Displays

Research in the river corridor commonly focuses in two study designs. One research strategy focuses on physical, chemical, and/or biological dynamics and feedbacks, emphasizing local variation and interaction over larger-scale context. A second study design focuses on gradients arising in response to non-local controls (e.g., climate, tectonic setting), with an emphasis on broad trends over smaller-scale “noise”. Here, we present a comprehensive set of measurements and calculated metrics describing physical, chemical, and biological conditions collected at 62 sites in the river corridor within a 5^th order basin including more than 150 variables at each site. The size and scope of this data set allows us to assess which variables have spatial structure in the basin using spatial semivariograms and regressions with discharge and drainage area. We ask how physical, chemical, and biological sub-systems co-vary using a principal components analyses. Next, we explain both spatial structure and local variance simultaneously using support vector machine regression techniques that reveal possible nonlinear, multivariate relationships that may direct future research. Key outcomes from this study include (1) an introduction to an open-source, comprehensive characterization of the river corridor, (2) interpretations of both broad trends and local variance in the river corridor, and (3) a summary of which metrics have the most explanatory power and why within the study system.

How to cite: Ward, A., Drummond, J., Li, A., Lupon, A., Kurz, M., Zarnetske, J., Stegen, J., Marti, E., Ouellet, V., Brekenfeld, N., Mao, F., Graham, E., Bernal, S., Krause, S., and Hannah, D.: An inductive approach to characterize physical, chemical, and biological system interactions in a 5th order river basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19379, https://doi.org/10.5194/egusphere-egu2020-19379, 2020.

The importance of riparian lowland buffer zones on surface and ground-water quality has recently received greater attention within Denmark. Significant funding has been made available to re-establish riparian lowlands to reduce nitrate loading to streams, as well as reducing GHG emissions. Surface water nitrate loads are currently estimated using the national nitrogen model, the scale of this model is unable to capture the flow dynamics of small-scale riparian lowlands. Therefore, the model is unable to account for the spatial and temporal variation in the nitrate reduction in the riparian lowlands. Consequently, the current focus is on upscaling the hydrological impacts of riparian lowlands so they may be incorporated into the national scale model in a consistent and transparent way.

Key to quantifying the impacts of riparian lowlands on the surface water nitrate loading is partitioning flow pathways, e.g., surface runoff, groundwater discharge, and drain flow. For example, the likelihood of nitrate reduction within a riparian lowland dominated by surface runoff is low, conversely if groundwater discharge dominants the likelihood is higher. Determining a relationship between the small-scale riparian flow pathways and larger scale landscape features, such as drainage area, slope, and aquifer geometry, may provide a means to upscale and quantify the reduction capacity of a lowland riparian zone.

Numerous field scale riparian lowland investigations have focused on describing the hydraulic processes, but very few investigations have attempted to quantify the flow pathways and/or provide insight into how this information may be used at a larger scale. This investigation will aim to simulate and quantify the observed flow pathways at the field scale for two field sites in Jutland (Fensholt and Holtum), Denmark. These simulations will aid in identifying the keys landscape features which can be used to determine the reduction capacity of riparian lowlands at the national scale.

How to cite: Noorduijn, S. and Højberg, A.: Upscaling riparian lowland buffer zone flow dynamics in the Danish nitrogen model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6585, https://doi.org/10.5194/egusphere-egu2020-6585, 2020.

Intermittent streams as well as extreme events are expected to become more common as the climate changes. Therefore, it is important to understand how drought affects the biofilms that are essential for nutrient and DOM processing within streams. Previous work has largely focused on Mediterranean streams. This project evaluates how drought affects the state and processes of the microbial biofilms in the hyporheic zone of temperate intermittent streams. The experiment was conducted with outdoor experimental hyporheic flumes (5 m long, 0.6 m wide, 1.2 m deep) that were allowed to fall dry for periods ranging from 4 to 100 days. Sediment was sampled before drying, during the drought, and at several time points after rewetting. Samples were analyzed for extracellular enzymatic activity, respiration, bacterial growth, live to dead cell ratios, bacterial abundances, and extracellular polymeric substances.

Extracellular enzymatic activities remained unaffected by the drought in the hyporheic zone but showed an increase on the surface during the dry phase. Upon rewetting, the enzymatic activities generally fell to pre-drought levels on the surface. Extracellular polymeric substances also remained unaffected by drought in the hyporheic zone. However, surface values for extracellular polymeric substances showed a similar pattern to enzymatic activities during the longer dry periods (70+ days) and subsequent rewetting. These results indicate that the hyporheic zone retained enough moisture in the sediment to continue functioning, while the surface sediment was impacted by the loss of flowing water.

How to cite: Coulson, L. E., Attermeyer, K., Griebler, C., Schelker, J., Hein, T., and Weigelhofer, G.: The impacts of drought on the microbial states and processes in the hyporheic zone of temperate streams, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13765, https://doi.org/10.5194/egusphere-egu2020-13765, 2020.

The stream hyporheic zone (HZ) represents the interface between streams and groundwater. Due to the mixing of organic matter and nutrients from groundwater and surface waters it is a hot spot of microbial activities and carbon processing within a stream network. The magnitude of terrestrial carbon degradation by microorganisms in the HZ influences the quantity and biochemical quality of terrestrial carbon as well as greenhouse gas concentrations in streams. One of the factors controlling microbial activities and terrestrial carbon degradation in the HZ are nutrients. However, major knowledge gaps exist regarding the control of nutrients on terrestrial carbon processing in the HZ among different streams.

We investigated the role of algal DOM (DOM_alg) and phosphorus (P) on the degradation of soil DOM (DOM_soil) by hyporheic microorganisms in a lab- and a field-based experiment. In the lab-based experiment, we focused on the influence of different DOM_soil:DOM_alg ratios on the DOM degradation at similar carbon concentrations in microcosms mimicking the HZ. One batch was incubated at ambient P concentrations and a second batch at increased P concentrations adapted to the highest levels found in the pure DOM_alg. We assessed microbial respiration and changes in DOM optical properties to examine quantitative and qualitative changes of the DOM pool. In the field-based experiment, we determined microbial respiration rates of HZ-sediments from 20 streams in Austria with differing ambient nutrient and organic carbon concentrations. The sediments were incubated with DOM_soil, with and without additional P.

Results from the lab-based experiment show that microbial respiration in the HZ decreased with increasing soil DOM fractions. When P levels were adapted to DOM_alg concentrations, microbial respiration rates were comparable between the different DOM mixtures and DOM_soil was degraded. However, in the field-based experiment, P addition only stimulated microbial respiration rates in one out of 20 HZ-sediments, suggesting that microbial respiration rates are not solely controlled by P.

In conclusion, nutrient pulses can stimulate microbial activities and thus terrestrial carbon degradation in the HZ. However, when using different stream HZ-sediments, it becomes evident that the nutrient stimulation is not a ubiquitous mechanism and terrestrial carbon degradation in the HZ is controlled by a multitude of factors.

How to cite: Attermeyer, K., Harjung, A., Schelker, J., Kainz, M., and Weigelhofer, G.: Is terrestrial carbon degradation in stream hyporheic zones stimulated by nutrients?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-301, https://doi.org/10.5194/egusphere-egu2020-301, 2020.

Rivers being influenced by treated wastewater are characterized by an altered water chemistry compared to their natural state. Downstream of the outlet of a wastewater treatment plant (WWTP), concentrations of dissolved organic matter (DOM) and trace organic compounds (TrOCs) in the receiving river are increased. As DOM might serve as a metabolic co-substrate during microbial TrOC degradation, DOM influences the attenuation of TrOCs. Due to steep biochemical gradients at the surface water - groundwater interface, the hyporheic zone is considered a hotspot for microbial activity. Therefore, turnover rates in the hyporheic zone of a stream are high in comparison to the turnover rates in the water column. The River Erpe is a sandy lowland river in the East of Berlin, Germany, which receives treated wastewater from the WWTP Muenchehofe. In order to study the simultaneous fate of TrOCs and DOM in surface water and the hyporheic zone, a field sampling campaign was conducted at a side channel of the River Erpe. Surface water samples were taken at site A and both surface and pore water samples from 25 cm sediment depth were taken at site B which is 850 m downstream of site A. The sampling interval was every three hours over 48 hours. Samples were analysed for 17 TrOCs (HPLC-MS/MS) and the molecular composition of DOM (FT-ICR MS). DOM compound classes were calculated semi-quantitatively as the percentage share of each class of the whole DOM composition. Mean concentrations of the TrOCs analysed changed by an increase of 200 % to a decrease of 29 % in the surface water between site A and B and by a decrease of 5 to 93 % in the hyporheic zone at site B. The molecular composition of DOM changed at most by a single digit percentage per compound group with the attenuation being larger in the hyporheic zone. The percentage share of two out of seven DOM compound classes significantly correlated with the concentration of at least ten TrOCs between surface water at site A and B. Such a correlation was observed for five compound classes in the hyporheic zone at site B. The present study shows that the attenuation of both TrOCs and DOM in a sandy urban river mainly takes place in the hyporheic zone but it is not capable of assuming a causal relationship between the attenuation of TrOCs and DOM.

