HS10.8

Increasingly, Instream wood  is (re)introduced into river systems to reverse decades of catchment mismanagement and to deliver nature-based solutions to contemporary water resource challenges such as flooding and pollutant attenuation. Most Research concerned with instream wood has focused on its ability to modify morphological and ecological conditions within a reach, but less work has considered the implications for hyporheic connectivity, a primary control of many ecosystem functions. Here, we investigate the impact of wood additions in river restoration on hyporheic exchange, at both the feature-scale and reach-scale,  with the application of a before-after-control-intervention experimental design.

Research was conducted over a 200m long reach of Wood Brook (Staffordshire, UK), a lowland river, which drains a 3.1km² catchment dominated by mixed-arable farmland and deciduous woodland. The experimental reach included 3 treatment sites where channel-spanning wood features were installed, 2 sites with natural wood features, and 3 control sites that were appropriate for treatment but received no intervention. High-resolution-temperature-sensors (HRTS) were installed at these sites to capture the temperature in the surface water and at 3 hyporheic depths, up to 25cm, at 3-minute intervals. Furthermore, a series of smart tracer injections allowed us to estimate (metabolically active) transient storage before and after intervention, in both the treatment sub-reach which had received wood additions and the control sub-reach which had not.

Results indicate, once background conditions are excluded from the dataset, that the mean difference between hyporheic and surface water temperatures across the treatment sites reduced by 31% over the course of the study whilst the control sites remained unchanged. Further examination determined that the daily mean temperatures observed at treatment sites were significantly different to those witnessed at the control sites. This suggests that the introduction of instream wood fostered an increase in the magnitude of hyporheic exchange. This is supported by the analysis of before-after intervention data, where a smaller deviation was observed between surface water and hyporheic temperatures across the treatment sites when compared with the control group. Preliminary analysis of smart tracer injections suggests that wood additions increase reach-scale residence times of surface water and reach-scale metabolism.

The current research supports observations previously derived from flume and model-based studies, suggesting that the addition of instream wood alters the magnitude of localised hyporheic exchange. Enhanced hyporheic exchange can offer numerous benefits to a reach including: increased habitat diversity, improved primary production, and greater attenuation and transformation of pollutants. Therefore, research within this area offers valuable insights for water resource managers who are increasingly under pressure to improve the health of our riverine environments as stipulated by international policies such as the European Unions’ Water Framework Directive. While our research has contributed to advancing current knowledge surrounding how instream wood alters hyporheic connectivity, there remains numerous questions which need addressing prior to its widespread application to global watersheds.

How to cite: Lugg, N., Howard, B., Kettridge, N., Ullah, S., Dixon, S., and Krause, S.: Examining the impact of instream wood additions on hyporheic exchange in a lowland agricultural catchment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8470, https://doi.org/10.5194/egusphere-egu22-8470, 2022.

Riparian zones are known for their role in regulating water quality in stream corridors. Specifically, riparian zones can act as buffers for high-concentration nutrient inputs into the stream, which can be harmful for the aquatic ecosystem. This natural attenuation capacity is controlled by variable water and solute exchanges bringing together different reactants via the mixing of stream water (SW) and local groundwater (GW). The degree and the extent of this mixing can regulate the potential for turnover processes for certain solutes. Here, we couple a previously calibrated transient and fully-integrated 3D numerical flow model with a Hydraulic Mixing Cell (HMC) method to map the different water sources in the stream corridor of the 4^th-order Selke stream and track their spatio-temporal evolution. This allows us to identify areas where waters from different sources mix enhancing the potential for turnover of groundwater-borne solutes such as nitrate. We evaluate HMC results with hydrochemical field data, and outline mixing hot-spots defined by high degrees of mixing (i.e. balanced volume fractions of the mixing endmembers in a model cell) expressed in terms of a threshold mixing degree (d=d_h) within the stream corridor. Our results show that around 50% of the water in the aquifer originates from infiltrating SW. Especially around the stream (within 250m from the stream), aquifer water is almost exclusively made up of infiltrating SW with minimal amount of water from other sources being mixed in. On average, 9% of the floodplain aquifer are characterized by high degrees of mixing (d=d_h), but this value can be nearly 1.5 time higher following big discharge events. Our modeling results further suggest that peak intensity of events is more significant for the increase of mixing degrees than event duration. We also found that discharge events mainly facilitate high mixing degrees at greater distances from the stream; while near the stream growing SW influxes dominate water composition in the aquifer and decreasing water transit times reduce exposure-times of water and solutes to the conditions in mixing hot-spots. With this easy-to-transfer modeling framework we seek to show the applicability of the HMC method as a complementary tool for the identification of SW-GW mixing hot-spots at the floodplain-scale, when simulating the spatio-temporal patterns of SW-GW exchange in stream corridors.

