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Over the past fifty years, thermal waters with temperatures of up to 65 °C have served as the basis for heating, health and recreational tourism in Topusko town. Natural thermal springs with temperatures of up to 53°C, which emerge in the discharge area of the Topusko hydrothermal system, are located at the junction of two major tectonic units: the SW margin of the Pannonian Basin System and the Dinarides. Initial research data were very scarce, so the presented results comprise the most recent findings gathered in the scope of the HyTheC research project. Hydrochemical and geothermometric research of thermal waters was done together with the application of geophysical investigations to improve the existing conceptual model of the Topusko hydrothermal system. The two main objectives of the presented work are the hydrogeochemical characterisation of the naturally emerging thermal waters and the identification of a fault damage zone in the spring area by electrical resistivity tomography. The hydrogeochemical facies Ca-HCO3 of thermal water was determined, the equilibrium reservoir temperature was estimated to be 88 °C using a SiO2-quartz geothermometer, and the assumed fault damage zones were identified for the first time in the discharge area of thermal springs.

Otok Vis jedan je od rijetkih hrvatskih otoka koji javnu vodoopskrbu osigurava isključivo iz vlastitog vodonosnika. Najvažniji su vodoopskrbni objekti centralno smješteno crpilište Korita s pet radnih bušenih zdenaca, kaptirani izvor Pizdica i bušeni zdenac Brojkovica iznad Komiže. Infiltracija, nakupljanje i istjecanje podzemnih voda odvijaju se u okršenoj stijenskoj masi, a hidrogeološki odnosi uvjetovani su geološkom građom terena. Najvažniji je utjecaj dviju hidrogeoloških barijera – vulkanogeno-sedimentno-evaporitnog kompleksa komiškog zaljeva zapadno od vodonosnika i krških polja južno od njega. U radu je prikazan pregled hidrogeoloških istraživanja na otoku Visu tijekom povijesti, te srodnih istraživanja bez kojih se hidrogeološka ne bi mogla provoditi. Prikazat će se geološka istraživanja, kao najvažnija podloga, te hidrogeološka istraživanja različitim metodama: kartiranja u različitim mjerilima, strukturnogeološke analize, izračuni hidrauličkih parametara vodonosnika, trasiranje podzemnih tokova, hidrokemijska istraživanja i analize trendova, razina, temperatura i elektrolitičke vodljivosti podzemne vode. U nepovoljnom klimatskom scenariju vjerojatno bi se na otoku moralo (1) povećati crpne količine i dodatno snižavati razine podzemne vode na vodocrpilištima, (2) izraditi dodatne zdence izvan neposrednog područja utjecaja trenutnih crpilišta, (3) desalinizirati blago bočatu vodu ili (4) razmotriti mogućnosti umjetnog napajanja vodonosnika. Svaka opcija zahtijeva hidrogeološka istraživanja, uz primjenu novijih metoda te kontinuiran monitoring više parametara uz razrađen sustav ranog upozoravanja i intervencije. Iako je otok Vis, s hidrogeološkog aspekta, najistraživaniji otok u Hrvatskoj (pogotovo od 1950.-ih do 1980.-ih godina, zbog boravka mornarice tadašnje države), postoje potrebe za brojnim dodatnim istraživanjima, prvenstveno novim pristupima, opremom i metodama kojima prethodni istraživači nisu raspolagali.

