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Hyporheic flow and nutrient turnover in hyporheic systems are strongly influenced by in-stream bedforms. An accurate representation of topographical variations of the stream-streambed interface is therefore essential in analytical models in order to represent the couplings between hydrological and biogeochemical processes correctly. The classical Toth approach replaces the streambed surface topography by a flat surface which is identical to a truncation of the original physical flow domain into a rectangle. This simplification can lead to biased estimates of hyporheic flow and nutrient cycling within hyporheic systems. We present an alternative analytical modeling approach for solving hyporheic problems without domain truncation that explicitly accounts for topographical variations of the streambed. The presented approach is based on the application of perturbation theory. Applications of the method to hyporheic systems, ranging from the centimeter-scale of rippled bedforms to riffle structures of 10 m and larger scale, indicate a high accuracy of the approach.

Microplastic particles (MPs) are a major threat to ecosystems worldwide, resulting in a great need to investigate their impacts on ecosystems. To date, most studies dealing with the effects of MPs on organisms used commercially available polystyrene spherical particles, with no to little characterization of their physicochemical properties. However, MPs occurring in the environments are a composition of various polymer types with different properties. Therefore, it is hard to tell how and if the polymer type and its associated properties determine their impact on organisms. Here we show how different polymer types of MPs in the same shape, concentration, and size range (20-120µm) affect the freshwater mussel Dreissena bugensis in comparison to mussel shell fragments as natural particle control. By using hall sensor-based real-time valvometry, we studied behavioural responses via the movement of the mussels' valves and show that mussels cannot distinguish between natural particles and MPs. Furthermore, we performed an in-depth characterization of the used MPs. Different types and quantities of additives and residual monomers were found in the different polymer types, which can be linked to polymer type-dependent adverse effects on the molecular level. Recycled PET elicited the most substantial adverse effects on D. bugensis, likely caused by anthranilamide, anthranilonitrile and butylated hydroxytoluene within the MP fragments, which have been described as toxic to aquatic organisms. Since PET is among the most abundant MPs found in nature, sublethal effects may gradually manifest at the population level, leading to irreversible ecosystem changes. In summary, adverse effects in organisms caused by MPs are dependent on specific physical and chemical properties of the particles. Also see: https://micro2022.sciencesconf.org/428254/document, In MICRO 2022, Online Atlas Edition: Plastic Pollution from MACRO to nano

Coastal ecosystem health and sustainability is tightly coupled to submarine groundwater discharge (SGD) and associated nutrient, carbon and pollutant fluxes. However, there are few studies that systematically analyse the interaction between the terrestrial aquifer system, catchment morphology and coastal SGD. The objective of this study was to evaluate the role of catchment morphology and how this influences the spatial distribution, timing and volume of the SGD flux to a branched tidal creek system, on the barrier island Spiekeroog, Germany. The subsurface salinity was mapped using electrical resistivity tomography (ERT) and a hydrogeochemical survey of the shallow groundwater. Temporal and spatial analysis of geochemical tracers (radon-222, chloride) in the tidal creek water was integrated into a transient 222Rn mass balance model for quantification of SGD rates during two field campaigns that encompassed spring and neap tides. The ERT mapping indicated that fresh groundwater dominated under the dune ridges down to about 15 m depth, but became progressively more brackish seawards. This is likely due to frequent tidal and storm flooding of low-lying areas. The highest groundwater fluxes into the creek were indicated by high 222Rn activities (average 468 Bq m−3) towards the dune ridge. Chloride concentrations (up to 17.3 g L−1) increased seawards showing the progressive salinization of water in the creek. The freshwater component of SGD was highly variable in time but was highest at low tide, while the total SGD flux (saline + fresh) was highest when tides changed from inflow to outflow as the rapid pressure release on the local aquifer caused a large hydraulic gradient towards the creek. Comparing the freshwater component of mean daily SGD to the creek with estimated groundwater recharge rates in the catchment (665 m3 d−1) shows that the fresh groundwater discharge exceeds fresh recharge during spring tides (~120%) but is was lower than recharge during neap tides (~27%). In this study we show that tidal creeks and their relation to catchment morphology are relevant for understanding the spatial and temporal exchange of fresh and saline water between the catchment and coastal zone.