How to cite: Mueller, B. M., Schulz, H., Putschew, A., and Lewandowski, J.: Simultaneous fate of trace organic compounds and dissolved organic matter in surface water and the hyporheic zone of an urban river, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7729, https://doi.org/10.5194/egusphere-egu2020-7729, 2020.

Perennial low order streams are normally well connected to shallow groundwater and therefore, they are among the first receptors of agricultural effluents. Understanding the processes governing the water quality in agricultural areas requires identifying sources of potential pollutants (such as nitrate), hotspots of biogeochemical reactivity and defining the different hydrologic flowpaths connecting groundwater and surface water. To this end, we have equipped an agricultural drainage system (Schönbrunnen) in south-western Germany with 3 stream gauging stations along a test segment of approximately 550 m and 33 piezometers in the adjacent shallow aquifer. Hydrological, hydrochemical, isotopic and microbiological variables have been monitored between August 2017 and December 2019 to spatially and temporally identify the controls of nitrogen cycling dynamics in our stream.

The Schönbrunnen generally loses water in its mid-segment and it gains in the lower part of the catchment, although this behavior showed strong seasonal variations, considering winter and summer as the two main annual seasons. The groundwater-streamwater (GW-SW) exchange flux, and the replacement of streamwater lost to the aquifer over a reach by shallow groundwater, defined as hydrologic turnover, was found to influence streamwater chemistry. The main groundwater flow directions were determined based on hydraulic head contour maps. We used them to characterize the nitrogen (N) species’ behavior along the flowpaths under two different hydrologic conditions: losing and gaining. Even though the losing condition at the midstream provides more favorable condition for N-species reduction at the GW-SW interface, reduction occurred also along gaining reaches. The isotope analyses of nitrate yielded data points plotting along the denitrification trend (slope of 0.5) in a dual isotope plot (¹⁵N-NO₃^- versus ¹⁸O-NO₃^-) for some of the sampling locations within the losing area. Comprehensive molecular approaches suggested a hotspot for denitrifying microbial communities in sediments of the losing stream reach. Along the GW flow path to the gaining area a depletion of nitrate was identified in concert with increasing sulfate and declining sulfide (H₂S) concentrations. Sulfide-driven nitrate reduction was likely to occur under anoxic conditions in this part of the aquifer. In summary, the findings demonstrate, that hydrologic turnover does not only mean hydrological exchanges, but also triggers variations in water composition along the transition zone between groundwater and streamwater by linking both, mixing and reactive processes.

How to cite: Jimenez Fernandez, O., Osenbrück, K., Wang, Z., Fleckenstein, J., Schmidt, C., Lüders, T., and Schwientek, M.: How alternating gaining and losing conditions along a low order agricultural stream govern the behavior of nitrogen species, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18530, https://doi.org/10.5194/egusphere-egu2020-18530, 2020.

Ecosystems role in preserving water resources is acknowledged by EU Water Framework Directive 2000/60/EC and by Groundwater Directive 2006/118/EC. In this view nutrient concentration represent a critical aspect with regard to water quality and eutrophication. Nitrate pollution is specifically addressed by the Nitrate Directive 91/676/EEC, which provides nitrate management rules for farming activities, defines nitrate vulnerable zones and gives indications on nitrate monitoring in water bodies.

To preserve water quality levels an important role is played by Riparian Buffer Zones (RBZ) through the provision of the Ecosystem Service (ES) nutrient retention, the uptake process operated by plant roots. Research has shown how ES valuation is a very effective approach to support land management process, as it allows a better understanding of the importance of the role of ecosystems in guaranteeing wellbeing conditions for human communities and for the environment.

In order to perform a complete assessment of ES, the use of maps only providing information on processes occurring at/above the surface (e.g land use maps, or ecosystem maps, or ecosystem function maps) does not allow a complete analysis of underground dynamics.

In the case of the valuation of the ES “nutrient retention” provided by RBZ it is necessary to include the hydrogeological model and its links with the riverine network. The simple presence/absence of vegetation, or even a detailed vegetation classification map, cannot provide a complete description of all conditions required to fully assess this ES, as information on groundwater flow, sediments and soils characteristics is needed.

For the Riparian Vegetation Management Plan of the Gesso Stura Riverine Park a specific index to study vegetation contribution to nutrient retention was introduced: the VEgetation NUtrient-retention Service (VENUS) Index. This index provides land managers a semi-quantitative indication on the spatial distribution of “best conditions” (suitability map) for nutrient retention by vegetation; it is based on a simple set of parameters representing the main factors controlling the interactions between vegetation roots and water flows (runoff and groundwater) from surrounding areas towards water bodies.

The application of the VENUS Index improved the definition of management measures for riparian vegetation and allowed reaching the following results: (1) identification of homogeneous sectors in terms of RBZ relevance for nutrient retention; (2) assessment of different RBZ management scenarios with regard to nutrient retention performance; (3) definition of specific management measures to preserve and/or foster the provision of nutrient retention.

How to cite: Murgese, D.: VEgetation NUtrient-retention Service (VENUS) index: an indicator to assess favourable conditions for nutrient retention by vegetation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20080, https://doi.org/10.5194/egusphere-egu2020-20080, 2020.

Stream tracer injection experiments are widely used  to characterize reach-scale transport and reaction in rivers. Results from tracer injection experiments (i.e., concentration vs. time profiles, or breakthrough curves) are often used to estimate reach-averaged processes controlling solute fate. Advances in both tracer technology have greatly improved our ability to infer finer scale processes from the integrated, reach-scale result. However, to better meet the demands of improved tracer technology and the small-scale processes they elucidate, we need a model that incorporates process-based understanding of solute transport and reactivity. In brief, smarter tracers require smarter models. 

A noteworthy example of the disconnect between measurement and modeling capabilities is the resazurin-resorufin (Raz-Rru) tracer system. Raz is a fluorescent chemical that transforms irreversibly to Rru at a rate proportional to the local rate of aerobic metabolic activity. Co-injections of Raz and a conservative tracer provide experimentalists with a “smart tracer” system that is commonly used to estimate aerobic metabolic activity in streams, particularly within the hyporheic zone. At present, aerobic respiration rates are challenging to estimate from breakthrough curves for two reasons. First, multiple reaction pathways are possible beyond the target parent-to-daughter transformation of Raz to Rru. This implies that aerobic respiration rates inferred from breakthrough curve concentrations may be confounded by additional, zone-specific reactions such as the abiotic degradation of Rru after it is created. Second, field campaigns using the Raz-Rru system have demonstrated that aerobic respiration rates vary strongly with depth in the hyporheic zone. Nevertheless, existing reach-scale models assume uniform reaction rates throughout the hyporheic zone for analytical tractability. This assumption biases the rates of metabolic activity inferred from tracer injection experiments in streams where metabolic rates are spatially variable. 

Here, we present recent advances in reach-scale analytical modeling that address both challenges. We generalize a classic mobile-immobile model to account for multiple reaction pathways of Raz (e.g., via aerobic metabolic activity and abiotic decay) and Rru (e.g., via Raz transformation and abiotic decay). We then extend this framework to account for spatial variability in the hyporheic zone, and we validate semi-analytical model solutions against reach-scale simulations for reactive transport. Together, these advances provide a simple way to estimate reactivity of the benthic biolayer – a known hotspot of reach-scale ecosystem respiration – using established methods. The new framework also opens the door for modeling other chemical constituents transformed through reaction cascades in streams. 

How to cite: Roche, K., Drummond, J., Sund, N., Schumer, R., and Dentz, M.: Reach-scale modeling of reaction cascades and spatially-dependent reactions in the hyporheic zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6057, https://doi.org/10.5194/egusphere-egu2020-6057, 2020.

