The presented study focuses on quantifying the impact of the anthropogenic heat input from residential buildings on the subsurface temperature regime, employing an innovative approach that combines building physics simulations with heat and groundwater flow modelling. To enhance the applicability of the approach, sensitivity analyses of various parameters that govern the heat transfer from the investigated buildings are performed. The investigated parameters took hydrogeological and meteorological conditions, building properties (including different insulation standards and building types) as well as petrophysical rock properties into account.

The findings contribute to a comprehensive understanding of the subsurface temperature regimes within densely settled areas, which is particular significant for the impact assessment of shallow geothermal applications. Results of the study show that neglecting anthropogenic heat input may lead to an underestimation of the effects of shallow geothermal applications on the underground temperatures.

How to cite: Hastreiter, N. and Vienken, T.: Anthropogenic heat input into the subsurface: Influencing factors and its importance during shallow geothermal impact assessment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16623, https://doi.org/10.5194/egusphere-egu24-16623, 2024.

Worldwide shallow groundwater is increasingly exposed to anthropogenic impacts. The thermal state of this important resource is affected not only by global warming but also by various local structures that release heat into the subsurface. This additional heat can accumulate and lead to local hotspots or - mostly urban - areas of elevated groundwater temperatures. The consequences of this warming for groundwater quality and ecology are widely unknown. Groundwater ecosystems are embedded in a naturally relatively stable environment, where temperature changes can affect the highly specialized, cold-stenotherm invertebrate community and meso- to psychrophilic microorganisms. In this study, we examine whether and how a groundwater temperature hotspot impacts groundwater ecology. We identified such a thermal anomaly in Hockenheim, Germany, caused by a water park with heated swimming pools and basements. The thermal impact was monitored over the course of a year by temperature data loggers in nine wells – four upstream and downstream of the structure each and one inside the basement. The same wells were sampled for chemical and microbiological parameters, such as the microbial total cell count and the cellular ATP content, as well as groundwater fauna. We additionally tested three wells in a nearby forest to obtain reference values that are mostly unaffected by anthropogenic interference. The measurements were repeated every three months in order to account for seasonal variations. The preliminary results show a local heat plume and an increase in groundwater temperatures by up to 8 K. However, there is no significant deterioration in the ecological parameters. Regarding the fauna, which generally shows low abundance due to oxygen depletion in the study area, we observed only a minor decrease within the thermally affected zone. Finally, the outcome of this study will improve our understanding of the vulnerability of groundwater ecosystems in the context of subsurface warming.

How to cite: Noethen, M., Becher, J., Menberg, K., Blum, P., Schüppler, S., Metzler, E., and Bayer, P.: Does an anthropogenically induced subsurface temperature hotspot affect groundwater ecology?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1879, https://doi.org/10.5194/egusphere-egu24-1879, 2024.

The subsurface Urban Heat Island effect has been proposed as a shallow geothermal energy resource, however, annual near-subsurface temperature variation may result in unexpected system performance. Understanding heat transport processes in the urban subsurface is key to managing and modelling city-scale thermal regimes for geothermal energy resource development. Existing studies have focussed on analysis of repeat temperature-depth profiles rather than long-term groundwater temperature time-series. We show here how time-series analysis can complement temperature-depth profiles and offer additional insights into the controls on subsurface thermal transport processes.

Annual variations in temperature time-series from 49 boreholes in the Cardiff Geo-observatory (UK), recorded between 2014-2018, fall into several distinct shape categories. We hypothesise these shapes are indicative of the dominance of particular flow and heat transport mechanisms such that sinusoidal profiles are associated with conduction-only settings, while ‘right-skewed’ profiles denote the influence of advection. Short-lived temperature events are observed on the cooling limbs of such profiles and are correlated with groundwater level rises, indicative of recharge events. These winter temperature drops have the effect of cooling groundwater faster in winter than it is warmed in summer. The short timescales of these events suggest recharge is localised and may be controlled by preferential flow paths within the superficial deposits overlying the aquifer. While these events do have an overall cooling effect on the seasonal temperature profile, groundwater temperatures following these events recover quickly to levels near what they were before the recharge event, suggestive of the presence of local thermal non-equilibrium with the gravel aquifer. More complex behaviours observed in boreholes located close to the city’s rivers indicate recharge responses coupled with the influence of stream-aquifer interactions. Thus, temperature time-series data have potential as a tool to identify subsurface hydraulic and thermal processes, with implications for geothermal exploration and the wider field of hydrogeology.