The flux of water, nutrients, carbon and salt through the subsurface at the land-sea interface is an important control on coastal nutrient processes, salinization of coastal aquifers and carbon balances of the coastal zone. However, these fluxes are often spatially and temporally complex and difficult to quantify, especially in high-energy mesotidal systems. Here we use vertical temperature profiles along a morphologically complex mesotidal high-energy beachface to map and quantify water infiltration and exfiltration on the island of Spiekeroog, Germany. Water fluxes were quantified using heat transport calculations from three solutions to the 1D heat transport equation, and include 1) a steady state analytical solution, 2) a non-steady state numerical model and 3) a non-steady state analytical solution. The temperature profiles could clearly map areas of upwelling warm (up to 10 °C) groundwater during the winters of 2018 and 2019. These upwelling zones were focused on an intertidal runnel system and at the low water line, consistent with the current understanding of the site based on visual observations and hydrogeological models. The steady state model provided good fits to the measured data in the winter when the seawater temperatures were not changing significantly, but was less able to reproduce the measured profiles in spring when seawater was warming. The steady state flux rates ranged from −110 to −43 mm d−1 in the runnel and low water line to +43 mm d−1 towards the high water line. The dynamic numerical model successfully captured the propagation of the seawater temperature signal into the subsurface and was able to reproduce the temperature profiles during both seasons. The flux estimates tended to be larger with the numerical model, with up to −150 mm d−1 in the runnel and +110 mm d−1 towards the high water line. The non-steady state analytical solution could only be applied to a limited time series due to the difficulty of logging temperatures in the subsurface at this highly dynamic site. Up to 1.5 days of data suggested fluxes that were considerably higher than the other two methods with best-estimates of −400 to −900 mm d−1. Thermal Peclet numbers ranged from 0.2 to 2 suggesting that both conduction and advection of heat is important. This study demonstrates that the morphology of the beach face is an important control on spatial distribution of down-welling and upwelling zones along the beach and that temperature measurement combined with heat modelling are potentially useful methods for understanding the interactions between groundwater and the sea.

Summary The concept of Damkohler numbers have been extensively used in the discipline of chemical engineering and lately increasingly found its application into environmental science in order to describe the integrated behavior of hydrological systems with respect to their physical transport and biogeochemical transformation capabilities. Defining characteristic time scales of transport and reaction, as part of the Damkohler concept, however is not trivial especially for non-well mixed systems like catchments where physically controlled transport and biogeochemical moderated reactions can be highly variable among individual flow paths. Often, system specific residence times alone are not useful to describe the timescales of transport in the Damkohler concept, because it neglects that degradation of redox-sensitive compounds depend on dynamically changing and non-uniformly distributed hydro-biogeochemical boundary conditions that either facilitate or suppress biogeochemical reactions. In this study an approach is presented that highlights the importance to specifically distinguish between residence and exposure times if system specific transformation efficiencies are evaluated. We investigate the inter-relationship between residence and exposure time distributions for different biogeochemical processes in a virtual wetland environment that is exposed to different hydrological conditions. The relationship between exposure and residence times is mathematically described by a composition matrix that linearly relates the two identities to each other. Composition matrices for different hydrological conditions are analyzed by using the singular value decomposition technique. Results show that especially the type of couplings between the surface and subsurface flow domain control how exposure and residence times are related to each other in the wetland system and that timescales of residence and exposure typically differ by orders of magnitude. Finally, results also indicate that the assessment of system specific transformation efficiencies can be very error-prone if residence instead of exposure times are being used to derive system specific Damkohler numbers.

