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Microplastics (MPs) have been detected ubiquitously in fluvial systems and advective transfer has been proposed as a potential mechanism for the transport of (sub‐) pore‐scale MPs from surface waters into streambed sediments. However, the influence of particle and sediment properties, as well as the hydrodynamic flow regime, on the infiltration behavior and mobility of MPs in streambed sediments remains unclear. In this study, we conducted a series of flume experiments to investigate the effect of particle size (1–10 μm), sediment type (fine and coarse sand), and flow regime (high/low flow) on particle infiltration dynamics in a rippled streambed. Quantification of particles in the flume compartments (surface flow, streambed interface, and in the streambed) was achieved using continuous fluorescence techniques. Results indicated that the maximum infiltration depth into the streambed decreased with increasing particle size (11, 10, and 7 cm for 1, 3, and 10 μm). The highest particle retardation was observed in the fine sediment experiment, where 22% of the particles were still in the streambed at the end of the experiment. Particle residence times were shortest under high flow conditions, suggesting that periods of increased discharge can effectively flush MPs from streambed sediments. This study provides novel insights into the complex dynamics of MP infiltration and retention in streambed sediments and contributes to a better understanding of MPs fate in fluvial ecosystems. Quantitative data from this study can improve existing modeling frameworks for MPs transport and assist in assessing the exposure risk of MPs ingestion by benthic organisms. Plain Language Summary Microplastics (MPs) (small plastic particles) are present in river systems worldwide. The processes that lead to their transport and retention in rivers are not fully understood. Scientists have proposed that the infiltration of surface water into the streambed can carry MPs with it. In this study, we conducted experiments in a controlled environment that resembles a stream and its streambed. We investigated how different sizes of plastic particles (1, 3, and 10 μm), the types of sediment (fine and coarse sand), and water flow rates (low and high) affect how far particles travel in a streambed. We found that the size of MPs played a significant role in their depth of infiltration. Larger particles did not infiltrate as deeply as smaller particles, and were also retained in the streambed. Fine sand trapped particles for a longer time than coarse sand, and 22% of the particles remained in the streambed until the end of the experiment. Faster flowing water quickly removed MPs from the streambed. Our research helps understand how MPs spread in river systems and how long they remain in the streambed. The data can be used to improve transport models and assess the risk MPs pose to aquatic organisms. Key Points (Sub‐) Pore‐scale microplastics were advectively transferred from the surface water into the streambed sediments in flume experiments Infiltration patterns depend on microplastic size, streambed sediment type and surface flow velocities Microplastic retention was observed for 10 μm beads, 1 μm beads were considerably retarded in fine sediments

Biogeochemical reactions occur unevenly in space and time, but this heterogeneity is often simplified as a linear average due to sparse data, especially in subsurface environments where access is limited. For example, little is known about the spatial variability of groundwater denitrification, an important process in removing nitrate originating from agriculture and land use conversion. Information about the rate, arrangement, and extent of denitrification is needed to determine sustainable limits of human activity and to predict recovery time frames. Here, we developed and validated a method for inferring the spatial organization of sequential biogeochemical reactions in an aquifer in France. We applied it to five other aquifers in different geological settings located in the United States and compared results among 44 locations across the six aquifers to assess the generality of reactivity trends. Of the sampling locations, 79% showed pronounced increases of reactivity with depth. This suggests that previous estimates of denitrification have underestimated the capacity of deep aquifers to remove nitrate, while overestimating nitrate removal in shallow flow paths. Oxygen and nitrate reduction likely increases with depth because there is relatively little organic carbon in agricultural soils and because excess nitrate input has depleted solid phase electron donors near the surface. Our findings explain the long-standing conundrum of why apparent reaction rates of oxygen in aquifers are typically smaller than those of nitrate, which is energetically less favorable. This stratified reactivity framework is promising for mapping vertical reactivity trends in aquifers, generating new understanding of subsurface ecosystems and their capacity to remove contaminants.

Background 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. Results We demonstrate that reservoir-scale retention efficiency rises sharply with increasing MP settling velocity, with near-complete retention (> 95%) achieved at settling velocities of 0.9 m d⁻ 1 or higher. Conversely, slower-sinking particles (≤ 0.3 m d⁻ 1 ) exhibit significant downstream export, indicating that weak sedimentation can negate the reservoir’s inherent trapping capacity even under long residence times (~ 1 year). Furthermore, episodic phenomena such as stratification breakdown or shortcut currents can rapidly redistribute or mobilise MP particles, bypassing much of the reservoir volume and potentially delivering MP particles directly to outflows. These findings highlight the critical roles of both hydrodynamics (stratification, mixing, and lateral transport) and particle-specific traits such as settling velocity in determining MP fate. Conclusions By integrating comprehensive field-derived meteorological inputs and a validated numerical framework, this study provides novel insights into MP retention in drinking water reservoirs and underscores the vulnerability of such systems to episodic transport events. Our approach offers a robust tool for reservoir managers and policy-makers to anticipate MP contaminant pathways, optimise withdrawal strategies, and develop early warning systems for drinking water preparation. This work thus advances both the scientific understanding of MP dynamics in lentic systems and supports more informed, adaptive water-resource management.