How to cite: Patton, A., Cleall, P., and Cuthbert, M.: Identifying urban subsurface thermal and hydraulic processes from time-series groundwater temperature data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1319, https://doi.org/10.5194/egusphere-egu24-1319, 2024.

Offshore freshened groundwater reserves have been identified in numerous regions worldwide. These reserves were often deposited during past sea level lowstands and are therefore non-renewable and slowly salinized by infiltrating seawater. However, in some cases these offshore freshened groundwater reserves can be connected to inland groundwater systems and can be recharged by fresh groundwater inflow from the landward direction. It has recently been suggested that these offshore freshened groundwater reserves could provide an additional source of fresh (and brackish) water for coastal communities that often face increasing fresh water stress. The feasibility, both economic and physical, of offshore freshened groundwater extraction is investigated in this study. To assess this feasibility from a physical point of view we built a set of 3D semi-conceptual groundwater flow models using the imod-wq code which allows us to estimate the offshore groundwater salinity development over large time scales (i.e. one glacial-interglacial cycle). The result of these large time scale models can be interpreted as estimations of the current offshore groundwater salinity conditions and thus provide a better picture of the current presence and magnitude of the offshore freshened groundwater resources in the model domain. In the next modelling stress period we introduce a set of pumping wells into the offshore domain and simulate several offshore freshened groundwater extraction scenarios. In such way we can evaluate the time it takes for these offshore freshened groundwater reserves to be fully salinized and exhausted. Additionally, we can also assess any potential negative impacts on the groundwater system in the coastal hinterland such as decreasing groundwater levels and/or increased salinization.

In the second part of our study we evaluate the economic feasibility of the offshore freshened groundwater pumping and use as additional fresh water resource for coastal communities. Several coastal areas located in south and south-east Asia (e.g. Pearl River delta) were selected since this region is identified as a region with high possibility and magnitude of offshore freshened groundwater resources. The economic parameters that are taken into account as favourable for offshore freshened groundwater exploration are (i) the overall economic development (e.g. GDP, HDI), (ii) the presence of groundwater pumping and desalination plants inland meaning the technology is already present in the region and (iii) costs of fresh water and groundwater pumping and desalination infrastructure in the region. Our study is only the first step in assessing the feasibility of offshore freshened groundwater exploration and hopefully our approach will be improved and tested in other coastal regions around the world to evaluate the full potential of these still untapped fresh groundwater resources.

How to cite: Zamrsky, D., van Lith, J. J. H., and van Beek, R.: Conceptual 3D groundwater models of offshore freshened groundwater extraction and its economic viability assessment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10761, https://doi.org/10.5194/egusphere-egu24-10761, 2024.