Summary Coupling surface and subsurface water flow in fully integrated hydrological codes is becoming common in hydrological research; however, the coupling of surface–subsurface solute transport has received much less attention. Previous studies on fully integrated solute transport focus on small scales, simple geometric domains, and have not utilised many different field data sources. The objective of this study is to demonstrate the inclusion of both flow and solute transport in a 3D, fully integrated catchment model, utilising high resolution observations of dissolved organic carbon (DOC) export from a wetland complex during a rainfall event. A sensitivity analysis is performed to span a range of transport conditions for the surface–subsurface boundary (e.g. advective exchange only, advection plus diffusion, advection plus full mechanical dispersion) and subsurface dispersivities. The catchment model captures some aspects of observed catchment behaviour (e.g. solute discharge at the catchment outlet, increasing discharge from wetlands with increased stream discharge, and counter-clockwise concentration–discharge relationships), although other known behaviours are not well represented in the model (e.g. slope of concentration–discharge plots). Including surface–subsurface solute transport aids in evaluating internal model processes, however there are challenges related to the influence of dispersion across the surface–subsurface interface, and non-uniqueness of the solute transport solution. This highlights that obtaining solute field data is especially important for constraining integrated models of solute transport.

The dynamic response of groundwater discharge to external influences such as rainfall is an often neglected part of water and solute balances in wetlands. Here we develop a new field platform for long-term continuous 222 Rn and electrical conductivity (EC) measurements at Sale Wetland, Australia to study the response of groundwater discharge to storm and flood events. The field measurements, combined with dynamic mass-balance modelling, demonstrate that the groundwater flux can increase from 3 to ∼20 mm d −1 following storms and up to 5 mm d −1 on the receding limb of floods. The groundwater pulses are likely produced by activation of local groundwater flow paths by water ponding on the surrounding flood plains. While 222 Rn is a sensitive tracer for quantifying transient groundwater discharge, the mass-balance used to estimate fluxes is sensitive to parameterisation of gas exchange ( k ) with the atmosphere. Comparison of six equations for calculating k showed that, based on parameterisation of k alone, the groundwater flux estimate could vary by 58%. This work shows that neglecting transient processes will lead to errors in water and solute flux estimates based on infrequent point measurements. This could be particularly important for surface waters connected to contaminated or saline groundwater systems.

An adequate representation of micro-topography in spatially explicit, physically based models can be crucial in modeling runoff generation, surface/subsurface flow interactions or subsurface flow patterns in hydrological systems with pronounced micro-topography. However, representation of micro-topography in numerical models usually requires high grid resolutions to capture relevant small scale variations in topography at the range of centimeters to meters. High grid resolutions usually result in longer simulation times, especially if fully integrated model approaches are used where the governing partial differential equations for surface and subsurface flow are solved simultaneously. This often restricts the implementation of micro-topography to plot scale models where the overall model domain is small to minimize computational cost resulting from a high grid resolution. In this study an approach is presented where a highly resolved digital elevation model (DEM) for a hummocky topography in a plot scale wetland model (10 m x 21 m x 2 m), is represented by spatially distributed rill/depression storage zones in a numerical model with a planar surface. By replacing the explicit micro-topography with spatially distributed rill/depression storage zones, important effects of micro-topography on surface flow generation and subsurface transport characteristics (e.g. residence time distributions) are being preserved, while at the same time the number of computational nodes is reduced significantly. We demonstrate that the rill/depression storage concept, which has been used for some time to represent time delays in the generation of surface runoff, can also be used to mimic subsurface flow patterns caused by micro-topography. Results further indicate that the rill/depression storage concept is an efficient tool to represent micro-topography in plot scale models because model computation times drop significantly. As important aspects of surface and subsurface flows induced by micro-topography can be mimicked adequately by applying the rill/depression storage concept on a coarser grid, it may also be a useful tool to represent micro-topography in numerical flow models beyond the plot scale.