Topusko hydrothermal system is located at the junction of two major tectonic units: the southwestern margin of the Pannonian Basin System (PBS) and the Dinarides. The PBS is known for its high geothermal gradient and surface heat flow. Thus, the north-eastern part of Croatia has favorable geothermal characteristics (average geothermal gradient 49 °C/km ; average surface heat flow 76 mW/m2), where natural thermal water springs, with temperatures up to 65 °C, emerge at two dozen localities. In the past fifty years, these waters served as a basis for developing tourism and health care centers. In the last forty years thermal water has also been used for heating in the city of Topusko. Thermal springs are part of hydrothermal systems, including recharge areas in the mountainous hinterlands of the springs, geothermal aquifers – in Croatia mostly Mesozoic carbonate rocks, and discharge areas in places with favorable structural characteristics of higher permeability. Refining of the available conceptual model will be based on multidisciplinary research which is currently underway. Hydrogeological, geothermal, hydrogeochemical, structural, and geophysical investigations and remote sensing will be used to construct an improved conceptual model. Its physical validity will be tested through numerical simulations supported by 3D geological reconstructions and local hydrogeological and thermal parameterization of the hydrostratigraphic units. Sustainable use of natural thermal springs cannot be planned or ensured without understanding at the system level. Having good conceptual and numerical models enables the delineation and protection of recharge areas and the determination of sustainable pumping rates. Preliminary results of ongoing research will be presented: geochemical characterization of thermal water, stabile and radioactive isotope signatures, geothermometry, as well as the identification of fault damage zones in the spring area using electrical resistivity tomography.

In the field of urban hydrogeology, it is often needed to define the permeability, as the most important hydrogeological parameter for interpretation of impact of pollutant on groundwater quality. Calculating permeability from grain size data is fast, relatively simple and cheap, and is often used for stated purpose. Causeless modifications of original methods for determination of hydraulic conductivity or permeability from grain size data have been, in the last three decades, widely disseminated in scientific and expert articles and published in numerous scientific journals. Modification of referential grain size, which is generally integrated in the calculations as square value, is especially a sensitive issue. Such and similar modifications strongly affect the results compromising the valuable utilization of grain size data for the permeability assessment in loose deposits and similar porous materials. This problem was demonstrated in this research though the presentation of original versions of four permeability calculation methods from grain size distribution: Hazen’s, as a representative of extremely experimental method, USBR /USCRO (Urumović et al., 2020) as empirical method, and Slichter’s and Kozeny – Carman (Urumović and Urumović, 2016) method as theoretical methods. Validity and proposal for corrections were demonstrated as their validity ceases to depend on the grading of the analyzed materials. This is especially important for Kozeny – Carman method that integrates formulation of specific surface area, making it widely applicable in soil science disciplines. In this research, samples of loess and loess-like sediments (Pola et al., 2020) were analyzed in laboratory within the ISSAH project (funded by Croatian Science Foundation), and later mathematically processed for hydraulic conductivity calculation. Statistical analyses show high accuracy of mentioned methods when properly used, opposed to recently (mis)used formulae.

Loess is a terrestrial sediment, formed by aeolian transport and deposition of silt-sized particles during Pleistocene glacial periods. Loess, loess-derived or loess-like materials cover significant areas of the Earth, being important for numerous human activities and scientific disciplines. Given that some of the most fertile soils are formed on loess, its importance for agriculture is especially high. Much of continental Croatia - the main agricultural region of the country - is covered by loess(-derived) deposits. These deposits are spread over important regional aquifers, as semi-permeable layers through which atmospheric water infiltrates into groundwater. At the same time, loess deposits protect groundwater from contamination, which can be pronounced in agricultural areas. Croatian loess(-derived) sediments and soils formed on them have not yet been investigated using an interdisciplinary approach that would involve hydrogeology, hydrology, agronomy, soil science and geology/sedimentology. Thus, the 5 year interdisciplinary scientific project ISSAH (lead researcher: Kosta Urumović, Ph.D.) had started in 2018. Main focus is on the specific surface area of mineral particles in loess and loess-derived soils, as it is the property with the key effect on various hydrogeological, geomechanical and pedophysical parameters. Project will yield a numerical model of groundwater-flow through the covering aquitard and water-bearing complex, helping the efficient use and protection of soils/groundwater in the study area. Also, it will establish an interdisciplinary team that should be able to tackle different and complex environmental issues, particularly in relation to the climate change. Study area is eastern continental Croatia, in which all important aquifers are loess-covered. Climate is semi-humid to semi-arid continental. Across the area, three representative locations were chosen for hydrogeological, meteorological and pedological research/monitoring (Figure). Each location is flat, found below 200 m asl, on loess(-derived) deposits. Two locations are grasslands (Kanovci and Livade) and one is a vineyard (Radoš). Performed pedological tasks comprise the following: a) available literature on soils and soil-forming factors of the study area was analyzed, reviewed and synthesized ; b) on the three locations, soil reconnaissance/augering was performed and soil pits were opened (Figure) ; c) soil profiles were described, sampled by genetic horizons (disturbed samples + triplicate 100 cm3 cores) and classified (WRB-2014): Kanovci - Calcic Kastanozem (Loamic, Novic, Transportic), Livade - Calcic Kastanozem (Loamic), Radoš - Hortic Anthrosol (Epiloamic, Endosiltic, Calcic) ; d) three gravitational lysimeters were installed in each soil pit to collect soil water percolating through the rhizosphere ; e) on each location, soil temperature and moisture sensors were installed in topsoil and subsoil layers. Currently, soil analyses are in progress (pH, organic matter and carbonate content, particle size distribution, density and porosity, aggregate stability, Atterberg limits, water retention across 0-15 bar range, hydraulic conductivity) and first results will be presented herein.

