Beach faces form the interface between terrestrial and marine systems. They act as a reactive zone between these two compartments, transporting and biogeochemically modifying chemical constituents such as nutrients, pollutants and carbon. Re-circulation of sea water through beach sediments is largely driven by tidal pumping and pressure gradients caused by tides, wave setup, and storm events that pile sea water up on the beach face. In contrast, terrestrial groundwater systems provide a source of low salinity and often nutrient rich water to the coastal zone. Mixing between these water sources is complicated by catchment morphology, variable density flow and very dynamic boundary conditions across temporal scales (e.g. tides, storms, yearly variations in terrestrial groundwater levels). Thus tracing water and nutrients fluxes through the subterranean estuary is not trivial. In this work we use a combination of point and long-term (7 months) temperature profile measurements and heat modelling to estimate water fluxes through the beach sediments into the Königshafen, on Sylt Island, Northern Germany. Temperature measurements were complemented by stable isotope and pore water chemistry measurements to infer the origin of discharge into the bay. The results showed that flow paths are complex, with dune morphology influencing the focal point for fresh groundwater discharge, with fluxes up to 20 cm d^-1. Moreover it appears that either the islands fresh groundwater isotopic signature is either variable or at least two end-members contribute to the freshwater signature. Seaward, saline and brackish discharge occurs into the tidal creek draining the bay. Overall temperature measurements and heat modelling combined with pore water chemistry show potential to understand water and chemical exchange through the subterranean estuary and thus help to understand water and material fluxes at the terrestrial-ocean interface.

How to cite: Gilfedder, B., Riedel, R., List, J., Böttcher, M. E., and Frei, S.: Mapping and quantifying water fluxes at the land-sea interface using temperature, stable isotopes and pore water chemistry: An example from Königshafen, Sylt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16274, https://doi.org/10.5194/egusphere-egu23-16274, 2023.

The occurrence of trace organic compounds (TrOCs) such as pharmaceuticals and personal care products in aquatic ecosystems has been a growing concern in urban environments in recent decades. Incomplete removal of organic compounds in conventional wastewater treatment along with increasing use of chemicals, especially due to urbanization, increase the contamination of rivers and groundwaters with high loads of TrOCs. Different strategies to combat this type of environmental pollution have been researched, leading to a better understanding of the occurrence and fate of these contaminants. For instance, the natural attenuation of TrOCs by processes occurring in the hyporheic zone has been recognized as an efficient alternative for contaminant removal. However, little is known about the effects of stream flow velocity and bedform celerity on TrOCs removal, although bedform migration is a common process in many natural and urban streams. Our research project evaluates the influence of bedform migration on the fate of sixteen organic contaminants and their transformation products due to dynamic hyporheic zone exchange. Both controlled flume experiments and field measurements in a side channel of the River Erpe in Germany contribute to understanding the effects of dynamic flow conditions on the biotransformation of TrOCs. The knowledge obtained may be applied to enhance water management remediation programs, considering water bodies' ecological and chemical status.

Keywords: TrOCs, hyporheic zone, bedform migration, flume experiments, attenuation, biotransformation.

How to cite: Villa Arroyave, M. A., Spahr, S., Arnon, S., and Lewandowski, J.: Dynamic hyporheic zone: Impact of non-steady stream flow and moving streambeds on the fate of trace organic contaminants, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13178, https://doi.org/10.5194/egusphere-egu23-13178, 2023.

Mixing between water column and sediment is a crucial physical process for the fate of nutrients and for the biogeochemical reactions in the stream ecosystem. There are two factors that characterize this mixing process: the mixing magnitude at the sediment water interface, which controls the amount of solutes exchange between water column and its underlying sediment layer; and the mixing extent that defines how quickly the mixing rate decays with depth of the sediment laye.  The vertical mixing process has been shown to be well represented by a 1-D equivalent diffusivity model with exponentially decaying profile in the vertical direction. This exponential profile is controlled by two parameters: the effective diffusion at the sediment water interface, that is equivalent to the mixing magnitude, and the decay coefficient of the exponential, that represents the reciprocal of the mixing extent.

