Hydraulic, thermal, chemical and mechanical processes (coupled or uncoupled) in saturated media are of increasing interest in many hydrogeological contexts and their understanding is a major challenge in modern hydrogeology. These processes play a major role in natural systems, such as in thawing rock and soil and volcanic environments, but also in anthropogenic systems where human activities are intensifying the pressure on groundwater and subsurface space use. The understanding of these processes is of paramount importance to:
• Ascertain the role of groundwater in the context of geothermal energy and mitigating its impacts. It is also needed to consider the seasonal and long-term development of thermal and mechanical conditions in aquifers, heat transfer across aquifer boundaries and between fluid and rock are focus points, and the influence of precipitated fluid compounds.
• Enhance the development of underground constructions improving its efficiency and minimizing impacts.
• Achieve an accurate characterization of subsurface flow, transport and heat transfer, which require observations of induced or natural variations of the thermal regime. There are many ongoing research projects studying heat as a natural or anthropogenic tracer, for characterizing aquifers, flow conditions, and crucial transport processes, such as mechanical dispersion.
• Determine the fate and evolution of pollutants and micropollutants introduced in natural systems by the leakage from sewers and the discharge of wastewaters.
• Explain the influence of urbanized areas and sealed surfaces in the aquifer behavior.
• Improve predictions concerning groundwater extraction, injection and/or both, and their associated impacts.
• Understand the consequences and risks associated with CO2 storage.
• Identify and quantify aspects that may impact the urban groundwater and investigate methods for minimizing their influence (e.g., artificial groundwater recharge, improvement of the “natural” recharge in urban areas, reuse of pumped groundwater, redesign of geothermal systems, etc.) and to enhance groundwater management strategies.
This session welcomes contributions that deliver new insight in the field of hydraulic, thermal, chemical and mechanical processes in saturated media, specially, but not restricted to, in urban aquifers, including experimental design, reports from new field observations, demonstration of sequential or coupled physical and numerical modelling concepts or case studies.

Co-organized by ERE6
Convener: Estanislao Pujades | Co-conveners: Martin Bloemendal, Anna Jurado Elices, Olivier Bour, Victor Vilarrasa, Kathrin Menberg, Guillaume Attard
| Attendance Mon, 04 May, 14:00–15:45 (CEST)

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Chat time: Monday, 4 May 2020, 14:00–15:45

Chairperson: Martin Bloemendal, Victor Vilarrasa, Anna Jurado, Estanislao Pujades
D516 |
Kent Novakowski

Heat transfer experiments conducted in the subsurface are usually interpreted using either analytical or numerical models, which incorporate first-type boundary conditions (specified temperature) to introduce the heat into the solution domain. An alternative approach is to use a third-type boundary condition, often refereed to as a convection bc in the heat transfer literature, which includes a heat transfer coefficient to accommodate the exchange of heat between fluid flowing outside the domain to that inside the domain under potential. To explore the impact of this boundary condition, a semi-analytical model was developed for a linear flow system in a discrete rock fracture with advective heat transfer in the fracture and conductive heat transfer in the matrix. To illustrate the influence of the heat transfer coefficient, the model is applied to the results of a heat tracer experiment conducted in a discrete fracture connecting two boreholes in a crystalline rock, with warm fluid injection in one borehole and passive temperature measurement in the other.  The experimental results were also simulated using a similar model having a first-type condition at the injection borehole for comparison. The simulations show that the heat transfer coefficient has a significant influence on the shape of the breakthrough curve and allows for an excellent match with the field data, whereas the model with the first-type condition cannot obtain a match of similar quality. 

How to cite: Novakowski, K.: The Use of a Convection Boundary Condition in the Simulation of a Heat Tracer Experiment Conducted in Bedrock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3592, https://doi.org/10.5194/egusphere-egu2020-3592, 2020.

D517 |
Johannes Nordbeck, Jens-Olaf Delfs, Malte Schwanebeck, Christof Beyer, and Sebastian Bauer

High temperature aquifer thermal energy storage (HT-ATES) is a promising technology for mitigating the temporal disparity between availability and demand for heating energy supply. By applying seasonal storage, renewable or alternative sources like waste heat can be used, reducing the dependency on fossil fuels and avoiding CO2-emissions.

HT-ATES uses external heat sources and stores heat in suitable formations in the geological underground by injecting hot water at temperatures of up to 90°C. Balanced energy injection and extraction however, is usually not feasible due to energy losses, leading to residual heat in the subsurface and maybe changing groundwater composition and quality. This study shows that numerical simulations can be used to quantify the thermal impact of heat storage on the geological storage formations as well as the subsurface space demand of such storage sites.

In a hypothetical scenario, a HT-ATES system is designed to store about 35 GWh/a of excess heat from solar thermal installations and a waste incineration plant, which would cover about 20 % of the heat energy needs of a typical city district. For this purpose, a three-dimensional numerical model of the HT-ATES is set up, which consists of six well doublets placed at 100 m depth in a typical northern German Pleistocene formation, a sand aquifer bounded by till layers at the top and bottom. The screen lengths of all wells cover the entire storage formation thickness of 20 m. The daily excess heat storage demand is derived from the estimated daily heat demand for space heating and hot water production for the city district, based on an available 3D building stock model and daily outside temperature data for 2018, combined with a supply curve for solar thermal heat production, which is based on available roof and open space area in the district and daily global radiation data for the location of the district from 2018.

Injection flow rates vary between 0 and 45 m³/h, while the injection temperature is assumed constant at 70°C. The extraction flow rates are controlled by a well doublet control module, which iteratively adapts the extraction flow rates according to the heat demand curve.

Results show that during the heating period from October to May, at least 21 GWh and up to 26 GWh after 30 years of operation or 12 - 15 % of total district heat demand can be supplied each year by the HT-ATES. Supply temperatures range from 70 to 39 °C at the start and at the end of the heating period, respectively. The storage efficiency increases from 65 to 74 to 78 % after 5, 15 and 30 years of operation, respectively. After 30 years, the HT-ATES operation affects an ellipsoid shaped volume of 28 Mio m³ with temperature increases of > 1 °C, which corresponds to the volume of a cube of approximately 300 m side length.

