Human activities negatively impact water quality by supplying excessive nutrients to streams. To investigate the capacity of streams to take up nutrients from the water column, we usually add nutrients to stream reaches, calculate the fraction of added nutrients that is taken up, and identify the environmental conditions controlling nutrient uptake. A common idea is that nutrient uptake increases with increasing water residence time because of increased contact time between solutes and organisms. Yet, water residence time only partially explains the temporal and spatial variability of nutrient uptake, and the reasons behind this variability are still not well understood. In this talk I’ll present a study which shows that good characterization of spatial heterogeneity of surface-subsurface flow paths and bioactive hot spots within streams is essential to understanding the mechanisms of in-stream nutrient uptake. The basis of this study arises from the use and interpretation of nutrient uptake results from the Tracer Additions for Spiraling Curve Characterization (TASCC) method. This model has been rapidly adopted to interpret in-stream nutrient spiraling metrics (e.g, nutrient uptake) over a range of concentrations from breakthrough curves (BTCs) obtained during pulse solute injection experiments. TASCC analyses often identify hysteresis in the relationship between spiraling metrics and concentration as nutrient concentration in BTCs rises and falls. The mechanisms behind these hysteresis patterns have yet to be determined. We hypothesized that difference in the time a solute is exposed to bioactive environments (i.e., biophysical opportunity) between the rising and falling limbs of BTCs causes hysteresis in TASCCs. We tested this hypothesis using nitrate empirical data from a solute addition combined with a process-based particle-tracking model representing travel times and transformations along each flow path in the water column and hyporheic zone, from which the bioactive zone comprised only a thin superficial layer. In-stream nitrate uptake was controlled by hyporheic exchange and the cumulative time nitrate spend in the bioactive layer. This bioactive residence time generally increased from the rising to the falling limb of the BTC, systematically generating hysteresis in the TASCC curves. Hysteresis decreased when nutrient uptake primarily occurred in the water column compared to the hyporheic zone, and with increasing the distance between the injection and sampling points. Hysteresis increased with the depth of the hyporheic bioactive layer. Our results indicate that the organisms responsible for nutrient uptake are confined within a thin layer in the stream sediments and that the bioactive residence time at the surface-subsurface water interface is important for nutrient uptake. I will end the talk illustrating how these findings can have important implications for in-stream nutrient uptake within the context of restoration practices addressed to modify the hydro-morphological characteristics of stream channels.

How to cite: Martí, E., Li, A., Bernal, S., Kohler, B., Thomas, S. A., and Packman, A. I.: Residence Time in Hyporheic Bioactive Layers Explains Nutrient Uptake in Streams, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16075, https://doi.org/10.5194/egusphere-egu21-16075, 2021.

Low order streams drain a big proportion of river catchments. They are not only fed by groundwater, but may also lose water to their connected aquifers and, in turn, can contribute to a substantial fraction of groundwater recharge. In such cases, these streams are typically characterized by the coexistence of gaining and losing stream reaches. Along the bidirectional exchange flow paths large biogeochemical gradients can evolve so that the exchange zones can function as hotspots for biogeochemical processes (such as the important (de)nitrification processes in croplands), which can substantially change under these two conditions. An agricultural first order stream (Schönbrunnen) in south-western Germany was equipped with stream gauging stations and piezometers were installed in the adjacent shallow aquifer, in order to find out how these biogeochemical processes change under losing versus gaining conditions. Hydrological and hydrochemical variables within the immediate vicinity of the stream, as well as stable nitrogen isotopes have been monitored between August 2017 and May 2020 to spatially and temporally identify the controls of nitrogen cycling dynamics in the Schönbrunnen.

Gaining and losing conditions at the Schönbrunnen were determined by salt tracer experiments and the flow direction (upwelling groundwater or downwelling streamwater) of the exchanging fluxes was determined based on hydraulic head contour maps.

Hydrochemical data suggests that nitrate reduction occurs within the first 20 cm of the streambed in the losing reaches. In these reaches, isotopic data depicts that nitrate is reduced along the flow path between stream and groundwater. Ammonium and organic electron donors (DOC) were found at greater depths in these reaches. By contrast, increasing nitrite and nitrate concentrations were observed also along the last 20 cm of the upwelling flow paths (gaining reaches). In summary, assuming that the transition zone between groundwater and streams is only a hotspot for denitrification might not always be true, as our field data suggests that redox conditions in the streambed, and in turn, the resulting biogeochemical processes differed substantially between losing and gaining reaches.

How to cite: Jimenez-Fernandez, O., Osenbrück, K., Schwientek, M., Knöller, K., and Fleckenstein, J.: Controls of nitrogen cycling under gaining and losing conditions in a first order agricultural stream, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5058, https://doi.org/10.5194/egusphere-egu21-5058, 2021.

Human wastewater emissions can cause amongst other impacts a nutrient surplus in the connected river systems. Nutrient uptake in the river system is driven by the interaction of hydraulic, ecological, and biogeochemical conditions and processes. Hence, information about these complex interactions would allow better predicting the metabolism of fluvial environments. 

