The exchange of stream water and groundwater (hyporheic exchange) plays an important role in hydrological and biogeochemical processes in rivers. Hyporheic flow comprises a distribution of subsurface flow paths characterized by distinct transit times and flow path lengths. Much of the previous research relied on the interpretation of slug tracer experiments that only capture a portion of the overall hyporheic exchange, given their relatively short (minutes to hours) duration. Therefore, there is a need to go beyond the characterization of shorter flow paths in hyporheic research to understand flow paths of the entire transit time distribution. We hypothesize that environmental tracers provide complementary information into longer hyporheic flow paths. Here we derive and compare commonly used transport metrics for hyporheic exchange derived from radon and slug tracer injections and aim to identify combinations of model parameters that predict the concentrations of both tracers along experimental stream reaches. For this purpose, we measured the environmental tracer radon (²²²Rn), that increases with time along hyporheic flow paths, at several stream sections along Oak Creek, Oregon (USA). We conducted slug tracer (sodium chloride) injections at the same stream sections. We employed a transient storage model that includes radon specific processes such as radioactive decay to ensure comparability in the information acquired on hyporheic exchange from radon and the typically applied slug tracer experiments. We calibrated final stream discharge and hyporheic exchange metrics through a global identifiability analysis and subsequently calculated relevant transport metrics using the refined parameters. Results with the field data will be obtained soon. Hence, this study will contribute to a more holistic understanding of hyporheic flow paths and related processes, such as the biogeochemical turnover processes in rivers.

How to cite: Bacher, M., Klaus, J., Ward, A., Krause, J., Segura, C., and Glaser, C.: On the value of slug tracer injections and the naturally occurring radon for observing hyporheic exchange, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1037, https://doi.org/10.5194/egusphere-egu24-1037, 2024.

River systems, such as the Himalayan, consist of three zones: source (high mountains, glaciers), transition (lower mountains, agriculture), and floodplain. About 800 million people in the highlands and Indo-Gangetic plains depend on the Himalayas, hence called the "Water towers of Asia," for freshwater (Kulkarni et al., 2021). While the transition zone contains lower mountains and sustains agricultural activities, the source zone has severe gradients, high peaks, and deep valleys (Nepal et al., 2014). Hence, it is imperative to understand the river systems. It is important to explore methods that have the potential to increase the water quality by inherent natural processes of lotic systems that can assimilate contaminants. One such process is Hyporheic exchange (HE). Keeping in mind the expenses associated with field testing and with the goal of facilitating the development of a deeper understanding over a greater spatial extent, a cost-effective approach is sought. Thus, my study aims to develop a preliminary understanding of hyporheic exchange in the pristine Himalayan headwaters over a meandering section of the Ringali gad stream flowing through the Mussoorie Wildlife Sanctuary. Wherein mini drive point piezometers were used to measure vertical hydraulic gradient (VHG) variations in response to rainfall events over a period of 30 days, and it is observed that the downwelling zones that were giving VHG values of around -0.1 also converted to upwelling zones with VHG values of around +0.1.

How to cite: Reddy, P. K. and Sen, S.: Understanding Hyporheic exchange flows around a meandering section of pristine Himalayan headwaters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1154, https://doi.org/10.5194/egusphere-egu24-1154, 2024.

In water table aquifers, evapotranspiration can be reduced due to the groundwater pumping. These changes must be evaluated to assess how much water can be used for groundwater withdrawals. Although this salvage phenomenon was well understood a long time ago since pioneering studies of Theis and Bredehoeft, the models always applied numerical techniques to treat their non-linearity, most commonly using MODFLOW. However, in cases of limitations on watershed data properties, analytical methods were used successfully for practical evaluation of water balance components (e.g., stream depletion rates). Previously we made a breakthrough in solving the Bredehoeft problem analytically by linearization. This solution permits evaluating stream depletion and loss of storage in addition to salvage. Using compiled ranges of input parameters, the detailed analysis of solution using the COMSOL software indicates that linear approximation of the problem has good accuracy in practical ranges. Diagrams for evaluation of accuracy for broad ranges of parameters defining stream-aquifer connection and evapotranspiration are presented. Results can be used for analyses of the watershed water balances.