Many of the most important ecosystem services performed by streams occur in the benthic biolayer, the biologically active upper layer of the streambed. Here we develop and test a rigorous modeling framework for the unsteady one-dimensional transport and mixing of a solute in the benthic biolayer of a turbulent stream. Our framework allows for depth-varying diffusivity profiles, accounts for the change in porosity across the sediment-water interface and captures the two-way feedback between evolving solute concentrations in both the overlying water column and interstitial fluids of the sediment bed. We apply this new modeling framework to an extensive set of previously published laboratory data, with the goal of evaluating four diffusivity profiles (constant, exponentially declining, and two hybrid models that account for molecular diffusion and enhanced turbulent mixing in the surficial portion of the bed). The exponentially declining and enhanced mixing profiles are superior and their reference diffusivities scale with a dimensionless measure of stream turbulence and streambed permeability called the Permeability Reynolds Number, Re_K. The dependence on Re_K changes abruptly at Re_K = 1, reflecting different modes of mixing below (dispersion) and above (turbulent diffusion) this threshold value. Because our modeling framework can be applied to open systems, it should inform the prediction and management of pollutant migration through a diverse array of aquatic ecosystems.

How to cite: Gomez-Velez, J. D., Grant, S. B., Ghisalberti, M., Guymer, I., Boano, F., Roche, K., and Harvey, J. W.: A Novel Modeling Framework to Represent Turbulent Mixing in the Benthic Biolayer of Streams , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12183, https://doi.org/10.5194/egusphere-egu2020-12183, 2020.

The interactions between streamwater and hyporheic or riparian porewater are tightly linked to the biogeochemical and ecological processes within fluvial ecosystems. The analyses of hyporheic biogeochemical cycles or hyporheic exchange fluxes often involves manual sampling of porewater or measurement of temperature time series in the streambed. Here, we compare these two techniques with electrical conductivity time series from a cluster of small, hyporheic EC sensors and discuss their implications on the interpretation of groundwater – surface-water interactions in a first-order boreal stream.

Based on repeated measurements and the co-located small, hyporheic EC sensors, we found that even small sampling-rates of sediment porewater alter the hyporheic flow at some locations significantly. However, since porewater samples are necessary for the analysis of hyporheic biogeochemical cycles and water source partitioning, we recommend for future experiments to either co-locate small, continuous sensors with the sampling ports or to conduct experiments quantifying the induced flux.

Calculated 1D fluxes based on profiles of temperature time series are often integrated over many centimetres to a few decimetres, if the sensors spacing is not very small. This might be the reason, why fluxes based on our temperature and EC measurements do not always have the same direction, especially if the fast hyporheic exchange fluxes are very shallow.

We conclude, that using several, co-located techniques together can compensate for the limitations of each technique and reduce the risk of misleading conclusions.

How to cite: Brekenfeld, N., Schneidewind, U., Comer-Warner, S., Schulz, H., Kettridge, N., Blume, T., Laudon, H., Bishop, K., Hannah, D., and Krause, S.: A Comparison of Field Techniques for the Analysis of Groundwater-Surface-Water Interactions: Porewater Sampling and Hyporheic Temperature and EC Time Series, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13426, https://doi.org/10.5194/egusphere-egu2020-13426, 2020.

Hyporheic exchange represents the interactions between surface and subsurface flows occurring at various geophysical scales. Its importance to the riverine ecological health and the fate of contaminants has long been recognized. Traditionally, the behaviors of hyporheic exchange are explained by the emergence of geomorphological features, such as dune-shaped bedforms, that usually introduce significant pressure differences along the channel bed and, therefore, facilitate exchanges by pumping the flow inward and outward the bed. In addition to this advective mechanism, near-bed turbulence has also been identified as another driver of flow exchange through the turbulent diffusive processes. This study, on the other hand, highlights the decisive control of surface waves on the hyporheic exchange at depth-limited flow conditions, especially for those unbroken standing waves commonly encountered in river riffle areas. It is hypothesized that the presence of surface waves will reshape the distribution of near-bed hydrodynamic pressures, thus altering the properties of advective flows along the channel bed. The validity of this hypothesis is carefully examined through the laboratory experiments using Refractive-Index-Matched (RIM) liquid and solid materials and Particle Tracking Velocimetry (PTV) techniques. This experimental setting helps to simultaneously resolve the surface and subsurface flow patterns to a sufficient detail; the hydrodynamic pressure field can then be derived from the obtained flow velocity fields. Further analysis in a Double-Averaged Navier-Stokes framework indicates that, among different contributing factors, pressure gradient is found to be the most dominant driver of interface exchange. The variations of this driving mechanism, interestingly, can be further decomposed into two parts, namely, the surface wave associated (global-scale) and the bed grain associated (local-scale) components, respectively.

How to cite: Shih, W. and Wu, F.-C.: Effects of surface waves on hyporheic exchange over a permeable gravel bed, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4778, https://doi.org/10.5194/egusphere-egu2020-4778, 2020.

Flood hazard and groundwater resource management in karst catchments require a better understanding of the groundwater/surface water interactions. Due to their high infiltration capacity, karst outcrops limit runoff production on hillslopes, but promote in the same time lateral gains to rivers due to fast transfers underground. This topic is a great challenge, as 30% of Europe is cover by such karst areas.

The aim of this communication is to present recent developments on the characterization of karst-river interactions through two main approaches: a spatial approach aiming at localizing karst areas promoting surface flows, and a temporal approach aiming at modelling lateral flows from karst units in rivers.

The first spatial approach is based on the GIS index IDPR (Index of Development and Persistency of River networks, developed by BRGM©), quantifying the hydrological connectivity to the hydrographic network. From the standard version of the IDPR over France (25 m resolution), we have compare IDPR calculations differentiating intermittent and perennial reaches of rivers in order to detect infiltrations zones that contributive temporary to rivers, as many karst units. Results show that the presence of large karst units promote infiltration, but also temporary runoff that illustrate fast groundwater flows to rivers during floods.

The second approach is a modelling framework based on the inverse problem for the diffusive wave model, to simulate lateral flow during floods on a river reach between two stations. Knowing the upstream and downstream hydrographs, the lateral one is simulating, given informations on the hydrological processes involved in the intermediate catchment (losses, gains, or both processes during flood events). A new development has been tested, by adding continuous solute data (electrical conductivity) in order to track the origin of the water during floods. Applying such approach on river reaches crossing karst areas is a new way to quantify river losses, river gains from surface runoff or groundwaters, characterizing localized recharge and aquifer drainage to rivers, respectively.

We propose to illustrate these two approaches through several case studies in France, where a better characterization of groundwater-surface water interactions and flood risk management are critical issues.

How to cite: Charlier, J.-B., Moussa, R., Pinson, S., Narbaïs, D., and Desprats, J.-F.: Coupling geomorphology and hydrological modelling to characterize the spatio-temporal variability of groundwater-surface water interactions in karst rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18661, https://doi.org/10.5194/egusphere-egu2020-18661, 2020.

The importance of hydrological interactions between groundwater and surface waters and the consequential transport of mass and energy across the streambed – water interface has gained significant research attention lately. In this phenomenological study we investigated the transient nature of hyporheic exchange as a response to flood events by performing a stream manipulation experiment in a small boreal stream within the Krycklan catchment, Sweden. The stream flow was manipulated in order to create a flood event and investigate the responding dynamically changing spatial extent of the hyporheic zone. The artificial flood caused an approximately 5-fold increase in stream discharge.

The experimental set-up consisted of both geophysical and hydrological methods, including time-lapse Electrical Resistivity Tomography (ERT) along the thalweg of a 6.3 m long stream section, with a 0.1 m longitudinal spacing of the electrodes. A constant stream water electric conductivity (EC) was obtained throughout the experiment by using a variable rate tracer injection of chloride. Additional measurements of background EC in the streambed sediments as well as streambed topography (from a total station) and subsurface structures (from Ground Penetrating Radar) were used to support the results from the ERT.

With combined experimental and numerical modeling approaches, the hyporheic response to transient hydrologic boundary conditions and small scale streambed heterogeneities were investigated. Results indicated that a quick response of the hyporheic zone to the changing pressure distribution on the streambed was strongly controlled by the shape of the flood hydrograph. Moreover, the response resulted in an alteration of the hyporheic flowpaths, which increased the hyporheic zone depth and contributed to a dynamically-changing residence time distribution within the hyporheic zone. This alteration was further complicated by the local streambed heterogeneities. The observed substantial variabilities in the hyporheic fluxes over the time span of a flood hydrograph and longitudinally over the measured stream section has direct consequences on the biogeochemical and hydro-ecological functioning of the hyporheic zone, which would be inadequately estimated using homogenous, steady-state approaches.

