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.

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.

Beach faces form the interface between terrestrial and marine systems. They act as a reactive zone between these two compartments, transporting and biogeochemically modifying chemical constituents such as nutrients, pollutants and carbon. Mixing between saline seawater and fresh terrestrial groundwater in the subsurface is complicated by catchment morphology, variable density flow and very dynamic boundary conditions across temporal scales (e.g. tides, storms, yearly variations in terrestrial groundwater levels). Thus, tracing water and nutrients fluxes through the subterranean estuary is not trivial, especially when attempting to quantify temporal dynamics on time scales from days to weeks. In this work we use long-term (months) temperature profile measurements and numerical heat modelling to investigate the dynamics of water fluxes through the beach sediments into the Königshafen Bay, Sylt Island, North Germany. Temperature measurements were complemented by stable isotope (δ18O, δ2H) and pore water chemical measurements to infer the origin of water discharging into the bay. The results showed that the temporal fluxes vary considerable depending on season, location and catchment characteristics. The freshwater flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge. Moreover, it appears that either the isotope signature of the islands fresh groundwater is variable or there are at least two end-members contribute to the freshwater signature. Seaward, saline and brackish discharge occurs into the tidal creek draining the bay. Overall temperature measurements and heat modelling combined with pore water chemistry show potential to understand the dynamics in water and element exchange through the subterranean estuary and thus help to understand local water and material fluxes and transformations at the land-ocean interface.

Soluble reactive phosphorus (SRP) concentrations in agricultural headwaters can display pronounced seasonal variability at low flow, often with the highest concentrations occurring in summer. These SRP concentrations often exceed eutrophication levels, but their main sources, spatial distribution, and temporal dynamics are often unknown. The purpose of this study is therefore to differentiate between potential SRP losses and releases from soil drainage, anoxic riparian wetlands, and stream sediments in an agricultural headwater catchment. To identify the dominant SRP sources, we carried out three longitudinal stream sampling campaigns for SRP concentrations and fluxes. We used salt dilution tests and natural 222Rn to determine water fluxes in different sections of the stream, and we sampled for SRP, Fe, and 14C dissolved organic carbon (DOC) to examine possible redox-mediated mobilization from riparian wetlands and stream sediments. The results indicate that a single short section in the upper headwater reach was responsible for most of the SRP fluxes to the stream. Analysis of samples taken under summer low-flow conditions revealed that the stream water SRP concentrations, the fraction of SRP within total dissolved P (TDP), and DOC radiocarbon ages matched those in the groundwater entering the gaining section. Pore water from the stream sediment showed evidence of reductive mobilization of SRP, but the exchange fluxes were probably too small to contribute substantially to SRP stream concentrations. We also found no evidence that shallow flow paths from riparian wetlands contributed to the observed SRP loads in the stream. Combined, the results of this campaign and previous monitoring suggest that groundwater is the main long-term contributor of SRP at low flow, and agricultural phosphorus is largely buffered in the soil zone. We argue that the seasonal variation of SRP concentrations was mainly caused by variations in the proportion of groundwater present in the streamflow, which was highest during summer low-flow periods. Accurate knowledge of the various input pathways is important for choosing effective management measures in a given catchment, as it is also possible that observations of seasonal SRP dilution patterns stem from increased mobilization in riparian zones or from point sources.

Microplastic (MP) is potentially harmful to lake ecosystems, with its uptake into the food web largely controlled by its residence time in the lake water column. Here we combine laboratory and virtual experiments to quantify residence times of small MP (<15 μm) in two contrasting model lakes; Lake Constance (large lake) and Esthwaite Water (a small lake). We compare MP residence times in a purely physical system with MP transport controlled by sinking and mixing to a model where, in addition to physical processes, zooplankton package MP into faecal pellets that are then egested into the water column. The laboratory experiments showed that MP settling velocities increased from ~5 × 10-6-10-3 mm s-1 for pristine MP to ~1 mm s-1 for MP embedded faeces. Modeled lake residence times for the 0.5 and 5 μm particles were >15 years in the abiotic models, while in the biotic simulations they were reduced to ~1 year. There was little difference between abiotic and biotic simulations for the 15 μm particles. The ratio of the MP zooplankton uptake velocity to the sinking velocity (v_up/vs_epi) was used to classify biological vs. physical transport pathways. For the 0.5 and 5 μm particles v_up/vs_epi was ≫1 in all cases for both lakes, while for the 15 μm MP there was a transition between biological and physical processes dominating residence times depending on zooplankton numbers. Our results suggest that packaging of small MP in faecal pellets by zooplankton will control its residence time in lakes. Moreover, the majority of small MP will cycle through organisms before reaching the sediment, increasing the likelihood of negative ecological effects and transfer in the food web.