How to cite: Zlotnik, V., Rabinovich, A., and Cardiff, M.: The Bredehoeft Problem: Verifying Accuracy of Linearized Salvage Evaluation in Water Table Aquifers due to Groundwater Pumping, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2581, https://doi.org/10.5194/egusphere-egu24-2581, 2024.

Predicting in-stream transport and transient storage processes is crucial for determining biogeochemical turnover and protecting stream ecosystems. One common way to study these processes consists in analysing slug injections of artificial tracers into the stream. The explanatory power of processes inferred from these experiments depends on the quality and completeness of the recorded tracer signal, i.e., the breakthrough curve (BTC). The stream settings strongly influence the explanatory power of the BTC. For instance, an increase in transient storage zones can result in a more pronounced tailing of the BTC. It is well-known that different tracers such as sodium chloride or dye tracers exhibit different detection limits. However, limited guidance exists if and how the choice of the selected tracer or the stream settings biases the conclusion drawn from tracer experiments. To address this research gap, we carried out numerical experiments generating BTCs from slug injections through 10,000 randomly selected parameter combinations mimicking stream conditions. We employed these randomly selected parameter combinations from a predefined range for the dispersion parameter, flow velocity, cross-sectional area, and with and without consideration of transient storage exchange processes. The BTCs were truncated based on the detection limits for sodium chloride and the dye tracer uranine. We calculated transport metrics such as the temporal moments of BTCs and the transient storage index (TSI) to identify differences between the BTCs truncated based on the detection limits of both tracers. We found that the different tracers resulted in clearly different transport metrics. Specifically, the BTCs of the dye tracer exhibited higher TSI values compared to those resulting from BTCs derived from the salt tracer. The absolute and relative differences between the transport metrics of both tracers increased with higher values for the transient storage parameters, particularly for higher flow velocity and a higher dispersion coefficients representative for small streams. Our results revealed that analyzing BTCs from small streams is clearly biased when relying on sodium chloride in the experiments. These finding raise caution considering the importance of the choice of tracer, and we recommend the use of dye tracers over salt tracers for small streams where a high impact of transient storage processes is expected.

How to cite: Glaser, C. and Klaus, J.: Different tracers result in different storage and transport parameters in transient storage models , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6022, https://doi.org/10.5194/egusphere-egu24-6022, 2024.

A new conceptualisation describing surface water-groundwater exchange for braided rivers and their associated alluvial aquifers has been developed (Wilson et al., 2023 Preprint). This conceptualisation recognises that braided rivers create their own high-permeability shallow aquifer system through the process of mobilising bed sediments during flood events. A braided river can therefore be considered a “river system” consisting of surface channels and an intrinsically linked subsurface gravel reservoir, the “braidplain aquifer”. This conceptualisation implies that for settings where the river system is hydraulically disconnected to the regional aquifer, groundwater recharge is largely governed by braidplain aquifer width. Additionally, for settings where the river system is hydraulically connected to the regional aquifer, river bed levels will have a large control on recharge rates since they determine the hydraulic gradient. Depending on the hydraulic status, groundwater recharge can be compromised by narrowing the active braidplain, and bed degradation caused by extracting gravel from the braidplain aquifer at a rate that exceeds natural replenishment. To test the impact of river management on groundwater recharge, long-term records of river mean bed level from surveyed cross sections were compared to groundwater levels for the Wairau and Ngaruroro rivers in New Zealand. Scenario testing for different river system widths and elevations was also conducted in MODFLOW based on shorter term monitoring records.

In New Zealand, groundwater monitoring commenced after the river flood engineering schemes of the 1960’s, so the impact of river narrowing is not captured by groundwater records. However, hydraulically connected recharge reaches of the Wairau and Ngaruroro river systems have both been subject to more recent bed degradation caused by gravel extraction. The long-term groundwater level decline in the regional aquifers clearly mimics the drop in mean bed levels in the recharge reaches of the rivers. The drop in river bed elevations can also account for the decline in groundwater levels in MODFLOW scenario modelling.

Observation data for the Wairau and Ngaruroro systems show that the dynamic component of recharge pulses from flood flows propagate rapidly through their associated highly transmissive alluvial aquifers. For both hydraulically connected and disconnected braidplain aquifer-regional aquifer settings, maintaining a steady rate of recharge is therefore most beneficial for sustaining groundwater levels throughout the year. The observation data and modelling results confirm that gravel extraction in the braidplain aquifer is having the largest impact on the hydrological function of the regional aquifer in the two hydraulically connected systems studied here. In both cases, the decline in bed levels offsets the benefit of recharge sourced from flood flow events.