How to cite: Riml, J., Wu, L., Earon, R., Krause, S., and Blume, T.: Identifying flow transience in the hyporheic zone by Electric Resistivity Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16594, https://doi.org/10.5194/egusphere-egu2020-16594, 2020.

The influence of heterogeneity occurring in natural porous media on spreading, dilution and mixing processes has been widely recognized in the literature, while less attention has been dedicated to investigating the influence of highly transient boundary conditions on the transport of contaminants in the subsurface. Mixing in groundwater flows is inefficient, but it can be greatly enhanced by transient conditions, which are commonly encountered in the environment. For example, surface water-goundwater interaction can be significantly affected by rapid fluctuations of the river stage caused by hydropower production (hydropeaking). In this work, we focus on the Adige valley aquifer, where river discharge of the Adige River and of the Noce River varies at multiple temporal scales due to seasonal (snow and glacier melt) and anthropogenic (i.e., hydropeaking) causes. To show how such fluctuations may affect the interaction between surface water and groundwater we present both experimental data of groundwater level, temperature and electrical conductivity and a model based interpretation of the data. Our study shows that the effect of hydropeaking on surface water - goundwater interaction varies for dry and wet years with important consequeces for solute transport.

How to cite: Chiogna, G., Basilio Hazas, M., and Galli, M.: Impact of hydropeaking on groundwater flow and its implication for mixing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2673, https://doi.org/10.5194/egusphere-egu2020-2673, 2020.

Massive production, extensive use and poor disposal practices of organic chemicals from industries and fuel facilities have polluted soil and water resources. Non-aqueous phase liquids (NAPLs), including oils and fuel hydrocarbons are immiscible in water, moving as separate layers based on their density. Due to their high persistence, they are seldom flushed from the system, potentially making it unfit for use later. Along with resource consumption, pollution also poses a threat to dependent aquatic life. NAPLs have the potential to disturb the physiological, behavioral and breeding performance of aquatic organisms, by affecting their oxygen and nutrient uptake from the system. To analyze these effects, this study examined the response of two common European freshwater macroinvertebrates: Gammarus pulex (freshwater shrimp) and Baetis rhodani (mayfly larvae) to controlled exposure of mineral oil (a representative NAPL). The experiments were conducted in the laboratory for a 24-hour period to observe any changes in their locomotion and survivorship at different concentrations of mineral oil. Results indicate that mineral oil has a serious effect on the organism’s movement and survivorship. Gammarus pulex displayed a high tolerance compared to the mayfly (Baetis rhodani). Most mayfly larvae died due to NAPL exposure. The results indicate that both organisms respond rapidly to NAPL contamination, providing the potential for the development of bio-monitoring tools for water quality studies.

Keywords: Aquatic macroinvertebrates, NAPLs, pollution, response

How to cite: Anuradha, A., Wood, P., Das, D., and Yadav, B.: Assessment of the effects of non-aqueous phase liquid (NAPL) pollution on aquatic macroinvertebrates survivorship, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-688, https://doi.org/10.5194/egusphere-egu2020-688, 2020.

India is one of the fastest economic growing and second-largest country by population. More than 75% people are living in rural areas and engage with agricultural activities for livelihood. A significant portion of the revenue comes from agriculture which cause ignorance in follow the guideline to get more yield. The supply of good quality food and drinking water are the necessity for economic and social health welfare of urban and rural population. In this study, we have observed that the groundwater quality is being degrading due to improper implementation of the rules and regulation. Twenty three groundwater sample were analyze for arsenic and trace elements contamination. The arsenic content in groundwater ranging from 10 to 780 µg/L, which is far above the levels for drinking water standards prescribed by World Health Organization (WHO). For identify the provable source of the contamination, four soil sample were analyzed and observed arsenic content ranging from 110 to 190 mg/kg. Rice is the staple food and ultimately cultivating the paddy crop on more over 80% of the agricultural land. The Paddy crop requires a large amount of water, ultimately maintain the waterlogging condition in the agricultural field. This waterlogging condition is providing a long time to get dissolution of the arsenic bearing minerals present in the soil. This study concluded that the traditional practicing of continuous growing paddy crop in the same field leading to groundwater contamination. The crops cycling could be a better option for reducing the contamination at a local scale.

How to cite: Kashyap, C. S. A. and Singh, S.: Source and mobilization mechanism of arsenic contamination in Manipur, Northeast India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1055, https://doi.org/10.5194/egusphere-egu2020-1055, 2020.

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.

How to cite: Kaule, R., Gilfedder, B., and Frei, S.: Nailing the hyporheic zone: Rusting metal rods as a proxy for the depth of the oxic zone in hyporheic sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2302, https://doi.org/10.5194/egusphere-egu2020-2302, 2020.

The effect of hyporheic fluxes on deep groundwater flow field was investigated in a numerical modelling framework over a spectrum of spatial scales ranging from local bed forms to landscape structures in a Swedish boreal catchment. The groundwater modelling was conducted for the whole catchment in which the site-specific landscape morphology and geological heterogeneity were accounted for. Deep groundwater discharge was quantified through conducting particle tracing analysis for 10,000 inert particles (grid of 100 × 100) released from a flat horizontal surface located 500 meter below the minimum topographical elevation. Further, the streambed scale modelling was performed independently by applying an exact spectral solution to the hyporheic fluxes in streambeds based on fluctuations of the streambed topography. Monte Carlo simulations were used in the streambed scale modelling to cover uncertainties in hydrostatic and dynamic head contributions, as well as topographic fluctuations. Through superpositioning of the two model results, we found that the magnitude of deep groundwater vertical velocity at the stream-water interface was generally lower than the hyporheic exchange velocity at the streambed interface. Finally, the deep groundwater particles’ travel time and the fragmentation of groundwater upwelling zones used as the main metrics to evaluate the impact of hyporheic fluxes on deep groundwater flow field. The results showed that the regional groundwater travel time distribution near the streambed surface was influenced by hyporheic fluxes, an impact that was  substantial for the particles with longer travel times. The size of coherent groundwater upwelling zone at the streambed interface was also affected by hyporheic fluxes. Almost half the superimposed cases were found to be more fragmented due to the presence of hyporheic flow field, which shifted the cumulative distribution function for upwelling regions towards smaller areas. This study, highlights the role of hyporheic fluxes in groundwater modelling, which controls the streambed sediment ecosystem as well as fate and transport of contaminations between aquifer and streams.

How to cite: Mojarrad, B. B., Wörman, A., Riml, J., and Xu, S.: The role of hyporheic fluxes in regional groundwater modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3591, https://doi.org/10.5194/egusphere-egu2020-3591, 2020.

Hyporheic exchange is transient in nature, considering the temporal fluctuations in hydrological and/or biogeochemical conditions in surface water and groundwater (SW/GW).  Efforts are needed to further identify the patterns and driving mechanisms of transient hyporheic exchange.  This study combined a reach-scale field survey and numerical modeling analysis to reveal the pattern of transient hyporheic exchange during rainfall events in the Zhongtian River, southeast of China. Field observations revealed hydrodynamic properties and temperature variations in SW/GW, suggesting that the regional groundwater recharged the study reach.  A one-dimensional heat transport solution was built and used to generate the planar and cross-sectional hyporheic flow fields. A two-step numerical modeling procedure, including a hydraulic surface flow model and a groundwater flow model, was then used to simulate the observed flow system. The hyporheic exchange exhibited strong temporal evolution, as indicated by the rainfall event-driven hyporheic exchange, the depth-dependent hysteretic response to rainfall, and the area of local downwelling flow increasing with rainfall. Dynamics of the hyporheic exchange in the study reach, therefore, significantly changed in space and time due to rainfall. The reversal of hydraulic gradient and transient hyporheic exchange were observed and validated using the numerical simulation. Anisotropic hydraulic conductivity is the key to generate transient hyporheic exchange. A revised conceptual model was used to interpret the observed temporal patterns in hyporheic exchange  The pattern of transient hyporheic exchange indicates that transient hyporheic exchange only appears after an increased phase of river stage but does not last for a long time. The temporal pattern of hyporheic exchange can significantly affect the evolution of biogeochemical processes in the hyporheic zone for a gaining stream by, for example, temporally facilitating special biogeochemical processes.

How to cite: Lu, C., Ji, K., Zhang, Y., Fleckenstein, J., Zheng, C., and Salsky, K.: Transient hyporheic exchange during rainfall events in a gaining stream: Field investigation, conceptual model, and numerical interpretation , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4386, https://doi.org/10.5194/egusphere-egu2020-4386, 2020.