The analysis of twenty geophysical well logs covering a shelf area of about 3,500 km² in front of the Emilia-Romagna coast (Italy), has shown apparent resistivity (ρ_a) values consistent with important Offshore Freshened Groundwater (OFG) reserves stored in the first 450 m of the Middle-to-Upper Pleistocene succession, and extending seaward about 60 km from the modern shoreline. Four classes with different ρ_a intervals (i.e., different salinities) were identified. The first three (1, 2 and 3) classes are characterized by ρ_a ranges of 7-28 Ω m, 4-7 Ω m and 2-4 Ω m, respectively. These values are higher than seawater resistivity (< 2 Ω m - i.e., class 4) and, based on the OFG definition (i.e., “the water stored in the sub-seafloor with a total dissolved solid concentration below that of seawater”), they have been used for OFG identification. Class 1 ρ_a is coherent with fresh-to-brackish water content, whereas classes 2 and 3 have been interpreted as transitional to seawater.
The correlation of offshore wells (spontaneous potential and ρ_a profiles) with onshore data (stratigraphic and lithological) from water wells and additional geophysical well logs, led to the stratigraphic architecture reconstruction of the Plio-Pleistocene siliciclastic succession along onshore-offshore transects, up to 60 km-long, from the Apennine front to the Adriatic shelf. The uppermost (first 450 m) Middle to Upper Pleistocene interval displays a vertical alternation of high-permeability (amalgamated and laterally continuous gravel to sand bodies) and low-permeability (mud-dominated) strata made of fluvio-deltaic, coastal and shelfal deposits. The high-permeability bodies represent the offshore extension of the onshore aquifer systems, whereas the low-permeability units make the aquitards. Along the transects, different stratigraphic intervals characterized by the four ρ_a classes have been identified. The highest ρ_a values (class 1) have been documented in the first 300 m of the succession, despite its deposition mostly occurred in deltaic to marine (i.e., saline water) conditions. This interval wedges out seawards, with ρ_a progressively decreasing down to class 3 values at about 35 km from the coast. Similarly, ρ_a decreases vertically, between about 300 and 450 m depth. Such a vertical gradual decrease may suggest that locally aquitards do not completely prevent water exchange, and transitional classes 2 and 3 likely resulted from freshwater and seawater mixing through space and time. Below 450 m depth, ρ_a drops to < 2 Ω m (class 4), thus defining the lowermost limit of the OFG reserves.    
Onshore-offshore reconstructions additionally revealed how OFG aquifers are actively recharged in correspondence of the Apennine front, where the topographic gradient is higher and permeable units are subaerially exposed. Their extremely high degree of amalgamation even allows the topographically-driven recharge of the deeper (and marine) strata.
The relatively shallow depth (< 350 m) of the northern Adriatic aquifers and the presence of several and abandoned oil&gas platforms in the area, provide a good opportunity to further investigate these OFG reserves that are strategic for the densely populated Emilia-Romagna coastal plain.

How to cite: Campo, B. and Antonellini, M.: Offshore freshened groundwater reserves identification as revealed by geophysical and stratigraphic data: insights from the Northern Adriatic shelf (Italy) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5599, https://doi.org/10.5194/egusphere-egu24-5599, 2024.

The work focused on the Oligo-Miocene Ragusa Formation, a known regional shallow aquifer in the Hyblean Plateau in southern Sicily, made of medium to high porosity carbonates deposited in the ramp environment, also investigated in the adjacent offshore by deep well drilling.

The main objective was to investigate if and how this known shallow onshore aquifer extend in the coastal area and possibly offshore.

A detailed methodology was defined for the quantitative use of geophysical logs from about five deep Oil & Gas wells to characterize groundwater in the Ragusa Formation in terms of pressure, piezometry and salinity distribution, as it follows:

• A first step was the digitization of the full suite of logs required for the application of petrophysical workflow for each well analysed, for a total of about 25 km of digitized logs, such as SP (Spontaneous Potential), GR (Gamma Ray), DT (Sonic log) and Resistivity logs.
• At the same time a synthetic lithological log for each selected well was built, to support the understanding of lithological influence of electrical logs.
• A customised petrophysical workflow to calculate porosity and salinity (concentration of salts in TDS) was applied, considering: lithotypes, BHT (borehole temperatures), porosity (derived to DT – sonic log), pore fluid resistivity.
• A comparison of TDS results with salinity data from DST and composite logs was performed.
• A detailed well correlation and comparison between onshore shallow water wells and deep Oil&Gas wells, both onshore and offshore, was carried out.

By applying this petrophysical approach, it was possible to identify and quantified key indications of the presence of fresh groundwater in the Ragusa Formation carbonates both onshore and offshore in southern Sicily (Italy). Indeed, has been demonstrated that the onshore outcropping aquifer appear likely connected with the deep offshore aquifer due to positive indications in the same geological formation 10 km offshore from the coastline.