The interest of scientific and stakeholder communities on the exploration of deep carbonate geothermal resources is rapidly increasing due to their widespread occurrence and the possibility of local scale uses for hydrothermal heat and power generation. Within the framework of the HotLime project, funded by Horizon2020 GeoERA program, the pilot area of Zagreb (Croatia) was chosen for detailed geological and thermal reconstructions of the thermal reservoir hosted in up to 400 m thick Miocene and Triassic carbonate rocks at a depth of approximately 1 km. Despite promising temperature (up to 82 °C) and significant number of wells drilled during 1980s (27 wells on 54 km2), geothermal exploration of this area did not continue in the following decades. Notwithstanding the number of wells, some of which are fully equipped for production and reinjection, and the location of the wells in the national capital with numerous possible users, only one doublet system is in operation. Stratigraphic logs of the wells, borehole logging data and 30 seismic sections were collected, digitalized and arranged in a database including chemical analyses of the waters, porosity and permeability measurements and bottom hole temperatures. These data allow obtaining a comprehensive picture of the geological, hydro-geochemical and thermal settings of the reservoir. In particular, seismic and borehole data are used to perform a 3D geological reconstruction of the subsurface consisting of 5 layers and highlighting the principal fault systems. 3D geological model will be used to numerically reconstruct the subsurface thermal regime. Coupled fluid flow and heat transport numerical simulations will be performed using data on the hydrodynamic and thermal properties of the formations. First, the simulations will be run at the steady state to assess the contribution of conductive processes to the thermal regime. Further transient simulations will be performed to evaluate the impact of the most likely convective processes occurring in the fractured carbonate reservoir and contributing to the local temperature increase. The obtained results of temperature model will be compared and calibrated with available bottom hole temperatures and thermal logs. Contemporary approach to the research of the geothermal resource will serve to demonstrate the resource potential to the decision-makers and decrease the risks for future development.

Hydrothermal resources have been used for tourism and energy purposes in Pannonian part of Croatia for decades. Due to favourable geothermal and structural characteristics, natural thermal water springs emerge at two dozen localities, with temperatures up to 65 °C. Thermal springs are part of hydrothermal systems (HTS) which include: recharge areas in the mountainous hinterlands of the springs ; geothermal aquifers (in Croatia mostly Mesozoic carbonate rocks) in which water resides and gets heated due to heat flow from the Earth ; and discharge areas in places with favourable structural characteristics of higher permeability. The continuous functioning of such systems depends on a delicate balance between groundwater flow velocities, precipitation/dissolution processes and structural framework. The increase in thermal water utilisation is foreseen by many European and Croatian strategic documents regulating energetics, tourism, environmental protection and sustainable development. Sustainable utilisation of thermal water resource from natural springs demands understanding on a system level. It is necessary to investigate the whole cycle ‐ from recharge to discharge of the systems. Multidisciplinary methodology (structural geology, hydrogeology, geothermal, hydrogeochemical and geophysical research and remote sensing) will be used to develop and physically validate the conceptual models of HTSs, perform 3D geological modelling, hydrogeological and thermal parametrisation of the geological units involved in the thermal fluid flow, and conduct numerical simulations of system functioning in undisturbed conditions and with different extraction scenarios. The aim is to delineate geothermal resources, locate aquifers or structures that may control aquifers, or assess the general properties of the HTS. Methodology will be applied to three HTS pilot areas in the Pannonian part of Croatia, all of which are well known for the utilisation of their thermal water resources: Daruvar, Hrvatsko zagorje and Topusko. These three systems have a different level of initial data availability and it shall therefore be determined which methodology and order of application of different methods should be applied while researching the systems with considerable existing data, medium amount of data and very scarce data, respectively.

