention due to its ubiquity in marine and freshwater systems, as well as its potential—though still uncertain—risks to human health. While MP concentrations in drinking water remain relatively low, safeguarding reservoir-based drinking water supplies against potential contamination remains a pressing concern. In this study, we applied a rigorously validated, two-dimensional hydrodynamic model (CE-QUAL-W2) to Germany’s largest drinking water reservoir, the Rappbode Reservoir, to examine MP retention under realistic inflow, meteorological, and operational conditions. Our primary aim was to quantify how varying particle settling velocities (0.1–1.0 m d⁻1) influence MP transport, sedimentation, and breakthrough to the raw water outlet over a 2-year simulation period. We demonstrate that reservoir-scale retention efficiency rises sharply with increasing MP settling velocity, with near-complete retention (> 95

Pore-scale microplastics (< 10 μm) are emerging contaminants whose behavior and fate in aquatic environments remain poorly understood. While the transport and retention of spherical microplastics (SMPs) have been studied in fluvial systems, irregularly shaped microplastics (IMPs) and microplastic fibers (MPFs) remain poorly understood. This study investigates how IMPs and MPFs differ from SMPs in their transport and retention in open stream and hyporheic flows under controlled flume conditions using fluorescently labeled particles. We compared the transport dynamics of IMPs (d90 = 7.68 μm) and MPFs (diameter 5–10 μm, length 60–250 μm) with reference SMPs (1, 3, and 10 μm in diameter) by continuously monitoring microplastic concentrations in surface water and streambed sediments. IMPs exhibited mobility similar to SMPs, with minor retention in the system. In contrast, MPFs showed markedly higher retention, preferentially accumulating at the sediment–water interface, where 9

The exchange between surface water (SW) and groundwater (GW) influences water availability and ecosystems in stream networks. Assessing GW-SW interactions can be based on various methods at different scales, such as point scale (e.g., local head differences, temperature profiles), reach scale (e.g., environmental tracers, water mass balance), and catchment scale (topographical-driven groundwater flow), which all have distinct advantages and limitations. In this study, we combined the analysis of local hydraulic head differences with regional topographical-driven groundwater flow to robustly reveal gaining and losing stream patterns in two study regions in Central Germany (Bode catchment and Free State of Thuringia). To evaluate local hydraulic gradients, we developed a method for estimating surface water levels across stream networks by modifying surface elevations from a coarse digital elevation model (25 m) and compared these to measured groundwater levels. Our results reveal prevalent occurrences of losing streams. Numerous stream locations are characterized by mismatching classifications from the two methods providing additional insights for understanding water cycles. The most notable discrepancy is the classification as losing based on head differences and gaining from topographic analyses accounting for 37% and 47% of the stream locations in Thuringia and in Bode catchment. This mismatch indicates anthropogenically lowered groundwater levels, typically occurring in urban and mining areas in the study areas. Our approach, combining local hydraulic head analysis and topographical-driven groundwater flow enhances the understanding of gaining and losing stream patterns at catchment scale, revealing widespread occurrences of losing streams and highlighting the significance of anthropogenic influences on water cycles.

Microplastic (MP) particles are assumed to be potentially harmful to organisms in the hydrosphere. To better assess the exposure and the associated risk it is essential to quantify the transport and sedimentation behavior of MP particles in aquatic environments.Using the Delft3D Flexible Mesh Suite we set up a three-dimensional hydrodynamic and MP transport model for lakes and reservoirs. Our focus is on modeling polymers with different densities and particle sizes to identify patterns of particle residence time and sedimentation. The reservoir Großer Brombachsee in Germany serves as the research site with realistic forcings and boundary conditions.We present first results for horizontal and vertical distribution patterns for different polymer types. We found that the distribution of MP in the computational domain is strongly affected by both particle density and particle size. Smaller, lighter particles are spread over the entire horizontal extent of the reservoir, but particles of higher density or of larger size settle within a limited area around the inflow location, indicating a much higher settling velocity.

Groundwater is a crucial resource for society and the environment, e.g., for drinking-water supply and dry-weather stream flows. The recent severe drought in Europe (2018–2020) has demonstrated that these services could be jeopardized by ongoing global warming and the associated increase in the frequency and duration of hydroclimatic extremes such as droughts. To assess the effects of meteorological variability on groundwater heads throughout Germany, we systematically analyzed the response of groundwater heads at 6626 wells over a period of 30 years. We characterized and clustered groundwater head responses, quantified response timescales, and linked the identified patterns to spatial controls such as land cover and topography using machine learning. We identified eight distinct clusters of groundwater responses with emerging regional patterns. Meteorological variations explained about 50 % of the groundwater head variations, with response timescales ranging from a few months to several years between clusters. The differences in groundwater head responses between the regions could be attributed to regional meteorological variations, while the differences within the regions depended on local landscape controls. Here, the depth to groundwater best explained the timescale of the observed head response, with shorter response times in shallower groundwater. Two of the clusters showed consistent long-term trends that were not explained by meteorological controls and could be attributed to anthropogenic impacts. Our study contributes to a better understanding of the regional controls of groundwater head dynamics and to the classification of groundwater vulnerability to hydroclimatic extremes.