How to cite: Chiacchieri, D., Lipparini, L., Micallef, A., and Quiroga, E.: Offshore freshened groundwater identified in southern Sicily (Italy) by applying well logs petrophysical interpretation. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4575, https://doi.org/10.5194/egusphere-egu24-4575, 2024.

Before extraction of offshore freshened groundwater (OFG) begins, ownership must first be determined in order to confirm the right to access, possess and distribute of the resource.  Since the geological formations sheltering OFG extend beyond the coastline, the UN Convention on the Law of Sea, which has been ratified by most countries in the world, will apply, and it provides that the nation having rights to the continental shelf where the OFG is located will have sovereign rights to the resource.  However, political boundaries do not often respect geologic formations, and some deposits of OFG will straddle national boundaries.  The Law of the Sea Convention is silent on transboundary resources, so policymakers must look to other legal principles that address governance of natural resources in order to develop a governance regime.  This presentation will summarize the applicable international law principles and will provide guidance on how transboundary OFG may be governed.

How to cite: Martin-Nagle, R.: Transboundary Offshore Freshened Groundwater: What Law Applies?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21423, https://doi.org/10.5194/egusphere-egu24-21423, 2024.

Dual porosity flow is an important mechanism for groundwater transport in fractured rock aquifers. However, quantification and characterization of fracture flow systems remains challenging, as it often involves complex procedures such as the injection of tracers. In this study we conducted single-well pumping tests in 11 uncased wells in a coastal fractured rock aquifer while monitoring in-well salinity and temperature gradients through downhole casts using a Conductivity-Temperature-Depth (CTD) logger. In this way, we aimed to observe how naturally occurring salinity gradients in the well become disturbed by induced groundwater flow to the well, and if these gradients may serve as natural tracers for fracture flow. Since natural temperature gradients in the wells are minimal, we applied point electrical heating at the bottom of the well to create a plume of slightly warmer water to migrate up the wellbore during pumping from the top. During the pumping tests in this set-up, repeated CTD casts suggest that groundwater flow to these wells is strongly focused along narrow zones and is occurring at various rates over a range of salinities and temperatures. Hence, the observed patterns in both salinity and temperature presumably reflect the presence of fracture zones, which could indeed be confirmed by downhole camera observations for some wells. Further data analysis resulted in detailed hydrogeological characterization of the 11 wells, comprising an assessment of the fracture density and hydraulic conductivity of the aquifers, as well as the origin of the inflowing water being meteoric mostly fresh water or deeper saline groundwater.

How to cite: Kruijssen, T., Wit, M., Akkermans, S., Leusink, J., van Breukelen, B., van der Ploeg, M., and Bense, V.: Turn up the heat to locate and quantify groundwater flow in fractured rock aquifers in coastal zones of the tropical island of Curaçao, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13973, https://doi.org/10.5194/egusphere-egu24-13973, 2024.