How to cite: Nogueira, G., Schmidt, C., Partington, D., Brunner, P., and Fleckenstein, J.: Spatio-temporal variations of water sources and mixing in a riparian zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11651, https://doi.org/10.5194/egusphere-egu22-11651, 2022.

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.

How to cite: Engel, M., Mischel, S., Quanz, S., Frei, S., Gilfedder, B., Radny, D., and Schmidt, A.: Quantification of groundwater inflow along Moselle River by using a multiple tracer approach (222-Rn, Tritium, and δ18O), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6273, https://doi.org/10.5194/egusphere-egu22-6273, 2022.

The groundwater-surface water interface, which is known as hyporheic zone, is a dynamic system that plays an important role in the hydrological functioning of catchments. During flow exchange, there are many chemical attenuation processes due to the variation of physical and chemical properties including surface water flow and the morphology of the riverbed. The fluctuations may depend on the seasonality, climatic zone, as well as due to the local variations such as soil structure, geology, land use, water sources etc.

The goal of the study was to gain a depth orientated insight into the hyporheic exchange functioning, mainly focusing on the difference between upstream and downstream conditions. We collected water samples from 4 different depths until 0.5 m below the stream bed surface of the stream Ahna in Kassel, Germany, using multi-level interstitial probes. The samples were taken at different locations in the stream during a whole year. Water samples were analyzed for stable isotopes of water (δ²H and δ¹⁸O) and some major ions. Results indicate that the variations following the depth, sampling time and the location. Isotopic signatures show the summer enrichment and the winter depletion, mirroring the seasonality of stable isotopes in rainfall. We also observed an isotopic enrichment at the downstream sites and significantly higher ion concentrations, especially for K⁺, Na⁺, Mg²⁺, Cl^-, NO₃^-, than the upstream. Water extraction flow rates decreased along the depth profile due to the changes in effective porosities and the hydraulic conductivities which were controlled by the sediment structure and clogging processes.

How to cite: Mahindawansha, A. and Gassmann, M.: Evaluation of the hydraulic exchange in the hyporheic zone: A depth-oriented analysis focusing on upstream and downstream conditions., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1199, https://doi.org/10.5194/egusphere-egu22-1199, 2022.

How to cite: Noack, M., Negreiros, B., Kunz, M., Aybar Galdos, A., Schwindt, S., Haun, S., and Wieprecht, S.: De-clogging riverbeds with artificial flushing in a near-natural bypass channel, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7006, https://doi.org/10.5194/egusphere-egu22-7006, 2022.

Although rivers and streams are major transport vectors of microplastics into the marine environment, little research has been conducted to understand the transport behavior of microplastic particles in fluvial systems. This work contributes to the understanding of these transport processes, specifically focusing on the interface of the surface water flow and the hyporheic zone.

Transport of microplastic particles in fluvial systems is currently modeled mainly at larger, river- or basin-wide scales using existing hydrodynamic and sediment transport models. To investigate the transport behavior of microplastic particles along the interface between the hyporheic zone and the open water flow domain, smaller-scale models are required so that the complex processes in this region can be adequately represented and analyzed.

To this end, a novel modeling technique will be presented based on the open source CFD toolbox OpenFOAM. It combines a new coupling approach for the hydrodynamic processes in the surface water and hyporheic zone with transport modeling of microplastics.

The methodology considers the latest findings regarding deposition and resuspension of microplastic particles as well as the hyporheic exchange in a fully coupled model. The model is validated by accompanying flume experiments and relevant transport processes are identified from the presented scenario simulations.

How to cite: Dichgans, F., Boos, J.-P., Frei, S., and Fleckenstein, J. H.: Integrated numerical modeling of microplastic transport in fluvial systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7445, https://doi.org/10.5194/egusphere-egu22-7445, 2022.

In the coastal plains of New Zealand, braided rivers lose a considerable amount of water to the groundwater system, and for many aquifers are the largest source of recharge. Quantifying the recharge rates/transmission losses and how they relate to river stage and flow is particularly challenging. A commonly used approach to estimating recharge rates/transmission losses is differential flow gauging, where river discharges are measured simultaneously at multiple locations along a given reach. However, differential flow gauging is labour-intensive and limited by the accuracy of river discharge measurements, particularly in braided river systems.