Background The dataset contains data on Microplastic transport experiments run in an experimental flume of the University of Bayreuth. It was analysed in the paper by J.P. Boos, F. Dichgans, J.H. Fleckenstein, B.S. Gilfedder and S. Frei, "Assessing the Behavior of Microplastics in Fluvial Systems: Infiltration and Retention Dynamics in Streambed Sediments", currently under review in Water Resources Research Description of the dataset This dataset contains data used for individual particle detection, and is a companion of the main dataset (10.5281/zenodo.10083568). The files need to be downloaded and merged into the given folder structure. Put the folder "1Pix" along with the folder "10Pix" to the folder in "210812/Data-FIS/matlab/2_Experiment/exp/". Disclaimer The data and code are provided as is without any warranty. Funding Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) -– Project Number 391977956 –- SFB 1357.

&lt;p&gt;Beach faces form the interface between terrestrial and marine systems. They act as a reactive zone between these two compartments, transporting and biogeochemically modifying chemical constituents such as nutrients, pollutants and carbon. Re-circulation of sea water through beach sediments is largely driven by tidal pumping and pressure gradients caused by tides, wave setup, and storm events that pile sea water up on the beach face. In contrast, terrestrial groundwater systems provide a source of low salinity and often nutrient rich water to the coastal zone. Mixing between these water sources is complicated by catchment morphology, variable density flow and very dynamic boundary conditions across temporal scales (e.g. tides, storms, yearly variations in terrestrial groundwater levels). Thus tracing water and nutrients fluxes through the subterranean estuary is not trivial. In this work we use a combination of point and long-term (7 months) temperature profile measurements and heat modelling to estimate water fluxes through the beach sediments into the K&amp;#246;nigshafen, on Sylt Island, Northern Germany. Temperature measurements were complemented by stable isotope and pore water chemistry measurements to infer the origin of discharge into the bay. The results showed that flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge, with fluxes up to 20 cm d&lt;sup&gt;-1&lt;/sup&gt;. Moreover it appears that either the islands fresh groundwater isotopic signature is either variable or at least two end-members contribute to the freshwater signature. Seaward, saline and brackish discharge occurs into the tidal creek draining the bay. Overall temperature measurements and heat modelling combined with pore water chemistry show potential to understand water and chemical exchange through the subterranean estuary and thus help to understand water and material fluxes at the terrestrial-ocean interface.&lt;/p&gt;