Within this study, we attempt to quantify in-stream nitrogen transformation processes with regard to hydraulic system characteristics as well as ecological characteristics such as vegetation cover, water temperature, dissolved oxygen concentrations and solar radiation. From 07 - 09/2019, four nutrient-addition-experiments were carried out in a continuous flow open air stream-mesocosm, comprising a 32.5 m highly aerated stream section (rifle) with a mean slope of 23 %, where low water levels and fast flow velocities characterize the hydraulic boundary conditions leading into a 9 m³ slowly flowing pool section (pool) with a mean depth of about 0.3 m and a spur dike increasing the residence time. The circulation of the system is driven by an electrical pumping system at the downstream end of the pool covering a flow range of 1 - 3.5 l/s. Floating algae and saturated oxygen conditions characterize the rifle section while the pool section is partly vegetated by algae, phragmites, typha and others and shows diurnal cycles of dissolved oxygen concentrations remaining most of the time below the oxygen saturation concentration. The system as a whole is decoupled from the underground with a tarp that is covered with a gravel-layer of about 3 - 8 cm depth.  Additionally, the ground of the pool section is covered by an organic litter layer of about 5 cm depth. Depending on the flow rates, the residence time in the rifle section varies between 5 - 15 min while the residence time in the pool changes from 25 – 75 min, accordingly. After nutrient-additions (Ammonium chloride and Monobasic potassium phosphate) at 10:00 water samples were taken at the downstream end of both sub-systems, with an increasing frequency of 30 min to 3 hours for the next five days. Interpolating the outlet concentrations of each system as input concentrations for the next system continuous changes in ammonium, nitrate and phosphate concentrations were identified for each system separately.

The results show that the combined ecosystems promote different types of reactions and processes in different parts of the system. The rifle induced highly aerated oxic conditions, promoting biological oxidation of ammonium consistently. On the other hand, the pool section produced limited oxic environments and longer residence times where denitrification occured, reaching the highest rates when the vegetation cover increased. Throughout the complete experimental period, phosphate transformation presented a stable behavior regardless of the environmental conditions. Therefore, spatial decoupling allowed us to  demonstrate that in-stream nitrogen cycling depends on the enduring variation and combination of local ecological and hydrological factors which occur in natural streams frequently.

How to cite: Gallo Tavera, P. and Schuetz, T.: Spatial decoupling of in-stream nitrogen cycling observed in an open air stream mesocosm., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3214, https://doi.org/10.5194/egusphere-egu21-3214, 2021.

The hyporheic interstitial as interface between surface water and groundwater offers a unique environment for contaminant attenuation and nutrient cycling, with steep chemical gradients and high retention times. Disentangling the effect of seasonal dynamics in exchange flux intensities and directions, we carried out 19 measurement campaigns where we sampled the continuum surface water - hyporheic zone - groundwater and the climatic and hydraulic boundary conditions of a whole year. Groundwater, surface water and hyporheic zone pore water from four depths were sampled at two vertical profiles in a second order stream about 150 m downstream a municipal waste water treatment plant effluent. Samples were analyzed for physical water parameters, major anions, ammonium, iron, manganese, NPOC and five selected pharmaceuticals (diclofenac, carbamazepine, caffeine, ethinylestradiol and clofibric acid). Surface water and groundwater levels as well as river discharge were measured to quantify the hydraulic boundary conditions. In addition, three vertical profiles, each equipped with five newly developed probes (Truebner AG) allowed a parallel monitoring of continuous bulk water temperatures and bulk electrical conductivity dynamics over two years. Furthermore, continuous hyporheic exchange flux intensities and exchange depths were calculated using analytical and numerical model schemes to allow distinguishing between small scale transport and attenuation processes.

The typical behavior of the redox sensitive metals and nutrients with depth is visible in each single profile snapshot. The picture is not as clear for the examined pharmaceuticals, because dilution has a major effect on the observable low concentrations. However, a clear seasonal variation driven by hydraulic and climatic processes can be observed for all substances. We were able to trace the organic pollutants down to the groundwater. Furthermore, the influence of hyporheic exchange flux intensities and directions on nutrient and contaminant depth profiles is shown.

How to cite: Schmitgen, L.-M. and Schuetz, T.: Seasonal variations in surface water groundwater interaction alter the relation of solute transport and biogeochemical processes in the hyporheic zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2949, https://doi.org/10.5194/egusphere-egu21-2949, 2021.

Until recently, Lake Stechlin was one of the few oligotrophic lakes in North-Eastern Germany. Lake Stechlin is located in a nature reserve and its catchment is nearly completely forested, there is no agriculture and only one small settlement. About 10 years ago there were the first indications in the lake’s hypolimnion for changes of the trophy. In the last 3 years the lake is experiencing a rapid eutrophication and phosphorus (P) concentrations quadrupled compared to the concentrations 10 years ago. It is generally agreed that the origin of this P is internal P cycling which is a self-reinforcing process. However, the trigger that started the intense internal P cycling is still unknown. There are several different hypotheses and we focused on investigating the role of groundwater for the eutrophication of Lake Stechlin. Groundwater is a crucial component of the water balance of Lake Stechlin because there are basically no surface inflows and outflows, i.e. besides precipitation and evaporation, both lacustrine groundwater discharge and infiltration of lake water into the aquifer are the only other relevant terms of the water balance. Anthropogenic and climate change-induced alterations in groundwater inflow and outflow might have triggered the rapid eutrophication by different processes and we present a conceptual model of the involved processes. Main findings are (1) At a few locations we measured P concentration in the aquifer which were up to two orders of magnitude above the P concentrations of the lake water. (2) Due to several years of low precipitation in a row, the volume of lacustrine groundwater discharge decreased and with that the input of important P binding agents decreased, thus influencing the lake's internal P cycling. (3) Warmer average annual temperatures increase evaporation and simultaneously lead to a concentration of phosphorus in the lake. Local reversals of groundwater flow directions could also prevent lake water and with it P from leaving the lake. Thus, groundwater might be an important factor for the degradation of Lake Stechlin.

How to cite: Lewandowski, J., Mehler, F., Bhardwaj, H., and Jäger, A.: The relevance of groundwater-lake interactions for the rapid eutrophication of Lake Stechlin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2152, https://doi.org/10.5194/egusphere-egu21-2152, 2021.

