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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 (th) 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 (222Rn), a radioactive noble gas of the 238U 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, 222Rn-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 222Rn introduce uncertainties due to difficulties of extracting sufficient water volume required for the measurement of 222Rn. 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 222Rn and can be directly installed in the riverbed sediment. Laboratory experiments where passive samplers were installed in water samples of known 222Rn activities were conducted to calibrate the number of tracks per passive sampler [tracks cm-2] with 222Rn activities [Bq m-3] to traditional measurement techniques. Results showed a good correlation between number of tracks and radon activities (R² = 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 Tübingen (southwestern Germany). Temperature was used as comparative measurement. Number of tracks increased with depth (between 20 and 240 tracks cm-1) and, after converting tracks to 222Rn activities using preceding calibration, led to activities between 450 and 5390 Bq m-3. Depth profiles of temperature had lower temperatures at the bottom and higher water temperatures in the upper sediment layers, which matched to 222Rn increase and reflect the expected longer hyporheic flowpaths at depth. Evaluation of th 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 222Rn activities can be determined in-situ at high spatial resolution, at relatively low cost while avoiding common difficulties of obtaining water samples of gaseous tracers from the streambed sediment.

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.

Mercury accumulation in lake sediments is a widespread environmental problem due to the biomagnification of Hg in the aquatic food chain. Soil Hg concentrations, catchment vegetation, erosion, and lake productivity are major factors controlling the accumulation of Hg in lakes. However, their influence on the Hg mass balance in lakes with different catchment characteristics and trophic state is poorly understood. In this multilake study, we decipher the effects of catchment vegetation (coniferous vs. deciduous forest), soil Hg content, and trophic state on Hg sedimentation at six lakes in Germany. We investigated Hg concentrations in leaves, soils, and the lake's water phase. Soils under coniferous stands show slightly higher Hg concentrations than under deciduous forest. Hg concentrations in the water phase were higher in the oligotrophic brown water lakes (8.1 ± 5.6 ng L−1 vs. 3.0 ± 1.9 ng L−1). Lower Hg concentrations in sediment trap material indicate dilution by algae organic matter in the mesotrophic lakes (0.12–0.17 μg g−1 vs. 0.57–0.89 μg g−1). However, Hg accumulation rates in sediment traps were up to 14‐fold higher in the mesotrophic lakes (113–443 μg m−2 yr−1) than in the brown water lakes (32–144 μg m−2 yr−1), which could not be explained by higher Hg fluxes to the productive lakes. Hg mass balance calculation reveals that water phase Hg scavenging by algae is the major reason for the intense Hg export to the sediments of productive lakes which makes them significantly larger sedimentary sinks than oligotrophic brown water lakes., Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Understanding groundwater–surface water (GW–SW) interactions is vital for water management in karstic catchments due to its impact on water quality. The objective of this study was to evaluate and compare the applicability of seven environmental tracers to quantify and localize groundwater exfiltration into a small, human‐impacted karstic river system. Tracers were selected based on their emission source to the surface water either as (a) dissolved, predominantly geogenic compounds (radon‐222, sulphate and electrical conductivity) or (b) anthropogenic compounds (predominantly) originating from wastewater treatment plant (WWTP) effluents (carbamazepine, tramadol, sodium, chloride). Two contrasting sampling approaches were compared (a) assuming steady‐state flow conditions and (b) considering the travel time of the water parcels (Lagrangian sampling) through the catchment to account for diurnal changes in inflow from the WWTP. Spatial variability of the concentrations of all tracers indicated sections of preferential groundwater inflow. Lagrangian sampling techniques seem highly relevant for capturing dynamic concentration patterns of WWTP‐derived compounds. Quantification of GW inflow with the finite element model FINIFLUX, based on observed in‐stream Rn activities led to plausible fluxes along the investigated river reaches (0.265 m3 s−1), while observations of other natural or anthropogenic environmental tracers produced less plausible water fluxes. Important point sources of groundwater exfiltration can be ascribed to locations where the river crosses geological fault lines. This indicates that commonly applied concepts describing groundwater–surface water interactions assuming diffuse flow in porous media are difficult to transfer to karstic river systems whereas concepts from fractured aquifers may be more applicable. In general, this study helps selecting the best suited hydrological tracer for GW exfiltration and leads to a better understanding of processes controlling groundwater inflow into karstic river systems., Karst aquifers represent an increased complexity when aiming to measure the interaction between groundwater and river water. Combining field‐based measurements on catchment scale and modelling, the applicability of ‘classical’ environmental groundwater tracers was compared to selected organic (micro)pollutants often considered as conservative and originally arising from a wastewater treatment plant. This study demonstrates that the choice of an appropriate tracer is crucial when either aiming to quantify groundwater exfiltration into karstic river systems, or indicating hydrological processes, applying (globally) omnipresent pollutants., German Research Foundation (DFG) http://dx.doi.org/10.13039/501100001659

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.

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.

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%.

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

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.

International audience; Key Points: Lumped parameter modeling of hyporheic nitrate removal by applying the exposure time concept.  Exposure time distributions are derived from analytical residence time distributions and reaction kinetics for hyporheic oxygen consumption.  Using exposure times rather than residence times is likely to lead to more realistic estimates for nutrient removal in the hyporheic zone Abstract The rate of biogeochemical processing associated with natural degradation and transformation processes in the hyporheic zone (HZ) is one of the largest uncertainties in predicting nutrient fluxes. We present a lumped parameter (LPM) model that can be used to quantify the mass loss for nitrate in the HZ operating at the scale of river reaches to entire catchments. The model is based on using exposure times (ET) to account for the effective timescales of reactive transport in the HZ. Reach scale ET distributions are derived by removing the portion of hyporheic residence times (RT) associated with flow through the oxic zone. The model was used to quantify nitrate removal for two scenarios: 1) a 100 m generic river reach and 2) a small agricultural catchment in Brittany (France). For the field site hyporheic RT are derived from measured in-stream 222 Rn activities and mass balance modelling. Simulations were carried out using different types of RT distributions (exponential, power-law and gamma type) for which ET were derived. Mass loss of nitrate in the HZ for the field site ranged from 0-0.45 kg d-1 depending on the RT distribution and the availability of oxygen in the streambed sediments. Simulations with power law ET distribution models only show very little removal of nitrate due to the heavy weighting towards shorter flow paths that are confined to the oxic sediments. Based on the simulation results, we suggest that ET likely lead to more realistic estimates for nutrient removal.