How to cite: Nordbeck, J., Delfs, J.-O., Schwanebeck, M., Beyer, C., and Bauer, S.: High Temperature ATES: Thermal impact and efficiency assessment with numerical simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14085, https://doi.org/10.5194/egusphere-egu2020-14085, 2020.

D518 |
| solicited
Ashley M Patton, Gabriel C Rau, Corinna Abesser, David R James, Peter J Cleall, and Mark O Cuthbert

Urban environments often have highly variable and evolving hydrogeology. Coastal cities present even greater challenges to hydraulic and thermal conceptualisation and parameter estimation due to their complex dynamics and the heterogeneity of ocean-influenced hydraulic processes. Traditional methods of investigation (e.g. pump tests, invasive sampling) are time consuming, expensive, represent a snapshot in time and are difficult to conduct in built-up areas, yet properties derived from them are crucial for constructing models and forecasting urban groundwater evolution.

Here we present a novel approach to use passive sampling of groundwater head data to understand subsurface processes and derive hydraulic and geotechnical properties in an urban-coastal setting. This is illustrated using twenty years of high frequency (hourly) time-series data from an existing groundwater monitoring network comprising 234 boreholes distributed across Cardiff, the capital city of Wales, UK. We have applied Tidal Subsurface Analysis (TSA) to Earth, Atmospheric and Oceanic signals in groundwater time-series in the frequency domain, and also generated Barometric Response Functions in the time domain. By also observing the damping and attenuation of the response to ocean tides with distance from the coast and tidal rivers, this combination of analyses has enabled us to disentangle the influence of the different tidal components and estimate spatially distributed aquifer processes and parameters.

The data cover a period pre and post construction of a barrage across the coastline, impounding the city’s rivers. We were therefore able to observe a huge decrease in the subsurface ocean tide signal propagation following this human intervention, through the coastal and tidal river boundaries. These changes reveal variations in hydraulic responses and values of hydraulic diffusivity between different lithologies, notably with made-ground deposits being much less sensitive to ocean tides than the underlying sand and gravel aquifer. By being able to map the spatial variations in hydraulic response and barometric efficiency for the first time (and therefore formation compressibility and extent of aquifer confinement) we have been able to refine interpretations (and in some cases overcome misconceptions) derived from previous inferences made solely from borehole logs. We anticipate that linking the improved hydraulic characterisation, enabled by the new methodology, will also help better characterisation of the subsurface thermal regime, and management of shallow geothermal energy resources in coastal urban aquifers.

How to cite: Patton, A. M., Rau, G. C., Abesser, C., James, D. R., Cleall, P. J., and Cuthbert, M. O.: Characterising hydrodynamic controls on groundwater in a coastal urban aquifer using time and frequency domain responses at multiple spatiotemporal scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1235, https://doi.org/10.5194/egusphere-egu2020-1235, 2020.

D519 |
Milena Walaszek, Lise Cary, Gabriel Billon, Michaela Blessing, Aurélie Bouvet-Swialkowski, Justine Criquet, and Jean-Rémi Mossmann

The metropolis of Lille (more than 1 million inhabitants) produces 40% of its drinking water through well fields tapping the chalk aquifer to the agricultural, urban, and industrial region southwest of Lille. The groundwater quality is threatened by the presence of chlorinated solvents amongst others pollutants. In fact, many industries using chlorinated solvents are or were established on the well field’s territory as paint factories, industrial laundries and metallurgical plants. The chlorinated solvent concentrations in groundwater often exceed the regulation limit for drinking water usage in France ([perchloroethene] + [trichloroethene] < 10 µg.L-1) and then limit its use for drinking water production. The understanding of the chlorinated solvents dynamics and space distribution in the aquifer is a major issue for the metropolis of Lille. In addition, the quantities of available water with good quality is currently decreasing due to repeated annual droughts in the recent years. Thus, the metropolis of Lille, the French Geological Survey and the University of Lille explored the transfer and degradation mechanisms of the chlorinated solvents in the well fields in the two research projects RESEAU (2016-2019) and COHMET (2017-2020).

18 wells and 9 piezometers were sampled during 3 years in order to evaluate the chlorinated solvents concentrations. In order to assess a possible migration of the compounds, three piezometers were sampled along the water column using passive samplers. In addition, a more detailed hydrochemical characterisation of groundwaters (chemical elements markers of the reducing conditions, physico-chemical parameters) was performed in the same three piezometers. Furthermore, the possible chlorinated solvent sources were identified with the databases BASIAS and BASOL, which list the past and current industrial plants, polluted soils and sites on the French territory. Finally, the chlorinated solvent degradation mechanisms were investigated with a compound-specific carbon isotope analysis.

The three-year concentration monitoring highlights complex dynamics of the chlorinated solvents in the aquifer. A wide variety of compounds is detected in the well fields (perchloroethene, trichloroethene, cis and trans-1,2-dichloroethene, 1,1-dichloroethene, 1,1,1-trichloroethane, 1,1-dichloroethane, 1,2-dichloroethane and vinyl chloride) with maximal concentrations ranging from 1.2 (vinyl chloride) to 155 µg.L-1 (cis-1,2-dichloroethene). The highest concentrations are measured downstream three former industrial laundries in the south of the territory. The chlorinated solvent concentrations are stratified along the wells water columns and increased with depth. These concentration increases are consistent with water inlets along the wells originated from the fractured chalk. Despite the measure of favourable physico-chemical conditions, the δ13C ratios comparison do not indicate biodegradation of the chlorinated solvents, except in two wells. The concentrations changes are essentially due to the migration of compounds in depth. Then, the δ13C ratios indicate the presence of several major sources of chlorinated solvents.

How to cite: Walaszek, M., Cary, L., Billon, G., Blessing, M., Bouvet-Swialkowski, A., Criquet, J., and Mossmann, J.-R.: Transfer dynamics of chlorinated solvents in the chalk aquifer of northern France, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13163, https://doi.org/10.5194/egusphere-egu2020-13163, 2020.