An important prerequisite to better understand the transport of nutrients and contaminants across the river-aquifer interface and possible implications for biogeochemical transformations is to accurately characterize and asses the exchange fluxes. In this study we investigate how monsoonal precipitation events and the resulting variability in river discharge affect the dynamics of river-aquifer exchange and the corresponding flux rates. We evaluate potential impacts of the investigated exchange fluxes on local water quality. Hydraulic gradients along a piezometer transect were monitored at a river reach in a small catchment in South Korea, where the hydrologic dynamics are driven by the East-Asian Monsoon. We used heat as a tracer to constrain river-aquifer exchange fluxes in a two-dimensional flow and heat transport model implemented in the numerical code HydroGeoSphere, which was calibrated to the measured temperature and total head data. To elucidate potential effects of river-aquifer exchange dynamics on biogeochemical transformations at the river-aquifer interface, river water and groundwater samples were collected and analyzed for dissolved organic carbon (DOC), nitrate (NO3) and dissolved oxygen saturation (DOsat). Our results illustrate highly variable hydrologic conditions during the monsoon season characterized by temporal and spatial variability in river-aquifer exchange fluxes with frequent flow reversals (changes between gaining and losing conditions). Intense monsoonal precipitation events and the associated rapid changes in river stage are the dominant driver for the observed riverbed flow reversals. The chemical data suggest that the flow reversals, when river water high in DOC is pushed into the nitrate-rich groundwater below the stream and subsequently returns to the stream may facilitate and enhance the natural attenuation of nitrate in the shallow groundwater.

Radon ( R 86 222 n ) as a hydrological tracer offers a method for studying short to medium term groundwater - surface water interactions. These high frequency processes play an important role in wetland hydrology and biogeochemistry and may influence their contribution to the global carbon cycle. Therefore, there is a definite need for robust methods to measure high resolution 222Rn time series in-situ. In this study we adapted and improved a membrane system to measure 222Rn continuously with a primary focus on a rapid response time and low power consumption. The membrane system was constructed using a hydrophobic capillary membrane and laboratory experiments were conducted to quantify the systems' response time to predefined 222Rn pulses. It was then deployed in a stream draining a riparian wetland. The new membrane system could reduce the response time by ≈ 60 % in comparison to the established silicone membrane. We could identify the behaviour of the system in response to dynamically changing 222Rn activities and suggest a new method using simple linear regression to quantify the systems’ response when the response time concept is inapplicable. Finally, we were able to measure high temporal resolution 222Rn activities reliably over an extended field deployment (68 d). We conclude that the improved system fills a gap ensuring high temporal resolution while maintaining extended maintenance intervals. This allows the user to study high frequency hydrological processes in remote areas. This new membrane system can be used to detect fast changes in 222Rn activities improving the comprehension of the underlying hydrological processes.

Groundwater pollution by fertilizer NO3− is a major problem recognized in many parts of the world. The excessive use of mineral fertilizers to assure high yields in agricultural production intensifies the leaching problem especially in regions affected by a monsoon climate as in South Korea. The extent that leaching occurs depends on several factors such as climatic conditions, agricultural management practices, soil properties and the sorption characteristics of fertilizers and agrochemicals. In the South Korean monsoon season 2010, NO3− concentrations under varying nitrogen fertilizer rates were monitored in a plastic mulched ridge cultivation (RTpm) with radish crops (Raphanus sativus L.). Based on these findings we calibrated a three-dimensional water flow and solute transport model using the numerical code HydroGeoSphere in combination with the parameter estimation software ParallelPEST. Subsequently, we used the calibrated model to investigate the effect of plastic mulch as well as different fertilizer best management practices (FBMPs) on NO3− leaching. We found that cumulative NO3− leaching under RTpm was 26% lower compared to ridge tillage without coverage (RT). Fertilizer placement confined to the ridges resulted in 36% lower cumulative NO3−-leaching rates compared to broadcast applied fertilizer. Splitting the total amount of 150 kg NO3− ha−1 per growing season into three fertilizer applications (1–4–2.5 ratio) led to a reduction of NO3− leaching of 59% compared to the one-top dressing at the beginning of the growing season. However, the combination of a fertilizer rate of 150 kg NO3− ha−1, plastic mulched ridges, fertilizer placement only in the ridges and split applications of fertilizer resulted in the lowest cumulative NO3− leaching rate (8.14 kg ha−1) during the simulation period, which is equivalent to 5.4% of the total NO3− fertilizer input. Compared to RT with conventional one-top dressing fertilization in ridges and furrows, the NO3− leaching was reduced by 82%. Consequently, the combination of all FBMPs is highly recommendable to decrease economical costs for fertilizer inputs as well as to minimize nitrate leaching and its impact on groundwater quality.