Clay minerals are important constituents of porous media. To date, only little is known about the transport and retention behavior of nanoplastics in clay-containing soil. To investigate the effects of clay minerals on the mobility of nanoplastics in saturated porous media, polystyrene nanoplastics (PS-NPs) were pumped through columns packed with sand and clay minerals (kaolinite and illite) at different pH and ionic strengths (IS). Mobility of PS-NPs decreased with increasing clay content attributed to physical straining effects (smaller pore throats and more complex flow pathways). Variations in pH and IS altered the surface charges of both PS-NPs and porous media and thus affecting the interaction energy. An increase of IS from 10 mM to 50 mM NaCl decreased the maximum energy barrier and secondary minimum from 142 KBT to 84 KBT and from -0.1 KBT to -0.72 KBT, respectively. Thus, the maximum C/C0 ratio decreased from ~51% to ~0% (pH 5.9, 3% kaolinite). Among the two clay minerals, kaolinite showed a stronger inhibitory effect on PS-NPs transport compared to illite. For instance, at the same condition (3% clay content, pH 5.9, 10 mM NaCl), the (C/C0)max of PS-NPs in kaolinite was ~51%, while for illite, it was ~77%. The difference in transport inhibition was mainly attributed to amphoteric sites on the edges of kaolinite which served as favorable deposition sites at pH 5.9 (pHpzc-edge is ~2.5 for illite and ~6.5 for kaolinite). Besides, the morphology of kaolinite was more complex than illite, which may retain more PS-NPs in kaolinite. Results and conclusions from the study will provide some valuable insights to better understand the fate of NPs in the soil-aquifer system.

23 pags., 10 figs., 4 tabs., In this work, we describe the compilation and homogenization of an extensive data set of aerosol iodine field observations in the period between 1963 and 2018 and we discuss its spatial and temporal dependences by comparison with CAM-Chem model simulations. A close to linear relationship between soluble and total iodine in aerosol is found (∼80% aerosol iodine is soluble), which enables converting a large subset of measurements of soluble iodine into total iodine. The resulting data set shows a distinct latitudinal dependence, with an enhancement toward the Northern Hemisphere (NH) tropics and lower values toward the poles. This behavior, which has been predicted by atmospheric models to depend on the global distribution of the main oceanic iodine source (which in turn depends on the reaction of ozone with aqueous iodide on the sea water-air interface, generating gas-phase I2 and HOI), is confirmed here by field observations for the first time. Longitudinally, there is some indication of a waveone profile in the tropics, which peaks in the Atlantic and shows a minimum in the Pacific. New data from Antarctica show that the south polar seasonal variation of iodine in aerosol mirrors that observed previously in the Arctic, with two equinoctial maxima and the dominant maximum occurring in spring. While no clear seasonal variability is observed in NH middle latitudes, there is an indication of different seasonal cycles in the NH tropical Atlantic and Pacific. Long-term trends cannot be unambiguously established as a result of inhomogeneous time and spatial coverage and analytical methods, Juan Carlos Gómez Martín acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and the Ramon y Cajal Program (RYC-2016-19,570). Alfonso Saiz-Lopez acknowledges financial support from the European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation program (Project “ERC-2016-COG 726349 CLIMAHAL”). Rafael P. Fernandez would like to thank financial support from ANPCyT (PICT 2015-0714), UNCuyo (SeCTyP M032/3853), and UTN (PID 4920-194/2018).

Bank filtration is a cost-effective and sustainable method of improving surface water quality for drinking water production. During aquifer transit, natural biodegradation and physiochemical filtration improve the quality of the raw water by removing sediments, pollutants, and pathogens. Strict regulations prohibit the use of substances that can be used to estimate aquifer residence times to define water protection areas for bank filtration. In this study, we present a novel measurement and modeling framework for deriving mean aquifer residence times for bank filtrate using the natural tracer radon-222. The method is intended for application in the drinking water sector, where extraction wells are screened over the entire aquifer and pumps are operated at high production rates. Mean aquifer residence times are estimated using composite residence time distributions that account for flow path mixing and non-uniform residence times with multiple components including bank filtrate, shallow groundwater, and deep groundwater. The mathematical framework is demonstrated for a drinking water production facility. Radon activities for the six monitored extraction wells ranged between 4,400 and 8,400 Bq/m³. Estimated mean aquifer residence times for the wells range from5 days to 110 days and strongly depend on i) the type of residence time distribution model (exponential, gamma or piston flow), ii) the mixing ratio between bank filtrate and local groundwater, and iii) the heterogeneity in the groundwater endmember. By accounting for mixing processes, we can show that radon can be used beyond the "5-fold half-life" (~20 days) commonly described in the literature as the upper limit for age dating purposes for radon. This method provides a simple and cost-efficient way to quantify residence times of bank filtrate on a regular basis without any addition of external substances to the aquifer.

The increasing use of plastic products and its inevitable decomposition after improper disposal has led to large numbers of nano- and microplastic in aqueous environments. There is currently a critical need to investigate the transport and retention mechanisms of nanoplastic particles in water-saturated porous media (e.g., aquifers or sediments) to better understand residence times and ecosystem exposure of these particles in aqueous environments. In this study, we performed a set of column experiments in order to investigate and understand the primary controls on the mobility of nanoplastics in a controlled laboratory environment. As part of the experiments, we used polystyrene nanoplastic particles (PS-NPs, 50 nm) in combination with iron oxyhydroxide–coated sand, which is known for its high surface reactivity and often can be found in natural systems in environmentally relevant amounts. We also adjusted pore water chemistry (pH, ionic strength, cation species) to represent non-uniform geochemical conditions in nature and to understand how these conditions quantitatively affect the transport of nanoplastics. Mobility and retention of PS-NPs were assessed by analyzing breakthrough curves. For negatively charged iron oxyhydroxide coatings (at pH > pHpzc), only little retention of PS-NPs could be observed. In contrast, positively charged iron oxyhydroxide coatings (pH < pHpzc) provided favorable deposition sites for the negatively charged PS-NPs. DLVO theory was used to show that high pH and low ionic strength increased the energy barriers between PS-NPs and the porous media. In contrast, low pH and high ionic strength decreased the barriers and thus increased retention in the columns. Finally, bridging agents, such as Ca2+ and Ba2+, resulted in the significant deposition of nanoplastics by forming bonds between O-containing functional groups on both the plastic and sediment surfaces. These findings indicate that the deposition and fate of nanoplastic particles are strongly affected by the water chemistry and soil components in subsurface environments.