D520 |
Abolfazl Poozan, ََAndrew Western, Meenakshi Arora, Matthew Burns, and Tim Fletcher

Urbanization leads to severe alterations to the flow regime of receiving waters, including increased frequency and magnitude of storm flows as well as reduced baseflows. Infiltration basins are among the most widely applied stormwater control measures worldwide, in part for their ability to intercept stormwater runoff and allow it to infiltrate into the ground, with the assumption that this will recharge groundwater and thus help in restoring clean, filtered baseflows to receiving waters. Recent research has highlighted that in fact, the fate of infiltrated stormwater is highly uncertain, particularly because of likely interactions with underground infrastructure—e.g. sewer pipes, telecommunication cables, etc. These infrastructures are typically surrounded by highly permeable material which has the potential to substantially alter the way infiltrated stormwater moves through the subsurface (a phenomenon known as the urban karst).

This study aimed to predict and generalize the impact of the urban karst on infiltrated stormwater as it can provide a preferential flowpath and thus may prevent infiltrated stormwater from reaching receiving waters or may short circuit subsurface storages that can increase routing time delays and thus baseflow. In doing so, a modelling study using HYDRUS-3D was undertaken. In addition, a novel approach to generalize the results was proposed based on groundwater level and the hydraulic conductivities of soil and gravel/sand. We predicted that the impact of the urban karst on infiltrated stormwater increases whit higher groundwater levels, and greater contrasts between the hydraulic conductivity of regional soil and gravel. The HYDRUS results for a wide range of scenarios are compared with the generalization, which captures the impact of Urban Karst well.

It is important to consider the impact of the urban karst where one of the goals of building infiltration basins is to recharge the baseflow of the stream downslope. This suggests that decision on basin location is important where urban infrastructure is located between potential infiltration basin sites and downslope stream. The impact of the urban karst should be investigated at each specific site before implementing infiltration systems and this study works towards simplified representations of impact for design.

How to cite: Poozan, A., Western, َ., Arora, M., Burns, M., and Fletcher, T.: The fate of infiltrated stormwater from infiltration basins to the stream: quantifying the impact of the urban karst, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1928, https://doi.org/10.5194/egusphere-egu2020-1928, 2020.

D521 |
Brian Thomas

The urbanization of a watershed radically impacts how watersheds store, transmit and discharge water. Although urbanization’s effects on floods, droughts, and water supply have been explored in recent decades through land-use modeling, hydrological modeling, remote sensing, and empirical approaches, clarification of these effects remains a challenge due to limited availability and accessibility of useful data. Streamflow records for three neighboring watersheds in Baltimore, an urbanized watershed, an urbanizing watershed and a natural watershed, provide a unique opportunity to study the influence of urbanization on watershed function.  The 5-minute instantaneous discharge records span an increase in residential development of the urbanizing watershed.  Coupling the streamflow and development records allows direct comparison of hydrologic changes with spatial patterns of land use change.  Recession analysis was used to evaluate altered hydrologic response, particularly relationships between watershed storage and streamflow that may occur during urbanization.  Recession approaches were applied using variable time steps to estimate the time derivative of streamflow (dQ/dt) to avoid known issues in parameter estimation driven by the time derivative of a noisy time series. Several hypotheses are tested, including comparisons to conceptual models of hydrologic change that would be expected in urbanizing watersheds.  Preliminary results suggest that hydrologic changes are notable during periods of intense development, with recession plot characteristics markedly variable in urbanizing and urban watersheds as compared to the natural watershed. Analysis of streamflow records during the process of urbanization reveals groundwater-surface water interactions driven by urban development previously only observed over relatively shorter time periods. These findings can inform implementation of sustainable design of storm water management and future development planning.

How to cite: Thomas, B.: What urban streamflow can tell us about changes in water storage and streamflow due to development, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10776, https://doi.org/10.5194/egusphere-egu2020-10776, 2020.

D522 |
| Highlight
emmanuel berthier, jérémie sage, emmanuel dumont, marie-laure mosini, fabrice rodriguez, and michael toriel

The urbanisation leads to modifications in the water budget, not only at the surface but in groundwater as well. Few urban modelling studies deal with this topic, due to the lack of appropriate models. The URBS (Urban Runoff Branching Structure) model has been developed since several decades to simulate water transfers at the scale of an urban district. An integrated modelling approach is deliberately adopted to account for the numerous elements that influence urban hydrology: the spatial distribution of the sealed surfaces, interactions between the urban soil and water networks or underground, sustainable drainage systems…. In URBS, the spatial discretization of a catchment is based on Urban Hydrologic Elements (UHE) constituted by cadastral parcels and the adjacent streets, connected to the drainage network. URBS is able to perform continuous and long-period simulations (typically several years) of water fluxes in urban districts for small time-steps (typically few-minutes), with rainfall and potential evapotranspiration as input data.

The URBS model is adopted to study the hydrological impact of the Moulon district layout, a 200 ha development operation of the Paris-Saclay Cluster (currently underway). The project should result in an increase of sealed surfaces from 14% to 35% and a densification of underground constructions such as networks and basements. A shallow unconfined aquifer extends on the whole area. The fluctuations of ground-water levels have been monitored at an hourly time-step with 8 piezometers since 2012. Water-table levels exhibit significant variations, with near-saturation levels during winter and several meters depths during summer, although the piezometers do not all exhibit the same dynamics.

A calibration of the URBS model is first conducted for a 2-year period using only piezometric data and no flowrate data. The calibration is solely performed for the parameters influencing the soil compartment: soil permeability and parameters of the sewer infiltration process. Model performances are rather satisfactory with good representation of the observed levels for several piezometers, despite some difficulties for two piezometers exhibiting atypical variations. Once the URBS model is calibrated for the initial situation, simulations are conducted for the project layout (accounting for land-use modification and underground constructions) so as to evaluate the hydrological impacts of the development. Simulation results suggest that an increase of water table levels might be expected after the development of the district (this somehow surprising result may partly originate from the decrease of evapotranspiration fluxes associated with the increased of sealed surfaces).