&lt;p&gt;The exchange of stream water and groundwater in the hyporheic zone (HZ) plays an important role for the hydrological and biogeochemical functioning of rivers. Hydrological parameters of the hyporheic zone such as storage volume and water fluxes, as well as biogeochemical turnover are a function of the residence times (t&lt;sub&gt;h&lt;/sub&gt;) of water since infiltration. Accurate determination and good resolution of residence times with depth is strongly coupled to the choice of tracer and the quality of the field data. In particular, radon (&lt;sup&gt;222&lt;/sup&gt;Rn), a radioactive noble gas of the &lt;sup&gt;238&lt;/sup&gt;U decay chain that is enriched in groundwater by contact with minerals in the aquifer matrix and degasses to the atmosphere in surface water, is a suitable residence time tracer as activities increase with time along the hyporheic flow path. Unlike many other environmental tracers traditionally used to trace water age, &lt;sup&gt;222&lt;/sup&gt;Rn-based residence time estimates can resolve residence times on the order of a few hours, allowing fast and small-scale exchange processes to be studied. However, sampling of HZ water to measure &lt;sup&gt;222&lt;/sup&gt;Rn introduce uncertainties due to difficulties of extracting sufficient water volume required for the measurement of &lt;sup&gt;222&lt;/sup&gt;Rn. In the study presented here, we demonstrate the applicability of a novel passive-sampler that has never been applied in hydrologic studies before and which is based on tracing the tracks of alpha particles formed during radioactive decay of &lt;sup&gt;222&lt;/sup&gt;Rn and can be directly installed in the riverbed sediment. Laboratory experiments where passive samplers were installed in water samples of known &lt;sup&gt;222&lt;/sup&gt;Rn activities were conducted to calibrate the number of tracks per passive sampler [tracks cm&lt;sup&gt;-2&lt;/sup&gt;] with &lt;sup&gt;222&lt;/sup&gt;Rn activities [Bq m&lt;sup&gt;-3&lt;/sup&gt;] to traditional measurement techniques. Results showed a good correlation between number of tracks and radon activities (R&amp;#178; = 0.97). Passive-samplers were installed in a vertical profile of a losing stream segment in the streambed of a first-order stream close to T&amp;#252;bingen (southwestern Germany). Temperature was used as comparative measurement. Number of tracks increased with depth (between 20 and 240 tracks cm&lt;sup&gt;-1&lt;/sup&gt;) and, after converting tracks to &lt;sup&gt;222&lt;/sup&gt;Rn activities using preceding calibration, led to activities between 450 and 5390 Bq m&lt;sup&gt;-3&lt;/sup&gt;. Depth profiles of temperature had lower temperatures at the bottom and higher water temperatures in the upper sediment layers, which matched to &lt;sup&gt;222&lt;/sup&gt;Rn increase and reflect the expected longer hyporheic flowpaths at depth. Evaluation of t&lt;sub&gt;h&lt;/sub&gt; based on measured tracks led to residence times between 1.9 hours in the upper 5 cm of the sediment and 31.2 hours in 20 cm depth. The residence times are reasonable given the expected slow flow velocity in fine-grained riverbed sediment material. These results suggest that the new-passive samplers have a great potential for hydrologic studies, and especially in the hyporheic zone as &lt;sup&gt;222&lt;/sup&gt;Rn activities can be determined &lt;em&gt;in-situ&lt;/em&gt; at high spatial resolution, at relatively low cost while avoiding common difficulties of obtaining water samples of gaseous tracers from the streambed sediment.&lt;/p&gt;

The majority of microplastics (MPs) in marine environments originate from terrestrial sources and are transported by rivers and streams. In fluvial systems, interactions between particles, biota and sediments influence particle mobility and retention. It has been shown that the transport behaviour of MPs differs from the one of natural sediments. However, specific transport mechanisms for MPs are not yet fully understood, mainly because of the limited availability of reliable experimental data. As part of this study, transport mechanisms for MPs in fluvial systems (surface flow and hyporheic sediments) were investigated using an experimental flume environment. Realistic flow conditions were represented by varying sediment characteristics (e.g. glass beads or sand) and bedform structures (e.g. riffle-pool sequences, ripples and dunes). To track MPs in i) surface flow, ii) at the streambed interface and iii) within hyporheic sediments, we developed a quantitative method for fluorescent MP particles (1-10 mu m) based on state-of-the-art fluorometric techniques. Particle velocities in surface flow were measured by Particle-Image Velocimetry and LaserDoppler-Velocimetry. With this setup, for the first time it was possible to quantitatively track advective MP transfer from surface flow into the streambed sediments. Empirical results obtained from this study can be used in a next step to establish and validate transport models for MPs.

Rivers and streams are a primary transport vector for microplastics (MPs), connecting terrestrial sources to marine environments. While previous studies indicated that pore-scale MPs can accumulate in streambed sediments, the specific MPs transport and retention mechanisms in fluvial systems remain poorly understood. We present a novel method for a quantitative analysis of the spatiotemporal transport and retention of pore-scale MPs in an experimental flume. A continuous mass balance for MPs in surface water was achieved using two online fluorometers, while a laser-induced Fluorescence-Imaging-System was developed to track and quantify the spatial migration of MPs through the streambed sediments. The detection limit was <1 μg/L for 1 μm polystyrene microbeads with the fluorometers and 3 μg/L for the Fluorescence-Imaging-System. The system was able to quantitatively track the advective transfer of MPs into the streambed sediments: a process that has yet not been observed experimentally. Results showed that MPs infiltrated into the streambed sediments up to a depth twice the bedform amplitude. This work provides a novel experimental method to quantitatively monitor MP transport through porous media and advective exchange of MP across the streambed interface.