Plastics are among the most widespread contaminants on Earth. They build up in fresh water bodies with high concentrations and migrate between different environmental compartments. In thermally stratified lakes, in summer, MPs pollutants can migrate between epilimnion, metalimnion and hypolimnion. This increases the probability of that microplastic will be filtered by filter feeders allowing MPs to migrate through different trophic levels. In this study, the transport of MPs in lake systems is presented through laboratory experiments as well as numerical modelling. The settling velocities of various biodegradable and non-biodegradable particles with various shapes and sizes were measured in the settling column under laminar conditions using particle image velocimetry (PIV). The particles used ranged between 150 to 2400 µm in diameter. The experimental results presented that shape, size and density of a particle are the key parameters controlling the sedimentation behavior of the particles. The measured settling velocities ranged between 0.4 to 50 mms-1. In parallel, the transport of the particles used in the laboratory experiments was simulated using CFD. The laboratory experiments and CFD have shown consistent results. Subsequently, the same MPs used in the first lab experiments were incubated in a pond at the University of Bayreuth for 6, 8 and 10 weeks. The formation of biofilm on the incubated particles was investigated using confocal laser scanning microscopy. Also, the effect of biofouling of microplastics on the physical properties and thus settling velocity was investigated experimentally. It was observed that biofilm-building organisms has only colonized few regions on the surface of MPs and the whole surface was not coated with biofilm as it was anticipated. In addition, no changes in shape, size and density of the incubated were detected. After 6, 8 and 10 weeks of incubation, no significant change in the settling velocity of the incubated particles was observed. The detected changes in the settling velocity ranged between ± 5 % which was considered as a measurement error. Finally, the residence time in suspension and the distribution of MPs throughout a virtual lake water column was simulated using a simplified model. The effect of turbulences and the temperature gradient on the settling velocity were considered during the simulations. The model presented that turbulences, water temperature and layer depth control the settling velocity and thus the residence time of the MPs.

Rivers and streams are the dominant transport vectors for microplastic (MP) input into marine environments. During transport, complex physicochemical interactions between particles, water and river sediments influence particle mobility and retention. The specific transport mechanisms of MP in fluvial systems are not yet fully understood, and the main reason lies in the limitation in reliable data derived from experimental analysis.In our subproject of the ‘CRC 1357 Microplastics’, we investigate the hydrodynamic mechanisms that control the transport and retention behavior of MP in open channel flows and streambed sediments. In an experimental flume environment, we create realistic hydrodynamic and hyporheic flow conditions by using various porous media (e.g. glass beads or sand) and bedform structures (e.g. riffle-pool sequences, ripples and dunes), modelled from real stream systems.The method developed here can quantitatively analyze the transport of pore-scale particles (1-40 µm) based on fluorometric techniques. Particle velocity distributions and particle transport are measured using Particle-Image-Velocimetry and Laser-Doppler-Velocimetry. With our setup, we can quantitatively investigate time-resolved MP transport and retention through the aqueous and solid phase in a flume scale experiment.

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.

Dissolved organic carbon (DOC) constitutes the biggest portion of carbon that is exported from soils. During the last decades, widespread increases in DOC concentrations of surface waters have been observed, affecting ecosystem functioning and drinking water treatment. However, the hydrological controls on DOC mobilization are still not completely understood.We sampled two different topographical positions within a headwater catchment in the Bavarian Forest National Park: at a steep hillslope (880 m.a.s.l.) and in a flat and wide riparian zone (770 m.a.s.l.). By using piezometers, pore water samplers (peepers) and in-stream spectrometric devices we measured DOC concentrations as well as DOC absorbance (A254/A365 and SUVA254) and fluorescence characteristics (fluorescence and freshness indices) in soil water, shallow ground water and stream water in order to gain insights into the DOC source areas during base-flow and during precipitation events.High DOC concentrations (up to 80 mg L-1) were found in soil water from cascading sequences of small ponds in the flat downstream part of the catchment that fill up temporarily. The increase of in-stream DOC concentrations during events was accompanied by changing DOC characteristics at both locations, for example increasing freshness index values. As the freshness index values were approaching the values found in the DOC-rich ponds in the riparian zone, these ponds seem to be important DOC sources during events. Our preliminary results point to a change of flow pathways during events.

Understanding the controls on event-driven DOC export is crucial, as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesize that topography is a key driver of DOC export because it influences hydrologic connectivity, which can inhibit or facilitate DOC mobilization. To test this 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 continuously measured using in-situ UV-Vis spectrometry from June 2018 until October 2020 at two topographically contrasting sub-catchments of the same stream: at a steep hillslope (888 m.a.s.l.) and in a flat and wide riparian zone (771 m.a.s.l). We focus on four events with contrasting antecedent hydrological conditions and event size. During 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 anti-clockwise 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 total DOC load up to 522 kg per event. The total 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 lack of 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 hydrological conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of different catchment parts depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different catchment parts to DOC export may change in the future, when hydrological connectivity will occur less often.

Summary We demonstrate the application of the induced polarization (IP) imaging method to map biogeochemical hot-spots at the catchment scale. Measurements were collected at the Hydrological Open Air Laboratory (HOAL), a 66 ha. research field. Data collected across the entire site reveal relative modest electrical conductivity values (between 60 and 80 mS/m) and low polarization values (quadrature conductivity 1 mS/m and conductivity phase above 20 mrads). Mapping IP lines permitted to delineate the geometry of the IP anomaly and select the location for the drilling of boreholes. Analysis of the sediments revealed high carbon and iron concentrations, as expected for a biogeochemically active area; however, negligible concentration of sulfides or magnetite which are the common IP targets. Although the polarization mechanism is open to debate, our results demonstrate the applicability of the IP method as a diagnosis tool to delineate in-situ biogeochemical activity in metal-free media.

Investigation of river water/groundwater interaction aims generally at: (i) localizing water migration pathways, and (ii) quantifying water and associated matter exchange between the two natural water resources. Related numerical models generally rely on model-specific parameters that represent the physical conditions of the catchment and suitable aqueous tracer data. A generally applicable approach for this purpose is based on the finite element model FINIFLUX that is using the radioactive noble gas radon-222 as naturally occurring tracer. During the study discussed in this paper, radon and physical stream data were used with the aim to localize and quantify groundwater discharge into a well-defined section of a small headwater stream. Besides site-specific results of two sampling campaigns, the outcomes of the study reveal: (i) the general difficulties of conducting river water/groundwater interaction studies in small and heterogeneous headwater catchments, and (ii) the particular challenge of defining well constrained site- and campaign-specific values for both the groundwater radon endmember and the radon degassing coefficient. It was revealed that determination of both parameters should be based on as many data sources as possible and include a critical assessment of the reasonability of the gathered and used datasets. The results of our study exposed potential limitations of the approach if executed in small and turbulent headwater streams. Hence, we want to emphasize that the project was not only executed as a case study at a distinct site but rather aimed at evaluating the applicability of the chosen approach for conducting river water/groundwater interaction studies in heterogeneous headwater catchments.