We have developed an alternative method for ephemeral rivers using river stage monitoring and widely available satellite photography. The method was applied to the upstream part of the Selwyn River (Canterbury, New Zealand), which is perennial in its mountainous environment, but becomes ephemeral once it crosses its alluvial plain. The river stage near the downstream boundary of the perennial reach was monitored for the period of March 2020 to May 2021 and a stage-discharge rating curve was developed. Downstream of the monitoring station, the river becomes ephemeral with the drying front location changing over time. On a number of 146 suitable satellite photographs, taken within the stage recording period, and retrieved from the Planet application program interface, we identified the position of the drying front, and used this to determine the length of the active (wet) river channel. This enabled us to calculate the average river transmission losses by dividing the river discharge at the monitoring station by the downstream active river length. The transmission losses estimated using the satellite photography correspond well with the losses estimated using seven sets of independent differential flow gauging surveys, given the respective uncertainties of both methods. The average estimated transmission losses range from 0.2 to 1 m³/s/km. Most of the estimated losses are below 0.4 m³/s/km and correspond to baseflow periods. The highest losses occur shortly after peak flows and decrease exponentially with time after the peak.

We hypothesize that the high losses, occurring shortly after peak flows, are due to the replenishment of the shallow braid plain aquifer associated with the river. Lower losses, occurring during baseflow periods, represent groundwater recharge to the deeper regional aquifer. Groundwater recharge to the deeper regional aquifer appears to be linearly correlated with the groundwater head in the shallow aquifer. This response is consistent with the presence of an unsaturated zone that has been identified between the shallow (riverine) and deeper (regional) aquifers. Furthermore, we have successfully trained a random forest regression model to reconstruct the transmission losses for every day of the study period. The daily transmission loss dataset can now be used to evaluate our physically-based groundwater – surface water interaction models, currently under development, as well as support water management in the Selwyn basin.

How to cite: Di Ciacca, A., Wilson, S., Kang, J., and Wöhling, T.: Quantification of groundwater recharge from an ephemeral braided river using satellite photography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7538, https://doi.org/10.5194/egusphere-egu22-7538, 2022.

In the absence of precipitation or any other artificial input source, base-flow, a component of streamflow, sustains natural surface water bodies like rivers and streams. Therefore understanding, identifying, and extracting baseflow from streamflow measurement is essential for many hydrological studies, e.g., estimating watershed characteristics, long-term groundwater storage trends, flow regulations or water policy, water quantity, quality, supply, habitat and informing management of regional water resources. We aim to understand the morphologic factors that are known to influence groundwater outflow, for example, slope, length of the stream, drainage density, stream order, stream frequency on the baseflow recession characteristic or storage delay constant (K) in the watershed. We study how (K) varies with the choice of different estimation methods like using streamflow recession analysis by Brutsaert, (2008) algorithm, our newly developed algorithm for baseflow analysis, and using one of the solutions of Bossinesq’s groundwater flow equation. Using the aforementioned three techniques, the influence of significant morphologic characteristics is found for 56 small watersheds within the large watershed of the Rock River basin over the study period of 1990-2021. Using factor analysis we rank morphologic parameters in terms of their relative influence on (K). The findings of our study suggest that the morphologic parameters that influence the storage delay constant are intercorrelated and play a complex role in shaping the (K) values.

How to cite: Hameed, M., Nayak, M. A., and Ahangar, M. A.: Baseflow recession characteristic variation with the Basin Morphology., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11464, https://doi.org/10.5194/egusphere-egu22-11464, 2022.

Declining summer streamflow is observed in Pacific Northwest catchments, impacting endangered salmon species which need sufficient flow to reach their spawning grounds. Groundwater pumping for irrigation is generally considered the cause of low summer flow. However, it is unclear, how much water is lost due to water use or climatic factors, as there often is no data on pumping-volume. In this study we assess the lost amount of streamflow during summer low flows and quantify the shares attributable to climate change and agricultural water-consumption, only using streamflow data. As a case study we focused on the Scott River catchment, California, having 7% agricultural land use. We compared summer streamflow, snow water equivalent and precipitation between historic (1940-1976), intermediate (1977-1999) and modern (2000-2020) timeframes. Snow water equivalent showed negative significant trends at lower elevations (1600-1800 m). We also observed significant negative trends in mean and minimum streamflow as well as earlier starting and longer lasting low flow season. Using a paired-basin approach we were able to detect a mean 38.5% (37.5 +/- 3 Mm³) streamflow decrease from historic to modern timeframe years, where 14.6% (14.25 +/- 1.4 Mm³) were attributable to agricultural water consumption and 23.9% (23.2 +/- 1.4 Mm³) to climate change. These results demonstrate that agriculture substantially impacts streamflow; however, the influence of climate change dominates. Therefore, stopping water use in summer to increase low flows is insufficient. A possibility to ensure enough flow for endangered salmon could be artificial aquifer recharge during high flows to top of low flow season.