Dissolved organic matter (DOM) comprises a large and complex range of molecules with varying mass, elemental arrangements, conformation, and polarity. These diverse molecules interact with the environment resulting in changes to their molecular character and reactivity over time. Significant advances in our understanding of the molecular character of reactive and recalcitrant DOM have been made throughout the past decade, largely due to the development of ultra-high resolution techniques such as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). This understanding, however, is almost entirely based on surface water environments. Here, we investigate how the molecular properties of DOM change due to reactions occurring in a groundwater environment over time. We use FT-ICR MS combined with liquid chromatography organic carbon detection (LC-OCD), fluorescence and radiocarbon (¹⁴C) dissolved organic carbon (DOC) for a range of groundwater DOM samples, including the oldest DOC reported from a site which is not impacted by sedimentary organic carbon inputs (25,310 ± 600 years BP). Our results indicate that polarity and nominal oxidation state of carbon (NOSC) play a major role in the reactivity of groundwater DOM, with a preferential removal of hydrophilic, high oxygen to carbon (O/C) ratio molecules over time (r_s = 0.91, p = 2.4 x 10^-6). We also note an increase in likely bio-produced molecules containing low numbers of O atoms in deep methanogenic groundwater environments. These molecular formulae appear to accumulate due to the prolonged anoxic conditions which would not be experienced by surface water DOM. The decline in NOSC with increasing average bulk groundwater DOC age contrasts with findings from marine environments where NOSC has been reported to increase over time. Furthermore, the proportion of specific molecular formulae which are stable in marine waters, decline in groundwater as ¹⁴C_DOC decreases (r_s = 0.68, p = 6.9 x 10^-3) suggesting that current indicators of DOM degradation state derived from marine environments are not applicable to groundwater environments. Our research shows that the molecular character of reactive DOM in groundwater differs from that of surface water due to exposure to different environments and processing mechanisms, suggesting that it is the interaction between external environmental factors and intrinsic DOM molecular properties which control DOM recalcitrance.

How to cite: McDonough, L., Behnke, M., Spencer, R., Marjo, C., Andersen, M., Meredith, K., Rutlidge, H., Oudone, P., O'Carroll, D., McKenna, A., and Baker, A.: Molecular insights into the unique degradation trajectory of natural dissolved organic matter from surface to groundwater, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1845, https://doi.org/10.5194/egusphere-egu21-1845, 2021.

Biomolecular quantities like gene, transcript or enzyme concentrations related to a specific reaction promise to provide information about the turnover of nutrients or contaminants in the environment. Particularly transcript-to-gene ratios have been suggested to provide a measure for reaction rates but a relationship with rates currently lacks validation.
We applied an enzyme-based reactive transport model for denitrification and aerobic respiration at the river-groundwater interface to simulate the temporal and spatial patterns of transcripts, enzymes and biomass under diurnal dissolved oxygen fluctuations.
Our analysis showed that transcript concentrations of denitrification genes exhibit considerable diurnal fluctuations, whereas enzyme concentrations and biomass are stable over time. The daily fluctuations in denitrification rates yielded a poor correlation between rates and transcript and enzyme concentrations. Daily averaged reaction rates, however, show a close-to-linear relationship with enzyme concentrations and mean transcript concentrations.
Our findings suggest that, under dynamic environmental conditions, single-event sampling may result in the misinterpretation of biomelucular quantities as these relate to reaction rates. A better representation of rates can be achieved via sampling that captures the temporal variability of a particular system.

How to cite: Störiko, A., Pagel, H., Mellage, A., and Cirpka, O. A.: Relating biomolecular data to denitrification rates in infiltrating river water – insights from enzyme-based reactive transport modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12179, https://doi.org/10.5194/egusphere-egu21-12179, 2021.

At the interface between aquifers and rivers, hyporheic zones are shallow sediment layers where surface and subsurface waters mix and react. In these zones, the dynamic of solute transport and mixing is a crucial and limiting component for many biogeochemical reactive processes (arsenic and nitrates degradation for instance). In particular, the understanding of the consequence of flow path heterogeneity on solute mixing and reactivity is key to develop physically-based upscaled models of the hyporheic function. By simulating the evolution of reacting fronts under simple 2D and 3D heterogeneous hyporheic flows created by bed superficial pressure gradients, we show that incomplete mixing of reacting solutes systematically precludes the use of macro-dispersion models as upscaled models of the hyporheic function, both in steady and unsteady flow conditions.
Based on these simulations, we propose an alternative theoretical framework, based on the concept of solute lamellae stretched by flow velocity gradients, to correctly upscale local reaction rates at the reach and basin scale. Finally, we compare our numerical and theoretical results to reacting fronts in a laboratory scale hyporheic mixing experiment.

How to cite: Rousseau, G., Le Borgne, T., and Heyman, J.: Reaction rates in the hyporheic zone explained by the lamellar theory of mixing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2760, https://doi.org/10.5194/egusphere-egu21-2760, 2021.

In-stream environments, many biogeochemical processes occur in the benthic biolayer, i.e., within sediments at a very shallow depth close to the sediment-water interface (SWI). These processes are important for stream ecology and the overall environment.

Here, a 1D diffusive model is used to analyze the vertical exchange of solutes through the SWI and in the benthic biolayer. The model is applied to an extensive set of previously published laboratory experiments of solute exchange with different bed morphologies: flatbeds, dunes, and alternate bars. Although these different bed features induce mixing that is controlled by different physical processes at the SWI, overall mixing within the sediment is well represented by a parsimonious diffusive model, provided that the diffusivity profile declines exponentially with sediment depth.