The analysis of these first simulations also suggests that large uncertainties might be expected regarding the water levels computed by URBS. A simplified uncertainty analysis (based on Monte-Carlo simulations) is thus conducted to evaluate and distinguish uncertainties associated with model parameters and the total uncertainties in model outputs. While the results clearly evidence the importance of total uncertainties (although the uncertainties due to the model parameters remain low), they also confirm that groundwater depths could be reduced by the construction of the Moulon district.

How to cite: berthier, E., sage, J., dumont, E., mosini, M., rodriguez, F., and toriel, M.: Assessing the impact of the development of an urban district on shallow groundwater using the integrated urban hydrological model URBS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10436, https://doi.org/10.5194/egusphere-egu2020-10436, 2020.

D523 |
| Highlight
Wenfu Chen

Temperature depth profiles has been applied on many topics, e.g. climate change, groundwater velocity and saltwater-groundwater interface. In this study, temperature depth profiles are used to identify the origin of groundwater salinization in Pingtung coastal plain, southern Taiwan. Some monitoring wells in the coastal area have reached salty groundwater. Even some of the deeper aquifers, down to 300 m are saltwater. There are two arguments for the origin of those saltwater. One theory is those saltwater were Holocene transgression relics. The other theory is that those saltwater were sea water intrusion due to over-pumping of groundwater. Using the measured temperature depth profiles, a 2D numerical model is developed to simulate the heat transfer of sea water intrusion. The preliminary results show that the cause of salinization is not likely by the modern sea water intrusion. The sea water below a depth of 100 m is a cooler source and the intrusion of sea water should decrease temperature in aquifer. However, the measured temperature data of those salty aquifer are higher.

How to cite: Chen, W.: Using temperature depth profiles to identify the origin of groundwater salinization in Pingtung coastal plain, Southern Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2070, https://doi.org/10.5194/egusphere-egu2020-2070, 2020.

D524 |
Kosuke Nagano, Maki Tsujimura, Ryu Suzuki, Hiroko Asakura, and Kazunori Tabe

     In 1960s, the groundwater level dropped with serious land subsidence due to the excessive pumping of groundwater for economic development in Tokyo Metropolitan area, whereas the groundwater level has been recovered after 1990’s because of the strict groundwater use regulation by the government. A few studies have reported long-term changes in the groundwater including a groundwater level dropdown and the recovery in Southeast Asia, such as Ho Chi Minh City in Vietnam and Jakarta in Indonesia. However, there are not enough investigations to monitor the groundwater flow covering the water level dropdown and the recovery during more than 50 years at the megacities in Asia regions. Therefore, we investigated the change of groundwater flow system at the Tokyo Metropolitan area with a special concern on the lowland area where the impact of land subsidence was particularly large due to an excessive pumping in the 1960's.

     First, we observed a spatial distribution of hydraulic head and the chemical and stable isotopic compositions in the groundwater and the river water to understand a current groundwater flow system in the whole Tokyo area in 2019, in which the groundwater level is stable. Then, we compared those results with that monitored from 1960’s to 1990’s.

     Groundwater was sampled from May to October 2019 at multiple boreholes installed at whole of Tokyo area with the depths ranging from 5 m to 260 m, and the main inorganic dissolved ions, stable isotopes (δD, δ18O) and the dissolved gas (CFCs, SF6) were determined on all samples. The high Cl- concentration more than 500 mg/L is limited in the groundwater and the river water in the coastal area with the average distance of 6 km from the sea, whereas the contour line of 500 mg/L intruded inland area with the average distance of 15 km from the coastal line in 1965 and 12 km in 1971, then 8 km in 1994 (Institute of Civil Engineering of the Tokyo Metropolitan Government, 1996). The groundwater hydraulic head is -6 m (m.s.l) at the lowland area in 2019, whereas that was -58 m in 1965, -52 m in 1971, and - 14 m in 1994.

     The depleted stable isotopes and the higher solute concentrations are observed in the lowland in 2019. Also, SF6 is not detected in the groundwater at the low land area, whereas we observe the SF6 concentration ranging from 0.8 pptv to 78 pptv in the upland area. These suggest that the groundwater in the upland has an apparent age of approximately less than one year to 40 years, whereas the groundwater in the lowland is recharged with an age more than 80 years at the higher elevation.

How to cite: Nagano, K., Tsujimura, M., Suzuki, R., Asakura, H., and Tabe, K.: Groundwater Flow System in Tokyo Metropolitan Area, Japan: Focusing on Changes in the Last 60 Years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6506, https://doi.org/10.5194/egusphere-egu2020-6506, 2020.

D525 |
| Highlight
Anna Jurado and Enric Vázquez-Suñé

This abstract aims to present the project CARE. Often, urban areas must pump water resources to cover various aspects of the growing urban water demand and as a strategic resource at specific times (e.g., droughts). These considerations lead one to wonder whether urban groundwater can be safely used, including its potential use as drinking water because urban aquifers usually contain a wide range of pollutants (e.g., heavy metals, nutrients, pathogens, and organic contaminants). Currently, there is a growing interest for the contaminants of emerging concern (CECs) (i.e., pharmaceuticals, personal care products, illicit drugs, etc.,) because most of them are not included in the watch lists of priority pollutants due to existing regulatory gap. Moreover, even detected at trace levels (ng/L-µg/L), they might pose ecological risk such as interference with the endocrine system of high organisms, microbiological resistance, accumulation in soil, plants and animals and, the effects of CEC mixtures are assumed to have unforeseen consequences on ecosystems.

Since CECs reach groundwater environment, their attenuation occurs mainly through microbial degradation because adsorption is reversible and only retards the contaminants’ transport. Moreover, although the long residence time of water in aquifers might result in strong attenuation of some CECs, others are persistent in urban groundwater. This requires appropriate understanding of all the processes that control the fate of CECs at field scale but, so far, most research is conducted at the laboratory scale, which misses potential synergetic effects associated with the heterogeneous and complex hydrochemical conditions that are inherent in urban aquifers. Considering the raising demand of secure freshwater and the concurrent increase of CECs use, understanding the factors that most influence their efficient removal in urban aquifers are of paramount importance to assure adequate protection of human health and the environment.