Dissolved oxygen (DO) is crucial for aerobic life in streams and rivers and mostly depends on photosynthesis (P), ecosystem respiration (R) and atmospheric gas exchange (G). However, climate and land use changes progressively disrupt metabolic balances in natural streams as sensitive reflectors of their catchments. Comprehensive methods for mapping fundamental ecosystem services become increasingly important in a rapidly changing environment. In this work we tested DO and its stable isotope ( 18 O/ 16 O) ratios as novel tools for the status of stream ecosystems. For this purpose, six diel sampling campaigns were performed at three low-order and mid-latitude European streams with different land use patterns. Modelling of diel DO and its stable isotopes combined with land use analyses showed lowest P rates at forested sites, with a minimum of 17.9 mg m −2 h −1 . Due to high R rates between 230 and 341 mg m −2 h −1 five out of six study sites showed a general heterotrophic state with P:R:G ratios between 0.1:1.1:1 and 1:1.9:1. Only one site with agricultural and urban influences showed a high P rate of 417 mg m −2 h −1 with a P:R:G ratio of 1.9:1.5:1. Between all sites gross G rates varied between 148 and 298 mg m −2 h −1 . In general, metabolic rates depend on the distance of sampling locations to river sources, light availability, nutrient concentrations and possible exchanges with groundwater. The presented modelling approach introduces a new and powerful tool to study effects of land use on stream health. Such approaches should be integrated into future ecological monitoring.

Groundwater inflow into the Spree River and its tributaries is an important factor for the iron precipitation problem of the Spree in the Lusatian mining district (Eastern Germany). The input of dissolved iron into the Spree is difficult to estimate mainly because of unknown groundwater inflow. As part of this study, the radio-active isotope 222-Radon (222Rn) was used as a natural tracer to localize and quantify groundwater inflow into the Spree River and one of its tributaries ( Kleine Spree). Based on two 222Rn monitoring campaigns in the catchment and by applying the 222Rn mass balance model FINIFLUX, we were able to quantify local groundwater inflow for a 20 km long river section of the Kleine Spree and a 34 km long section for the Spree River. For the first campaign in May 2018 total groundwater inflow was estimated with ~3,000 m³/d for the Kleine Spree and ~20,000 m³/d for the Spree River. For the second campaign in August 2018 estimated total groundwater inflows were significantly higher with ~7,000 m3 d−1 (Kleine Spree) and ~38,000 m3 d−1 (Spree). Preferential groundwater inflow areas were identified (with up to 70% of total inflow) along the Spreewitzer Rinne, a local high permeable aquifer consisting of excavated mining materials. Based on a stoichiometric ratio calculation and by measuring instream sulfate and dissolved iron loadings, we additionally were able to estimate iron precipitation rates for the entire catchment of the Spree in the Lusatian mining area. According to our calculations, for the entire catchment of the Spree River in the Lusatian mining district total iron precipitation rates reach values as high as 120 tons/day; large quantities of iron (oxy)-hydroxides that are retained within the catchment as iron precipitates.

Beach faces form the interface between terrestrial and marine systems. They act as a reactive zone between these two compartments, transporting and biogeochemically modifying terrestrially sourced chemical constituents such as nutrients, pollutants and carbon towards the sea. This interface is referred to as the subterranean estuary. The subterranean estuary is important on global scales for modifying and processing dissolved and particulate matter sourced from the sea as it cycles through the beach sediments. Re-circulation of sea water through beach sediments is largely driven by tidal pumping and pressure gradients caused by tides, wave setup, and storm events that pile sea water up on the beach face. In contrast, terrestrial groundwater systems provide a source of low salinity and often nutrient rich water to the coastal zone. Mixing between these water sources is complicated by catchment morphology, variable density flow caused by saline sea water lying above and below the fresh terrestrial groundwater. Thus tracing water and nutrients fluxes through the subterranean estuary is not trivial. In this work we use a combination of temperature measurements and heat modelling to estimate water fluxes through the subterranean estuary and pore water stable isotopes and chemistry to infer the origin of this discharge in the Königshafen, on Sylt Island, Northern Germany. The results show that flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge. Seaward, saline and brackish discharge occurs into the tidal creek draining the bay. The complexity of heat transport modelling in the coastal zone depends on the boundary conditions, with very complex boundaries requiring more complex modelling structures (e.g. steady state vs. non steady state). Overall temperature measurements and heat modelling combined with pore water chemistry show potential to understand water and chemical exchange through the subterranean estuary and thus help to understand water and material fluxes at the terrestrial-ocean interface.