Wilson, S., Hoyle, J., Measures, R., Di Ciacca, A., Morgan, L. K., Banks, E. W., Robb, L., and Wöhling, T.: Conceptualising surface water-groundwater exchange in braided river systems, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-2767, 2023.

How to cite: Wilson, S., Measures, R., Hoyle, J., Stecca, G., Durney, P., Di Ciacca, A., and Woehling, T.: Impact of river management on groundwater recharge from braided rivers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6843, https://doi.org/10.5194/egusphere-egu24-6843, 2024.

Large in-stream structures such as dams can regulate the dynamic of the river stage, which potentially alters the patterns of hyporheic exchange flow (HEF) and mass transfer between the river and adjacent aquifer. However, current studies haven’t focused on how a large dam affects the evolution of HEF and residence time distribution (RTD), especially in the upstream area. In this study, we conducted the geophysical survey and groundwater stage monitoring of three monitoring transects around the XingLong Water Conservancy Dam (XLD) in China, which covers both upstream and downstream riparian areas. Based on these field data, a two-dimensional, horizontal numerical model was built to assess the spatiotemporal evolutions of lateral HEF, hyporheic zone (HZ) and groundwater RTD under the regulation of the XLD. The monitoring and simulation outcomes highlighted the different impact patterns of the XLD in upstream and downstream regions. For instance, the groundwater in the regions upstream of the dam was found to be recharged for the most of time, while the groundwater immediately downstream of the dam was significantly discharged. As the river stage fluctuates, the XLD significantly enhanced the HZ along the upstream river boundary whereas substantial HZ downstream was mainly observed in response to rising or high river stage. Furthermore, the XLD resulted in flow paths around the XLD with short lengths and high flow velocity, which consequently resulted in a significant HZ in the perimeter surrounding the XLD. Results of RTD show that water in the HZ downstream of the XLD was rejuvenated, whereas the HZ upstream of the XLD comprises a mix of aged and rejuvenated waters. These findings emphasized the need for full consideration of river stage dynamics surrounding the dam in analytical or numerical analysis when aiming to assess the bank storage and the aquatic environment in a dam-regulated river corridor aquifer, and have potential implications for the decision of the construction or removal of the dam, river restoration and purification of pollutants in aquifer.

How to cite: Li, Y., Wen, Z., Schneidewind, U., Liu, H., and Krause, S.: Field monitoring and numerical investigation on the regulation of a large dam structure on the patterns of lateral hyporheic exchange and residence time distributions - The Xinglong Water Conservancy Dam, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14520, https://doi.org/10.5194/egusphere-egu24-14520, 2024.

The coupling transformation of iron and phosphorus in the riparian zone is of great significance on the biogeochemical cycle of iron and pollutants in surface water-groundwater interaction system. However, the spatial and temporal distribution, biochemical transformation and its pollution interception in the interaction zone is poorly understood. In this study, we used the sand tank experiment to investigate the migration and the transformation of iron and phosphorus under the redox fluctuation forced by the river stage in the riparian zone. Results show that there is a good correlation between the changes of Fe/Al coupled P and amorphous total Fe. Additionally, more attention should be paid to the effect of organic carbon rather than dissolved oxygen on the redox condition in the underground environment. It was also found that aqueous phosphorus usually accumulates in the transition area of the riparian zone regardless of the recharge or discharge relationship between river and groundwater. This study thus revealed the distribution, migration and transformation mechanism of iron and phosphorus in the typical fine sandy riparian zone, providing theoretical support for tracing and controlling the source of phosphorus pollution in riparian aquifer.

How to cite: Guo, Z., Wen, Z., Bu, X., Liu, H., and Yuan, S.: A sand tank experimental study of distribution, migration and transformation mechanism of iron and phosphorus species under redox fluctuation in a simulated riparian zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14653, https://doi.org/10.5194/egusphere-egu24-14653, 2024.