How to cite: Pyschik, J., Harter, T., and Stahl, K.: Assessing Climate Impacts Against Groundwater Pumping Impacts on Stream Flow with Statistical Analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13465, https://doi.org/10.5194/egusphere-egu22-13465, 2022.

The protection of water resources is a challenging task and requires a detailed understanding of surface water and groundwater dynamics and interactions. Integrated groundwater and surface water models represent useful tools to assess how groundwater affects the quantity and quality of surface water, which is often a source of drinking water. It is the case for the City of Quebec, Canada, where surface water is the only source of drinking water and where water managers must assess its vulnerability to contamination and depletion. This work focuses on the Nelson River catchment (70 km²), located within the larger catchment of the main drinking water source in Quebec City. The objective is to quantify the links between groundwater and surface water with the 3D integrated hydrological model HydroGeoSphere and simulate coupled surface/subsurface water flow and contaminant transfer. The Nelson catchment model has been calibrated to reproduce observed surface discharges and water table level measurements. Coupled surface water and groundwater flow is then simulated over multiple years using daily meteorological data. Output variables such as distributed infiltration, preferential flow pathways, inter-seasonal changes of surface water volumes, unsaturated and saturated groundwater volumes, are analysed to assess the link between surface water and groundwater. Since this urban area is undergoing growing urbanisation, future scenarios of urban development are also simulated to evaluate the impact of soil sealing on surface/groundwater interactions. The understanding of surface/subsurface interactions in this particular context aims at assessing the vulnerability of the surface drinking water source.

How to cite: Gatel, L., Tremblay, Y., and Therrien, R.: Integrated surface and subsurface hydrological modelling to support the assessment of the vulnerability of surface water supplies., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6397, https://doi.org/10.5194/egusphere-egu22-6397, 2022.

Stream dunes have been widely recognized among the major morphological features driving water exchange between a stream and its sediments, and many modeling studies have been performed to characterize hyporheic exchange induced by this type of bedforms. Despite of these efforts, the high number of factors that affect hyporheic exchange has not been completely addressed yet, mainly because of the simplifying assumptions that are unavoidably required to reduce the complexity of the problem. For instance, the effect of the limited extent of the thickness of the sediment layer on the Residence Time Distribution (RTDs) of hyporheic exchange has not been fully explored. This incomplete knowledge is particularly relevant due to the paramount role of RTDs in controlling biogeochemical reactions in microbiologically active sediments.

In this context, this study presents a modeling analysis of RTDs in dune-shaped streambeds of finite depth in the presence of groundwater flow. Numerical simulations of particle tracking have been performed to determine the combined influence of sediment depth and horizontal underflow on the shape of RTDs. Moreover, different analytical distributions (Exponential, Gamma, LogNormal, Fréchet) have been fitted to the numerical RTDs, and the best distribution for each range of dimensionless sediment depth and underflow velocity have been identified on the basis of Anderson-Darling tests.

How to cite: Boano, F., Monofy, A., and Grant, S.: Influence of sediment depth and groundwater underflow on residence time distributions in dune-shaped streambeds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6480, https://doi.org/10.5194/egusphere-egu22-6480, 2022.

Under moving bedform conditions, the shape of the sediment-water interface (SWI) is known to evolve over time. However, existing mathematical models of bedform-induced hyporheic exchange flux (HEF) assume a fixed bedform shape in determining the pressure boundary condition at the SWI. This simplifying assumption is adopted because there is no established method for prescribing head along an arbitrary, changing sediment-water interface (SWI). This gap has prevented most flow modeling efforts from accounting for the dynamics of bedform sizes and shapes, and it is currently not well understood how such dynamics are expected to affect transport and biogeochemical processes in streams. Previously, measurements of head along the SWI have been taken under stationary bed conditions using pressure sensors installed within bedforms, but installing sensors to take the same measurements under moving-bedform conditions is impractical. We propose a method to quantify the dynamics of hydraulic head at the SWI using timelapse photos of dye tracer tests, without installing any sensors in the flume. For every photo, an initial guess of head along the SWI is generated using established methods from the literature. Flow paths in the bed are calculated using the steady-state groundwater flow equation and Darcy’s Law. The predicted evolution of the dye plumes in the photo is compared against the dye plumes from the subsequent photo. This comparison is used as the objective criterion in an optimization procedure, which is run until the estimate of head at the SWI converges. Preliminary results show agreement with experimental observations from dye penetration tests. In providing a new way to estimate head under moving-bed conditions, this work is an important advance in realistic modeling of bedform-induced HEF and its effect on flow, transport, and biogeochemical processes in streams.