Although land‐water carbon (C) transport represents a critical link in the global C cycle, rare attempts have been made to compare hydrologic controls over storm pulses of dissolved organic C (DOC) and particulate organic C (POC) in mountainous watersheds. An immersible UV/Vis spectrophotometer was used to comparatively investigate the rapid storm responses of stream water DOC and POC in a small mountainous forested watershed in South Korea. High‐frequency measurements at 5‐min intervals during 42 hydrologic events, including monsoon storms and winter snowmelts, showed consistent patterns: POC concentrations were lower than DOC concentrations during base flow and small storm events but exceeded them during the peak flow periods of intense storm events. Although both the DOC and POC concentrations had hysteretic relationships with discharge, the POC concentrations showed larger increases and variations after crossing a threshold discharge on the rising limb of the storm hydrograph. Stronger responses to intense storms resulted in a disproportionately large export of POC at high flow, whereas a large portion of the total DOC flux was exported under prevailing low‐flow conditions. The results demonstrate the potential of in situ optical measurements for investigating fine‐resolution dynamics of the DOC and POC export during storm events. Stronger storm responses of the POC export compared to the limited response range of the DOC export suggest that erosion‐induced POC export will become more important as a major pathway for the hydrologic soil C loss from mountainous watersheds in response to an increasing occurrence of extreme storm events. Key Points In situ optical monitoring captured differential storm responses of DOC and POC POC export was more variable on rising discharge than limited DOC responses Strong storm responses lead to disproportionately large POC export at high flow

Increased anthropogenic inputs of nitrogen (N) to the biosphere during the last few decades have resulted in increased groundwater and surface water concentrations of N (primarily as nitrate), posing a global problem. Although measures have been implemented to reduce N inputs, they have not always led to decreasing riverine nitrate concentrations and loads. This limited response to the measures can either be caused by the accumulation of organic N in the soils (biogeochemical legacy) – or by long travel times (TTs) of inorganic N to the streams (hydrological legacy). Here, we compare atmospheric and agricultural N inputs with long-term observations (1970–2016) of riverine nitrate concentrations and loads in a central German mesoscale catchment with three nested subcatchments of increasing agricultural land use. Based on a data-driven approach, we assess jointly the N budget and the effective TTs of N through the soil and groundwater compartments. In combination with long-term trajectories of the C–Q relationships, we evaluate the potential for and the characteristics of an N legacy. We show that in the 40-year-long observation period, the catchment (270 km2) with 60 % agricultural area received an N input of 53 437 t, while it exported 6592 t, indicating an overall retention of 88 %. Removal of N by denitrification could not sufficiently explain this imbalance. Log-normal travel time distributions (TTDs) that link the N input history to the riverine export differed seasonally, with modes spanning 7–22 years and the mean TTs being systematically shorter during the high-flow season as compared to low-flow conditions. Systematic shifts in the C–Q relationships were noticed over time that could be attributed to strong changes in N inputs resulting from agricultural intensification before 1989, the break-down of East German agriculture after 1989 and the seasonal differences in TTs. A chemostatic export regime of nitrate was only found after several years of stabilized N inputs. The changes in C–Q relationships suggest a dominance of the hydrological N legacy over the biogeochemical N fixation in the soils, as we expected to observe a stronger and even increasing dampening of the riverine N concentrations after sustained high N inputs. Our analyses reveal an imbalance between N input and output, long time-lags and a lack of significant denitrification in the catchment. All these suggest that catchment management needs to address both a longer-term reduction of N inputs and shorter-term mitigation of today's high N loads. The latter may be covered by interventions triggering denitrification, such as hedgerows around agricultural fields, riparian buffers zones or constructed wetlands. Further joint analyses of N budgets and TTs covering a higher variety of catchments will provide a deeper insight into N trajectories and their controlling parameters.

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.

Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3D + time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface–subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space–time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space–time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components.