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.

How to cite: Gilfedder, B., Wismeth, F., and Frei, S.: Mapping and quantifying groundwater inflow to the Spree River (Lusatia) and its role in Fe fluxes, precipitation and coating of the river bed. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4975, https://doi.org/10.5194/egusphere-egu2020-4975, 2020.

Near-stream groundwater table dynamics, subsurface flow pathways, and streamflow dynamic control near-stream groundwater-stream water exchange. These processes are critical for evaluating the extension of the hyporheic zone and water travel time in the stream corridor. This in turn, influences riverine pollutant transport, biotic activity, and nutrient cycling, making near-stream water exchange crucial when addressing the ecological status of a riverine environment.

However, near-stream groundwater hydrology has rarely been investigated in the past as shallow groundwater studies often focus on subsurface water dynamics along hillslopes. As a result, detailed understanding of near-stream groundwater table oscillations, fluxes, and their changes over time across a various spectrum of rainfall and streamflow conditions in the stream corridor is currently missing. This study aims at the following questions:

To answer these questions, we set-up a monitoring network with 36 near-stream wells and 7 in-stream piezometers along a headwater stream section of 55 m in Luxembourg. Based on the recorded data we calculated the changes of the groundwater table and groundwater flux direction. Subsurface structure was evaluated with an ERT survey. We related the GW level dynamics to stream discharge and rainfall event characteristics.

Results reveal that the changes of near-stream groundwater table and groundwater flux direction across different hydrological conditions display three modes of water exchange between near-stream groundwater and the stream channel. During dry conditions, the groundwater table lays in the weathered bedrock up to 0.8 m below the stream channel. During these conditions, near-stream groundwater fluxes direction are strongly affected by sporadic rainfall events and shift between gaining-stream conditions (pre-event) to losing-stream conditions (post-event). Once the system wets-up, the groundwater table rises in more conductive subsurface layers. This leads to near-stream groundwater fluxes to be mostly parallel to the stream channel, with stream sections displaying variable direction of exchange with the groundwater, passing from losing- to gaining-conditions and vice-versa. During wet conditions, near-stream groundwater level rises above the stream channel, which shows persistent streamflow during this period. The groundwater table points constantly toward the stream channel, now displaying continuous gaining-conditions for the stream.

How to cite: Bonanno, E., Blöschl, G., and Klaus, J.: Groundwater dynamics in a near-stream domain: variability of flow directions and subsurface connectivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5039, https://doi.org/10.5194/egusphere-egu2020-5039, 2020.

The hyporheic zone (HZ), the region beneath or alongside a streambed where active groundwater and surface water mix, plays a vital role in the stream ecosystem. Reactions in the HZ such as denitrification and nitrification have been examined in previous studies. However, those numerical models are lack of consideration for the reaction zones for aerobic and anaerobic changes due to the reactions consuming the dissolved oxygen (DO) in hyporheic flux. In order to simulate nitrogen concentration changes in the HZ more accurately, this study proposes a method of evaluating the nitrogen removal rate in the HZ through numerical modeling. Firstly, a basic two-dimensional numerical model following previous simulation models in the HZ, which only couple flow conditions with biochemical reactions, is proposed to consider both nitrification and denitrification, but ignoring the changes generated by the reactions. Next, the zones for different reactions are determined in an improved model under the assumption that related environmental variables (i.e., the DO) will be considered to delineate the boundary between nitrification and denitrification zones and to identify a transition zone where either reaction might take place. The changes of reaction zones through the whole process can be controlled by the characteristic variable of hyporheic flux, and this variable can be selected differently for different reaction processes. In this study, the characteristic variable is determined as median residence time. Using this model, the accuracy of the nitrogen simulation in the HZ can be improved. To overcome the shortcoming that more information about biochemical reactions in the HZ is required to use the improved model, a new model that couples the basic model and genetic programming (GP) is proposed to optimize the simulation results of the basic model and allow for real-time forecasting. The results show that the improved model performs better than the basic model, but the model coupling the basic model with GP performs best. In addition, the function of the HZ in nitrogen removal is examined through a case study of four scenarios, leading to the conclusion that the HZ has a higher nitrogen removal rate when water quality is neither too poor nor too good. Therefore, even though the HZ facilitates nitrogen removal, sewage should still be treated to a certain level before being discharged into rivers. Overall, this study enhances our understanding of the HZ, and can benefit the restoration and management of HZs and streams in the face of the continual degradation caused by human activity.

How to cite: Liu, S., Chui, T. F. M., and Kuang, X.: Numerical modelling for evaluation of the nitrogen removal rate in hyporheic zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6365, https://doi.org/10.5194/egusphere-egu2020-6365, 2020.

As a major component of the hydrological cycle, submarine groundwater discharge (SGD) has been widely recognized as a significant source of water and an important pathway for dissolved material transport from land to ocean. Natural radium isotopes are recognized as ideal tracers for effective and efficient assessment of SGD in local scales and global scales since they are conservative chemically and widely enriched in SGD. Here we report the estimates of coastal mixing rates and SGD in Guangdong-HongKong-Macau Greater Bay Area, China using radium isotopes. The distributions of short-lived ²²³Ra, ²²⁴Ra and long-lived ²²⁸Ra in seawater and coastal groundwater were investigated. Based on the horizontal distribution of short-lived Ra and a mixing model, the horizontal eddy diffusion coefficient in the region was estimated to be 230-1085 m²/s. The offshore fluxes of ²²⁸Ra can be derived from their across-shelf activity gradients and the eddy horizontal diffusion coefficient. Such ²²⁸Ra fluxes require a substantial volume of groundwater discharge to balance Ra removal, and thus SGD can be estimated via radium mass balance model.

How to cite: Wang, X., Li, H., and Zheng, C.: Estimating coastal mixing rates and submarine groundwater discharge (SGD) in Guangdong-HongKong-Macau Greater Bay Area, China using radium isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6580, https://doi.org/10.5194/egusphere-egu2020-6580, 2020.

With an area of about 77,000 km², the Bohai Sea includes three bays: Laizhou Bay, Bohai Bay and Liaodong Bay. In this study, ²²⁸Ra, δD, δ¹⁸O and salinity data were collected from surface seawater in the entire Bohai Sea, river water, and groundwater along its coastline in August 2017. Based on the spatial distributions of δD, δ¹⁸O, and salinity in surface water in the entire Bohai Sea and δD-salinity relations, the marine hydrological processes were investigated and the members of river water and groundwater for δD and δ¹⁸O were determined. The steady-state mass-balance models of δD, δ¹⁸O and salinity are given and used to estimate submarine fresh groundwater discharges and the flushing times of the entire Bohai Sea and its three bays. Based on the results of the flushing times, the steady-state mass-balance model of ²²⁸Ra is used to estimate submarine groundwater discharges of the entire Bohai Sea and its three bays.

How to cite: Li, H., Zhang, X., Wang, X., Xiao, K., Zhang, Y., and Jiao, J. J.: Estimating submarine groundwater discharges of the Bohai Sea, China using radium, hydrogen and oxygen isotopes and salinity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6644, https://doi.org/10.5194/egusphere-egu2020-6644, 2020.

Distributed temperature sensing (DTS) is improving rapidly and provide opportunity for high spatial and temporal resolution. It has emerged as a unique and powerful tool for ecological application. The first-order stream of Chichiawan Creek in Taiwan is the crucial habitat for the endangered species of Formosan land-locked salmon, but the stream fragmentation, no surface streamflow, seriously reduced the salmon population, hampering the rehabilitation work. The utility of combining DTS, long-term water level data, temperature profiles, and electrical resistivity tomography (ERT) were demonstrated to comprehensive understand the exchange process beneath the first-order stream. Stream heat budget modeling with HFLUX has been developed through the field measurements along with local meteorological data and unmanned aerial vehicle (UAV) image to simulate the stream temperature and evaluate the exchange rate of energy gain and loss. The results show that the spatial and temporal variations of cold water inflows has been observed and the groundwater and hyporheic inflow have been differentiated using the statistical method. The significance of groundwater and hyporheic inflow can contribute certain amount of water as the ecological base flow at the downstream and reduce the water temperature during the summer time. If the river restoration is conducted to prevent the stream fragmentation, the model results indicate that the amplitude of daily stream temperatures can be reduced. Less stream surface and steeper stream slope are also projected to decrease stream temperatures. The quantitative evaluation method demonstrated here, based on extensive measurements and numerical models, is able to predict the precise level of impact of river restoration on the key environmental objective before actually conducting the efforts.