The two primary problems currently facing agriculture are drought and the availability of mineable phosphorus minerals used for fertilization. More frequent and longer drought periods are predicted to threaten agricultural yields in future. The capacity of soils to hold water is a highly important factor controlling drought stress intensity for plants during the growing phase. High phosphorus availability in soils is necessary for high agricultural yield. For both drought and phosphorus availability in soils amorphous silica (ASi) has been suggested to be able to mitigate these problems. Amorphous silica pools in natural soils are in the range of 0-6%. However ASi pools have declined in agricultural soils since the development of high intensity agriculture to values of 40% and >60%, respectively. Additionally, we fertilized soils with ASi and measured phosphorus mobilization from the solid phase into the soil pore waters. We found a strong mobilization of phosphorus by ASi. In a lysimeter experiment we found that ASi strongly increased the WHC of soils. Furthermore, as expected from the batch experiments the ASi is decreased phosphorus sorption to soil minerals and consequently increased its mobilization. Our results suggest that ASi addition to soils enhances the water availability, potentially decreases drought stress for plants as well as increasing phosphorus mobility in soil of terrestrial ecosystems.

With the increasing use of nanoplastic products in our daily life, these particles will invariably enter into the subsurface environment. It is, therefore, vital to understand the transport and retention of nanoplastic particles in groundwater systems. Surface charge heterogeneity is one of the basic chemical-physical characteristics of aquifer materials, but little research has been conducted on this topic. This study aimed to understand how the interaction between the porous media, solution chemistry, and NP surface charge influences the transport and retention of PS-NPs in the subsurface. 25 mg/L polystyrene nanoplastic particles (PS-NPs) were injected into columns packed with iron oxyhydroxide-coated sand. In addition, factors such as the content of iron oxyhydroxide-coated sand (λ), pH, ionic strength (IS), and cation valence were systematically studied. DLVO theory was used to evaluate the interactions between PS-NP and the porous media. By comparing the breakthrough curves (BTCs) of PS-NPs, it was clear that all these variables exerted a significant influence on the mobility of PS-NPs in the columns. These effects could be explained by the following: Firstly, by applying the DLVO theory, it was possible to model the electrostatic interaction between quartz sand and PS-NPs. For instance, at different IS (NaCl), the maximum energy barrier (Φmax) decreased with an increase in IS, which meant PS-NPs could more easily overcome the energy barrier to deposited on the sand surface at higher IS. Secondly, the positively charged iron oxyhydroxide coating provided additional favorable deposition sites for negatively charged PS-NPs. However, when the pH of the solution exceeded the iron oxyhydroxide pHpzc (~pH 9), the iron coating became negative and increased the mobility of PS-NPs. Finally, bridging agents, such as Ca2+ and Ba2+, resulted in the significant deposition of PS-NPs on the sand due to the bridging effect connecting the porous media and PS-NPs through the O-containing functional groups on both plastic and mineral surfaces. This study provides a better understanding of how the charge heterogeneity on aquifer materials and groundwater hydrochemistry affect the transport of PS-NPs in aquifers.

Microplastics (MPs) have been found ubiquitously in oceanic and terrestrial environments. As the production and consumption of plastic polymers increases the amount of plastic evading accepted disposal pathways and entering natural systems is also expected to increase. To date the focus of plastic and MP research in particular has been on the ocean, there has recently been a rapid increase in interest in MP levels and distribution in terrestrial systems. However, the focus of existing studies has mostly been on the quantification and distribution of MP contamination in the sediment or on the water column of rivers and lakes. The aim of this project is to investigate the fundamental physical and biological influences on the transport of microplastics (MPs) in lake systems. In particular, we will focus on an understanding of the migration and distribution of MPs, and a systematic investigation on transport and sedimentation of MP in the lake water column. Lab and field experiments are planned to investigate the behavior of different MPs polymers, shapes and sizes under different conditions and determine how this influence the MP transport.The settling velocity of MPs in stationary water was measured in the laboratory using particle image velocimetry (PIV) which was then compared to manual timing of the sinking velocity. The trajectories of the settling MPs have also been tracked during weak turbulences. In addition, the results were compared with theoretical calculations.To investigate microbial colonization and biofilm formation on the surface of MPs, samples were exposed on a natural lake environment for varying time periods. The colonization of MP surfaces by microorganisms and their excretion of extracellular polymeric substances (EPS) were examined by laser microscopic techniques and subsequently the effect of the microbiological colonization of settling velocity was determined. In this work we show that the transport of MP is complex, as it is influenced by plastic type, shape, and biological colonization as well as the hydrodynamic conditions in the lake water column.

Biogeochemical hot spots are spatially confined areas where biogeochemical processes take place with anomalously high reaction rates. On the landscape scale, biogeochemical hot spots are of major interest due to the possible emission of greenhouse gases (carbon dioxide) and high nutrient turnover. Such hot spots are sensitive environments and given their environmental impact, there is a growing demand for noninvasive methods to investigate such hot spots without disturbing the biogeochemical settings. Classical geochemical sampling methods (e.g., piezometers or suction cups) often disturb the redox-sensitive settings by bringing oxygen into anoxic areas. Induced polarization (IP) is a noninvasive geophysical method that was initially developed to explore metal-ore deposits but more recently developed into a versatile tool for environmental studies. Here, we present imaging results from a geophysical survey using the IP method to characterize hot spots in a wetland located in the Lehstenbach catchment in Southeastern Germany. We collected IP imaging data sets along 64 profiles using 64 electrodes deployed with a spacing of 20 cm. Our highly resolved measurements aimed at delineating hot spots within a thin layer (approximately 20 cm) heterogeneous peat material on top of the local granite bedrock. To validate the field-scale IP signatures, geochemical analyses (e.g., dissolved and solid iron concentration) were performed on freeze-core samples obtained in areas characterized by anomalous high and low IP responses. Furthermore, the thickness of the peat was measured with a dipstick along every fifth profile to evaluate the IP imaging results. Our imaging results reveal an increase in the IP response within the upper 20 cm of the subsurface, which correlates with the variations in the iron concentrations and variations in the geochemical composition of groundwater accompanying microbial activity within the biogeochemical hot spots observed in the soil samples. The IP response decreases in the deeper regions, which can be associated with the granite bedrock.