In humid upland catchments wetlands are often a prominent feature in the vicinity of streams and have potential implications for runoff generation and nutrient export. Wetland surfaces are often characterized by distinct micro-topography (hollows and hummocks). The effects of such micro-topography on surface–subsurface exchange and runoff generation for a 10 by 20 m synthetic section of a riparian wetland were investigated in a virtual modeling experiment. A reference model with a planar surface was run for comparison. The geostatistically simulated structure of the micro-topography replicates the topography of a peat-forming riparian wetland in a small mountainous catchment in South-East Germany (Lehstenbach). Flow was modeled with the fully-integrated surface–subsurface code HydroGeoSphere. Simulation results showed that the specific structure of the wetland surface resulted in distinct shifts between surface and subsurface flow dominance. Surface depressions filled and started to drain via connected channel networks in a threshold controlled process, when groundwater levels intersected the land surface. These networks expanded and shrunk in a spill and fill mechanism when the shallow water table fluctuated around the mean surface elevation under variable rainfall inputs. The micro-topography efficiently buffered rainfall inputs and produced a hydrograph that was characterized by subsurface flow during most of the year and only temporarily shifted to surface flow dominance (> 80% of total discharge) during intense rainstorms. In contrast the hydrograph in the planar reference model was much “flashier” and more controlled by surface runoff. A non-linear, hysteretic relationship between groundwater level and discharge observed at the study site was reproduced with the micro-topography model. Hysteresis was also observed in the relationship between surface water storage and discharge, but over a relatively narrow range of surface water storage values. Therefore it was concluded that surface water storage was a better predictor for the occurrence of surface runoff than groundwater levels.

Summary The parallel physically-based surface–subsurface model PARFLOW was used to investigate the spatial patterns and temporal dynamics of river–aquifer exchange in a heterogeneous alluvial river–aquifer system with deep water table. Aquifer heterogeneity at two scales was incorporated into the model. The architecture of the alluvial hydrofacies was represented based on conditioned geostatistical indicator simulations. Subscale variability of hydraulic conductivities ( K ) within hydrofacies bodies was created with a parallel Gaussian simulation. The effects of subscale heterogeneity were investigated in a Monte Carlo framework. Dynamics and patterns of river–aquifer exchange were simulated for a 30-day flow event. Simulation results show the rapid formation of saturated connections between the river channel and the deep water table at preferential flow zones that are characterized by high conductivity hydrofacies. Where the river intersects low conductivity hydrofacies shallow perched saturated zones immediately below the river form, but seepage to the deep water table remains unsaturated and seepage rates are low. Preferential flow zones, although only taking up around 50% of the river channel, account for more than 98% of total seepage. Groundwater recharge is most efficiently realized through these zones. Subscale variability of K sat slightly increased seepage volumes, but did not change the general seepage patterns (preferential flow zones versus perched zones). Overall it is concluded that typical alluvial heterogeneity (hydrofacies architecture) is an important control of river–aquifer exchange in rivers overlying deep water tables. Simulated patterns and dynamics are in line with field observations and results from previous modeling studies using simpler models. Alluvial heterogeneity results in distinct patterns and dynamics of river–aquifer exchange with implications for groundwater recharge and the management of riparian zones (e.g. river channel-floodplain connectivity via saturated zones).