How to cite: Chiu, Y.-C., Lee, T.-Y., Hsu, S.-Y., Pan, T.-X., and Huang, P.-S.: Coupling Temperature Measurement and Stream Heat Budget Model to Evaluate Impact of River Restoration on the First-order Alpine Stream in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7754, https://doi.org/10.5194/egusphere-egu2020-7754, 2020.

Until recently lake eutrophication was generally associated with phosphorus loads of surface inflows, surface runoff, direct sewage discharge, atmospheric deposition, bathers, waterfowls or other well visible nutrient sources. On the contrary, phosphorus (P) imports by groundwater were systematically disregarded because P was considered immobile in aquifers. This applies also to Lake Arendsee where we started our research about the relevance of lacustrine groundwater discharge (LGD) a decade ago. Lake Arendsee is a 50 m deep, monomictic lake in north-eastern Germany; the lake suffers intense eutrophication problems. We quantified P loads of groundwater to Lake Arendsee using volume fluxes of LGD and near-shore measurements of groundwater P concentrations at a high spatial resolution. Results have shown that LGD accounts for about 50% of the overall external P load, thus fueling the eutrophication of the lake. We assume that both, ancient sewage pits and leaking sewer pipes are major sources of the extremely high P concentrations in the aquifer (above 4000 µg/L). Now, after the renewal of many sewer pipes we present a long-term data set (last decade) of P concentrations close to the groundwater-lake interface. Groundwater is still the major source of the high P concentrations of 180 µg/L in the lake.

How to cite: Lewandowski, J., Meinikmann, K., and Hupfer, M.: Groundwater-borne phosphorus import to eutrophic Lake Arendsee (Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8060, https://doi.org/10.5194/egusphere-egu2020-8060, 2020.

Gotjawal is forest that was formed by the lava flow along the slopes as a result of the volcanic activity of the parasite volcanoes which distributed over Jeju Island, the Republic of Korea. Since Gotjawal has very shallow soil layer and wetland in Gotjawal does not have water for a long time, infiltration characteristics are very important. Study wetland is 48m in diameter and 2.2m in depth and water level change with respect to time was measured in 2019. The results showed that annual rainfall in this area was measured to 2,748mm and 5 rainfall events with daily rainfall more than 100mm were recorded in 2019. When the wetland was full of water, the period of drainage was 12 days. Water level started to change when the rainfall was 43~70mm taking 10~15 hours and this seems to vary depending on the period of no rainfall. This result will be helpful for estimating groundwater sources in lava flows area and for developing conservation plan of Gotjawal.

How to cite: Kim, J., Choi, H., Song, K., Kim, Y., Choi, B., and Seo, Y.: Study on water infiltration characteristics in Gotjawal (forest on lava flow) wetland area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8947, https://doi.org/10.5194/egusphere-egu2020-8947, 2020.

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.

How to cite: Hiller, C., Kaule, R., Gilfedder, B., and Peiffer, S.: Effect of drought on deposition of ferric hydroxides in the HZ of a small upland stream in Northern Bavaria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10005, https://doi.org/10.5194/egusphere-egu2020-10005, 2020.

Exchange of water and nutrients between a river and the surrounding hyporheic zone is controlled by multiple factors, including river morphology, streamflow variability, connection with groundwater, and sediment properties. Among these factors, the heterogeneity of river sediments is known to strongly affect the fate of nutrients exchanged with the hyporheic zone, but this influence has received relatively little attention compared to other factors. Moreover, sediments are heterogeneous in terms of both physical properties (i.e., hydraulic conductivity) and chemical composition (e.g., organic carbon content), but studies about heterogeneity have mostly focused on variations of hydraulic conductivity compared to the variations of chemical properties of sediments.

This contribution presents a modeling analysis of the influence of physical and chemical heterogeneity of alluvial sediments on lateral hyporheic exchange in meandering rivers. Sediments are treated as a binary mixture of mineral sand and organic silt, and a coupled hydraulic and biogeochemical model is employed to simulate the effect of different silt/sand ratios on exchange and reaction of organic carbon, oxygen, nitrate, and ammonium. Model results show that sediments with higher content of silt are characterized by lower exchange fluxes, but their higher carbon availability fosters higher rates of biochemical reactions and hence leads to higher nitrogen removal by net denitrification. These results indicate the importance of improving the description of sediment heterogeneity in modeling studies of hyporheic exchange.

How to cite: Boano, F., Pescimoro, E., Sawyer, A., and Soltanian, M. R.: Modeling effects of physical and chemical heterogeneity of alluvial sediments on hyporheic exchange, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10086, https://doi.org/10.5194/egusphere-egu2020-10086, 2020.

Groundwater surface water interactions can greatly impact the ecohydrology on a wide range of spatial scales, ranging from biogeochemical reactions under local bedforms to alteration of regional groundwater discharge patterns. Hyporheic exchange fluxes (HEF) are controlled by the streambed geology and driven by hydraulic head fluctuations at the stream bottom, consisting of a static and a dynamic part. Currently, few studies have investigated the relative importance of these two drivers of HEF in the field, which hinder a holistic understanding of the governing processes and may affect predictions of hyporheic exchange intensities.

This study is based on an extensive field survey of 9 stream reaches located in small, pristine streams in Sweden, with varying hydromorphological characteristics such as slope, bottom material, morphological complexity and stream discharge. The field survey included distributed measurements of the hydraulic head and the hydraulic conductivity along the streambed, as well as tracer tests with Rhodamine WT. The overall aim of the study was to evaluate the relative importance of HEF driven by dynamic and static head fluctuations in streams by usage of a spectral model that decomposes the observed hydraulic head fluctuations on distinctive spatial scales. As a validation, the advective storage path (ASP) transport model was calibrated against the conducted in-stream tracer tests and its parameters compared to the equivalent gained from the spectral model.

The results showed that the average exchange velocity evaluated by the two models were comparable in most observed cases, validating the usage of the spectral model in small alluvial streams with high slope, and low discharge and stream depth. However, a sensitivity analysis of the two models revealed some degree of equifinality for some of the independent model parameters. Detailed results from the spectral model indicated that the static head was dominating the HEF in all reaches, both on average and when distributed over separate spatial scales. Uncertainty in the results was found, predominantly effecting calculations of the dynamic HEF and connected to (1) the approximation of streambed topography at spatial scales <0.5 m, where the dynamic exchange is assumed to dominate and (2) the use of Fehlman’s constant for estimating the hydrodynamic exchange under complex streambed topographies. Despite those uncertainties, the spectral model approach gives a deeper understanding of the phenomena of HEF by incorporating its multiscale nature and illustrating the fact that static and dynamic drivers might be equally important, only acting on different scales.

How to cite: Morén, I., Riml, J., and Wörman, A.: The importance of static and dynamic head drivers for hyporheic exchange: evaluation of a spectral modelling approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11455, https://doi.org/10.5194/egusphere-egu2020-11455, 2020.

Dynamic coastal forces, such as waves and tides which are typically coexisting in coastal environments, impact groundwater flow and salt transport in the intertidal zone. In this study, firstly, an iterative least-squares fitting method for tidal level and wave height was introduced, and the wave height can be acquired from measured sea level and then further verified by wind speed. Groundwater flow and salt transport were then simulated using a code called MARUN under different seaward boundary conditions with and without wave effects. Comparison of measured and simulated water level and salinity indicates that the model which included wave setup can accurately reproduce the measured data in the observation wells. Simulation results show that water and salt fluxes across the aquifer-ocean interface are increased and the groundwater circulation in the intertidal zone is more active after considering wave setup. Most of the influx occurs in the intertidal zone, while a considerable amount of efflux occurs in the subtidal zone, and the maximum influx of water and salt moves toward the high tide line compared to the model results without wave setup. The water influx and efflux rates increase greatly especially during the period of high wave height. After wave effects considered, fresh submarine groundwater discharge only takes up a small proportion of submarine groundwater discharge, which is dominated by recirculated seawater. It is concluded that the presence of waves significantly increases the amount of seawater circulation.

How to cite: Yu, S., Wang, C., Luo, X., Jiao, J. J., and Li, H.: Wave effect from sea level dynamics on density-dependent groundwater flow at a sandy beach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12454, https://doi.org/10.5194/egusphere-egu2020-12454, 2020.