Changing climatic drivers significantly endanger the function of the hyporheic zone (HZ). Increased temperature leads to an increase in respiration and expansion of anoxic areas in river sediments. Under conditions of low stream discharge (with high proportion of groundwater) reduced substances enter the upper parts of the HZ and surface water. Contact with dissolved oxygen (DO) leads to formation of areas of preferential oxidation of reduced substances (e.g. iron precipitation and clogging of pore spaces, nitrification). Thus oxygen levels in the hyporhic zone are an important parameter for understanding biogeochemical cycles in the stream sediments. However, oxygen measurements are time consuming, prone to error and instruments are expensive.In this work we hypothesize that iron nails can be used as a very simple and inexpensive tool for mapping the depth of oxygen penetration into hyporheic sediments. The experiments compared iron oxidation of nails inserted in a laboratory scale sand tank model for hyporheic flow as well as a riffle-pool sequence in northern Bavaria, Germany. We have combined this with a mathematical model that simulates flow and biogeochemical reactions in the subsurface and with direct oxygen measurements in the sediment.Oxygen measurements showed that oxygen concentrations decrease from over 8 mg l-1 to less than 1 mg l-1 in the first 15 cm of the tank model. This agreed well with the mathematical model, which predicted the decrease of oxygen to approximately the same depth. The nails showed a clear rust (iron oxide) crust down to sub-oxic DO levels (at ~11 cm) while below this we observed a black precipitate indicating reducing conditions. In some cases, we found an area between the oxic and anoxic zones where there were no obvious signs of reaction on the nails surface. The mathematic model indicated that the water residence time in the oxic part of the tank was ~5 h while it took up to 19 h to pass through the anoxic zone and out of the tank.The field results confirms that rust on the nails can be used to indicate oxic depths, but with a somewhat more complex pattern along the riffle-pool sequence.Iron oxidation and associated rust on the metal nails gives us only qualitative data on the presence or absence of Oxygen. Quantitative data may be derived by determining the iron mineral phases on the nails (using e.g. Raman spectroscopy). Overall this appears to be a very inexpensive method for gaining high spatial resolution information of oxygen levels in the hyporheic zone, which is important for stream ecosystems as well as biogeochemistry.

Global warming forecasts predict an increase in extreme weather events like droughts, heavy rainfalls and floods. Such events act as multiple stressors for streams through extreme discharges, temperature increases and also enhanced erosion of fine sediments into the system. In the last years, Northern Bavaria suffered from severe droughts with extremely dry and hot summers, a situation which is expected to become even worse in the future. We hypothesize that increased periods of drought lead to an increase in oxygen consumption rates in the hyporheic zone (HZ) and subsequent anoxia along with microbial iron reduction and formation of Fe(II). At the redoxcline, in the presence of oxygen, oxidation of Fe(II) and subsequent precipitation of ferric (hydr)oxides occurs. Such processes may have severe effects on stream water ecology causing clogging of interstitial spaces within the riverbed with subsequent habitat loss for benthic organisms as well as clogging of fish gills and trachea, coverage of fish eggs and oxygen depletion.In this contribution we will present first results from a field study on the understanding of the effects of drought on biogeochemical processes within the hyporheic zone. To these ends we have installed tube bundles into the HZ of a small upland stream in Northern Bavaria, Germany. Stream Mähringsbach (3rd order stream) is located south east of the city Hof and runs through silicate rich areas. Mähringsbach is home to the endangered pearl mussel. Sampling is conducted in the upper stream reach starting at N 50°14.884 E°012°05.743. Eight sampling points are installed including a transect covering a length of about 20 m. To allow sampling at the same spot tube bundles were inserted in HZ and left for the whole sampling campaign. Tube bundles have mesh covered (pore size about 160 x 310 μm) openings at 5, 15, 25 and 40 cm depth. A luer lock-three-way valve combination is attached to the opening at the surface to enable sampling. Water samples are taken using a 60 ml syringe with an attached oxygen flow through cell. This sampling approach allows to investigate the dynamics of dissolved Fe(II), Fe(III) and O2 in the porewater at different depths of the HZ. For a better depth resolution (2 cm) custom made dialyses samplers are used. A membrane allows only small components < 0,2 μm to enter. To account for the exchange dynamics between surface water and the hyporheic zone, we determine the residence time of water within the HZ using radon as a tracer. First results from filtered (0,45 µm) tube bundle samples indicate high concentrations of Fe(II) up to 216 µmol/l and of Fe(III) up to 7 µmol/l. Peeper samples have Fe(II) concentrations up to 408 µmol/l and Fe(III) concentrations up to 215 µmol/l.

The spatial distribution and temporal dynamics of groundwater inflow to rivers is often poorly defined but central to understanding water and matter fluxes. This is especially true for the Spree River which drains the Lusatia mining district, Brandenburg Germany. In the Spree catchment iron and sulphate fluxes to the river stem from the pyrite rich groundwater system, and the area’s history of open-pit lignite mining and re-flooding of many of these mines at the end of their lifetime. This iron flux threatens the river ecosystem, tourism in downstream communities (Spreewald) and the drinking water of Berlin. Iron is often observed as precipitates along the river bed, as well as colouring the river water yellow-brown, indicating the presence of iron (oxy)hydroxides such as ferrihydrite and goethite. In this work we have used radon as a natural groundwater tracer to delimited areas of active groundwater discharge to both the main Spree River and the Kleine Spree River to better understand the spatial destitution of groundwater input to the system. This was combined with mass-balance modelling to quantify the groundwater flux along the river using the FINIFLUX model. This was complemented by measurement of iron and sulphate concentrations in the steam and stream-near groundwater. During two measurement campaigns during 2018 the total groundwater inflow for a 20 km long reach of the Kleine Spree and a 34 km long reach of the Spree ranged between ~3,000 and ~7,000 m³ d-1 (Kleine Spree) and ~20,000 and ~38,000 m³ d-1 (Spree). Particularly high groundwater inflow was identified (up to 70% of total inflow) along the Spreewitzer Rinne, a local aquifer consisting of excavated mining materials. For the Kleine Spree the dominant groundwater and Fe flux occurred shortly before the confluence with the Spree. For these river reaches large amounts of dissolved iron and sulphate enters the rivers with inflowing groundwater as calculated from the radon data. Using the measured iron and sulphate loadings we calculated that up to 120 tons/day of iron (oxy)hydroxide was retained in the combined Spree and Klein Spree catchments, a large amount of which remains in the mining lakes.