The Vjosa River and its tributaries represent a large and dynamic river network characterized by a near natural flow regime and largely undisturbed hydromorphological dynamics. Due to the high connectivity (e.g. the lack of dams and regulation in the main river stem), the Vjosa River is a hotspot of natural biodiversity and may serve as an ecological reference system.

The self-purification potential of an ecosystem describes the resistance and resilience to contamination (disturbances). In rivers, the contaminants are attenuated and degraded not only in the visible river channels, but most importantly in the hyporheic zone and the riparian corridor. The subsurface is a vital bioreactor, that hosts an immense river- and groundwater-borne biodiversity.

Here we target the key biogeochemical processes involved in the cycling of carbon and nutrients in a qualitative and quantitative manner. The spatio-temporal distribution and transformation of different carbon (e.g. DOC, DIC, CO₂, CH₄), nitrogen (e.g. NO₃^-, NO₂^-, NH₄⁺), and phosphorus (e.g. PO₄^3-, P_tot) species are studied in detail at different spatial scales. Extrapolation from flow-through sediment microcosms to natural river sections of various dimensions (mesoscale to macroscale) will allow a good estimation of material import, transformation, attenuation, and export. The role of the microbial community, in the water and attached to the sediments, will be analyzed via high throughout sequencing to determine its composition and functions.

The outcome of this study will help to better understand the functioning river ecosystems from the micro to the catchment scale as a basis for in-depth evaluation of future sustainable management options. The Vjosa River has been proclaimed a national park recently, and options for ecotourism are currently developed by its management authorities. In the light of global change, the Vjosa River network shall serve as a reference system for other rivers in Europe and a unique field laboratory for assessing biogeochemical processes in an intact environment.

How to cite: Hoxha, S., Griebler, C., Karwautz, C., Singer, G., and Beqiraj, S.: Characterization of biogeochemical self-purification processes in the Vjosa River network focusing on different spatial and temporal scales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18397, https://doi.org/10.5194/egusphere-egu24-18397, 2024.

Surface-groundwater interaction is an important link between hydrology and hydrogeology and can contribute considerably to groundwater recharge. However, quantification and continuous observation of water flows is challenging in practice. This may explain why surface-groundwater interaction is disregarded in many hydrogeological models so far.     
In this work we test several field methods in three medium-sized streams (average discharge at outlet: 1.9-10.9 m³/s) close to the city of Freiburg, south-west Germany. We subdivide the streams into sections and monitor gains and losses by a combination of different methods. Continuous discharge data is obtained by capacitance water level recorders combined with repeated runoff measurements by electromagnetic current meter. As an alternative, we apply particle tracking algorithms to drone footage and compute surface velocities and discharge volumes. Here we also analyse different types of seeding material. Additionally, thermal drone images show surface temperature anomalies which we combine with discharge measurements to estimate groundwater intrusion. Our first data shows that continuous data collection under field conditions is challenging and can suffer from drawbacks such as flooding or tree fall. We therefore recommend redundant methods. Discharge measurements via electromagnetic current meter are generally robust but limited to medium flow conditions. Here, remote sensing via drones can provide labour-efficient alternatives. With their help, discharge measurements are possible also during high flow periods but in turn limited to areas with little or no tree cover, whereas thermal imagery is more efficient during low flow periods. Then it is well suited to locate point sources of groundwater inflow, particularly during times of strong temperature gradients between rivers and aquifers in summer or winter. Quantification of these inflows remains uncertain, though.

How to cite: Glaser, D., Krämer, A., Lange, J., and Weiler, M.: Comparing different field methods to quantify surface-groundwater interaction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18662, https://doi.org/10.5194/egusphere-egu24-18662, 2024.