SGD can significantly alter salinity and temperature of coastal waters and also deliver abundant nutrients and anthropogenic substances, such as pathogens, toxins and other pollutants. Contaminated groundwater has been proven to be the sources of fecal indicator bacteria (FIB), which is the sum of total coliform, fecal coliform (i.e., E.coli) and Enterococcus. FIB is highly influential to the public health and fishery management. This study sought to investigate the effects of SGD on the E.coli concentrations in a representative surf zone. A total of 82 nearshore coastal water and 78 coastal groundwater samples were monthly collected from July, 2019 to January, 2020 for radon (Rn), E.coli and nutrients analyses, together with the measurements of concurrent water temperature and salinity. A good positive correlation between the coastal water Rn activity and E.coli concentrations was found with a coefficient of determination (R²) of 0.56, indicating that the SGD mounts a positive effect on the E.coli over the study period including wet and dry seasons. For each sampling period, the slope (K) of each regression model represents the increasing rate of E.coli per unit of Rn activity. The averaged E.coli concentrations (11.5 – 65.5 cfu/100 ml) for each month were inversely correlated to the averaged coastal water salinity (29.8 – 34.0 PSU) with a R² of 0.66; and the inhibited temperature of E.coli was observed to be 30 ℃. For groundwater, a stronger positive linear correlation between the Rn activity and E.coli concentrations was reported with a R² of 0.87. As the temperature and salinity of groundwater were almost constant, the K was not much changed. Also, compared to the coastal seawater, groundwater showed a better correlation as the temperature and salinity in groundwater were relatively constant. On the other hand, the regression model of Rn activity and E.coli in groundwater showed that the E.coli was not changed significantly when the Rn activity reached to near 1400 dpm L^-1, which means high SGD inputs may not always lead to lasting growth of E.coli.

How to cite: Cheng, K., Luo, X., Jiao, J. J., and Yu, S.: The effects of submarine groundwater discharge on fecal indicator bacteria in a fish culture zone in Tolo Harbour, Hong Kong, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12612, https://doi.org/10.5194/egusphere-egu2020-12612, 2020.

Submarine groundwater discharge (SGD) can be a significant terrestrial input of nutrients to the coastal ocean. The mixing between nearshore groundwater and seawater in coastal aquifers modifies the chemical composition of the water prior to discharge. Agricultural, aquaculture and leaky urban sewers may elevate the land-derived contaminates in the near-shore areas. The nutrient structure of Daya Bay has been strongly changed with the economic and urban development. In this study, the spatial distribution of nutrients (e.g. NO₃^-, NH₄⁺, PO₄^3-, SO₄^2-, S^2-), groundwater salinity and level were systematically investigated along an intertidal beach transect. Two-dimensional variable density and saturation, and nutrient reactive transport simulations were developed using the finite-element model MARUN. Tidal and meteorological data were also collected from local weather station to correct the model boundary.  Besides, surface air evaporation and precipitation were considered in this model to better match the field observations. The results showed that the distribution pattern of nutrients both of field observation and simulation was similar to that of salinity. For example, the concentration of NO₃^--N from the landward side towards the seaward side of the beach decreased and then increased, presenting an upper NO₃^--N plume, lower location NO₃^--N saltwater wedge and NO₃^--N discharge tube. It can approximately correspond to the upper salt plume, classical salt wedge and freshwater discharge tube. In stead of using flow velocity or simple attenuation constant to calculate the nutrient fluxes, this study used complex coupled solute reaction transport to improve the computational accuracy.

How to cite: Hu, W., Li, H., and Xiao, K.: Reactive Transport Simulations of Groundwater-derived Nutrients in a Sandy Beach in Daya Bay, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13212, https://doi.org/10.5194/egusphere-egu2020-13212, 2020.

The hyporheic zone (HZ) is the region of saturated sediment surrounding a stream which connects surface water and groundwater flow. The overlying water with dissolved matters infiltrates into the HZ, stays there for some time and interacts with groundwater, and exfiltrates out of the HZ, resulting in hyporheic exchanges (HEs). The HEs support physicochemical and biological reactions that are essential to river ecosystem functions. In recent decades, more and more stream restoration projects involve the recovery of HE, however, effective guidance in restoring HE is still missing. Therefore, this study aims to examine the effectiveness of different engineering baffle designs in restoring HZ in a straight channel with floodplain. Both flume and numerical models coupling stream and groundwater flow were built. The flume model was built in a recirculating box to simulate different hydrological conditions (e.g., streamflow and groundwater flow) and baffle designs (e.g., baffle amplitude, interval). Tracer experiments were performed, and results were used to quantify the impacts of baffles designs on the HE fluxes. For the numerical models, the surface flow was simulated by solving Reynold-average Navier-Stokes (RANS) equations in two phases using volume of fluid method (VoF) in Fluent, while the groundwater flow was simulated by solving Richard’s equation in COMSOL. The numerical models were calibrated with experiments, and could output the flux, scale and median residence time (MRT) of the HE. For fixed baffle interval of four times the stream width, the flux and scale of HE peaked at baffle amplitude of around one third of stream width, while the MRT increased with increasing amplitude. For fixed baffle amplitude of one third of the stream width, the flux of HE peaked at baffle interval of around four times the stream width, the scale of HE was positively correlated to interval while the MTR had the lowest value at the interval of around two times the stream width. The results of this study directly benefit the development of practical baffle designs of river restoration.  The coupled models developed are also generally applicable to investigate the efficiency of different stream rehabilitation designs in restoring HZ.

How to cite: Huang, P. and Chui, T. F. M.: Impacts of engineering baffles on the hyporheic exchange in a straight channel with floodplain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13224, https://doi.org/10.5194/egusphere-egu2020-13224, 2020.

Studies have indicated that the streambed with the clogging layer affects the interaction between surface water and groundwater. When the streambed covered by a clogging layer, the decrease of the groundwater table can transform the state of stream-groundwater from the connection into disconnection. When the stream-groundwater interaction reaches the state of disconnection, the infiltration rate is independent with the groundwater level beneath the streambed. In this study, we show the effects of the topography of the streambed and clogging layer on the infiltration and groundwater flow patterns beneath the streambed by numerical simulations. The results show that the clogging layer and the change of topography of streambed affect the development of the unsaturated zone, flow path, and residence time beneath the streambed.

How to cite: Lo, J.-H., Lin, H.-Y., Chiu, Y.-C., Lee, T.-Y., Tsai, Y.-Z., Lee, J.-D., and Hsu, S.-Y.: The influence of topography of stream bed with the clogging layer on the stream-groundwater interaction and the groundwater flow patterns beneath stream bed, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14976, https://doi.org/10.5194/egusphere-egu2020-14976, 2020.

Inverse modeling approaches based on tracer data are often used to characterize transport processes in streams and rivers. This generally involves the calibration of a one-dimensional transport model using concentrations measured in the surface water at one or multiple locations along a stream reach. A major concern is whether the calibrated model parameters are representative of the physical transport processes occurring in the water column and the underlying sediment bed. This study looks at the identifiability of the parameters of a physically based one-dimensional stream transport model that represents hyporheic exchange as a vertically attenuated mixing process in accordance with recent experimental evidence. It is shown that, if the average flow velocity and hydraulic radius are not predetermined, there are infinite sets of parameter values that generate the same space-time concentration distributions in the water column. The result implies that in-stream transport and hyporheic exchange parameters cannot be determined from sole measurements of solute breakthrough curves in the surface water unless stream discharge and average cross-sectional geometry can be independently estimated.

How to cite: Bottacin-Busolin, A.: Are in-stream transport and hyporheic exchange parameters identifiable from tracer test data?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17523, https://doi.org/10.5194/egusphere-egu2020-17523, 2020.