In this context, the main objectives of CARE are to: (1) identify the most suitable conditions that contribute to the natural bioremediation of selected CECs in urban groundwater at field scale and (2) propose and develop solutions for the sustainable management of urban groundwater resources by means of numerical modelling facilitating the decision making and improving its management. A suitable area for CARE is the pilot zone of Sant Adria del Besòs (Barcelona, Spain) because there is a huge amount of urban groundwater is routinely pumped (6 Hm3/y) and discharged into the sewage system. Moreover, our previous investigations have demonstrated the presence of a wide range of CECs in this aquifer reaching concentrations up to 2 µg/L. The main outcome of CARE  will be an integrated method for urban groundwater management using monitoring, measuring and modelling approaches that will support improved decision-making to ensure the long-term availability of water resources to the water authorities. This method can be applied in other urban aquifers.

How to cite: Jurado, A. and Vázquez-Suñé, E.: Contaminants of emerging concern in urban Aquifers: are they a pRoblem for groundwater usE? (CARE), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7385, https://doi.org/10.5194/egusphere-egu2020-7385, 2020.

D526 |
Luka Serianz, Nina Rman, and Mihael Brenčič

In Alps, a number of thermal springs are known, which represent the outflow of thermal water from low temperature geothermal systems in fractured rocks. Such dynamics is usually characterized with convection flow, derived either by fault intersection or hydrogeological barrier where the thermal water is uprising due to hydraulic pressure imbalance. When the water is uprising due to convection, it is very likely that the mixing processes between the deep thermal component and the shallow fresh groundwater are established. In Bled case study in Slovenia, the thermal water with average temperature of 21.5 °C, which is around 12 °C higher than average annual air temperature, is discharging from fractured carbonate rocks into glacial Quaternary sediments. Since they have relatively higher but heterogeneous permeability, the uprising thermal water drains into these deposits and, consequently, forms thermal plume which is extending parallel to prevailing fresh groundwater flow direction. Knowing the extent of the thermal plume is of crucial importance for sustainable exploration of the geothermal resource, since it provides answers also to the key issues related to its geothermal and hydraulic characteristics and the dynamics of the regional flow of groundwater, including its recharge area. By approximating the thermal water outflow as a planar source (since we assume it springs out from a fault zone), a planar advective heat transport model (PAHM) was used to evaluate its geometry and quantify the rates. Nine scenarios were applied accounting for different dimensions of the heat source. Each scenario was verified by calculating relative error between the analytical model results and measured borehole temperatures. The PAHM proved to be a useful tool in applying heat transfer as a planar source in groundwater flow. Still, it is necessary to consider or to introduce relatively rough assumptions (e.g. simple model geometry) leading to a very conservative approach. The heterogeneity of the medium has a significant influence on the temperature distributions obtained with different simulation scenarios. Therefore, the calculated temperature distribution within a thermal plume is a subject to uncertainty. In addition, some small portion of a relative error can be attributed to Lake Bled, since the thermal plume is extending in the zone of lake water temperature fluctuation influence. Nevertheless, the analytical model can be used as a tool for simulating spatial distribution of the observed values acquired from field measurements and thus more correctly evaluating the average natural conditions.

How to cite: Serianz, L., Rman, N., and Brenčič, M.: Assessment of thermal water outflow plume in heterogeneous glaciofluvial deposits: a case study from Julian Alps, Slovenia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7833, https://doi.org/10.5194/egusphere-egu2020-7833, 2020.

D527 |
Chaofan Chen, Wanlong Cai, Olaf Kolditz, and Haibing Shao

Recently, an Enhanced U-type Borehole Heat Exchanger (EUBHE) system has been installed in Xi’An, China. The EUBHE system is composed on one vertical and a second deviated borehole. The bottom ends of them are connected at a depth of 2.5 km and form a closed “U-type” loop system. During the heating season, water is circulated through the two boreholes to extract geothermal energy for building heating purpose. In this work, a numerical model was developed based on the OpenGeoSys (OGS) software, in which the boreholes of the EUBHE system are represented by the line elements and the soil/rock surrounding them is discretized with 3D prism elements. With this dual-continuum model, the operation of the EUBHE system can be efficiently simulated, particularly for long-term scenarios. The developed numerical model was verified against analytical solution in a benchmark proposed by Ramey. To simulate the long-term operation of EUBHE, a Direchlet-type boundary was imposed at the inlet of the system. Temperature difference between inlet and outlet were calculated based on the building thermal demand. The impact of parameters of the vertical and deviated boreholes and flow rate of the circulating water are further investigated. Preliminary modelling results showed that the sustainable specific heat extraction rate of the EUBHE system can reach up to 200 W/m. With higher grout and pipe conductivities, the system performance will be improved. The heat extraction efficiency of EUBHE system is higher than the traditional Deep Borehole Heat Exchangers (DBHE). However, electricity consumption from the circulating pump is elevated. The developed numerical model presented in this work can also be utilized for the design and optimization of the EUBHE system.

How to cite: Chen, C., Cai, W., Kolditz, O., and Shao, H.: Connecting deviated and vertical deep boreholes to enhance the extraction of geothermal energy - case study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8742, https://doi.org/10.5194/egusphere-egu2020-8742, 2020.

D528 |
Laura Scheiber, Enric Vazquez-Suñé, Thom Bogaard, Sílvia Bofill Mas, Sandra Pérez, Antoni Ginebreda Martí, Linda Luquot, Marta Rusiñol, Rotman Criollo, Rosina Girones, Eva Fores, Elena Gómez, Geoffroy Duporté, and Maria Garcia-Rios

According to the European Union, half of the human population will live in cities, one of the main water spend zones and focus of pollutants it is expected by 2050. Considering the growing pressure on water resources whole world, a better knowledge for its management is needed to face this situation.

URBANWAT is a project funded by the EU Commission under the call “Closing the Water Cycle Gap” of the Water JPI Strategic Research and Innovation Agenda. The goal of this project is to come up with an improvement of tools and criteria for groundwater management in urban areas to guarantee the urban water resources sustainability, identify their potential uses and their risks related to groundwater use from both environmental and human health perspectives by an integral approach developing novel technologies and methodologies.