The 1-D diffusivity model was applied to a large dataset of experiments from literature that were conducted with conservative solutes on three different morphology types: flat beds, dunes and ripples, and alternate bars. The mixing magnitude and extent appears to be the largest in alternate bars, the smallest in flatbed and intermediate in dunes and ripples. Afterwards, the dependence of the mixing magnitude and mixing extent on stream and sediment characteristics was studied to derive a regression model to infer the values of the mixing controlling parameters from stream and sediment characteristics. This regression model was developed in dimensionless form using the Multiple Linear Regression (MLR) technique.  The regression formulae demonstrate no dependence of the mixing magnitude on morphology type, while it is significantly correlated to the permeability Reynolds number (Re_k) that depends on sediment permeability, shear velocity and water viscosity. On the other hand, the mixing extent of solutes within the benthic biolayer can be categorized based on the morphology type. Specifically, for flat beds the mixing extent exhibits different trends of dependance on the permeability Reynolds number due to the dominance of different physical process for each interval of Re_k. Instead, for dunes and ripples the mixing extent is monotonically correlated to the bedform (ripple or dune) wave number, as expected from literature. Finally, due to the limited number of experiments on alternate bars we were not able to derive a robust and reliable regression formula for this morphology type. Therefore, more experiments under different conditions should be performed with alternate bars. These results help to unravel the influence on mixing processes of different characteristics of streams and their sediments.

How to cite: Monofy, A., Boano, F., and B.Grant, S.: Mixing magnitude and extent within the benthic biolayer for different morphology types, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3588, https://doi.org/10.5194/egusphere-egu23-3588, 2023.

The Wairau Plain in Marlborough, New Zealand, host a shallow, highly conductive gravel aquifer which is mainly recharged by the braided Wairau River. The groundwater of the Wairau Aquifer is an important source of drinking water in the region and provides irrigation water for viticulture. Managing the groundwater is increasingly challenging because of a decline in levels and spring flows combined with a natural seasonality and an expected high vulnerability to climate change and overexploitation.

A prerequisite for groundwater managment and limit-setting is the knowledge of the water supply (recharge) as well as the water discharge and takes from the aquifer. Both are spatialy and temporally highly variable and difficult to measure. In addition, the aquifer is influenced by ephemeral streams which could potentially lead to unknown groundwater sinks and sources and periodical shifts of boundary locations.

In order to investigate these aspects and to estimate the water balance components and fluxes in the associated groundwater system, a coupled surface water – groundwater flow model (MODFLOW) has been set up. A variety of observations, ranging from groundwater heads to spring/river flows and exchange rates, were built into the model. Metered groundwater abstraction data was used to test and adapt a spatio-temporally distributed soil water balance model that is coupled to the MODFLOW model.

The complex surface flow network of the Wairau Plain and strong contrasts of hydraulic conductivity (up to 4 orders of magnitude) between older hydrogeological units and more recent, interwoven fluvial sediments have been a major challenge in the model setup. An iterative procedure with step-wise increasing complexity of parameterization was adopted to derive a plausible model structure, that is commensurable with the various observation types. Model parameters were calibrated and (linear) uncertainty bounds estimated using the PEST software.

The model performs well with respect to the plausibility of the groundwater flow field and the overall water balance. Groundwater heads, river-groundwater exchange flows and spring flows are generally well covered by the predictive uncertainty bounds during the evaluation period (data not utilized for calibration). The model provides insights into the relative contributions and the seasonality of the various water balance components of the Wairau Aquifer. It has been confirmed that the Wairau River is the main contributor to groundwater recharge, but also that recharge in a given year is matched by equal levels of discharge to low-land springs and off-shore. Although large river flood events during the wet period lead to interim excess groundwater storage, the recession to pre-flood groundwater levels is surprisingly fast (less than one year). On the other hand, unremarkable (high return period) flood events in the summer period, which keep river flows at elevated levels, have a noticeable effect on groundwater storage. This effect can last up until the following dry season.