In dieser Studie wurde die Interaktion zwischen Grund- und Seewasser am Steislinger See, einem 11 ha grosen See nordwestlich des Bodensees, untersucht. Die Schwerpunkte lagen (1) auf der Kartierung und Quantifizierung des Grundwasserzuflusses mithilfe des Tracers Radon sowie (2) der Auswirkung des Grundwasserzustroms auf die Seewasserchemie. 222Rn, CH4, $$\text{NO}_{3}^{-}$$ und $$\text{NO}_{2}^{-}$$ wurden aus Tiefenprofilen und sedimentnahen Wasserproben im April und Juni 2016 gemessen. Die raumliche Verteilung der Radonaktivitaten (max. 944 Bq m−3) zeigte, dass das Grundwasser aus Tiefenquellen (11–14 m) in der Seemitte und am Nordostufer in den See stromt. Fur den Wasserhaushalt des Sees spielt der Grundwasserzufluss eine grose Rolle. Er betragt 11 l s−1 und macht damit etwa 70 % des gesamten Wasserzuflusses des Sees aus. Hinsichtlich der Wasserchemie zeigten sich signifikante positive Korrelationen von Methan und Nitrit mit der Radonaktivitat, was das Grundwasser als gemeinsame Quelle identifiziert. Der Zustrom von anoxischem Grundwasser lasst insgesamt auf eine Verschlechterung der Wasserqualitat schliesen.

Oxygen is one of the most crucial elements on earth and equally affects life and inorganic redox processes. After its transition to water with moderate solubility and slow diffusion rates, most aquatic organisms depend on permanent renewal of dissolved oxygen (DO). Recharge of this pivotal aqueous gas may become hampered by anthropogenic and climatic influences with so far unknown consequences for surface freshwater systems and entire ecosystems. Because rivers integrate biogeochemical information of catchments, their oxygen dynamics may also reflect ecosystem and landscape health. Here we summarize the most important sources and sinks of DO and its role in river systems. These considerations also extend to associated water compartments and fluxes including lakes, reservoirs, soils, groundwater and the hyporheic zone. In addition, for continental-scale considerations, we analysed the GLObal RIver CHemistry (GLORICH) database with 170,369 DO measurements. These analyses revealed that DO in rivers relates to water temperature, pH and nutrient availability. On larger scales, it is also influenced by catchment area, slope, ratios of forests to managed land and population density. Our review also highlights important links between physical and biological influences on DO transfer as well as its sources and sinks in streams and rivers. We conclude that DO monitoring should be combined with novel interdisciplinary tracing techniques such as stable isotope ratios, radon gas and biological analyses. Such combined analyses have the potential to improve our understanding of transfer and transformations of oxygen in rivers as essential integrators of landscapes.

Identifying groundwater discharge zones to lakes is important because the groundwater system can contribute a significant flux of water and solutes and potentially influence lake chemistry and ecology. We present a new instrument, Temperature Mapper (TM), to map groundwater discharge zones in lakes via temperature anomalies along the lake bottom, tested in Lake Constance at 2 former sand and gravel extraction sites. TM is essentially a 4 m wide sled with an array of 19 high-resolution sensors attached to an aluminum lattice towed along the lake bed behind a boat. It is able to map spatial temperature patterns representing groundwater discharge zones at a resolution of 20 cm laterally and 2.5 m in path direction. We conducted 2 field campaigns in February 2016 and February 2017, during which TM identified several temperature anomalies interpreted as groundwater discharging zones. Radon-222 was also analyzed at selected locations as an independent indicator of groundwater discharge. Overall, when comparing the different methods and different years, we observed similar spatial patterns of preferential groundwater discharge. TM offers a useful qualitative tool to identify and map potential groundwater discharge zones in lakes or coastal areas that can then be further studied by more quantitative, small-scale methods such as chemical tracers or seepage meters.

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.

Determining recharge rates is critical for understanding and managing groundwater systems. This study compares recharge rates estimated using different methods in a temperate catchment (the Ovens Valley, southeast Australia). A catchment water balance yields recharge rates of 110–410 mm yr−1, with the range reflecting uncertainties in evapotranspiration and the fraction of river water that represents surface runoff. The water table fluctuation method yielded recharge rates of 177–1296 mm yr−1 with a median estimate of 518 mm yr−1. Residual moisture in the unsaturated zone, which lowers the effective specific yield, probably results in this method overestimating recharge rates. Recharge rates estimated from chloride mass balance are also variable (5–712 mm yr−1) with a median value of 285 mm yr−1. Most uncertainties in those estimates arise from uncertainties in the mass of Cl exported directly by surface runoff. Recharge rates calculated from 3H activities using the well-mixed reservoir (renewal rate) model are 11 to 480 mm yr−1 with a median estimate of 159 mm yr−1. The lower recharge rates calculated from 3H may be due to the presence of older water in zones of restricted flow in the aquifers. Additional uncertainties in this technique arise from assumptions in the 3H input function and having to estimate the depth of the active recharge zone at the top of the aquifer. The wide range of recharge rate estimates from methods based on individual groundwater samples may also reflect spatially heterogeneous recharge, which creates difficulties for determining representative recharge rates. The Ovens catchment is typical of many globally in that recharge rates are estimated using infrastructure and data that were not specifically intended for that purpose. The catchment water balance and chloride mass balance are based on abundant commonly-available data and probably yield reasonable estimates of recharge in this and similar temperate catchments.

The potentially detrimental impact of groundwater discharge into rivers on the ecosystem services provided by the river makes the localization of groundwater discharge areas as well as the quantification of the associated mass fluxes an issue of major interest. However, localizing groundwater discharge zones and evaluating their impact are challenging tasks because of (i) the limited number of suitable tracers and (ii) the high spatio-temporal variability of groundwater/river water interaction in general. In this study, we applied the ubiquitous naturally occurring radioactive noble gas radon (&sup2, &sup2, Rn) as an aqueous tracer to localize and quantify groundwater discharge along a 60 km reach of the upper German part of the major river Elbe under drought conditions. All radon data processing was executed with the numerical implicit finite element model FINIFLUX, a radon mass balance-based approach, which has been developed specifically to quantify the groundwater flux into rivers. The model results were compared to the tritium (3H) distribution pattern in the studied river reach. The results of the study proved the applicability of both (i) the methodical approach (i.e., radon as tracer) and (ii) the application of FINIFLUX to drought conditions (with river discharge rates as low as 82 m3/s vs. a long time mean of 300 m3/s). Applying the model, the recorded dataset allowed differentiating between groundwater baseflow, on the one hand, and interflow and surface water runoff distributions to the river, on the other. Furthermore, the model results allowed assessing the location and the intensity of groundwater discharge into the river under low flow conditions. It was also shown that analysing discrete river water samples taken from distinct points in a major stream might lead to slightly incorrect results because of an incomplete mixing of river water and locally discharging groundwater. An integrating sampling approach (as applied for radon) is preferable here.

Understanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field-based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater ( 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re-established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low-salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m3/m/day compared with 26 and 3 m3/m/day for the same periods via mass balance using Radon (222Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.