The traditional territory of the Lù'àn Män Ku Dän (Kluane Lake People) is found along the Saint Elias Mountains in Yukon. It hosts the Burwash Landing community, home of the Kluane First Nation, which is one of eleven self-governing First Nations operating in tripartite with Yukon Government and Canada. Burwash Landing is primarily dependent on diesel for space heating and power generation. Cutting-edge technologies were deployed in the scope of geothermal resource assessment to evaluate the thermal state and properties of the subsurface. Active distributed temperature sensing was conducted with a composite heating and fiber-optic cable installed in the water column of two existing wells with the objective of quantifying the geothermal potential and groundwater flow along available wellbores. Heat injection tests were made in the 220 and 385 m deep wells located on the south and north side of the Denali fault, near a probable releasing bend that is favorable to permeability. Melting glacier water infiltrates in mountains and groundwater flows toward Kluane Lake, which is hypothesized to be a major groundwater discharge zone. The shallower well is at an altitude of 925 masl and intercepted 40 m of quaternary deposits before hitting fractured bedrock while the deeper well is at the valley bottom near the lake (altitude of 795 masl) and entirely drilled in quaternary deposits. Passive temperature monitoring was initially made and revealed a geothermal gradient of 34 ⁰C km^-1 and 47 ⁰C km^-1 in the shallow south side and deep north side wells. Heat was injected during active tests for 2 and 3 days and thermal recovery was monitored for 6 and 8 days, respectively. Temperature was measured every 25 cm at 4-minute intervals. The infinite line source equation and the superposition principle were used to analyze data and calculate a thermal conductivity profile. Nearly continuous ground thermal properties and temperature profiles were combined to assess the Earth natural heat flux, considering paleoclimate and topographic corrections. Analysis indicated a heat flux above 90 mW m^‑2, thought to be favorable for geothermal resource development. Peclet number analysis was undertaken to infer horizontal groundwater flow in permeable horizons. Results are being used to develop a regional groundwater flow and heat transfer model to evaluate temperature at kilometer depth and assess the communities’ geothermal potential. This presentation will illustrate how active temperature sensing can be deployed to reduce geothermal exploration risks, acknowledging Kluane First Nation that allowed us to better understand groundwater flow in this magnificent territory.  

How to cite: Raymond, J., Chapman, F., Klepikova, M., Bour, O., and Soucy Laroche, R.: Active fiber-optic distributed temperature sensing to assess the geothermal potential and groundwater flow over the traditional territory of the Lù'àn Män Ku Dän, Yukon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20169, https://doi.org/10.5194/egusphere-egu24-20169, 2024.

Unconfined shallow aquifers are particularly exposed to the risk of contamination. Especially when exploited for drinking water production, for which water quality is of particular concern, careful monitoring of the physical processes and detailed characterization of the subsurface properties are crucial. Furthermore, the possible presence of heterogeneities, such as intricate networks of hydraulically conductive paleo-channels that are often inherent in alluvial aquifers, can establish preferential pathways. Consequently, monitoring activities in these complex environments pose serious challenges and raise the demand for advanced techniques and innovative approaches. In this context, recent advances have been made possible by employing Fiber Optics Distributed Temperature Sensing (FO-DTS). This technology combines the use of heat as a natural tracer with a detailed spatiotemporal resolution and has proven informative in a wide variety of applications. In this study, we applied downhole passive FO-DTS to a cluster of piezometers in a highly heterogeneous phreatic gravelly aquifer. The aquifer is exploited for irrigation and drinking water supply, and exhibits both natural and pumping-induced groundwater temperature fluctuations. Vertical transient water temperature profiles were acquired over a 1-month experiment. Borehole-dependent and depth-related features of the temperature measurements were ascribed to possible spatial structures having different hydraulic conductivity. The collected data were used to invert the three-dimensional saturated hydraulic conductivity field of a physics-based numerical model that couples flow and heat transport. Even without active heating, FO-DTS has demonstrated its ability to provide valuable insights at an unprecedentedly high resolution.

How to cite: Furlanetto, D., Camporese, M., Schenato, L., Costa, L., and Salandin, P.: Downhole passive fiber optics temperature monitoring for improved characterization of aquifer heterogeneities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7668, https://doi.org/10.5194/egusphere-egu24-7668, 2024.