The model proved useful for assessing the status quo of the Wairau Aquifer groundwater resources. This is a prerequisite for the search and testing of alternative management options that are imminent due to the observed trends in groundwater levels.

How to cite: Wöhling, T., Gosses, M., Wilson, S., Nguyen, H., and Davidson, P.: Surface Water – Groundwater Flow of the  Wairau Plains, New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7419, https://doi.org/10.5194/egusphere-egu23-7419, 2023.

Natural constraints and the damaged Meli gold mining: forecasting impact on the water resources quality of the Meli area and the surroundings, Tigray 

 Kaleab Adhena Abera ¹, Berhane Abrha ²,  Tesfamichael Gebreyohannes ², Abdelwassie Hussien ² ,  Miruts Hagos ², Gebremedhin Berhane ², and Kristine Walraevens ¹

¹ Laboratory for Applied Geology and Hydrogeology, Department of Geology, Ghent University, Belgium (kaleabadhena.abera@ugent.be)

² Department of Geology, School of Earth Science, Mekelle University, Ethiopia

Abstract: Meli is the only modern gold mining site in the Tigray region, Northern Ethiopia. Water resources of this area and the surroundings are currently very susceptible to pollution by toxic chemicals than ever before, due to both the natural geological setup of the area and anthropogenic impacts, specifically because of the recent war in northern Ethiopia. The war that was started on November 03, 2020, resulted in the complete destruction of the mining company and tailings dam due to bombing. This potentially could lead to the uncontrolled movement of wastewater from the dam to the environment. The area is characterized by quite complex geology and associated geological structures. In addition to the direct flow of contaminant plumes to downstream areas as surface water, the naturally existing geological fractures, as well as faults, could also act as conduits and increase the infiltration rate of the pollutants to the groundwater resource. In this research, integrated geological, structural, and remote sensing methods were applied. Mapping of geology and geological structures was compiled using both Spot and Landsat satellite images and a physical field survey conducted before the war started. Metavolcanics, metasediments, granite, and sandstone are the identified lithologies in the area. The detailed fracture measurement helps determine the possible flow direction of water and the pollutants. Totally, 110 structural measurements were taken, and the area is affected by a series of Neoproterozoic structures. These include WNW–ESE striking compression, NE –SW striking exfoliation fractures, and variably oriented faults. Moreover, structures such as folds, minor strike-slip faults, and joints were observed. The chemicals used in the gold mining company were evaluated. The Meli area tailing dam contains wastewater with a very high concentration of cyanide, caustic soda, heavy metals, and salts which are very toxic. The possible impact of these pollutants on water resources was forecasted and threat-solving mechanisms were proposed. The result of this research work will serve as a baseline for further pollution impact studies at a larger catchment scale and as an input for groundwater resource pollution modeling works of the Meli area and the surroundings.

How to cite: Abera, K. A., Abrha, B., Gebreyohannes, T., Hussien, A., Hagos, M., Berhane, G., and Walraevens, K.: Natural constraints and the damaged Meli gold mining: forecasting impact on the water resources quality of the Meli area and the surroundings, Tigray , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1724, https://doi.org/10.5194/egusphere-egu23-1724, 2023.

Groundwater flow and stream-aquifer interactions are the most fundamental processes that control the physical and biological characteristics of entire hydrographic networks during low-flow periods.

Our study focuses on these complex flows represented in a physically based, regional, and fully distributed hydrological model. We use the J2000 hydrological model, applied to the case study of the Saône river in France. This 30,000 km² watershed has a contrasted lithology, with karstic sectors, which makes it possible to study the performance of the model according to the karstification of its sub-watersheds. It is also a heavily monitored watershed with daily flow measures at 227 hydrological stations evenly distributed over the study area and 587 hourly temperature measurement stations.