A peatland from subarctic Canada (Handle Lake 62°29’26.44”N, 114°23’18.23”W) is a degrading permafrost peatland chosen for detailed study due to a legacy of regional arsenic (As) contamination as a result of almost 8 decades of gold mining. The fate of permafrost peatlands and their element stores is unknown due to complex feedbacks between peat accumulation, hydrology, and vegetation that affect redox state and element mobility. We combine palynology with study of plant macrofossils, testate amoebae, organic matter composition, and bulk geochemistry preserved in a ca. 4180-4972 cal year old peat monolith retrieved from the Handle Lake peatland to reconstruct the ecohydrological dynamics to assess future trajectories of permafrost peat, and contaminant storage or release, in response to current and future warming. Sphagnum riparium macrofossils are rare in modern peat habitats and sub-fossils are rare in paleoecological records. Plant macrofossils of this taxon occur in an 11-cm thick layer together with Sphagnum angustifolium between 43 cm (ca.  3390-3239 cal BP) and 25 cm depth (ca. 2755-2378 cal BP) in the monolith. The S. riparium sub-fossils are present with the hydrophilous testate amoebae species Archerella flavum, Hyalosphenia papilio and Difflugia globulosa that are used to quantitatively reconstruct a water table depth of 0-4 cm below the peat surface. Sub-fossils of S. riparium disappear at ca. 2755-2378 cal BP, likely due to an autogenic trophic shift and succession towards more acidophilic conditions favourable to species such as Sphagnum fuscum and Sphagnum russowii. We interpret the occurrence of S. riparium as an indicator of wet and minerotrophic conditions linked to peatland development form rich fen to oligotrophic bog.  Because S. riparium is a key pioneer species of disturbed peatlands that have experienced permafrost degradation it will likely be favoured in northern regions experiencing rapid climate warming. In the palynological record the proportion of Sphagnum-type A spores increases (up to 80%) between ca.  3390-3239 cal BP and ca. 2755-2378 cal BP concurrent with a decline in other Sphagnum-type spores. A peak in micro- and macroscopic charcoal occurs between ca. 3557-3286 cal BP and ca. 3275-2771 cal BP, concurrent with a decline in Picea pollen and an increase in Alnus pollen. Regionally, between ca. 3500 and ca. 2500 cal BP Neoglacial climate prevailed with post-Neoglacial warming at ca. 2500 cal BP. It is therefore possible that regional fire occurrence stimulated permafrost degradation at ca. 3500 cal BP. Background As in the active layer monotlith is ~20-30 ppm. The upper 10 cm of the peat are impacted by aerial deposition of As from ore processing and concentrations range up to ~360 ppm. An increase in the concentration of As in the monolith from ~15-20 ppm at the base of the monolith to ~30-40 ppm during this interval may reflect water table depth dynamics that affected the mobility and fate of this redox sensitive element and/or downward mobility from layers impacted by contamination from mineral processing. Degradation of this permafrost within the Handle Lake peatland will release the currently stored As and other contaminants to the regional environment.

How to cite: Galloway, J., Gałka, M., Swindles, G., Amesbury, M., Wolfe, S., Morse, P., Patterson, T., and Falck, H.: Ecohydrological dynamics of a degrading subarctic peatland: Implications for Arsenic mobility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19666, https://doi.org/10.5194/egusphere-egu2020-19666, 2020.

Distribution of the hyporheic streamlines and residence time (HRT) is a crucial factor under streambed to understand the transport phenomena of environmental toxins, sediment metabolic rates in fluvial ecology as well as hydrological water budget. To quantify HRT, both the laboratory and numerical approach could serve as discerning tools. However, due to high heterogeneity in natural streambed sediment and topography, an efficient numerical model setup can prove to be pragmatic in comparison to tedious laboratory experiments for tracing streamlines. Moreover, repeatability of results, high amount of variation in the laboratory flumebed setup, greater insight into the 3D flow system and investigation possibilities with regard to individual streamlines or particular areas of HRT distribution cannot be well executed in laboratory. On the other hand, an automated generation of hyporheic streamlines with a range of various flumebed setups could propel a better understanding of the process and behavior of hyporheic streamlines and HRT distribution. Therefore, a robust numerical method could bestow to trace a large number of particles from various seeding locations at the flumebed. All of these facts enforce the necessity of numerical modeling of flume experiments to perceive the hyporheic exchange mechanisms at fieldwork and research, which are difficult to segregate under natural in-stream conditions. Keeping these issues in mind, we developed an automated numerical  method for quantifying the hyporheic exchange, where the surface water modeling software, HEC-RAS 5.0.5 and the subsurface flow and reactive transport code, MIN3P are coupled. A channel segment with a longitudinal dimension of 1 m and water surface elevation of 0.02 m is used for generating the hydraulic head distribution over the flumebed. A groundwater model domain of the dimensions of x:y:z = 1m:0.1m:0.1m is considered for the investigation of hyporheic exchange. A simple code for computing streamlines based on 4th order Runge-Kutta technique with the adaptive time integration method is developed using Matlab. Sensitivity analysis of streamline distribution and HRT to small scale changes (e.g. changes in dimension, distribution, and shape of the flumebed material) was performed, assuming a sand-gravel material mix. Various geometric shapes of gravel pieces (e.g. triangle, rectangle, trapezoid, and sphere) were used to vary the elevation of flumebed on a 1 mm scale. The results of the automated process show that the size, shape and distribution of trapezoidal gravel and sand portion in the streambed have a significant impact over hyporheic streamlines and HRT. High number and length of streamlines thus high HRT are found in case of the higher length of ridges created by the elevated portion of gravel pieces. In case of the increase of the length of gravel pieces along the longitudinal direction of flumebed, the length of streamlines and HRT decrease whereas the number of streamlines increase. Small scale hyporheic exchanges are found in case of increasing length of gravel pieces. Similar outcomes are also found for triangular and spherical gravel pieces. Both the number and length of streamlines are significantly reduced in case of the high number of gravel and sand portion on the streambed.

How to cite: Mehedi, M. A. A., Reichert, N., and Molkenthin, F.: Sensitivity Analysis of Hyporheic Exchange to Small Scale Changes In Gravel-Sand Flumebed Using A Coupled Groundwater-Surface Water Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20319, https://doi.org/10.5194/egusphere-egu2020-20319, 2020.

The 20.000 km² swamp of the Delta is organized into islands, flood plains and permanent and seasonal channels. Most of these islands display a surprising vegetation distribution composed of tree rings surrounding the islands and limiting an inner domain with scarce vegetation. Whereas the hydrology of the Okanvango wetlands is governed by a series of drivers such as, sedimentation, climate, tectonic and biological processes, the potential of the use of multi-chemical tracing has not been so far much investigated. The conducted study as part of a multidisciplinary project dedicated to the understanding of the functioning of the Delta, involved water samples collected both upstream and downsream the river, close to one of these islands and also recovered within the island as well. The main objective of this geochemical investigation was to better constrain the interactions prevailing in between these islands and the water chemical record. pH, conductivity (C), dissolved organic and inorganic carbon (DOC & DIC) concentrations were measured as well as those of major anion and cation and trace cation concentrations as well. Whichever the tracers are considered, two contrasted groups of samples were evidenced depending on their sampling positioning regarding the island. The samples recovered only within the island displayed pH around and over 9 and higher conductivities, whereas the other showed lower circumneutral pH values and conductivities as well. The high conductivities of the water samples fom the island also correspond to the highest DOC and DIC concentrations. The strong relationship linking the high DIC values and the high pH in the island samples records probably alkaline CO₃^2- et HCO₃^- -rich waters resulting from water-rock interactions with carbonates. The marked DOC enrichment has mostly to be related to microbial or photo-degradation of plant-derived organic matter and/or hydrological condition variations promoting DOC release. Significant, Cl^-, SO₄^2-, NO₂^-enrichments as well as major cation ones were also evidenced in the same group of samples within the island. However, the most surprising results are sourced in the trace element fingerprinting. This latter includes huge enrichment in heavy, critical metals and metalloids as well (e.g. Cr, Pb, V, REE, U, Th or As). Beyond the only marked REE-spike, Upper Continental Crust-normalized REE patterns displayed markedly contrasted shapes exhibiting two types of waters with circumneutral pH ones with MREE-enrichment, whereas the alkaline waters evidenced a classical continuous enrichment throughout the whole series from LREE to HREE and a positive Ce anomaly. The use of such multi-tracing allowed an efficient fingerprinting of two distint types of waters to get clues to further constrain both the dynamics of such islands and the functioning of the water system. Still in progress, the study will be completed by (i) the stable isotope analysis, (ii) the modeling of the minerals possibly at equilibrium with the waters and of the organic matter-trace element interactions, (iii) the speciation analysis of some enriched elements, (iv) the comparison between water and solid samples analyses and (v) the understanding of the relations in between the concentrations and locations in the hydrological system.

How to cite: Dia, A., Dauteuil, O., Jolivet, M., Davranche, M., Bouhnic-le-Coz, M., Marsac, R., Pierson-Wickmann, A.-C., Petitjean, P., and Murray-Hudson, M.: Multi-chemical fingerprinting of contrasted waters flowing within the unconventional Okavango Delta: evidence of an original ‘island reactor’, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20352, https://doi.org/10.5194/egusphere-egu2020-20352, 2020.