Managed aquifer recharge (MAR) refers to a suite of methods by which excess surface water is diverted underground where it is stored for subsequent recovery. Although MAR has a long history of implementation in aquifers with intergranular porosity, its application in the karst environment presents major challenges due to the high heterogeneity of karst aquifers (Dillon et al., 2019). This research was focused on conducting a MAR feasibility study on the island of Vis, a small karstic island in the Croatian part of the Adriatic Sea, where favorable geological and hydrogeological conditions enabled the formation of high-quality karst aquifers, making the island autonomous in terms of water supply. The island’s main aquifer is protected from seawater intrusions by several hydrogeological barriers, however, climate change and high seasonal pressures related to tourism pose a threat to the future availability of freshwater. To investigate whether a MAR operation is feasible and suitable for the karst aquifer on Vis, detailed field and laboratory investigations were carried out. Field investigations included in-situ measurements of physicochemical parameters on water samples from springs and boreholes, groundwater monitoring (conductivity, temperature, and water levels), geophysical methods (ERT, magnetotellurics, and seismic refraction), and structural measurements. Laboratory analyses included measurements of stable water isotopes, principal cations and anions, tritium activity, and analyses of stable isotopes from dissolved sulphates. The results corroborated the available conceptual model of the water resources detailing the geometry of the carbonate aquifer and of the fracture systems driving the water circulation. A 3D numerical model (FEFLOW) of the island will be developed to simulate various scenarios of climate change, with and without MAR. The integration of the obtained results with historical data and previous research will be used for implementing efficient and sustainable management of the karst aquifer through MAR or alternative solutions on Vis island.

Implementation of managed aquifer recharge (MAR) systems in karst environments is generally considered challenging due to the high heterogeneity of karst aquifers and only a handful of global examples demonstrated optimal methods for research and sustainable operation of such systems. Throughout the project DEEPWATER‐CE, an integrated approach to the investigation of the feasibility of MAR systems in various geological and hydrogeological environments in Central Europe was fostered. In particular, our research was focused on conducting a MAR feasibility study on the island of Vis, a small karstic island in the Croatian part of the Adriatic Sea, where favourable geological and hydrogeological conditions enabled the formation of high‐quality karst aquifers, making the island autonomous in terms of water supply.

The presentation of research conducted within the ISSAH Project.