For all morphology types, mixing is better simulated by an exponential diffusivity model than a constant diffusivity approach. Two parameters define the exponential diffusivity model; the effective diffusivity at the SWI, and a depth scale over which the exponential profile decays. Using a combination of classification and regression trees (CART) and multiple linear regression approaches, we demonstrate that a single predictive model captures measured variability in the effective diffusivity coefficient at the SWI across all morphological types. The best predictive model for the decay depth scale, on the other hand, is tailored to each morphological type separately.

The predictive framework developed here contributes to our understanding of the physical processes responsible for mixing across the SWI,  and therefore the in-bed processes that contribute to the biogeochemical processing of nutrients and other contaminants in streams.

How to cite: Monofy, A., Boano, F., B. Grant, S., and A. Rippy, M.: A diffusive description of Vertical Mixing in the Benthic Biolayer., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-843, https://doi.org/10.5194/egusphere-egu21-843, 2021.

Riverine systems have a dynamic exchange of water with the hyporheic zone and groundwater. Exchange fluxes can be challenging to estimate because they vary spatially and temporally and depend on many geological and hydrological properties. Temperature as a tracer has become a low-cost and robust method to monitor such fluxes both at local and reach (several channel widths) scales. Here, we present the capabilities and functionality of a new graphical user interface (GUI) developed in Python which is operating system independent. The GUI integrates standard and state-of-the-art signal processing methods with data visualization and analysis techniques. The signal analysis library allows the user to select the important frequencies to improve result confidence while the advanced LPMLEn and window function in FFT to reduce leakage in the extraction process of the amplitude and phase of the signals. The GUI streamlines the entire analysis process, from evaluating the raw temperature data to obtaining end-user specified parameters such as flux and streambed thermal properties. It allows for the analysis of single-probe and multi-probe data from short to long-term data sets.

How to cite: Bertagnoli, A., van Berkel, M., Schneidewind, U., van Kampen, R., Krause, S., Tranmer, A., Luce, C., and Tonina, D.: Groundwater-surface water exchange: A New Graphical User Interface for temperature time-series analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9311, https://doi.org/10.5194/egusphere-egu21-9311, 2021.

Exchange flows at the water-sediment interface control river water quality and carbon cycling through microbial respiration. However, accurate quantification of these exchange flows and microbial respiration is still challenging in field surveys due in part to the dynamic turbulence generated by streambed topography. Using a framework that combines Structure-from-Motion (SfM) photogrammetry with a fully-coupled surface-subsurface computational fluid dynamics (CFD) model, this work studies the effects of streambed sediment structure on riverbed turbulence and its impact on exchange flows and microbial respiration. Specifically, the SfM photogrammetry is first applied to obtain mm- to cm-scale resolution riverbed topography over a meter scale domain at four sites; these high-resolution riverbed topography data are then used to generate meshes for use in hyporheicFoam, a fully coupled surface-subsurface model developed in OpenFOAM. Simulated time series of water depth and average flow velocity from a previously-developed 30-kilometer scale CFD model will be used to set the water depth and mean flow velocity conditions for high-resolution CFD models of the SfM-characterized locations. The modeling results will be used to investigate the dependence of riverbed exchange flows, concentration gradients, and the concentration profile from the water surface to riverbed on water depth, mean velocity, roughness size, sediment distribution, bed porosity, and subsurface permeability. The relative importance of flow advection, turbulence dispersion, and microbial reaction in both streambed and surface water will also be evaluated.

How to cite: Chen, Y., Bao, J., Li, B., Liu, X., DiBiase, R., and Scheibe, T.: Effects of natural streambed sediment on the riverbed exchange flows and microbial respiration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13878, https://doi.org/10.5194/egusphere-egu21-13878, 2021.

The exchange of water between streams and groundwater plays an important role for hydrologic and biogeochemical processes. Along a stream the composition of stream water is modified by sequential losses of stream water with the current in-stream chemical signature to the subsurface and gains of water with another signature from the subsurface. This process has been termed hydrologic turnover. To date, most studies on hydrologic turnover have been focused on small stream networks. Moreover, the influence of hydrologic conditions on hydrologic turnover has not been systematically investigated. Taking the lower Selke River in central Germany as an example, we evaluated the evolution of stream-groundwater exchange and the source composition of stream water under different precipitation and stream discharge conditions, based on a coupled stream-groundwater model built in MODFLOW using the Streamflow-routing (SFR1) package. The results show that the stream reaches could be classified into three types: permanently gaining reaches, permanently losing reaches, and transitional reaches. Transitional reaches range from losing condition at higher stream discharge or lower precipitation to gaining condition at lower stream discharge or higher precipitation. In the lower Selke River with a length of 30 km, transitional reaches account for nearly 30% of the total river length in the studied period from 2011 to 2018. Regardless of dry or wet hydrologic condition, nearly 80% of the total groundwater contribution to stream discharge at the catchment outlet were generated over 20% of the total river length. This indicates diffuse groundwater pollution such as from agricultural nitrate may enter the stream network predominantly at a few distinct reaches. Our analysis can help to prioritize areas in a catchment where reduction of diffuse groundwater pollution would have the highest impact on improving stream water quality.

How to cite: Zhang, Z.-Y., Schmidt, C., and Fleckenstein, J.: Effect of precipitation and stream discharge on the source composition of stream water, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11572, https://doi.org/10.5194/egusphere-egu21-11572, 2021.