Transit time-based water quality models using StorAge Selection (SAS) functions are crucial for nitrate (NO3-) management. However, relying solely on instream NO3- concentration for model calibration can result in poor parameter identifiability. This is due to the interaction, or correlation, between transport parameters, such as SAS function parameters, and denitrification rate, which challenges accurate parameters identification and description of catchment-scale hydrological processes. To tackle this issue, we conducted three Monte-Carlo experiments for a German mesoscale catchment by calibrating a SAS-based model with daily instream NO3- concentrations (Experiment 1), monthly instream stable water isotopes (e.g. delta 18O) (Experiment 2) and both datasets (Experiment 3). Our findings revealed comparable ranges of SAS transport parameters and median water transit times (TT50) across the experiments. This suggests that, despite their distinct reactive or conservative nature, and sampling strategies, the NO3- and delta 18O time series offer similar information for calibration. However, the absolute values of transport parameters and TT50 time series, as well as the degree of parameter interaction differed. Experiment 1 showed greater interaction between certain transport parameters and denitrification rate, leading to greater equifinality. Conversely, Experiment 3 yielded reduced parameters interaction, which enhanced transport parameters identifiability and decreased uncertainty in TT50 time series. Hence, even a modest effort to incorporate only monthly delta 18O values in model calibration for highly frequent NO3-, improved the description of hydrological transport. This study showcased the value of combining NO3- and delta 18O model results to improve transport parameter identifiability and model robustness, which ultimately enhances NO3- management strategies. A transit time-based water quality model was calibrated against nitrate (NO3-) concentrations alone and alongside with stable water isotopes (delta 18O). While both methods produced similar patterns in simulated median water transit times (TT50), the inclusion of delta 18O data resulted in a narrower uncertainty band (95PPU) and improved transport parameter identifiability. Hence, integrating delta 18O data in model calibration improved the description of hydrological transport. This ultimately can enhance NO3- management strategies. image

Stream water (SW) infiltration to the subsurface and subsequent mixing with groundwater (GW) is crucial for controlling water quality in river-corridors. Mixing of solutes from SW and GW triggers biogeochemical reactions, which can attenuate contaminant influx from upstream SW or from GW that eventually enters the river in downstream regions. It is known that geological heterogeneity affects SW-GW exchange fluxes (EF), however, the combined effects of both hydrological and geological aspects (e.g., sand fraction present in the aquifer material) on EF and on SW-GW mixing in strong contrasting bimodal aquifer units remain unclear. Here, we examined this gap by combining geostatistical realizations, fully-integrated numerical flow simulations, and a mixing-cell routine to assess the major controls on EF and to evaluate how mixing develops and is affected by stream discharge events with different magnitudes and durations. Results show that subsurface heterogeneity at the river-reach scale mainly affects EF magnitudes rather than EF patterns. Yet, both EF magnitudes and SW-GW mixing increased with the introduction of subsurface heterogeneity and with the increase of average hydraulic conductivity (K) values and sand fraction in the models. The simulations further indicated a larger potential for mixing under more frequent, short events regardless of the aquifer material, however, mixing values were generally higher for sandier and heterogeneous models. Lastly, for a high K contrast between subsurface units, these effects were more pronounced. This characterization is critical for river restoration strategies and for downstream management of dam-regulated rivers. Our study elucidates the interplay between hydrological and geological controls on the development of SW-GW mixing at intermediate scales and highlights the importance of considering aquifer characteristics in future studies.

Spatiotemporal distribution patterns of microplastic (MP) particles in lakes hinge on both the physical conditions in the lake and particle properties. Using numerical simulations, we systematically investigated the influence of lake depth and bathymetry, wind and temperature conditions, MP particle release location and timing, as well as particle diameter (10, 20, and 50 mu m). Our results indicate that maximum lake depth had the greatest effect on the residence time in the water column, as it determines the settling timescale and occurrence of hydrodynamic complexity such as density-driven flows in the lake. Increasing particle size from 10 to 20 and 50 mu m also significantly reduced the residence time making particle size the factor with the second strongest effect on the residence time and, in turn, on the availability of MP particles for uptake by organisms. Changing bathymetry from a uniform to a non -uniform had a less pronounced effect on particle residence time compared to maximum depth and particle size. Release location, wind conditions, and release time had comparably little effect on particle behavior but became more important as MP particle size decreased. The release of the 10 mu m MP particles in the deeper lakes with uniform bathymetry during summer with stable thermal stratification, resulted in a nearly month-long turnover phase in the fall in which both settling and rising of particles occurred simultaneously. This was caused by convective heat and water transport during this period. In these scenarios about 2.6 to 5.4 % of the released MP particles were held in or returned to the water layers near the lake surface. While acknowledging the dominant role of lake depth and MP particle size on the particle residence time, this study further emphasizes that it is ultimately a particular combination of different factors and their interactions that shape MP distribution patterns in lakes.