Understanding groundwater inflows to rivers is important in managing connected groundwater and surface water systems and for protecting groundwater-dependant ecosystems. This study defines the distribution of gaining reaches and estimates groundwater inflows to a 62 km long section of Deep Creek (Maribyrnong catchment, Australia) using 222 Rn. During summer months, Deep Creek ceases to flow and comprises a chain of ponds that δ 18 O and δ 2 H values, major ion concentrations, and 222 Rn activities imply are groundwater fed. During the period where the river flows, the relative contribution of groundwater inflows to total river discharge ranges from ∼14% at high flow conditions to ∼100% at low flows. That the predicted groundwater inflows account for all of the increase in discharge at low flow conditions lends confidence to the mass balance calculations. Near-continuous 27 week 222 Rn monitoring at one location in the middle of the catchment confirms the inverse correlation between river discharge and relative groundwater inflows, and also implies that there are limited bank return flows. Variations in groundwater inflows are related to geology and topography. High groundwater inflows occur where the river is at the edge of its floodplain, adjacent to hills composed of basement rocks, or flowing through steep incised valleys. Understanding the distribution of groundwater inflows and quantifying the contribution of groundwater to Deep Creek is important for managing and protecting the surface water resources, which support the endangered Yarra pygmy perch.

Knowledge of groundwater residence times and recharge locations is vital to the sustainable management of groundwater resources. Here we investigate groundwater residence times and patterns of recharge in the Gellibrand Valley, southeast Australia, where outcropping aquifer sediments of the Eastern View Formation form an "aquifer window" that may receive diffuse recharge from rainfall and recharge from the Gellibrand River. To determine recharge patterns and groundwater flow paths, environmental isotopes (3H, 14C, δ13C, δ18O, δ2H) are used in conjunction with groundwater geochemistry and continuous monitoring of groundwater elevation and electrical conductivity. The water table fluctuates by 0.9 to 3.7 m annually, implying recharge rates of 90 and 372 mm yr−1. However, residence times of shallow (11 to 29 m) groundwater determined by 14C are between 100 and 10 000 years, 3H activities are negligible in most of the groundwater, and groundwater electrical conductivity remains constant over the period of study. Deeper groundwater with older 14C ages has lower δ18O values than younger, shallower groundwater, which is consistent with it being derived from greater altitudes. The combined geochemistry data indicate that local recharge from precipitation within the valley occurs through the aquifer window, however much of the groundwater in the Gellibrand Valley predominantly originates from the regional recharge zone, the Barongarook High. The Gellibrand Valley is a regional discharge zone with upward head gradients that limits local recharge to the upper 10 m of the aquifer. Additionally, the groundwater head gradients adjacent to the Gellibrand River are generally upwards, implying that it does not recharge the surrounding groundwater and has limited bank storage. 14C ages and Cl concentrations are well correlated and Cl concentrations may be used to provide a first-order estimate of groundwater residence times. Progressively lower chloride concentrations from 10 000 years BP to the present day are interpreted to indicate an increase in recharge rates on the Barongarook High.

222Rn is an important tracer for quantifying groundwater inflows to rivers, especially where groundwater and surface water have similar major ion and stable isotope geochemistry. Uncertainties in the 222Rn mass balance arise, however, from not accurately estimating the degree of degassing of 222Rn to the atmosphere and the extent to which interaction within the parafluvial zone provides an additional source of 222Rn. This study estimates both 222Rn production in the parafluvial zone and degassing along a 75km stretch of the King River, Australia, in order to more precisely quantify groundwater inflows. The contribution of 222Rn from the parafluvial zone (Fp) was estimated using 222Rn emanation rates from near-river sediments and assessment of the residence time of water in the parafluvial zone from 222Rn activities and Cl concentrations in water the alluvial sediments. Values of Fp range from 40,400Bq/m/day in the upper King to 8500Bq/m/day in the lower King, corresponding to differences in the mineralogy and the volume of the parafluvial zone. The gas transfer coefficient (k) was estimated by matching groundwater inflows to observed increases in river discharge during a period of low discharge; k decreases from 25day-1 in the upper King to 3day-1 in the lower King. These k values are higher than those from most empirical formulations, probably due to the extensive pool and riffle sections that promote degassing. The King River is gaining in its upper and middle sections but several of the lower reaches are losing. Groundwater inflows on a reach scale are as high as 10m3/m/day and cumulative inflows along the 75km stretch are up to 19,000m3/day. Groundwater inflows increase proportional to total flow reflecting the response of both groundwater and surface water systems to rainfall. Uncertainties in the calculated groundwater inflows are reduced by independently estimating k from the discharge data; however, calculated inflows are up to 40% higher if parafluvial flow were not taken into account. •Predicted Rn activities from emanation measurements agree with those measured in groundwater.•Contribution of Rn from parafluvial zone is assessed.•Rn degassing is independently assessed by comparison with flow gauge data.•Rn degassing is higher than predicted from empirical formulations due to pool and riffle sections.•King River transitions from gaining in upper sections to losing in lower sections.

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.

A robust correlation between electrical conductivity (EC) values and Cl concentrations in river water from southeast Australia allows detailed Cl fluxes to be calculated from continuous EC and river discharge records. Many Victorian rivers export significantly more Cl than is delivered to their catchments by rainfall. Cl* is defined as the mass of Cl exported in the rivers relative to that input by rainfall over a multi-year period (Cl*. =. 100% indicates that the river exports the same mass of Cl as is input by rainfall). There is a systematic relationship between catchment type and Cl*. Rivers draining cleared plains have Cl* values between 50 and 750%, rivers draining volcanic plains have Cl* values of 770-1600%, whereas rivers with large forested upland catchments have Cl* values of 50-110%. These values are minima as they do not account for Cl exported by groundwater from the catchments. The calculations are based on long-term (up to 22. year) records that span drought and high rainfall periods. The magnitude of Cl* is far higher than can be explained by errors in the calculations or variability in rainfall and runoff, and Cl/Br ratios preclude halite dissolution as a source of Cl. The excess Cl reflects hydrological changes in the catchments. Land clearing on the cleared plains has caused the rise of regional water tables which results in the export of Cl from saline groundwater via increased baseflow to the river systems. Drainage systems on the volcanic plains are re-establishing following impoundment by recent (