How to cite: Teitelbaum, Y., Shimony, T., Saavedra Cifuentes, E., Packman, A., Arnon, S., and Hansen, S.: Estimating Head Induced by Moving Bedforms Using Dye Tracer Tests and Modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-314, https://doi.org/10.5194/egusphere-egu22-314, 2022.

Clay deposition in streambed sediment can cause partial or complete clogging of the streambed. It was shown that clogging reduces the hyporheic exchange flux (HEF) between the water column and the streambed and can negatively affect stream ecosystems. For example, by reducing the fluxes of nutrients to benthic microorganisms. It has been shown that flow from the stream towards the groundwater (losing) or in the opposite direction (gaining) also affects clay deposition patterns; however, this has only been investigated experimentally under stationary bedform conditions. Here, we investigated the dynamics of clogging during moving bedform conditions and under losing or gaining fluxes. We conducted a series of experiments in a 640 long and 29 cm wide flume packed with sand (D50 = 270 μm). The flume is equipped with a drainage system that can simulate losing or gaining conditions at prescribed flux. We conducted experiments under two different losing fluxes and two gaining fluxes (10 and 20 cm/day), while stream velocity was constant at 29 cm/day.  During the experiments, kaolinite was added as a discrete series of pulses. Each pulse was added after the kaolinite deposition stabilized (2 - 4 days). HEF and the vertical hydraulic conductivity were quantified before each kaolinite addition by salt tracer test, and the “falling-head” methods, respectively. Morphodynamic properties of the bed were measured using high-frequency acoustic doppler sensor and time-lapse photometry timeseries. The kaolinite deposition rate was measured with a turbidity sensor, while core samples were taken at the end of the experiment to analyze the vertical deposition patterns. Preliminary results showed that HEF and hydraulic conductivity in losing conditions decreased from their initial value by 95% and 37%, respectively, while in gaining conditions, HEF decreased by 72% and hydraulic conductivity by 23%. We also observed that under gaining conditions, most of the clay was deposited at the upper part of the sediment (in the moving fraction). In losing conditions, kaolinite was also found deeper in the bed and below the moving fraction of the streambed. In all cases, most of the kaolinite mass was deposited at a depth of less than five cm. The results show a significant effect of stream-groundwater interactions on HEF and on suspended particle deposition in situations where the bed is under movement. Therefore, the quantification and prediction of clay deposition patterns in streams with strong interactions with groundwater has to be included in models that predict clogging and transport processes in streams. 

How to cite: Shimony, T., Saavedra Cifuentes, E., Packman, A., Teitelbaum, Y., and Arnon, S.: Kaolinite deposition and clogging of moving streambeds under losing and gaining flow conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-401, https://doi.org/10.5194/egusphere-egu22-401, 2022.

Biofilms in river sediments play a key role in P retention in aquatic ecosystems. Most studies in freshwater ecosystems focus mainly on the autotrophic component of biofilms but little is known about the role of heterotrophic components on P removal. It is known that DOC in some streams is of low bioavailability, hence, resulting in severe DOC and P co-limitation of heterotrophic biofilm growth which could then constrain P removal efficiency. How DOC limitation affects P removal efficiency in the benthic zone and how it modifies P thresholds (i.e. concentration from which the removal efficiency decreases) are still open questions.

We performed an experiment in the MOBICOS (Streamside Mobile Mesocosms) in the Holtemme River (Germany) to study the role of labile DOC on P thresholds in P retention in the bed-sediment biofilm community. Our flume experiment followed a BACI design (before: no DOC addition; after: labile DOC addition at C:P molar ratios >100; control: basal P and DOC concentrations; impact: P concentrations ranging from 25 µg P/L to 420 µg P/L).

Our results show that labile DOC increases the P removal efficiency of the system (i.e. P water mass balances in the flumes) and shifts P thresholds for P removal towards higher P concentrations meaning that at a given P concentration higher P removal efficiency is achieved if the system is supplied with labile DOC. Labile DOC activated the heterotrophic component in the flumes and benthic biofilms receiving labile DOC show higher bacterial density and higher P accumulation compared to the ones not receiving labile DOC.

Our results demonstrate that the heterotrophic biofilm community plays a key role in in-stream phosphorus retention. As it relies on availability of labile DOC, the interaction of DOC and P dynamics need consideration in models for stream nutrient processing and retention.