Geothermal heat production and aquifer thermal energy storage have significant potential to contribute to the energy transition. However, due to higher temperature inside the wells used, it is known that this leads to heat loss through conduction to the surrounding cooler, shallower groundwater systems. Therefore it is important to be able to anticipates such impacts to allow effective monitoring and prevention or mitigation measures when needed. However the thermal impact on groundwater systems is expected to strongly depend on local conditions. Therefore, this study focused on the impact of operational conditions (e.g. effective well temperatures and intermittency) and aquifer conditions (e.g. permeabilities and heterogeneity) on the resulting heat transport processes into the aquifer by conduction and density driven flow. To evaluate the degree and variation of impact that may occur under field conditions, the heat loss to a shallow groundwater system was simulated using a 2D axisymmetric numerical MODFLOW 6 model for a wide range of conditions considering both the impact of conduction and density-driven flow. The results of this study indicate that the total thermal impact and its distribution (up to >10 m from the hot well in 10 years) in shallow groundwater systems is strongly impacted by the induced density driven flow in the relatively permeable layers of the groundwater system. Conduction is dominant in transfer of heat from the hot well in the low permeability confining layers and for mitigating temperature differences in the groundwater system induced by buoyancy flow. Overall, this study highlights the importance of considering local conditions in assessing thermal impact by heat losses from hot well casings, to allow distinguishing these thermal impacts from those induced by leakage and to allow efficient thermal groundwater impact monitoring.

How to cite: de Vries, E. and Hartog, N.: Thermal impact on shallow groundwater systems by heat loss from hot wells: the impact of operational conditions and subsurface heterogeneity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15116, https://doi.org/10.5194/egusphere-egu24-15116, 2024.

Tidal analysis of borehole pressure has become in the recent years’ literature an essential method to follow the evolution of the hydraulic conductivity of an aquifer over time. Most traditional methods (mainly pumping or slug tests) only produce a small number of observations, and come at a greater cost. However, groundwater level tidal analysis only requires monitoring data at a sampling rate of 1 hour, data which is extensively available. These solutions are applicable provided aquifers respond to at least one tidal phenomenon among oceanic, earth or atmospheric tides.

Martinique Island, in the Lesser Antilles, is a very interesting field to study these techniques, since 16 years of piezometric level data have been recorded on this volcanic island in a monitoring network of 29 boreholes. Here we focus our study on a closely monitored study site in the Galion plain, with three boreholes, a seismometer and past conducted pumping tests and seismic surveys. We compute amplitude and phase response of aquifers to atmospheric and earth tides. Then, the response of the semi-confined aquifers to different loading sources at the tidal frequencies (between 1 and 2 cycles per day) is modelled. A careful inversion is done to obtain the characteristics of the aquifer, including aquifer transmissivity and shear modulus.

Finally, we analyse the evolutions of these inverted parameters and decipher their reversible and irreversible changes. Between earthquakes, we show the dominant effect of effective stress to control aquifer hydraulic conductivity. At the time of the earthquake, with the help of seismic stress numerical simulation, we show that seismic shear stresses are the most probable cause of aquifer properties changes both in permeability and shear modulus.

How to cite: Thomas, A., Fortin, J., Vittecoq, B., and Violette, S.: Passive characterization of aquifer permeability and shear modulus and their evolution following earthquakes using tidal signals, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19115, https://doi.org/10.5194/egusphere-egu24-19115, 2024.

Groundwater monitoring is fundamental, especially for areas where there is a high dependency on groundwater use. Groundwater level (GWL) monitoring is poorly known in Ethiopia. The study focused on evaluating groundwater levels and their relation to precipitation in Ethiopia's Gilgel Gibe and Dhidhessa catchment areas. Groundwater levels (GWL), spring discharges, and rainfall data were collected from various points over the 2022/2023 hydrological year.  Rainfall varied across the region, increasing from April to September and decreasing from plateaus to lowlands with a value between 1539 mm to 1973mm annually. Groundwater levels showed significant spatial and temporal variation, influenced by precipitation and local topography.  Maximum water level varies between 17.6 and 5.75 m in the northwest, 11.6 and 6.2 m in the central part, 11.5 and 3.2 m in the east, 13.1and 4.2 m in the south. Minimum water level varies between 13.2 and 3.8 m in the northwest, 5.8 and 2.7 m in the central, 3.5 and 1.1 m in the east and 7 and 3. 6 m in the south of the study area. Groundwater level fluctuation in the automatically monitored well was 1.55m in the deep well and 3.99m in the shallow well. The spatial drop of the water table in the northwest and south is due to a hydraulic gradient to lowlands and depressions, and evapotranspiration from dense forest coverage. In the central and eastern study area, GWL is shallow and intermediate based on the positions of monitoring wells. Some wells are fully saturated during the rainy season between August and September. Shallow wells reacted swiftly to rainfall, but their levels declined in the dry season. Some wells in high elevation areas experienced minimal fluctuations due to their perched aquifer positions. Groundwater drawdown from usage in dug wells quickly recovered, suggesting potential for small-scale agricultural use. Long-term monitoring and increased data logging are recommended for future studies.