Groundwater is a major resource for drinking water supply and irrigation of crops in many parts of the world. Many groundwater aquifers are already fully allocated, but the demand is projected to increase further while, concurrently, climate change may cause more variability in the natural supply. This poses enormous challenges for the future management of groundwater resources and a paradigm shift from traditional, threshold-based management strategies to more flexible, adaptive management strategies.

For that purpose, operational forecasting tools are required that predict future states of groundwater aquifers under various scenarios and to predict the risk of critical states which would have adverse effects either for the environment or for water users, or both.

The mathematical description of the complex interactions particularly of shallow, unconfined river-fed aquifers typically requires the use of spatially explicit numerical models. These are, however, not suitable for operational forecasting due to lengthy run-times and extensive data requirements. This also poses strong limitations with respect to predictive uncertainty analysis – which should be an integral part of predictive management tools. Model simplification or model surrogates are the method of choice to circumvent the problem.

An operational forecasting tool is presented here to predict groundwater heads and groundwater storage in the unconfined Wairau Plain Aquifer in Marlborough, New Zealand, during flow recession times. The tool uses low-complexity “eigenmodels” to describe groundwater flow and to provide an early warning for critical groundwater storage levels to the Marlborough District Council, which manages the groundwater resource. These critical levels have been approached more frequently during the past years when the natural recession of groundwater storage in summer is exacerbated by groundwater abstraction to satisfy the irrigation water demand of the Plain’s viticulture.

The forecasting tool requires, amongst others, daily forecasts of Wairau River flows because the river is the major recharge source for the aquifer. Flow forecasts and their uncertainty are computed i) by using a master recession curve for predictions during flow recession times and ii) by a lumped rainfall-runoff model for times of aquifer storage recovery. This allows a broad evaluation of forecasting scenarios. The tool has been tested and is operational for recession times (worst-case scenario predictions; Wöhling & Burbery, 2020). The rainfall-runoff model performs reasonably well in predicting river flows and correspondingly in predicting groundwater storage recovery for historic data (hindcasting). The 30-day predictive uncertainty bounds generally cover the observations of river flows, groundwater levels and aquifer storage. The predictive accuracy of the tool largely depends on the predictive accuracy of the drivers, particularly of the areal estimates of precipitation that drives the rainfall-runoff model and the river-groundwater exchange function that describes aquifer recharge rates.

References

Wöhling T, Burbery L (2020). Eigenmodels to forecast groundwater levels in unconfined river-fed aquifers during flow recession. Science of the Total Environment, 747, 141220, doi: 10.1016/j.scitotenv.2020.141220.

How to cite: Wöhling, T.: Operational prediction of river-groundwater exchange, groundwater levels and aquifer storage: The Wairau Plain Aquifer , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9700, https://doi.org/10.5194/egusphere-egu21-9700, 2021.

Several distinct approaches to the one-dimensional modeling of river corridor transport at the macroscale have been developed as generalizations of the original Transient Storage Model (TSM).  We show that essentially all of them can be captured by simply restructuring the TSM so that the exchange coefficients are functions of residence time, because doing so converts the TSM to a general memory function form.  We use this generalized TSM approach to find novel closed-form expressions for the temporal moments of breakthrough curves resulting from river corridor tracer tests, when hyporheic zone exchange is governed by a memory function.  These expressions are useful because they can be used to test different hypotheses about the hyporheic zone residence time distribution based on analyses of the temporal moments of the tracer test breakthrough curves prior to detailed modeling work.  We demonstrate the application with a case study, and present extensions of the notion of making rate coefficients depend on residence time.

How to cite: Aghababaei, M., Ginn, T., Carroll, K., Gonzalez-Pinzon, R., and Tartakovsky, A.: The Remarkable Generality of the Transient Storage Model with Residence Time Dependence: Temporal Moments., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13942, https://doi.org/10.5194/egusphere-egu21-13942, 2021.

Groundwater - surface water interactions (SGI) fundamentally control groundwater recharge. The according dynamics are, thus, key for sustainable (drinking) water management. SGI are particularly relevant in the context of climate change and re-naturalization of canalized rivers, which might affect the availability and quality of groundwater pumped near streams. SGI are often not directly observable due to their complex spatial and temporal patterns. To complement the few available tracer methods (dye, electric conductivity, heat, etc.) to analyze SGI, we developed a novel method to quantify riverine groundwater recharge by using helium (He) as an artificial tracer.

We injected gaseous He into a Swiss pre-alpine river (river Emme, canton of Berne) through perforated tubing which was placed on the riverbed. Dissolved He (as well as Ar, N2 and O2) concentrations were continuously monitored in the river (200 m downstream of the injection point) and in a piezometer (30 m away from the river) using a portable mass spectrometer allowing quantitative gas determination under field conditions (miniRUEDI, gas-equilibrium membrane-inlet mass spectrometer (GEMIMS), Gasometrix GmbH, Brennwald et al. (2016)). The He injection consisted of two pulses, each lasting around 8 hours, during which dissolved He became supersaturated by up to three orders of magnitude compared to the natural (atmospheric) He abundance in surface waters (concentration of air saturated water (ASW)). The two associated He pulses were clearly identifiable in the groundwater and appeared in the piezometer approximately one day after the injection phases. The measured He concentrations in the groundwater were four to six times higher than ASW.

In conclusion, our experimental setup allows the identification of the freshly infiltrated river water in an adjacent groundwater body in a concise, robust and straightforward manner. Our new method is also non-toxic and can thus often be implemented with minimal constraints. Such tracer methods provide useful observations to constrain physically based, surface water/groundwater models.

How to cite: Blanc, T., Peel, M., Brennwald, M. S., Kipfer, R., and Brunner, P.: Use of helium as an artificial tracer to study surface water/groundwater exchange, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9005, https://doi.org/10.5194/egusphere-egu21-9005, 2021.