How to cite: Perujo, N., Neuert, L., Fink, P., Kamjunke, N., Sunjidmaa, N., Weitere, M., and Graeber, D.: Labile DOC increases P removal efficiencies in the benthic zone and shifts P thresholds towards higher P concentrations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5521, https://doi.org/10.5194/egusphere-egu22-5521, 2022.

Biogeochemical reactions along surface water flow paths mitigate nutrient inputs from agricultural land and can have large impacts on both the local water quality and the downstream export of nutrients from agricultural areas. Thus, stream restoration, in terms of engineered structures with the aim to increase the in-stream nutrient retention, is seen as an important strategy to restore the ecosystem functioning of degraded stream systems, mitigate excess nutrient concentrations and reduce the export to downstream recipients. Here, we propose a physically based model framework to assess the large-scale removal of Nitrogen (N) by denitrifications in the hyporheic zone along stream networks. The model framework, supported by an extensive dataset of hydromorphology and reach scale investigations, was used to estimate the current N removal in all local agricultural streams in Sweden defined as having a mean discharge < 1 m³/s and an agricultural N load > 0. Moreover, the theoretical potential to increase this removal by restoration structures that enhances the hyporheic removal efficiency and prolongs the stream residence times was assessed based on the Damköhler number, defined as the ratio between the hyporheic transport time scales and the reaction times scales.

The analyses comprised approximately 26000 stream reaches equivalent to ~75 000 km or 36% of the entire stream network in Sweden and revealed that both the N removal and the conditions limiting the hyporheic denitrification was highly dependent on the stream flow conditions. Specifically, during mean discharge conditions the aggregated results indicated that 13% of the N load to the assessed reaches was removed through hyporheic denitrification and that reaction limited conditions predominately occurred (72% of the assessed reaches). The theoretical potential of N removal, i.e. the N removal under the assumption of optimal hyporheic conditions, during mean discharge conditions was estimated to be 36% when all reaches were aggregated. Overall, the study shows that stream structures, especially if implemented over larger distances, could be a promising restoration strategy to enhance hyporheic removal and reduce terrestrial N export from agricultural areas.

How to cite: Riml, J., Wörman, A., and Morén, I.: Hyporheic nitrogen removal – assessing the potential for large scale stream restoration in Sweden, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11977, https://doi.org/10.5194/egusphere-egu22-11977, 2022.

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.

How to cite: Frei, S. and Gilfedder, B.: Quantification of local groundwater inflow into the Spree River and its relevance to the iron precipitation problem in the Lusatian mining area (Germany)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11492, https://doi.org/10.5194/egusphere-egu22-11492, 2022.

The interface between groundwater and surface water is a critical zone that influences ecohydrological and biogeochemical cycles within the surface water ecosystems. It is characterized by complex redox gradients. with groundwater-mediated inflow of reduced substances into the stream water. 
In this study, we have experimentally simulated the inflow of Fe(II)-rich groundwater with concentratrion of up to 1000 mmol L-1  into the open stream water of a flume system in order to quantify its effect on dissolved oxygen concentration in both the stream water and the hyporheic zone. The experimental setup consisted of 24 flumes, 12 of which were used for input of groundwater augumented with Fe(II), while the another 12 were used as controls, i. e., with inflow of Fe(II)-free groundwater. In addition, the experimental set-up provided the possibility to study the effects of fine sediment (coarse reference substratum (5% fine sediment content) vs. added fine sediment (35% fine sediment content) and low discharge (reference flow conditions vs. low discharge (drought) conditions within a threefold replicated, crossed design. All flumes had permanent groundwater input during the experiment. Fortnightly sampling campaigns were performed to analyze Fe(II), Fe(III),  and DO, concentrations in the porewater (hyporheic zone) and the open water over five consecutive weeks. 
Our results clearly indicate that Fe(II) inflow resulted in a decrease of DO concentrations both in the porewater and subsequently in the open water, with distinct effects of sediment porosity and discharge. Over the five weeks, the sustained decrease between 40 and 50% of DO concentrations was more pronounced in flumes with fine sediment than in flumes with coarse sediments. Our findings suggest that increasing the Fe(II) concentration in the hyporheic zone can affect the availability of oxygen, important in controlling biogeochemical and ecological processes, microbial activities, and aquatic life. The formation of oxygen-depleted subsurface and surface waters in freshwater ecosystems has been associated with nutrient-rich waters stimulating eutrophication and the subsequent reduction of river health. In conclusion, this study highlights the importance of considering the effects of hyporheic redox processes and Fe(II) in assessing the health of stream ecosystems.  