The project will involve a multidisciplinary approach integrating the research of the natural state of the hydrological cycle and pollutants identification (general chemistry, pollutants of emerging concern (CECs) and microorganisms, with emphasis in viruses).

To achieve that goal, URBANWAT proposes to use innovative approaches based on liquid chromatography high resolution mass spectrometry (HLC-RMS) to: (1) detect differences in degradation in different anoxic conditions employing CECs as indicators of contamination and their transformation products (TPs) as indicators of degradability; (i2)  analyse fate and transport of chosen contaminants in the soil-plant as a remediation system utilizing picked infrastructures; (3) understand the contaminants movement applying encapsulated DNA nanoparticles; (4)  scout the presence of new and emergent viruses in groundwater samples employing viral metagenomics. Viral concentration methods from water samples will be also optimized in this research. To do so, column experiments will be carried out.

URBANWAT project will focus on Barcelona city. In parallel, several studies will be carried out at full-scale in the demonstration facility called ‘WaterStreet’ at TUDelft. The expected results will help to provide novel and cost-efficient technologies for urban groundwater management with beneficial environmental, economic and societal impacts for the European Union (EU) facilitating their application worldwide.

How to cite: Scheiber, L., Vazquez-Suñé, E., Bogaard, T., Bofill Mas, S., Pérez, S., Ginebreda Martí, A., Luquot, L., Rusiñol, M., Criollo, R., Girones, R., Fores, E., Gómez, E., Duporté, G., and Garcia-Rios, M.: JPI_URBANWAT project. Tools and criteria for URBAN groundWATer management , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8958, https://doi.org/10.5194/egusphere-egu2020-8958, 2020.

D529 |
Jean-Marc Ballard, Cynthia Lee, Nataline Simon, Jerome de la Bernardie, Daniel Paradis, Jasmin Raymond, Olivier Bour, and Rene Lefebvre
Historically, heat and temperature observations have been occasionally used to help understand aquifer systems or constrain numerical flow models. However, the development of fiber optics (FO) as part of the Distributed Temperature Sensing (DTS) technology has spun a renewed interest in the use of heat as a groundwater tracer. Recent studies have shown the possibility to carry out an active heat tracer test using fiber optics and heating cables installed by direct push and to invert the resulting thermal responses to estimate a vertical profile of groundwater fluxes. However, a better understanding of how FO-DTS results compare to other aquifer characterization methods is needed to guide its future application and integration into a practical workflow. The objective of this study was thus to compare the information provided by FO-DTS with other direct and indirect measurements used to characterize the heterogeneity of granular aquifers at multiple scales. 
The multiscale integrated characterization was carried out at a heterogeneous deltaic aquifer located north of Quebec City, Canada. This aquifer has been the object of a complete hydrogeological characterization and thus provides a wide range of existing data against which the acquired data can be compared. This communication will focus on the multiscale methodology for the granular aquifer characterization including FO-DTS measurements. Based on an existing numerical hydrogeological model, three sites with a range of horizontal groundwater fluxes were selected for active FO-DTS heat tracer experiments. At one of the sites, direct push monitoring wells were also installed downstream to measure the hydraulic conductivity of the hydrofacies and the arrival of the thermal front from the heat tracer test. A previous study established a relationship between the hydrofacies of the deltaic aquifer to cone penetration test (CPT) response. As such, each FO cable and monitoring well direct-push installation was preceded by a co-located CPT. Soil cores were also taken for laboratory measurements of hydraulic and thermal properties. 
The vertical profiles of groundwater fluxes from FO-DTS were found to correlate well with the relative magnitude of permeability of the hydrofacies identified with CPT profiles. FO-DTS could thus provide a qualitative or quantitative proxy for hydraulic conductivity and allow the recognition of hydrofacies at a fine scale. At the aquifer scale, the total flux measured by FO-DTS can also be compared to fluxes obtained from numerical models and thus provide a constraint to validate models. Overall, this study shows that not only does FO-DTS provide coherent results with other characterization methods, but it also adds the key measurement of groundwater flux that cannot be easily obtained by other means. FO-DTS thus has the potential to become a significant addition to existing characterization methods for granular aquifers.

How to cite: Ballard, J.-M., Lee, C., Simon, N., de la Bernardie, J., Paradis, D., Raymond, J., Bour, O., and Lefebvre, R.: Multi-scale integrated characterization of heterogeneous hydraulic and thermal properties of a deltaic aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8984, https://doi.org/10.5194/egusphere-egu2020-8984, 2020.

D530 |
zhang qiang, sun jinyu, tamg jinping, zhang jiashen, and huang sishuang

Located at the southeast of the Minjiang alluvial-proluvial fan, the downtown area of Chengdu mainly composed of sand gravel layer. Now Chengdu has 8 subway lines operated; in the next 10 years, more than 34 routes will be constructed. Metro Line 7 forming a transfer relationship with multiple urban MRTS and urban commuter radiation built completly in downtown area, with depth of subway station 1.73-11.3 m, and the depth of interval tunnel 6.47-28.01 m. In order to study how the groundwater will be influenced, 3 3d groundwater numerical models in different scales have been constructed using FeFlow software, the results illustrated regional groundwater seepage field and local seepage field.

Baed on 1 regional model (417 km2 for downtown Chengdu ) and 2 models of typical underground space (Taipingyuan station and Yipintianxia station), at the same time with the basic geology and hydrogeology Analysis, shows that:

(1) The influence of metro line 7 on the seepage field is relatively limited in regional scale, and the change of groundwater level is very little(4-10cm) at several typical observation points; in the long-term, the raising of groundwater level will decrease gradually.

(2) Comparing the simulation results of Taipingyuan station and Yipintianxia station shows the impact of subway construction on the groundwater environment in the downtown Chengdu. In the big view, from northwest to southeast, the phenomenon of underground water interception or raising in subway stations decrease gradually, this is owing to the influence of aquifer thickness, groundwater flow direction and the direction of underground station structure.