Hydraulic engineering structures like locks affect the natural hydraulic conditions and have a relevant impact on surface water – groundwater interactions due to enlarging the hydraulic gradient. For this, these sites are excellent areas to study associated flow paths, mass transport and their spatial and temporal variability in higher detail. However, no large-scale study at an inland waterway is available in Germany until now.

Our work aims to close this gap by applying a multiparameter approach for analyzing surface water-groundwater-interactions by using pH, electrical conductivity, major ions in combination with various other tracers like stable water isotopes, 222-Rn, and tritium. In this context, we also investigate the usability of organic trace compounds and their associated transformation products as potential new tracers.

The main study approach is based on the hypothesis that i) gaining stream sections show relatively high 222-Rn concentrations originating from discharging groundwater and ii) losing stream sections which are characterized by low 222-Rn concentrations as well as lower tritium and organic trace compounds inventories compared to unaffected areas.

During different flow-scenarios of the river Moselle, we test these hypotheses by means of a high-resolution longitudinal sampling at 2 km intervals of the main stream (along 242 km) and its major tributaries in combination with groundwater sampling at numerous wells.

Here, we present the first results of the longitudinal sampling campaign of the river Moselle in October 2020, which took place during intermediate flow conditions (Q=200 m³/s). We used on-site and in-situ 222-Rn measurements and electrical conductivity as a tracer to immediately identify zones along the Moselle with increased groundwater inflow.

With the use of these tracers, we will deepen the conceptual process understanding of surface water – groundwater interactions occurring at larger streams and during different flow conditions, which may lead to a general river characterization of losing and gaining stream reaches. Moreover, understanding the sources of water compounds and the processes involved during transportation and transformation is crucial for maintaining a good quality of the water body, which is key for proper water management. The findings obtained in the region of the Moselle river might be further transferred to other waterways and support decision making.

How to cite: Mischel, S., Engel, M., Quanz, S., Radny, D., Schmidt, A., Schlüsener, M., and Wick, A.: Analysing surface water-groundwater interactions on selected sites of the River Moselle: Identifying transport processes along an important inland waterway in Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11973, https://doi.org/10.5194/egusphere-egu21-11973, 2021.

Targeting hypohreic exchange as well as gains and losses as the means of interaction between ground- and surface water in a stream leads forward to the consideration of both influencing the apparent hydrological turnover at the catchment scale i.e. the cumulative effect of gains and losses on physical water composition along a stream. The variability in hydrological turnover across a catchment is governed by the spatially varying connectivity between groundwater and the streambed. Especially under low flow conditions, expansion of turnover relative to stream flow is prominent and its spatial variability is intensified.

Studying the scaling behaviour of hydrological turnover processes, we measured hydrological turnover along two representative stream segments of about 500-600m length at a second order tributary to the river Mosel in Trier, western Germany by applying differential sault dilution gauging (after Payn et al., 2009) over 10 campaigns in summer and 7 in winter. Each stream reach represents a typical geomorphological setting in the catchment. The upstream reach is characterized by steep sloping terrain towards the stream with pastures and forest at higher elevations as the dominant land use. At the downstream reach the terrain is flatter with the stream meandering. The land use is diverse with meadow, pastures and forest as well as settlements. Each respective reach was split into two equidistant parts, resulting in three measurements of hydrological turnover, first and second section as well as the whole reach. Thus, acquiring data accounting for the spatial variability in each reach as well as between reaches. The measurements were carried out weekly, at the two stream reaches from August to September with stream flow ranging from ca. 2 l/s to 94 l/s and at the downstream reach from November to February with stream flow ranging from 200 l/s to over 1000 l/s.

The results show clearly the positive relationship between discharge and the relative volume of water exchanged between stream, hypohreic zone and groundwater as gains and losses at the reach scale. In addition to that, exchange processes vary independently at both investigated reaches. However, the dataset suggests a distinctive relationship between turnovers of an entire reach compared to the sum of the two sub-reach sections. The slope of this relationship may be a first step for the upscaling of observed exchange and turnover processes from the reach to the network scale.

How to cite: Bäthke, L., Ulrich, S., and Schuetz, T.: Quantifying spatial and seasonal variations of groundwater- surface water interaction for the prediction of hydrological turnover on the catchment scale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12616, https://doi.org/10.5194/egusphere-egu21-12616, 2021.

Ephemeral streams play a key role in supplying water and sediment which are of high ecological importance for their permanent tributaries. The upper River Ehen (the Lake District, North West England) is the focus of a habitat restoration initiative to conserve populations of the endangered freshwater mussel (Margaritifera margaritifera). A previously diverted ephemeral stream, Ben Gill, was restored to its original course over a small alluvial fan (0.075 km²) connecting to the River Ehen to reactivate sediment supply and improve the habitat for freshwater mussels. Like most temporary streams situated on alluvial fans, the flow regime and sediment dynamics of Ben Gill are strongly influenced by fan sedimentary characteristics and interactions with its shallow groundwater aquifer.

This study combined high spatial resolution, near surface geophysics and outcrop data with hydrological data to characterise the hydrogeological properties of the alluvial fan and further develop a hydrological conceptual model of the fan to understand Ben Gill stream flow regimes and sediment supply to the River Ehen.

The conceptual model showed the alluvial fan aquifer was highly productive at the fan apex and along buried palaeochannels, whilst reduced aquifer productivity occurred towards the fan margins. When the volume of water entering the fan apex (via a perennial stream) reached ~60l/s, the fan apex infiltration rate was exceeded resulting in ephemeral flows. This typically occurred following rainfall events >9-11 mm. During flows, significant infiltration occurred along much of the ephemeral channel, though a less permeable zone was observed in the mid-fan. In the lower reaches of the ephemeral channel, groundwater levels sometimes exceeded streambed levels resulting in groundwater discharge into the stream during prolonged wet periods. Connectivity between the ephemeral stream and the River Ehen occurs for approximately 20% of the year.