How to cite: Parra-Suarez, S., Wild, R., Barth, J. B., Gilfedder, B. S., Geist, J. G., Dichner, S., and Peiffer, S. P.: Groundwater born Fe(II) affects concentrations of  dissolved O2 in stream water, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11042, https://doi.org/10.5194/egusphere-egu22-11042, 2022.

Trace organic compounds (TrOCs) are frequently detected in surface waters such as rivers. Possible entry pathways into the environment include stormwater runoff, industrial effluents, and wastewater treatment plant (WWTP) effluents. Understanding the behavior of TrOCs and their transformation products (TPs) is important, as they represent a risk to ecosystem and human health. The hyporheic zone of a river shows high turnover rates for nutrients, dissolved organic carbon, metals, pathogens, and TrOCs. Turnover rates are dependent on both, hydrological and biogeochemical conditions. We conducted a high-frequency sampling campaign in the urban lowland River Erpe (Brandenburg, Germany) which receives treated wastewater from the WWTP Muenchehofe. The aim was to study the fate of TrOCs and respective TPs along specific hyporheic flow paths. The basic idea was to enable the sampling of water parcels along specific hyporheic flow paths by forcing the flow path with a pipe (diameter: 8 cm, length: 27 cm, maximum depth: 17 cm) onto a specific path similar to the natural one. Wood on top of the pipe should increase the hyporheic exchange flow through the pipe and mimic the effect of woody debris which is often used in river restorations. Samples from the hyporheic zone and the surface water were taken every 2 hours for 14 hours. The samples were analyzed for oxygen concentrations, redox parameters, nutrients, and TrOCs. We found a clear redox zonation along the flow paths inside the pipes and investigated its impacts on the fate of TrOCs and their TPs. The hyporheic zone proved as an important river compartment for the retention of TrOCs and their TPs.

How to cite: Reith, C. J., Posselt, M., Spahr, S., Putschew, A., Amann, F., Hinkelmann, R., and Lewandowski, J.: The fate of trace organic compounds and their transformation products along specific hyporheic flow paths, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1643, https://doi.org/10.5194/egusphere-egu22-1643, 2022.

The hyporheic zone (HZ) describes an interfacial zone of permeable sediments, located in river beds, riparian and floodplain areas, where surface water mixes with groundwater. It exerts major control over the quality of river water by impacting the exchange processes between surface water and the sediment compartment through dynamically exchanging water, temperature and compounds as well as demonstrating an intensive nutrient turnover, providing the stream with a self-purification capability. Due to these properties, engineered hyporheic zones that aim to increase the hyporheic exchange flux are of great interest in the context of river management and restoration. The spatial and temporal scales on which the hyporheic exchange processes occur are manifold: Small topographical features of the streambed like ripples or burrows of aquatic organisms have to be considered as well as larger geomorphological features like meanders. In addition to steady-state-like streams and rivers, events like rapidly moving floods have to be taken into account when investigating the HZ. Numerical models are often utilised to gain a more comprehensive understanding of the interacting processes in the HZ. In contrast to widely used two-domain concepts, which are based on coupling surface flow and groundwater flow models,  integral one-domain modelling approaches to improve the resolution of the exchange processes and better account for feedback effects have recently attracted more attention. In this contribution, such an integral surface water – groundwater model, extended by a transport model, is validated against the results of a field experiment conducted in a side channel of the urban lowland River Erpe (Brandenburg-Berlin, Germany), which receives effluent from the wasterwater treatment plant Münchehofe, containing trace organic compounds (TrOCs) like Carbamazepine. The experiment consisted of a high-frequency sampling campaign to study the fate of the TrOCs along specific hyporheic flow path. The investigated flow path was dictated by a U-shaped pipe inserted into the sediment parallel to the surface water flow direction. To increase the hyporheic exchange, wooden debris was placed on the sediment in between the pipe openings. The model is set up using porousInter, which extends the multiphase flow solver interFoam of the computational fluid dynamics software package OpenFOAM, to allow flow through porous media. Flow is simulated by solving the three-dimensional Navier-Stokes equations extended by a porosity coefficient and additional drag terms to account for porous media flow. Further, the solver is extended by a transport model for a conservative tracer, which is used to simulate the spreading of TrOCs in the surface water and porous sediment. We expect to compute tracer  residence times and flow velocities inside the pipe in accordance with the results of the experiment.