Understanding the controls on event-driven dissolved organic carbon (DOC) export is crucial as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesized that topography is a key driver of DOC export in headwater catchments because it influences hydrological connectivity, which can inhibit or facilitate DOC mobilization. To test this hypothesis, we studied the mechanisms controlling DOC mobilization and export in the Große Ohe catchment, a forested headwater in a mid-elevation mountainous region in southeastern Germany. Discharge and stream DOC concentrations were measured at an interval of 15 min using in situ UV-Vis (ultraviolet–visible) spectrometry from June 2018 until October 2020 at two topographically contrasting subcatchments of the same stream. At the upper location (888 m above sea level, a.s.l.), the stream drains steep hillslopes, whereas, at the lower location (771 m a.s.l.), it drains a larger area, including a flat and wide riparian zone. We focus on four events with contrasting antecedent wetness conditions and event size. During the events, in-stream DOC concentrations increased up to 19 mg L−1 in comparison to 2–3 mg L−1 during baseflow. The concentration–discharge relationships exhibited pronounced but almost exclusively counterclockwise hysteresis loops which were generally wider in the lower catchment than in the upper catchment due to a delayed DOC mobilization in the flat riparian zone. The riparian zone released considerable amounts of DOC, which led to a DOC load up to 7.4 kg h−1. The DOC load increased with the total catchment wetness. We found a disproportionally high contribution to the total DOC export of the upper catchment during events following a long dry period. We attribute this to the low hydrological connectivity in the lower catchment during drought, which inhibited DOC mobilization, especially at the beginning of the events. Our data show that not only event size but also antecedent wetness conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of subcatchments, depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different subcatchments to DOC export may change in the future when hydrological connectivity will be reduced more often.

In 2018–2019, Central Europe experienced an unprecedented 2-year drought with severe impacts on society and ecosystems. In this study, we analyzed the impact of this drought on water quality by comparing long-term (1997–2017) nitrate export with 2018–2019 export in a heterogeneous mesoscale catchment. We combined data-driven analysis with process-based modeling to analyze nitrogen retention and the underlying mechanisms in the soils and during subsurface transport. We found a drought-induced shift in concentration–discharge relationships, reflecting exceptionally low riverine nitrate concentrations during dry periods and exceptionally high concentrations during subsequent wet periods. Nitrate loads were up to 73 % higher compared to the long-term load–discharge relationship. Model simulations confirmed that this increase was driven by decreased denitrification and plant uptake and subsequent flushing of accumulated nitrogen during rewetting. Fast transit times (<2 months) during wet periods in the upstream sub-catchments enabled a fast water quality response to drought. In contrast, longer transit times downstream (>20 years) inhibited a fast response but potentially contribute to a long-term drought legacy. Overall, our study reveals that severe droughts, which are predicted to become more frequent across Europe, can reduce the nitrogen retention capacity of catchments, thereby intensifying nitrate pollution and threatening water quality.

Nitrate (NO3-) excess in rivers harms aquatic ecosystems and can induce detrimental algae growths in coastal areas. Riverine NO3- uptake is a crucial element of the catchment-scale nitrogen balance and can be measured at small spatiotemporal scales, while at the scale of entire river networks, uptake measurements are rarely available. Concurrent, low-frequency NO3- concentration and streamflow (Q) observations at a basin outlet, however, are commonly monitored and can be analyzed in terms of concentration discharge (C–Q) relationships. Previous studies suggest that steeper positive log (C)–log (Q) slopes under low flow conditions (than under high flows) are linked to biological NO3- uptake, creating a bent rather than linear log (C)–log (Q) relationship. Here we explore if network-scale NO3- uptake creates bent log (C)–log (Q) relationships and when in turn uptake can be quantified from observed low-frequency C–Q data. To this end we apply a parsimonious mass-balance-based river network uptake model in 13 mesoscale German catchments (21–1450 km2) and explore the linkages between log (C)–log (Q) bending and different model parameter combinations. The modeling results show that uptake and transport in the river network can create bent log (C)–log (Q) relationships at the basin outlet from log–log linear C–Q relationships describing the NO3- land-to-stream transfer. We find that within the chosen parameter range the bending is mainly shaped by geomorphological parameters that control the channel reactive surface area rather than by the biological uptake velocity itself. Further we show that in this exploratory modeling environment, bending is positively correlated to percentage of NO3- load removed in the network (Lr.perc) but that network-wide flow velocities should be taken into account when interpreting log (C)–log (Q) bending. Classification trees, finally, can successfully predict classes of low (∼4 %), intermediate (∼32 %) and high (∼68 %) Lr.perc using information on water velocity and log (C)–log (Q) bending. These results can help to identify stream networks that efficiently attenuate NO3- loads based on low-frequency NO3- and Q observations and generally show the importance of the channel geomorphology on the emerging log (C)–log (Q) bending at network scales.