(3) As the main body or long section of the underground structure is coincide with the groundwater flow direction, the cross-section blocking the groundwater is minimized, so its influence on the groundwater seepage field is not notable even with development of the underground space, this is also help avoiding the floatation effect on the building foundation due to the raising of the groundwater flow.

How to cite: qiang, Z., jinyu, S., jinping, T., jiashen, Z., and sishuang, H.: Influence of Subway Construction on Groundwater Environment in Downtown Area of Chengdu, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9641, https://doi.org/10.5194/egusphere-egu2020-9641, 2020.

D531 |
Victorien Djotsa Nguimeya Ngninjio, Bo Wang, Christof Beyer, and Sebastian Bauer

Seasonal or sub-seasonal large scale heat storage will be required for a switch of the heating market to renewable heat sources, due to the seasonality of the heating demand. Subsurface high-temperature heat storage (up to 90°C) is investigated here as a promising option for urban areas with strong land use pressure, as this technology provides the required high capacities. Surplus heat originating from solar thermal installations or industrial production can be stored and later on used when the heat demand is high. One technology option available is borehole thermal energy storage using borehole heat exchangers (BHE) to store the heat in the geological subsurface. However, storing heat at high temperatures in porous media can trigger convective density-driven flow. This interacting transport of heat and water may affect the storage efficiency of such storage systems. In this study, therefore, lab-scale experiments are numerically designed and experimentally conducted in order to identify, characterize and quantify the induced convective heat transport process at different storage temperatures.

A lab-scale analogon of a heat storage is constructed in a PP plastic barrel of 1.23 m height and 1.2 m diameter, consisting of water saturated homogeneous sand medium, with a hydraulic permeability of about 2.9x10-10 m² and a thermal conductivity of 2.042 W/m/K. Coupled thermo-hydraulic process simulation applying OpenGeoSys was used to design and optimize the experimental set-up and the test cycles. Hot water is circulated in a coaxial BHE at 70°C for seven days to heat the storage medium, while tab water is used to recover the stored heat. The side of the barrel is cooled using ventilators while the top and bottom of the barrel are insulated.

The experimental results show that after four days of heat injection, a steady state temperature distribution is reached. The temperature distribution in the storage medium is vertically stratified with an average temperature approximately 39°C and 26°C in the upper and lower part, respectively. Thus the centre of the mass of stored heat is shifted to the top part of the storage medium, and a larger convection cell is formed, with water rising at the BHE in the middle and sinking at the barrel wall. The vertical temperature gradient decreases from the grout surface to the barrel wall with a rate of 0.153 K/m. The decreasing rate of the radial temperature gradient from the upper to the lower part of the sand medium is 0.174 K/m. The Rayleigh number, which characterizes the magnitude of the convective heat transfer, is about 44.15 for this experiment and thus greater than the critical value. Heat transfer process in the sand medium hence is influenced by density driven convective flow. Additional laboratory experiments at inlet temperatures of 30°C, 50°C, and 90°C show an increase of convective heat transfer with increasing temperature.

The numerical model qualitatively reproduces the convective heat transfer within the storage. An inverse model adaption is currently carried out to determine the effective heat transfer parameters for the storage components and to quantitatively fit the experimentally observed temperature distributions.

How to cite: Djotsa Nguimeya Ngninjio, V., Wang, B., Beyer, C., and Bauer, S.: Numerical and experimental investigation of induced convective flow by high-temperature heat storage in water saturated sediments. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10938, https://doi.org/10.5194/egusphere-egu2020-10938, 2020.

D532 |
Martin Binder, Christian Engelmann, Afshin Sadeghikhah, Mario Schirmer, Peter Krebs, Rudolf Liedl, and Marc Walther

At the current time, cities harbor more than 4 billion inhabitants. According to the United Nations’ projections, an increment by 2.5 billion is expected until mid-century. This will create enormous stresses to the water resource management in urban regions, including detrimental impacts on both groundwater quantity and quality. For instance, leakages from aging urban sewer network systems may lead to uncontrolled recharge and contamination. Sewer-borne contaminants cover a broad bandwidth of substances including pathogenic microbiota, nutrients and emerging contaminants. These substances may be highly persistent and accumulate in the subsurface over time. This, in turn, may pose a long-term threat to urban ecosystems. Hence, understanding the spatiotemporal distribution of sewer-borne plumes within the subsurface is of strategic importance. Sewer failures may include, among others, pipe blockades, local collapses and smaller cracks, as well as leaking joints between pipe segments. The intensity of sewer exfiltration to the soil and eventually to the aquifers depends on a variety of influencing factors, including pipe diameter and failure type as well as pipe burial depth and distance to the groundwater table. In this context, this study’s specific aim is to investigate the effect of selected vadose zone and aquifer properties and of failure characteristics on the final shape of sewer contaminant plumes to eventually delineate groundwater contamination characteristics solely from sewer network properties.

Results from two numerical studies, employing the HYDRUS 3D software code for variably saturated flow and transport simulation, will be presented. First, a small-scale principal model setup with a single pipe defect was designed to investigate the effect of soil type, colmation layer properties, pipe water level, defect shape and natural groundwater recharge on the shape of the plume in the vadose zone and at the aquifer table. Hereby, the simulations included both constant and varying pipe water levels. To define a de-facto worst-case scenario, continuous water injection as well as conservative transport (i.e., no decay or sorption) were assumed for most simulation runs. Besides the pipe water level, the intensity of precipitation was found to be a major influencing factor on the contaminant plume dimensions. In a second step, an intermediate-scale model involving a long pipe was conducted to further investigate overlay effects of multiple contaminant plumes. Here, multiple defects were positioned along the pipe in various distances, starting from a quasi-continuous line source and ending at a rather broad interval. It was found that the plume shapes on larger scale were very similar for most defect positionings, if the averaged injection rate remains the same. The direction of groundwater flow was altered in addition to the variation of the defects’ positions. Here, the contaminant plumes became slightly skewed.