Mismanaged waste leads to inputs of microplastics into the environment and the aquatic system affecting rivers and lakes. The physical properties of microplastic (MP) particles affect their terminal settling velocity (TSV) in the water column and in turn their distribution patterns in aquatic systems. To evaluate the settling behavior and the TSV of MP particles we simulated the settling of a large range of MP particles with regular and irregular shapes in the water column using a computational fluid dynamics (CFD) model. To validate the results returned by our model, we compared CFD findings to the corresponding results obtained by semi-empirical relationships as well as the results from experiments for 120 irregularly shaped MP particles with sizes and densities ranging from 500 to 2000 µm and 1.03 to 1.38 grcm-3, respectively. The CFD results are in good agreement with the results from the laboratory and semi-empirical relationships with a 0.05 difference in the slopes of their linear regressions. In a next step, we defined scenarios to systematically investigate the influence of different particle characteristics such as roundness, density, and volume as well as water temperature on the TSV of regular and irregular MP particles. Our simulations revealed a dominant effect of particle density on the TSV compared to the effects of the other parameters. For example, doubling particle densities increased the TSVs of the MP particles up to 500%, while, doubling their volumes only led to a maximum increase in their TSV of 200%. Increasing the roundness of the MP particles, letting them evolve towards a perfect sphere, increased their TSVs by up to 15%, while seasonal changes in lake water temperatures typical for lakes in temperate climate regions, caused changes in TSVs by up to 32%.

Groundwater represents a major component for runoff generation of large rivers systems. Its quantification is of uttermost importance during low flow periods and in the context of changing runoff dynamics due to climate change. The present study focuses on the surface water-groundwater interaction using the example of the Moselle River, the second most important tributary of the Rhine. The river is classified as a federal waterway and has 12 barrages on German territory to ensure navigability all year round. The research approach is based on the assumption that local groundwater inflow into the Moselle is detectable by increased 222-Rn concentrations in the river and that the δ18O composition of the river water approximates that of the groundwater. Therefore, we applied a numerical model for solving the 222-Rn and Tritium mass balance and a mixing model of δ18O and electrical conductivity. For this purpose, water samples were taken at intermediate flow conditions (gauge Cochem: about 220 m³/s) in October 2020 along the Moselle on a stretch of 242 kilometers at high spatial resolution (every 2 km) to measure stable water isotopes and electrical conductivity. Integrated over the same spatial resolution, in-situ 222-Rn measurements were carried out. Tributaries and selected groundwater monitoring wells were sampled for the same analysis. Precipitation was collected at the station Trier of the German Meteorological Service on a monthly basis. In agreement with this measurement concept, another sampling campaign took place for selected reaches in August/September 2021 at lower discharges (Cochem gauge: about 94 m³/s). In autumn 2020, diffuse groundwater inflow (approx. 0.17 to 0.3 m³/s) was detected for the shell limestone of the upper Moselle reaches and locally increased groundwater inflow for the middle reaches in the transition area to the Rhenish Slate Mountains and the Detzem barrage (approx. 1.4 to 2.4 m³/s). These estimates translate into groundwater contribution of the total Moselle discharge of 0.3 and 1.2 % respectively, which is much lower than those calculated by the mixing model (about 10 and 5 %, respectively). For August/September 2021, higher groundwater inflows in these areas are expected for both methods. The evaluation to date indicates that 222-Rn is the most sensitive tracer to locations with increased groundwater inflow compared to tritium and stable water isotopes. While tritium results seem to strongly depend on the current flow conditions and the propagating river wave, stable isotope results are affected by the appropriate characterization of end-member hydrochemistry.