Rad opisuje istraživanja provedena u prve dvije godine projekta ISSAH

Urban areas represent the most important market for geothermal energy. The City of Zagreb is by far the largest urban agglomeration in Croatia with a population exceeding 800, 000, and population density higher of 1, 232 inhabitants per km2 (CROATIAN BUREAU OF STATISTICS, 2018), with heating season lasting 220 days annually, and a significant potential for both deep and shallow geothermal applications. Deep geothermal resources are aquifers comprising bioclastic (Lithotamnium) limestones of the Prečec Fm (M1-5) and dolostones, limestones and dolomitic limestones of the PT Fm (T2-3). Boreholes have penetrated the aquifer at depths generally ranging from 800 to 900 m in the central area of the geothermal field, and the water temperatures range from X to Y (Bošnjak, et al., 1998). Zagreb geothermal field had been investigated since 1980-s, but the utilisation level is poor considering the available resource inside densely populated area with existing district heating system. Vintage exploration data are currently being digitalised for the purpose of making a 3D model of the subsurface and thermal model in the scope of Horizon 2020 GeoERA project HotLime. Although thermal water occurrences are generally a spectacular phenomenon, 85% of the investment into geothermal in the EU is directed toward the shallow subsurface, i.e. heat pump utilisation (REF EGEC). Its advantages constitute almost omnipresent potential, lower investment risks and possibility of reversible regime (heating, but also cooling, which has high load in urban surroundings). In the alluvial part of Zagreb urban area it is more favourable to use groundwater heat pumps (GWHPs), while at the foothills of Medvednica Mt. ground source heat pumps (GSHPs) are successfully installed. Shallow geothermal potential of the area will be investigated in the scope of Horizon 2020 GeoERA project MUSE. The research will concentrate on GWHP applications because the same aquifer is utilised for water supply, and prospects for GSHP installation have been investigated by a previous project GeoMapping (Borović, et al., 2018, Soldo, et al., 2016). There exists a competition of multiple users in the shallow subsurface of urban areas (water supply and sewerage systems, geothermal systems, deep underground constructions like hydro-insulated underpasses, underground garages and deep foundations), which can become a conflict issue. That is why existing workflows must be adapted to the needs of local scale urban areas, which will represent the most important SGE market in the future and drive heating and cooling sector decarbonisation. Both of the mentioned projects have foreseen a wide outreach toward scientific, professional, and general public, as well as toward the regulatory bodies and decision makers, with the final aim to increase the uptake of this readily available resource in the common pilot area, as well as to transfer good practices to other urban areas. That will be achieved through knowledge and experience exchange between sixteen European geological survey organisations which participate in the projects and presented through web sites, GeoERA Information Platform (GIP-P), fact-sheets, workshops, and congress communications.

The sustainable use of natural renewable resources represents one of the main tasks of the twenty first century. The renewable geothermal resources can be profitably employed for energy production, heating and in several industrial processes allowing the reduction of the greenhouse gases and the human footprint. The knowledge of the main features that favor the development and the renewability of a geothermal system is the subject of detailed studies. In this context, numerical models reproducing the 3D geological setting of a geothermal system represent a tool to evaluate the involved geological and physical processes. The results can be used to perform site specific plans for a sustainable exploitation of the resource preserving it for the future generations. This procedure was adopted for the first time in the case study of the Euganean Geothermal System (EuGS ; Veneto Region, NE Italy). Approximately 15 million cubic meters per year of thermal waters with a temperature ranging from 63 to 87 ℃ are exploited by approximately 200 wells. The Euganean thermal water is of meteoric origin and infiltrates an elevation of about 1500 m a.s.l., as suggested by its stable isotope composition. The recharge area is located on the Veneto Pre-Alps, approximately 100 km to the north of the exploitation area. The high-angle NE-dipping faults that characterize the central part of the Veneto Region allow the groundwater flow from the recharge area to the exploitation area. In the latter, a pattern of local fractures deforming the bedrock favour the rising of the thermal fluids toward the surface. The waters are mainly used for recreational purposes, producing an income of 300 million euros per year. To maintain this natural resource a detailed numerical model based on coupled simulation of fluid flow and heat transport was performed. Being the existence of this geothermal system related to a particular geological and structural setting, a 3D geological reconstruction was achieved by the use of MOVE software. The geological model was implemented into the numerical model through an unstructured mesh with the aim of maintaining a high geological detail. The results achieved by the numerical model allowed both to explain the geological processes that characterize the EuGS and to evaluate its renewability. The unravelling of these processes and of their interplay will allow to perform a management plan for the sustainable exploitation of the Euganean thermal water.

In the twenty-first century the use of natural geothermal resources is significantly increased and their renewability help the reduction of greenhouse gases emissions and the human footprint. A strategy for the sustainable utilization and exploitation of these resources is required in order to preserve them for the future generations. A reliable way to evaluate the future exploitation scenarios is represented by the numerical models in which the geological features of the geothermal system are considered. This approach is used in the case of the Euganean Geothermal System (EuGS) in which approximately 150 wells are active and exploit an amount of 14.9x10⁶ m³ of thermal water per year with a temperature between 63°C to 87°C.