The presentation will also give a short outlook to future works which will include simulations on city district scale employing HPC-capable codes such as ParSWMS, ParFlow and/or OpenGeoSys, and a comparison to simplified modeling approaches.

How to cite: Binder, M., Engelmann, C., Sadeghikhah, A., Schirmer, M., Krebs, P., Liedl, R., and Walther, M.: Leaky sewer systems: Influence of soil properties and sewer failure characteristics on the shape of contaminant plumes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13696, https://doi.org/10.5194/egusphere-egu2020-13696, 2020.

D533 |
| Highlight
Hannes Hemmerle and Peter Bayer

Surface temperature variations have been well shown to transfer their thermal signature into the subsurface. This continuous heat transfer manifests in altered thermal conditions in the subsurface where temperature variations over a long lapse of time are more pronounced than shorter ones. Hence, repeated temperature depth profiles allow to investigate the effects of recent climate change on the subsurface. In this study we present recent temperature trends in more than 40 observation wells in Bavaria, Germany. Temperature depth profiles have been quarterly measured for one year between 1992-1994 and measurements have been repeated two times in 2019. The quarterly measurements reveal that the periodic seasonal temperature signal dampens to around 0.1 K at a depth of 15 m below ground surface. This implies that temperature variations below this depth can be used as climate archives as they store the temperature history of multiple years. The measurements span a time period of almost 30 years which is the most common period of reference for deriving climate normals according to the World Meteorological Organization. Therefore, the findings of recent subsurface temperature variations are assessed versus and complemented by 22 air temperature stations. Preliminary results show, that the linear regression of the annual mean air temperature since 1990 yields a slope of 0.35 ± 0.11 K 10a-1. In the subsurface, median temperature differences of the respective baselines from 1992-94 period and 2019 are 0.26, 0.13 and 0.07 K 10a-1 at 20, 40 and 60 m depth below surface, accordingly. Despite the common magnitude and continuous downward decrease, subsurface temperature differences exhibit a much higher variance compared to air temperature changes. This is due to local effects, such as varying thermal conductivities of the subsurface, latent heat transport caused by evapotranspiration, lateral and vertical groundwater flow, and anthropogenic influences. Our contribution will feature a comparison of this temperature change in response to recent atmospheric climate change in Bavaria and link these results with perceptions gained by similar investigations on local scale in other European regions.

How to cite: Hemmerle, H. and Bayer, P.: Recent trends of groundwater temperatures in Bavaria, Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18663, https://doi.org/10.5194/egusphere-egu2020-18663, 2020.

D534 |
Alberto Previati, Giovanni Battista Crosta, and Jannis Epting

Aquifers beneath big cities are considered a very important resource from an energy and water supply point of view and are increasingly exploited by means of groundwater extraction wells as well as by shallow open- and closed-loop geothermal systems. Moreover, the shallow subsurface of densely populated cities is increasingly hosting underground infrastructures such as tunnels and building foundations. These activities lead to thermal pollution of the shallow urban underground. This phenomenon has already been documented (urban heat island effect) in many cities worldwide with higher ground/groundwater temperatures in the city centers with respect to surrounding rural areas. The local thermal impact of various underground activities has been studied with analytical and local-scale numerical modeling. However, the resulting groundwater thermal regime at the city-scale is yet mostly unexplored.

In this work the effects of anthropogenic heat sources and subsurface infrastructures in the Milan metropolitan area is presented. To this aim a groundwater head/temperature monitoring network was established in 2016. Groundwater temperatures in the city center are up to 3°C higher with respect to less urbanized areas. A correlation between the urban density and the groundwater thermal regime was observed. In order to evaluate the spatial variability of the groundwater temperatures, a detailed analysis based on a 3D FEM groundwater flow and heat transport numerical model was carried out by means of the commercial code FeFlow. First, the variability of hydraulic and thermal properties as from borehole logs was spatialized into the model by means of 3D geostatistical techniques to account for aquifer heterogeneities. Complex thermal boundary conditions were assigned to the model including the effects of different land cover/sealing materials, building foundations, tunnels, shallow geothermal wells and the canal network. The thermal transport model was calibrated against high-resolution time-lapse groundwater temperature profiles and continuous measurements at fixed depth.

The modeling of the current thermal regime of the shallow aquifers was essential to understand the hydrogeological and thermal processes that are relevant at the city scale. The numerical results are a valuable tool to assess the impact of specific heat sources as well as of surface/subsurface infrastructures on the overall thermal regime and to test the long-term thermal potential of ground/groundwater heat exchangers under possible urban development scenarios. Thereby, the proposed approach can support the sustainable development of subsurface infrastructures at the city-scale and the management and assessment of the thermal potential of low enthalpy geothermal resources.

How to cite: Previati, A., Crosta, G. B., and Epting, J.: City-scale groundwater flow and heat transport modeling in the Milan Metropolitan Area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20306, https://doi.org/10.5194/egusphere-egu2020-20306, 2020.

D535 |
Estanislao Pujades, Anna Jurado, Jesús Carrera, and Enric Vàzquez-Suñé

Underground constructions in urban environments are more and more frequent, and usually, they are undertaken below the water table. The interaction between groundwater and underground constructions is a relevant topic that has to be carefully considered because unforeseen incidents may appear during these constructions. This paper shows the methodology used to design the dewatering system of a large excavation below the water table in a high populated urban area. The excavation was required for the construction of the assembly shaft of the tunnel boring machine that was used to excavate the high speed train tunnel below Barcelona (Spain). This methodology was useful to design an efficient dewatering systems that allowed constructing the assembly shaft in safe conditions and without producing appreciable impacts around the construction site. The most important step of the proposed method was the hydrogeological characterization of the soil because this allowed building a realistic and representative numerical model. This work shows the importance of interdisciplinary approach because the dewatering system was designed combining field work, classical analytical solutions and numerical methods.

How to cite: Pujades, E., Jurado, A., Carrera, J., and Vàzquez-Suñé, E.: On the design of the dewatering system of a large excavation in Barcelona (Spain) used for the construction of the high speed train tunnel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21729, https://doi.org/10.5194/egusphere-egu2020-21729, 2020.