Numerical hydrogeological modelling of the fan is underway to integrate data on groundwater and streamflow dynamics and associated sediment export from the ephemeral stream. This will help gain a predictive understanding of the streams flow regime and its long-term impacts on the River Ehen, which in turn, will determine the success of the restoration initiative.

How to cite: Blackburn, J., Comte, J.-C., Foster, G., and Gibbins, C.: Investigating the hydrogeological controls of an ephemeral stream’s flow regime on an alluvial fan in an ecologically important setting in North West England, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14709, https://doi.org/10.5194/egusphere-egu21-14709, 2021.

Groundwater dynamics and flow directions in the near-stream zone depend on groundwater gradients, are highly dynamic in space and time, and reflect the flowpaths between stream channel and groundwater. A wide variety of studies have addressed groundwater flow and changes of flow direction in the near-stream domain which, however, have obtained contrasting results on the drivers and hydrologic conditions of water exchange between stream channel and near-stream groundwater. Here, we investigate groundwater dynamics and flow direction in the stream corridor through a spatially dense groundwater monitoring network over a period of 18 months, addressing the following research questions:

• How and why does groundwater table response vary between precipitation events across different hydrological states in the near-stream domain?
• How and why does groundwater flow direction in the near-stream domain change across different hydrological conditions?

Our results show a large spatio-temporal variability in groundwater table dynamics. During the progression from dry to wet hydrologic conditions, we observe an increase in precipitation depths required to trigger groundwater response and an increase in the timing of groundwater response (i.e. the lag-time between the onset of a precipitation event and groundwater rise). This behaviour can be explained by the subsurface structure with solum, subsolum, and fractured bedrock showing decreasing storage capacity with depth. A Spearman rank (r_s) correlation analysis reveals a lack of significant correlation between the observed minimum precipitation depth needed to trigger groundwater response with the local thickness of the subsurface layer, as well as with the distance from and the elevation above the stream channel. However, both the increase in groundwater level  and the timing of the groundwater response are positively correlated with the thickness of the solum and subsolum layers and with the distance and the elevation from the stream channel, but only during wet conditions. These results suggest that during wet conditions the spatial differences in the groundwater dynamics are mostly controlled by the regolith depth above the fractured bedrock. However, during dry conditions, local changes in the storage capacities of the fractured bedrock or the presence of preferential flowpaths in the fractured schist matrix could control the spatially heterogeneous timing of groundwater response. In the winter months, the groundwater flow direction points mostly toward the stream channel also many days after an event, suggesting that the groundwater flow from upslope locations controls the near-stream groundwater movement toward the stream channel during wet hydrologic conditions. However, during dry-out or long recessions, the groundwater table at the footslopes decreases to the stream level or below. In these conditions, the groundwater fall lines point toward the footslopes both in the summer and in the winter and in different sections of the stream reach. This study highlights the effect of different initial conditions, precipitation characteristics, streamflow, and potential water inflow from hillslopes on groundwater dynamics and groundwater surface-water exchange in the near stream domain.

How to cite: Bonanno, E., Blöschl, G., and Klaus, J.: Groundwater dynamics and groundwater surface-water exchange in the near-stream zone across the hydrologic year, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9576, https://doi.org/10.5194/egusphere-egu21-9576, 2021.

The quantification of water fluxes across streambeds is an important aspect in the study of groundwater-surface water interactions. One way to deduce fluxes is to use heat as an environmental tracer and measure streambed temperatures. In this study we quantified vertical streambed fluxes over a small reach of the Hammer Stream (West Sussex, UK) that is characterized by a heterogeneous streambed morphology and large woody debris. The Hammer Stream is a meandering lowland stream draining a catchment of 24.6 km² with mixed agricultural and forest areas.

Our 40 m-long study reach is situated in a deciduous forested valley, characterized by sand-dominated streambed sediments and several large-scale instream woody structures. Previous geophysical measurements identified extensive clay lenses at 1-2 m depth within the streambed, in parts disconnecting the upper streambed from the connected aquifer. Nine high-resolution-temperature sensors (HRTS) were deployed in the upper streambed along the investigated reach to monitor streambed temperatures over several days in different seasons (summer, autumn, winter) and to cover the diverse stream morphology. Each HRTS comprised a fiberoptic cable wrapped around a PVC tube and connected to a XT-DTS (Silixa, single-ended mode with alternating measurement directions) system. This setup allowed us to measure streambed temperatures at an effective vertical resolution of less than 0.5 cm about every 2 min. HRTS measurements were accompanied by surface water and air temperature measurements while previous studies provided information on streambed grain size and hydraulic conductivity.

For analyzing the temperature time-series data we made use of the LPMLEn method, embedded in a newly developed GUI that allows for easy-to-use model setup and estimation of vertical exchange fluxes and thermal streambed parameters. By solving the heat transport equation in the frequency domain for a finite model domain, the LPMLEn method is a continuation of the LPMLE3 method but unlike the latter can make use of temperature information from multiple (n) vertically deployed sensors while optimally taking into account measurement uncertainty during flux estimation.

Results show that streambed temperatures are variable in space and time, with warming/cooling patterns primarily driven by seasonal hydrometeorological conditions. High-flow conditions in winter led to increased hyporheic exchange around the large woody structures.