How to cite: Kebede, A. B., Feyessa, F. F., Hermans, T., and Walraevens, K.: Groundwater Level Assessment using Data logger and Manual monitoring in developing Country, southwestern Ethiopia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17299, https://doi.org/10.5194/egusphere-egu24-17299, 2024.

The exchange between groundwater (GW) and surface water (SW) plays a crucial role for streamflow generation and the biogeochemical cycles within landscapes. However, accurately observing and predicting this exchange remains challenging due to the spatial heterogeneity and temporally dynamic fluxes of groundwater within the stream corridor. This presentation offers new insights into the characteristics of GW-SW interactions and hydrological processes within the hillslope-riparian-stream continuum, employing a combined experimental and modeling approach. The research builds on a comprehensive, long-term dataset obtained through baseline monitoring in the Weierbach Experimental Catchment (WEC) in Luxembourg that is a 45-hectare forested catchment. In addition to baseline monitoring, our approach involved (i) a network of 43 wells and piezometers along a selected stream reach for continuous monitoring and tracer experiments, (ii) a network of 13 wells along the riparian-hillslope interface, and (iii) ground-based thermal infrared imagery to observe spatiotemporal dynamics of surface saturation along the stream corridor. An integrated surface-subsurface hydrologic model served as a hypothesis-testing tool to examine whether surface saturation is predominantly driven by groundwater inflow or precipitation and how the relevance of the processes – surface ponding from precipitation or subsurface exfiltration – change in space and time.

We coupled the hydrological model with a hydraulic mixing-cell approach that enabled deciphering the contributions from different water sources to SW. The well network and associated artificial tracer experiments provided valuable insights into the direction of GW-SW exchange, revealing directional variability at scales of a few meters. Additionally, wells at the riparian-hillslope interface demonstrated a strong non-linearity of GW contributions to SW, influenced by GW table fluctuations. The observed and simulated surface saturation aligned well, suggesting that GW exfiltration primarily controls surface saturation in the stream corridor. Furthermore, the mixing-cell simulations revealed that subsurface water exfiltration is the dominant source for riparian surface water and intermittent streamflow, with distinct differences between stream water and riparian surface saturation. Overall, the combination of experimental techniques, hydrologic modeling, and well networks clearly improved our understanding of GW-SW interactions and revealed previously hidden exchanges in the WEC.

How to cite: Klaus, J., Blöschl, G., Bonanno, E., Glaser, B., Gourdol, L., Hissler, C., Hopp, L., Pfister, L., and Smettem, K.: Exploring the Hidden Exchanges: Groundwater-Surface Water Interactions in a Critical Zone Observatory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5269, https://doi.org/10.5194/egusphere-egu24-5269, 2024.

Springs offer insight into the sources and mechanisms of groundwater recharge and can be used to characterize fluid migration during earthquakes. However, few reports provide sufficient annual hydrochemical and isotopic data to compare the variation characteristics and mechanisms with both atmospheric temperature and seismic effects. As such, it is critical to obtain time series observations of stable isptopes (δ²H, δ¹⁸O and δ¹³C_DIC) to understand the complex interactions between hydrological processes, cycle, and relationship with earthquakes. In this study, we used continuous δ²H, δ¹⁸O, δ¹³C_DIC, and major ion data from four springs over 1 year to understand the groundwater origin, recharge sources, circulation characteristics, and coupling relationships with atmospheric temperature and earthquakes. We found that (1) the four springs are likely recharged by deep circulation of meteoric water from Bogda Mountain in the east, as well as long-distance runoff recharge from the Turpan Basin to the south. (2) atmospheric temperatures above and below 0 °C can cause significant changes in ion concentrations and water circulation depth, resulting in the mixing of fresh and old water in the aquifer, it can cause changes in δ¹³C_DIC but it doesn’t work in δ²H and δ¹⁸O. (3) Earthquakes of magnitude ≥ 4.8 within a 66 km epicentral distance can alter fault zone characteristics (e.g., permeability) and aggravate water–rock reactions, resulting in significant changes in δ²H, δ¹⁸O, and hydrochemical ion concentrations, accompanied by limited changes in δ¹³C_DIC. (4) Hydrogen and oxygen isotopes are the most sensitive precursory seismic indicators. The results of this study offer a reference for the establishment of long-term hydrochemical and isotopic monitoring, with the potential for use in earthquake forecasting.