Changes in population and agricultural development are increasing demands on available water resources in the Transboundary Rio Grande (TRG), an area defined by the Rio Grande River and the Mesilla aquifer between New Mexico, Texas, and Mexico. Due to the continued drought, surface water availability is continuously declining, increasing the reliance on groundwater to satisfy the water demands (mainly for agriculture and domestic uses). To simulate the conjunctive use and management of the surface water and groundwater in the TRG, a groundwater flow model implemented on Modflow-OWHM called the Rio Grande Transboundary Integrated Hydrologic Model (RGTIHM) has been previously developed. In this study, the RGTIHM model is used to assess the historical conditions and to project climate change driven scenarios on the water budget variables related to conjunctive use of surface water and groundwater in the TRG. The historical conditions were simulated from 1940-2014, while the future scenarios were simulated for the period 2015-2065 considering: i) inputs of precipitation and temperature from global climate change models, ii) management scenarios of land use and water demands for agriculture. The historical period of the model shows that due to the aquifer depletion, the river is permanently becoming a losing stream and at the same time is becoming a source of recharge for aquifer storage, the recharge from irrigated fields has a significant weight in the total recharge, and the diffuse recharge from precipitation is a small source compared to the previous two. The future period shows that maximum and minimum temperatures tend to increase, as well as the real evapotranspiration; precipitation does not change significantly, diffuse recharge decreases, and runoff increases. Water availability in the Rio Grande River decreases due to reduced snowpack in the Rocky Mountains, increasing the reliance on groundwater and posing uncertainty in future water supply management in the TRG.

How to cite: Perez, K. and Fernald, A.: Potential influence of climate change in the water budget variables related to the conjunctive use in the Transboundary Rio Grande , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21404, https://doi.org/10.5194/egusphere-egu24-21404, 2024.

Several studies have investigated the high reactivity of the hyporheic zone (HZ) and the interactions between moving bedforms and biogeochemical processes. However, the impact of bedform migration on the attenuation of trace organic compounds in the streambed has not yet been investigated. The oxygen distribution and dynamics in the HZ play a major role in this regard. So far, there have been no in-situ measurements of two-dimensional oxygen distributions in the HZ. To address this gap, we developed a novel device and tested it for the first time in the Erpe River, Germany. Our setup included a planar optode installed in the streambed to visualize the redox zonation within the HZ. Additionally, we tested five different stream flow velocities (from 10 to 50 cm/s) to investigate typical bedform celerities in lowland streams more thoroughly. By repeatedly sampling the surface and pore water, our aim was to determine how dynamic flow patterns and variable bedform celerities in sandy streams influence water constituents. The field experiment confirmed that changes in flow conditions can non-linearly influence bed movement and oxygen consumption, thereby affecting the fate of trace organic compounds.

Keywords: Biogeochemical processes, bedform celerity, in situ measurements, field experiments, redox zonation, hyporheic zone, oxygen distribution, planar optode.

How to cite: Villa, A., Schulz, H., Spahr, S., Arnon, S., and Lewandowski, J.: Bedform migration’s impact on streambed oxygen distribution: A novel field experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15043, https://doi.org/10.5194/egusphere-egu24-15043, 2024.

The hyporheic zone plays a critical role in nutrient cycling, biogeochemical processes, and overall stream ecosystem health. Variation of physical and chemical properties in the hyporheic zone affects the quality and quantity of the exchange process. To gain a depth-oriented insight into the hyporheic functioning, isotopic (¹⁸O and ²H) and chemical analysis (major ions such as K⁺, Na⁺, Mg²⁺, Ca²⁺, Cl^-, SO₄^2-, NO₃^-) was carried out focusing on the differences between upstream and downstream conditions. Multi-level interstitial probes were used to take subsurface water samples up to 0.6 m depth from two streams named Ahna and Losse in North Hesse, Germany.

Fast downward flow and higher mixing rates were observed for Losse compared to Ahna and no differences were detected between the two locations in Losse. In Ahna, due to changes in effective porosities and the hydraulic conductivities, the water extraction rates were decreased along the depth. Downstream water was more isotopically enriched than upstream being subject to more evaporation and mixing with other waters. Higher concentrations of  K⁺, Na⁺, Mg²⁺, Cl^-, and NO₃^- were found in downstream than the upstream due to the addition of wastewater, agricultural runoff, pollutants, and road salt while flowing. Denitrification and thus decrease in nitrate and increase in nitrite concentrations were more dominant in the upstream due to the high oxygen consumption. By combining isotopic and chemical analyses with detailed hydrogeological measurements, this study can provide valuable insights into the spatial dynamics of hyporheic flow exchange in a stream ecosystem.

How to cite: Mahindawansha, A. and Gassmann, M.: Assessing the variations in hyporheic flow exchange by isotopic and chemical analysis , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17630, https://doi.org/10.5194/egusphere-egu24-17630, 2024.