ISSAH PROJEKT – ISTRAŽIVANJA HIDROGEOLOŠKIH SVOJSTAVA PRAPORA ISTOČNE HRVATSKE

The Euganean Geothermal System (EuGS), located in the central part of the Vento Region (NE Italy), represents one of the most important low-enthalpy geothermal field in the southern Europe (Fabbri et al. 2017). This thermal water is of meteoric origin and infiltrates in the norther part of the system (Veneto Pre-Alps) an elevation of about 1500 m a.s.l. A particular structural setting allows the existence of the EuGS. As a matter of fact, the groundwater flow from the recharge area to the exploitation area is favor by a system of high-angle NE-dipping faults. Moreover, the rising of the deep thermal fluids in the exploitation area is favor by a pattern of local fractures that increase the permeability of the bedrock (Pola et al 2015). Approximately 15x10⁶ m³ per year of thermal waters with a temperature ranging from 63 to 87 °C are exploited from approximately 200 wells. This water is mainly used for balneotherapy and recreational purposes, feeding about 195 SPA, and the related tourism industry produces an income of about 300 million euros per year (Fabbri et al. 2017). These data testify the important role played by this natural resource on the economy of the Veneto Region. As a consequence, a strategy for its sustainable exploitation is required in order to preserve it for the future generations. Numerical simulations can be used as a tool to assess the processes driving the geothermal resource and its renewable component and to define the sustainability of its utilization (Monterrosa and Montalvo Lopez 2010). However, the reliability of the results is strictly related to the dataset that is implemented into the numerical model (Axelsson 2010). Typically, the geological features characterizing a system are reproduced in a very schematic way using some assumptions that reduce the necessary numerical effort. Consequently, the results can be partially useful to perform a reliable utilization strategy. In the EuGS case study, a detailed geological reconstruction of the analyzed geothermal system was carried out through MOVE software. Different type of data (geological and seismic section, wells data, gravimetric and structural maps) were implemented in the geological model to reproduce the main geological features allowing the existence of the geothermal system. This approach was adopted to increase the reliability of the numerical model, reducing the simplification of the conceptual model. To preserve the detail obtained through the geological reconstruction into the numerical model an unstructured grid approach was employed. The unstructured mesh allows to reproduce the high heterogeneity that characterize the geological setting of the analyzed geothermal system (Blӧcher et al. 2010 ; Passadore et al. 2012). As a matter of fact, these meshes can be deformed and distorted in zones characterizing by high variations (i.e. fault and fold zones). This procedure was possible using the open source MeshIt software (Cacace and Blocher 2015). The obtained unstructured mesh was implemented in the FeFlow code to perform coupled simulations of fluid flow and heat transport. The results allowed to define the interplay of geological features ensuring the existence of the geothermal system and its renewability. Starting from this point, particular attention will be paid to analyze the sustainability of the geothermal resource related to the current exploitation of the Euganean geothermal resource.

Loess is a predominantly silt-sized continental aeolian sediment covering 10% of the Earth’s land surface. Groundwater generally represents the most important source of potable water in loess areas, where loess is the aquitard overlying aquifers. Despite its importance in aquifer protection, its hydrogeological properties are partially investigated. This work reviews the well-established depositional and post-depositional processes affecting this deposit to elucidate its hydrogeological behaviour. The result is beneficial to propose a hydrogeological conceptual model of aquitard- aquifer systems in loess areas.

The Euganean Geothermal System (EuGS), located in the Veneto region (NE Italy), is one of the most important, economically profitable, low-enthalpy geothermal resource in southern Europe. From the Euganean Geothermal Field (EuGF) approximately 15 Mm³/y of thermal water are exploited with a temperature ranging from 63 to 87 C. The water is mainly used for balneotherapy and recreational purposes and the related tourism industry produce an income of approximately 300 million € per year. A conceptual hydrogeological model, describing the heating of groundwater during their flow from the recharge area to the exploitation field, was recently developed. In order to validate this conceptual model, a regional 3D numerical model was carried out. The model results, in agreement with the temperature measured by thermal logs, confirm the suggested conceptual model. However, several geological and hydrogeological features that characterize the geothermal system was not taken into account in its implementation (i.e. the real extension of the regional fault system and the hydrostratigraphic units geometries). Since the existingmodeling was essentially focused on the simulation of the processes producing the EuGS, this work aims to refine the existing 3D regional numerical model improving the geological and hydrogeological settings. For this reason, a detailed geological reconstruction by Move code is performed. In order to achieve the geological reconstruction geological and seismic section and stratigraphic logs of wells located in the Veneto plain are used. Furthermore, in the EuGF area approximately 500 stratigraphic logs are available that allow to define in detail the geological setting of the exploitation field. Subsequently, the geological reconstruction will be implemented in GOLEM code carrying out coupled simulations of fluid flow and heat transport.