How to cite: Amann, F., Reith, C. J., Lewandowski, J., and Hinkelmann, R.: Simulating TrOCs concentrations along specific hyporheic flowpaths using an integral surface water-groundwater model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13282, https://doi.org/10.5194/egusphere-egu22-13282, 2022.

Small postglacial depressions, such as kettle holes in northeastern Germany, are distributed across glacially embossed areas of the world. Water bodies forming in these depressions are distinct hydrological systems. Depending on the exchange fluxes with adjoining groundwater system, kettle holes are classified as recharge, flow-through-, and discharge-dominant systems. This classification is a result of their topographical position over an undulating landscape. The upland and lowland kettle holes across the undulating landscape are expected to represent recharge- and discharge-dominant systems, respectively. Hence, those located between these two are expected to be flow-through kettle holes. Nonetheless, due to the complexity of the geological setting of undulating postglacial landscapes, this topography-based classification may be wrong. Furthermore, the hydrological system of kettle holes varies in both time and space. Dynamic boundary conditions of kettle holes, resulting from extreme weather conditions such as severe or prolonged droughts and heavy storm events, may cause a discharge-dominant kettle hole to temporarily shift to a recharge-dominant or a flow-through system.

Many kettle holes of northeastern Brandenburg, Germany are scattered throughout croplands. As a result, fertilizers are transported via the surface runoff and/or groundwater to the kettle holes. Thus, distribution and redistribution of water and solute from each of the kettle hole types and their adjoining groundwater domain and vice versa would likely be different. As these three types of kettle holes have different roles in the context of the hydrological cycle, differentiation of them based on the aforementioned classification would be of paramount importance for their proper characterization and role within a landscape hydrological system. A better characterization will also help to reduce the uncertainty in tracing water and solutes in these hydrogeologically complex systems. An extensive monitoring network of piezometers, installed within kettle holes and around them, is probably the most accurate method to characterize their hydrological system. However, its implementation is expensive, labor-intensive, and time-consuming. Therefore, it cannot be used for the determination of a landscape scale hydrological system containing a great number of kettle holes. The evaporation-to-inflow ratio (E/I) — derived from the stable isotopes of water (H and O) — has been demonstrated to be a viable alternative. We will present a new approach to determine the hydrological system of kettle holes based on geochemistry. To that end, eight chemical species (Ca, Mg, K, Na, Br, Cl, NO3, and SO4), and four in-situ parameters (temperature, pH, electrical conductivity, and redox potential) were monitored for 36 kettle holes over a 17 months period. Based on this dataset, the geochemical characteristics of the kettle holes will be identified using an advanced multivariate statistical algorithm, i.e. Gaussian finite mixture modelling (GFMM) and these will be compared to the hydrological classification of the kettle holes based on the E/I ratios.

Keywords: Kettle Holes, Geochemical Characteristics, Groundwater System, Stable Water Isotopes, Germany

How to cite: Taie Semiromi, M., Steidl, J., Hayashi, M., van Meerveld, I., and Merz, C.: Identification of the Hydrological System of Kettle Holes of Northeastern Brandenburg, Germany based on their Geochemical Characteristics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8108, https://doi.org/10.5194/egusphere-egu22-8108, 2022.

Lake Stechlin was an oligotrophic lake but experienced rapid eutrophication in the last 12 years. The speed of the deterioration is surprising, especially since the lake is located in a nature reserve, its catchment is almost entirely forested, there is no agriculture and there are only two small settlements with a total of less than 1000 inhabitants. Undoubtedly, groundwater is a crucial component of the water and compound balances of Stechlin, as there are no surface inflows. However, if one considers the total amount of groundwater entering the lake and the maximum compound loads possible with it, groundwater alone cannot explain the rapid increase in P concentrations in the Stechlin. Thus, internal P cycling, i.e. the mobilization of previously imported P, is an important process in the biogeochemistry of the lake. However, even if it is true that most of the P is remobilized by internal processes, it can still originate originally from groundwater. And groundwater might be the decisive trigger of the eutrophication in recent years. We provide evidence that not only P import from groundwater is the trigger, but also changes in loads of compounds that are closely coupled to the P cycle, namely calcium, manganese, iron and sulphate. While there is some evidence of elevated concentrations in the aquifer below the two settlements, the main P import into the Stechlin probably originates from the eutrophic Lake Dagow, which drains into Lake Stechlin via the aquifer.

How to cite: Lewandowski, J., Jäger, A., Mehler, F., Goldhammer, T., and Hupfer, M.: Significance of lacustrine groundwater discharge for the rapid eutrophication of formerly oligotrophic Lake Stechlin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7277, https://doi.org/10.5194/egusphere-egu22-7277, 2022.