This work is financially supported by the Natural Science Foundation of China (Grant No. 42373067) and by the Science for Earthquake Resilience (grant number XH23048C).

How to cite: Zhou, Z., Ren, X., Zhong, J., and Feng, X.: Response Characteristics of Hydrogen, Oxygen, and Carbon Isotope Composition to Atmospheric Temperature and Seismic Activity in Spring Water Hydrogeochemistry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2499, https://doi.org/10.5194/egusphere-egu24-2499, 2024.

The geothermal resources in sedimentary basins have high potential for development and utilization, and have become an important research topic worldwide(Olasolo et al.,2016; Pasvanoğlu and Çelik., 2019; Duan et al.,2022). This paper focuses on the genetic mechanism and evolution process of deep geothermal water were explored through the analysis of hydrogeochemical and isotope geochemical data, which can provide technical and theoretical support for the sustainable development of geothermal fields in the Weihe basin. The study indicates that: (1)the hydrochemical type of geothermal water of Dongda geothermal field are predominantly HCO₃·SO₄-Na type. Meanwhile, the hydrochemical type of geothermal water of the northern Xi'an Depression are mainly SO₄·HCO₃-Na and SO₄·HCO₃·Cl-Na types. The ionic fraction is primarily influenced by the dissolution of silicate and evaporite minerals, as well as alternating cation adsorption. (2) Geothermal water is primarily recharged by atmospheric precipitation originating from the Qinling Mountains. The recharge elevation ranges from 677.94 m to 1467.65 m. (3) The Dongda geothermal field has a thermal storage temperature ranging from 50.19℃ to 80.29℃, and a depth of thermal circulation ranging from 1126.32 m to 2129.62m. Meanwhile, the northern Xi'an depression has a thermal storage temperature ranging from 73.17℃ to 109.50℃, and a depth of thermal circulation ranging from 1892.41 m to 3103.22 m. (4) The δ¹⁸O of the geothermal water in the northern Xi'an depression is more significantly shifted to the right of the atmospheric precipitation line than that of the Dongda geothermal water, indicating a significant “oxygen drift”.(5) The Dongda geothermal reservoir in the southern Xi'an Depression mainly experiences heat transfer through convection, while the geothermal reservoir in the northern Xi'an depression experiences heat transfer through conduction.

References

[1]Duan, R., Li, P., Wang, L., He, X., & Zhang, L. (2022). Hydrochemical characteristics, hydrochemical processes and recharge sources of the geothermal systems in Lanzhou City, northwestern China. Urban Climate, 43, 101152.

[2]Olasolo, P., Juárez, M. C., Morales, M. P., & Liarte, I. A. (2016). Enhanced geothermal systems (EGS): A review. Renewable and Sustainable Energy Reviews, 56, 133-144.

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How to cite: Liu, J., Ren, Z., Yu, Q., Yan, X., Qi, K., Wang, Z., Guo, S., Lan, H., Jia, M., and Liu, Y.: Hydrogeochemical Characteristics and Genetic Mechanisms of Geothermal Fields in the Xi'an Depression of the Weihe Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4855, https://doi.org/10.5194/egusphere-egu24-4855, 2024.