Excess of arsenic in groundwater is a worldwide problem threatening the health of the millions of people directly exposed to As-rich water intake. The problem is particularly acute in naturally occurring unconsolidated aquifers where As-rich groundwater is an easily-accessible resource of drinking water, such as in India, Bangladesh and Vietnam. Naturally occurring high concentrations of As in groundwater also notoriously mark the Venetian Alluvial Plain (VAP) in Italy, affecting in particular the shallower confined aquifers. The VAP is characterized by a patchy As distribution with variable extensions and concentrations. Although empirical evidences exist about the relationship between As occurrence and other factors controlling As, several aspects regarding the physical and geochemical processes controlling As in the VAP aquifers remain unclear. This study aims to elucidate the geochemical conditions that control As mobilization in the VAP, in order to improve the knowledge about As-controlling processes in the study area and use them as a proxy to evaluate occurrence of As in other world regions characterized by a similar hydrogeological and geochemical settings, such as the Bengal Delta Plain. To this end, we focused in detail on an agricultural zone nearby the Venice lagoon, affected by As contamination (called “Aree Agricole West”, AAW). The available data, collected by several hydrogeological surveys, show a spatial and temporal variability of As concentration, which can be associated to a variety of hydro-geochemical processes such as redox variations, sorption or reductive dissolution of As-rich iron oxy-hydroxides. Multicomponent 1D reactive transport modeling is applied to elucidate the leading As-controlling processes and their relevance on the AAW and VAP in general. The results provide the initial input for a 2D/3D reactive transport model aiming at highlighting As control by other mechanisms, including influence of recharge due to agricultural activities, supporting regional-scale environmental management and future stakeholders’ decision procedures.

Hydrogeological modeling, 3D simulation, Venice lagoon, reactive transport numerical mode., Flowpath 2017, National meeting on hydrogeology 2017

As one of the largest and most extensive utilized geothermal system in northern Italy, the Euganean Geothermal System (EGS, Veneto region, NE Italy) has long been the subject of still ongoing studies. Hydrothermal waters feeding the system are of meteoric origin and infiltrate in the Veneto Prealps, to the north of the main geothermal area. The waters circulate for approximately 100 km in the subsurface of the central Veneto, outflowing with temperatures from 65◦C to 86◦C to the southwest near the cities of Abano Terme and Montegrotto Terme. The naturally emerging waters are mainly used for bal-neotherapeutic purposes, forming the famous Euganean spa district. This preferential outflow is thought to have a relevant structural component producing a high secondary permeability localized within an area of limited extent (approx. 25 km2). This peculiar structure is associated with a local network of fractures resulting from transtentional tectonics of the regional Schio-Vicenza fault system (SVFS) bounding the Euganean Geothermal Field (EGF). In the present study, a revised conceptual hydrothermal model for the EGS based on the regional hydrogeology and structural geology is proposed. Particularly, this work aims to quantify: (1) the role of the regional SVFS, and (2) the impact of the high density local fractures mesh beneath the EGF on the regional-to-local groundwater flow circulation at depths and its thermal configuration. 3D coupled flow and heat transport numerical simulations inspired by the newly developed conceptual model are carried out to properly quantify the results from these interactions. Consistently with the observations, the obtained results provide indication for temperatures in the EGF reservoir being higher than in the surrounding areas, despite a uniform basal regional crustal heat inflow. In addition, they point to the presence of a structural causative process for the localized outflow, in which deep-seated groundwater is preferentially conducted to the surface, warming up on its way, by the high level of connected fractures beneath the EGF, thus corroborating the proposed conceptual model.