How to cite: Schneidewind, U., Folegot, S., van Berkel, M., Bertagnoli, A., van Kampen, R., Luce, C., Tonina, D., and Krause, S.: Quantifying vertical streambed fluxes around woody structures using high-resolution streambed temperature measurements., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9228, https://doi.org/10.5194/egusphere-egu21-9228, 2021.

Microbial metabolic activity (MMA) in streambeds drives greenhouse gas (GHG) production and nutrient turnover. Previous research has identified that the quantity and quality of organic matter (OM) are important controls on MMA. Instream wood may make a significant contribution to the total OM content of the streambed, especially in forested streams, but it has typically been ignored or explicitly omitted in previous research. By means of an incubation experiment, we investigate the impact of streambed wood on MMA, GHG production and nutrient turnover rates. By using three geologies (sandstone, chalk and limestone) and allowing temperatures to fluctuate with environmental conditions, we observe these impacts under a range of typical scenarios. These results could have implications for estimates of GHG emissions from streams and inform catchment management, for example the impacts of direct installation of instream wood in river restoration or the indirect input as a result of riparian planting.

How to cite: Howard, B., Ullah, S., Kettridge, N., Baker, I., and Krause, S.: The contribution of instream wood to streambed organic matter controls on microbial metabolic activity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14965, https://doi.org/10.5194/egusphere-egu21-14965, 2021.

In-stream faunal organisms constantly interact with their habitat to modify its physical and hydraulic properties. However, little is known about how sediment-organism interactions could modify the hyporheic exchange. Previous experimental work investigating the effects of the activities of faunal organisms on exchange across the sediment-water interface has been largely conducted in small mesocosms or infiltration columns that do not represent the lotic environment adequately. Therefore, the experimental findings from these studies may not be transferable to flowing water environments (e.g., streams). Our previous experimental work demonstrated that sediment reworking by macroinvertebrates could significantly alter the hyporheic flux, mean residence times, and depth of exchange in streambeds. In this work, we explore how sediment-organism contact time influence the effect of the activities of model organisms, Lumbriculus variegatus, on the hyporheic flow regime. We conduct laboratory experiments in re-circulating flumes subject to different sediment reworking times (5 and 10 days). The hyporheic flow characteristics in these flumes were studied by conducting dye tracer tests after the bed sediments were reworked. Deposition of fecal pellets and holes/burrows dug by sample organisms were visible at the bed surface in both the experimental flumes. The flume reworked for a longer time exhibited higher hyporheic flux, longer median/mean residence times, and deeper depth of solute penetration compared to the flume reworked for a shorter period. The modification of hyporheic flow regime to different degrees depending on the sediment reworking times has direct relevance to the biogeochemistry in hyporheic zones, and thus on the overall quality of surface and sub-surface waters. We advocate that more intensive laboratory experiments and field investigations must be conducted to support the findings from our study and advance our understanding of the role of the activities of faunal organisms on fluvial ecosystem functioning.

How to cite: Shrivastava, S., Stewardson, M., and Arora, M.: Effect of sediment-organism interactions on hyporheic exchange in streams: role of sediment reworking time, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12699, https://doi.org/10.5194/egusphere-egu21-12699, 2021.

In this study, the temporal variability of the hydrological exchange between stream water (SW) and groundwater (GW), colmation, hyporheic invertebrate fauna, organic matter (OM) and physicochemical parameters were examined for the period of one year. Sampling and measuring were conducted monthly from May 2019 to April 2020 at the Guldenbach river, a second order stream in Rhineland-Palatinate, Germany. All hyporheic samples were extracted from a depth of 15 cm below stream bottom. Colmation was measured quantitatively in the same depth.

Following the biotic and abiotic patterns found, three temporal stages of different hydrological conditions can be described:

• 1) Strong floods, in February and March 2020 caused hydromorphological alterations of the river bed, leading to a decolmation of the hyporheic zone, a wash out of OM and hyporheic fauna. Due to high GW tables the vertical hydrological gradient (VHG) was positive indicating upwelling GW.
• 2) In the months of Mai to August 2019 and April 2020, precipitation and stream discharge were lowest. Predominantly exfiltrating conditions were observed, while the amount of fine sediments (clay and silt) increased as well as colmation. High densities of hyporheic fauna, dominated by fine sediment dwelling taxa, were assessed.
• 3) From September 2019 to January 2020 stream discharge was low. The VHG became increasingly negative, indicating downwelling SW. In accordance, colmation increased continuously, while densities of hyporheic invertebrates decreased and sediment dwellers became more dominant.

Precipitation, discharge events and GW table were found to be the driving factors for the annual dynamics of the hydrological exchange as well as for colmation, fauna and hydrochemistry. Electric conductivity seems a suitable indicator for the origin of water with high values in months of low precipitation and lower values after extensive precipitation events, respectively. Hyporheic fauna displayed a significant seasonality and the community structure was correlated with colmation and changes in the VHG.

This pronounced seasonality seems to be typical of many streams and should be considered for the monitoring of sediments and hyporheic habitats: Seasons with lower stream discharge are probably the most critical periods for sediment conditions.

We assume that the basic patterns of the dynamics observed basically reflect the natural situation in the catchment. However, the strength of surface run-off and the amount of fine sediments are mainly the result of anthropogenic activities and land use in the catchment.

These findings underline the significance of dynamical processes for the assessment and implementation of the Water Framework Directive.

How to cite: Stein, H. and Hahn, H. J.: Periodic alterations of the hydrological exchange in hyporheic sediments: colmation, hyporheic fauna and abiotic parameters in a second order stream during one year , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15700, https://doi.org/10.5194/egusphere-egu21-15700, 2021.