J2000 model performance is estimated by calculating widely used hydrological signatures such as BFI (Base Flow Index), IGF (Interbasin Groundwater Flow), or FDC (Flow Duration Curve) characteristics. This set of signatures evaluates the performance of the model with respect to the representation of groundwater flows. In addition, we calculate thermal signatures derived from the relationship between air and water temperature (damping factor, time lag, slope). They are not used as a performance criterion but they give some more information about the spatial distribution of thermal regime and the type of groundwater contribution.

The analysis of observed and simulated hydrological signatures, and observed thermal signatures, revealed various hydrological and thermal responses (e.g. shallow and deep groundwater signature), depending of the lithology of the sub-basin. In our future work, we will couple the J2000 model with the process-based thermal model T-NET (Thermal NETwork). The work presented aims to increase the performance of the thermal regime determination, which has shown significant sensitivity to groundwater contributions.

How to cite: Rouchy, L., Branger, F., Hévin, G., and Moatar, F.: Hydrological and thermal signatures to characterize groundwater influence and improve a spatialized regional hydrological model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13883, https://doi.org/10.5194/egusphere-egu23-13883, 2023.

Wetlands are affected by and also have a significant influence on climate change because of their ability to regulate atmospheric concentrations of the greenhouse gases such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). This study aimed to characterize nitrogen dynamics in a groundwater-dependent wetland in South Korea. Soil textures and compositions, surface and groundwater constituents, groundwater levels, and air temperatures were determined in the field and laboratory in summer, 2021. Production rates and isotopic signatures of N₂O gases were measured using static chamber and novel kinetic cell methods. Production rates of N₂O gas were were up to 78.8 g N₂O N/ha/d, N₂O production was most active at 20-32 cm depths, and the source of N₂O production was identified as denitrification of nitrate in groundwater. Statistical analyses indicated N₂O flux generally increased with increased groundwater level and air temperature. Quantitative analyses via in situ N₂O gas isotopic transport modeling will provide novel approach for rapidly characterizing the sources and dynamics of nitrogen and other greenhouse gases in groundwater-dependent wetlands around the world.

How to cite: Lee, E. S., Johns, T., Linville, L., Moon, H. S., and Kim, Y.: Characterizing nitrogen dynamics in a groundwater-dependent wetland in South Korea using field and novel laboratory methods, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9626, https://doi.org/10.5194/egusphere-egu23-9626, 2023.

Understanding the fundamental mechanisms of solute transfer across the sediment-water interface (SWI) is essential for comprehending river functioning. Considering the stochastic and complex pollutant emissions across the SWI, we investigate the effects of release position on the solute transport in this work. Linking turbulent momentum and solute transport in the hyporheic zone through numerical modeling. Simulation results reveal that the location of the point source release determines the initial convection and dispersion intensity, which in turn affects the process of solute dispersion across the SWI. The location of solute release can affect hyporheic mixing processes more significantly than changes in streambed permeability. The penetration of turbulence into the streambed directly controls both interfacial exchange and mixing within a transition layer below the SWI, which is consistent with previous findings. In addition, the finite-time Lyapunov exponents are used to describe the lagrangian coherent structures of the flow field in the whole process, which provides a new perspective to reveal the mixing process of solutes across the SWI.

Acknowledgments

This research was funded by the National Key R&D Program of China (2022YFC3202605), the Fundamental Research Funds for the Central Universities (B200204044), the Research funding of China Three Gorges Corporation (202003251).

How to cite: Xu, P., Wang, L., Xu, J., Wang, Z., Chen, H., and Wu, M.: Effects of solute release position on transient solute dispersion across the sediment-water interface: A modeling study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2145, https://doi.org/10.5194/egusphere-egu23-2145, 2023.