The interaction between groundwater and surface water along the Tambo and Nicholson rivers, southeast Australia, was investigated using 222Rn, Cl, differential flow gauging, head gradients, electrical conductivity (EC) and temperature profiles. Head gradients, temperature profiles, Cl concentrations and 222Rn activities all indicate higher groundwater fluxes to the Tambo River in areas of increased topographic variation where the potential to form large groundwater–surface water gradients is greater. Groundwater discharge to the Tambo River calculated by Cl mass balance was significantly lower (1.48 × 104 to 1.41 × 103 m3 day−1) than discharge estimated by 222Rn mass balance (5.35 × 105 to 9.56 × 103 m3 day−1) and differential flow gauging (5.41 × 105 to 6.30 × 103 m3 day−1) due to bank return waters. While groundwater sampling from the bank of the Tambo River was intended to account for changes in groundwater chemistry associated with bank infiltration, variations in bank infiltration between sample sites remain unaccounted for, limiting the use of Cl as an effective tracer. Groundwater discharge to both the Tambo and Nicholson rivers was the highest under high-flow conditions in the days to weeks following significant rainfall, indicating that the rivers are well connected to a groundwater system that is responsive to rainfall. Groundwater constituted the lowest proportion of river discharge during times of increased rainfall that followed dry periods, while groundwater constituted the highest proportion of river discharge under baseflow conditions (21.4% of the Tambo in April 2010 and 18.9% of the Nicholson in September 2010).

Documenting whether surface water catchments are in net chemical mass balance is important to understanding hydrological systems. Catchments that export significantly greater volumes of solutes than are delivered via rainfall are not in hydrologic equilibrium and indicate a changing hydrological system. Here an assessment is made of whether a saline catchment in southeast Australia is in chemical mass balance based on Cl. The upper reaches of the Barwon River, southeast Australia, has total dissolved solids, TDS, concentrations of up to 5860mg/L and Cl concentrations of up to 3370mg/L. The high river TDS concentrations are due to the influxes of groundwater with TDS concentrations of up to 68,000mg/L. Between 1989 and 2011, the median annual Cl flux from the upper Barwon catchment was 17.8×106kg (~140kg/a/ha). This represents 340-2230% of the annual Cl input by rainfall to the catchment. Major ion and stable isotope geochemistry indicate that the dominant source of solutes in the catchment is evapotranspiration of rainfall, precluding mineral dissolution as a source of excess Cl. The upper Barwon catchment is not in chemical mass balance and is a net exporter of solutes. The chemical imbalance may reflect the transition within the last 100ka from an endorheic lake system where solutes were recycled producing shallow groundwater with high TDS concentrations to a better drained catchment. Alternatively, a rise in the regional water table following land clearing may have increased the input of groundwater with high TDS concentrations to the river system.

There is little known about the short-term dynamics of groundwater-surface water exchange in losing rivers. This is partly due to the paucity of chemical techniques that can autonomously collect high-frequency data in groundwater bores. Here we present two new instruments for continuous in situ (222)Rn measurement in bores for quantifying the surface water infiltration rate into an underlying or adjacent aquifer. These instruments are based on (222)Rn diffusion through silicone tube membranes, either wrapped around a pole (MonoRad) or strung between two hollow end pieces (OctoRad). They are combined with novel, robust, low-cost Geiger counter (222)Rn detectors which are ideal for long-term autonomous measurement. The down-hole instruments have a quantitative response time of about a day during low flow, but this decreases to12 h during high-flow events. The setup was able to trace river water bank infiltration during moderate to high river flow during two field experiments. Mass-balance calculations using the (222)Rn data gave a maximum infiltration rate of 2 m d(-1). These instruments offer the first easily constructible system for continuous (222)Rn analysis in groundwater, and could be used to trace surface water infiltration in many environments including rivers, lakes, wetlands, and coastal settings.

²²²Rn is a natural radionuclide that is commonly used as tracer to quantify groundwater discharge to streams, rivers, lakes, and coastal environments. The use of sporadic point measurements provides little information about short- to medium-term processes (hours to weeks) at the groundwater-surface water interface. Here we present a novel method for high-resolution autonomous, and continuous, measurement of ²²²Rn in rivers and streams using a silicone diffusion membrane system coupled to a solid-state radon-in-air detector (RAD7). In this system water is pumped through a silicone diffusion tube placed inside an outer air circuit tube that is connected to the detector. ²²²Rn diffuses from the water into the air loop, and the ²²²Rn activity in the air is measured. By optimizing the membrane tube length, wall thickness, and water flow rates through the membrane, it was possible to quantify radon variations over times scales of about 3 h. The detection limit for the entire system with 20 min counting was 18 Bq m⁻³ at the 3σ level. Deployment of the system on a small urban stream showed that groundwater discharge is dynamic, with changes in ²²²Rn activity doubling on the scale of hours in response to increased stream flow.

Bromine was historically termed a cyclic salt in terrestrial freshwater environments due to its perceived conservative cycling between the oceans and the continents. This basic assumption has been challenged recently, with evidence that bromine is involved in dynamic chemical cycles in soils and freshwaters. We present here a study on dissolved bromine species (bromide, organically bound bromine, DOBr) concentrations and fluxes as well as sediment trap bromine levels and fluxes in Lake Constance, a large lake in southern Germany. Water samples were obtained from all major and some minor inflows and outflows over one year, where-after dissolved bromine species were measured by a combination of ICP-MS and ion chromatography coupled to an ICP-MS (IC-ICP-MS). Sediment traps were deployed at two locations for two years with Br, Ti and Zr levels being measured by μ-XRF. 190 t yr−1 of total dissolved bromine (TDBr) was delivered to the lake via 14 rivers and precipitation, with the rivers Alpenrhein (84 t TDBr yr−1) and the Schussen (50 t TDBr yr−1) providing the largest sources. The estimated particulate bromine flux contributed an extra 24–26 t Br yr−1. In comparison, only 40 t TDBr yr−1 was deposited to the lake’s catchment by precipitation, and thus ∼80% of the riverine TDBr flux came from soils and rocks. Bromide was the dominant species accounting for, on average, 78% of TDBr concentrations and 93% of TDBr flux to the lake. Despite some high concentrations in the smaller lowland rivers, DOBr was only a minor component of the total riverine bromine flux (∼12 t yr−1, 7%), most of which came from the rivers Schussen, Bregenzer Ach and Argen. In contrast, most of the bromine in the sediment traps was bound to organic matter, and showed a clear seasonal pattern in concentrations, with a maximum in winter and minimum in summer. The summer minimum is thought to be due to dilution of a high Br autochthonous component by low bromine mineral and organic material from the catchment, which is supported by Ti, Zr and Br/Corg data. In the lake bromine was irreversibly lost to the sediments, with best flux estimates based on mass-balance and sediment trap data of +50–90 μg Br m−2 d−1. Overall, it appears that bromine is not simply a cyclic salt in the case of Lake Constance, with a clear geological component and dynamic lacustrine biogeochemistry.