Groundwater represents a major component for runoff generation of large rivers systems. Its quantification is of uttermost importance during low flow periods and in the context of changing runoff dynamics due to climate change. The present study focuses on the surface water-groundwater interaction using the example of the Moselle River, the second most important tributary of the Rhine. The river is classified as a federal waterway and has 12 barrages on German territory to ensure navigability all year round. The research approach is based on the assumption that local groundwater inflow into the Moselle is detectable by increased 222-Rn concentrations in the river and that the δ18O composition of the river water approximates that of the groundwater. Therefore, we applied a numerical model for solving the 222-Rn and Tritium mass balance and a mixing model of δ18O and electrical conductivity. For this purpose, water samples were taken at intermediate flow conditions (gauge Cochem: about 220 m³/s) in October 2020 along the Moselle on a stretch of 242 kilometers at high spatial resolution (every 2 km) to measure stable water isotopes and electrical conductivity. Integrated over the same spatial resolution, in-situ 222-Rn measurements were carried out. Tributaries and selected groundwater monitoring wells were sampled for the same analysis. Precipitation was collected at the station Trier of the German Meteorological Service on a monthly basis. In agreement with this measurement concept, another sampling campaign took place for selected reaches in August/September 2021 at lower discharges (Cochem gauge: about 94 m³/s). In autumn 2020, diffuse groundwater inflow (approx. 0.17 to 0.3 m³/s) was detected for the shell limestone of the upper Moselle reaches and locally increased groundwater inflow for the middle reaches in the transition area to the Rhenish Slate Mountains and the Detzem barrage (approx. 1.4 to 2.4 m³/s). These estimates translate into groundwater contribution of the total Moselle discharge of 0.3 and 1.2 % respectively, which is much lower than those calculated by the mixing model (about 10 and 5 %, respectively). For August/September 2021, higher groundwater inflows in these areas are expected for both methods. The evaluation to date indicates that 222-Rn is the most sensitive tracer to locations with increased groundwater inflow compared to tritium and stable water isotopes. While tritium results seem to strongly depend on the current flow conditions and the propagating river wave, stable isotope results are affected by the appropriate characterization of end-member hydrochemistry.

Our study discusses imaging results from a spectral induced polarization (SIP) survey to identify concurring processes (such as aerobic respiration, denitrification, or sulfate- and iron reduction) in a biogeochemically active peat in a wetland located in the Lehstenbach catchment in Southeastern Germany. Terrestrial wetland ecosystems such as peatlands are a critical element in the global carbon cycle. Due to their role as natural carbon sinks and ecological importance for an array of flora and fauna, there is a growing demand to conserve and restore degraded peatlands. Biogeochemical processes occur with non-uniform reaction rates within the peat, making the environment sensitive to physical disturbances. To investigate biogeochemical processes in-situ, it is important to avoid disturbing the redox-sensitive conditions in the subsurface by bringing oxygen into anoxic areas. Our previous study demonstrated that the induced polarization (IP) was able to identify biogeochemically active and inactive areas of the peat. The IP response was sensitive to the presence of carbon turnover and P release in the absence of iron sulfide. These highly polarizable areas have high iron concentrations, but most likely in an oxidized form. As most iron oxides are poor conductors, the strong polarization response is unlikely related to an electrode polarization process.Here we also analyzed the frequency dependence of the SIP data to investigate whether iron oxides and carbon-iron complexes, two possible mechanisms for the high polarization response, can be distinguished. SIP imaging data sets covered the frequency range between 0.06 and 225 Hz and were collected with varying electrode spacing (20 and 50 cm) at different locations within the Waldstein catchment characterized by different properties, e.g., saturated and non-saturated soils. Our imaging results reveal variations of the IP effect within the peat layer, indicating substantial heterogeneities in the peat composition and biogeochemical activity. The frequency dependence allowed us to resolve a sharper contrast between the different features of the peat. Geochemical analyses on a freeze core and pore water samples are used to validate our results and find correlations between the Cole-Cole parameters of the SIP response and the geochemical parameters.

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 degrees 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.