Surface water and groundwater interactions between the stream and the hyporheic zone profoundly affect river intermittency and biogeochemical processes, yet they are rarely quantified. As an excellent natural tracer, temperature was used to quantify unknown patterns of hydrologic fluxes and to understand their impact on heat budget over time and space. The Yousheng Creek (a first-order upstream of Chichiawan Creek in Taiwan) is one of the crucial habitats for the endangered species of Formosan land-locked salmon. In recent years, stream disconnection seriously limited the expansion of the salmon habitat, hampering the rehabilitation work. This study takes heat as a tracer to examine exchange processes in the intermittent reach of Yousheng Creek, with the application of fiber-optic distributed temperature sensor (FO-DTS). The combined use of FO-DTS, stream heat budget model (the HFLUX computer program), drone imagery, meteorological measurements, and field surveys allowed for identifying, quantifying, and mapping groundwater inputs beneath the 1924 meters reach. Analysis of the temperature traces measured from June 23rd to June 25th, 2019, have identified several active hyporheic zones and groundwater discharge points, providing significant cooling in the study section. HFLUX successfully modeled the river temperature through time and space with a normalized root mean square error of 3.12%. Inference from the model indicates a series of high infiltration zones at midstream whereas primary groundwater discharge from the downstream. The results suggest that different groundwater contributions along the Yousheng Creek significantly impact river temperatures. These insights of groundwater-surface water interactions can be applied to improve the knowledge of hydrology processes and energy budgets in headwaters

How to cite: Lee, T.-Y., Pan, Y.-W., and Chiu, Y.-C.: Modeling stream heat budget with HFLUX in a subtropical alpine intermittent river, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2177, https://doi.org/10.5194/egusphere-egu23-2177, 2023.

Nitrogen pollution is an important cause of lake eutrophication, and the deterioration of lake water quality caused by nitrogen pollution has attracted wide attention worldwide. Honghu Lake is a large lake located in the middle and lower reaches of the Yangtze River in China, which has been threatened by nitrogen pollution for a long time. We studied the evolution of Hong Lake from 1990 to 2020. From 1990 to 2005, the natural water area of Hong Lake decreased by 72%, and most of the natural water was converted into aquaculture water, resulting in the rapid deterioration of lake water quality. Farming was gradually banned after 2005 and the area of natural water has been restored, but the lake's water quality has not improved, indicating that the lake is also threatened by other sources of pollution. According to the results of two groundwater surveys in 2011-2012 and 2019-2020, we found that the nitrate nitrogen content in groundwater in the study area increased from 1.29mg/L to 3.58mg/L. We also collected sediment samples from the lakeshore zone, and the test results showed that a large amount of exchangeable nitrogen was adsorbed on the sediment. Groundwater is one of the important factors causing nitrogen pollution in lake.

How to cite: Tang, L.: Nitrogen accumulation in a big lake along wetland evolution affected by groundwater and surface water interaction in central Yangtze River Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4778, https://doi.org/10.5194/egusphere-egu23-4778, 2023.

Groundwater is a major source of water supply over most regions, accounting for nearly one-third of all water abstraction globally. However, due to overreliance on this resource, many aquifers around the United States have experienced rapid depletion, while some aquifers have seen increasing water levels due to extensive recharge efforts. It is essential to have a comprehensive understanding of the rate at which groundwater storages are changing to assess the potential availability of groundwater in the future. In this work, we estimate groundwater storage changes across more than 1000 watersheds over the US from a proposed algorithm for baseflow extracted using streamflow and precipitation observations. We also study the spatial and temporal variations in the characteristic of baseflow recession. We compare the storage change estimated from baseflows with those obtained from the estimates from the Gravity Recovery and Climate Experiment (GRACE) and observations from monitoring wells. The results help in validating the application of the proposed baseflow-based storage estimates in different aquifers and climatic regions. The proposed approach is simple and computationally efficient for estimating baseflows and groundwater storage changes in poorly-gauged watersheds.

How to cite: Hameed, M., Nayak, M., and Ahanger, M.: Groundwater Storage Changes in the United States using Baseflow Recession Method: Comparison with GRACE, and Observation Well Data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15784, https://doi.org/10.5194/egusphere-egu23-15784, 2023.