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Abstract The geological formation of the Muschelkalk is widespread in the center of the North German Basin (NGB) and is increasingly attracting interest for application of geothermal energy extraction or high-temperature aquifer thermal energy storage (HT-ATES). This study investigates the Middle Triassic “Rüdersdorfer Schaumkalk”, which was the former injection horizon of the natural gas storage facility in Berlin, Germany. For the first time, detailed chemical and microbiological analyses of formation water of this Lower Muschelkalk limestone formation were conducted and hydrogeochemically characterized. In addition, a hydrogeochemical model was developed to quantify the potential reactions during HT-ATES focusing on calcite dissolution and precipitation. The main objectives of this study are: (1) to determine the origin of the water from the three wells targeting the Muschelkalk aquifer, (2) to understand changes in hydrochemistry after system operation, and (3) to evaluate the long-term sustainability of a potential HT-ATES system with increasing temperature. The target formation is encountered by several wells at about 525 m below the surface with an average thickness of 30 m. Two hydraulic lifting tests including physical, chemical, and microbial groundwater as well as gas monitoring were carried out. In addition, several downhole samples of formation fluid were collected from the aquifer at in situ pressure and temperature conditions. Fluid analysis of the saline formation water indicate a seawater origin within the Muschelkalk with subsequent evaporation and various water–rock interactions with anhydrite/gypsum, dolomite, and calcite. With a salinity of 130 g/L, dominated by Na–Cl, a slightly acidic pH between 6 and 7, and a low gas content of 3%, the formation water fits to other saline deep formation waters of the NGB. Gas concentrations and microbial communities like sulfate-reducing bacteria and methanogenic archaea in the produced water indicate several geochemical alterations and microbial processes like corrosion and the forming of biogenic methane. Geochemical simulations of calcite equilibrium over 10 HT-ATES cycles indicated a pronounced propensity for calcite precipitation up to 31 mg/kgw, within the heat exchanger. At the same time, these models predicted a significant potential for calcite dissolution, with rates up to 21 mg/kgw, in both the cold and hot reservoirs. The results from the carbonate aquifer characterized in this study can be transferred to other sites in the NGB affected by salt tectonics and have provided information on the microbiological-chemical processes to be expected during the initial use of old wells.

The geological formation of the Muschelkalk is widespread in the center of the North German Basin (NGB) and is increasingly attracting interest for application of geothermal energy extraction or high-temperature aquifer thermal energy storage (HT-ATES). This study investigates the Middle Triassic "Rüdersdorfer Schaumkalk", which was the former injection horizon of the natural gas storage facility in Berlin, Germany. For the first time, detailed chemical and microbiological analyses of formation water of this Lower Muschelkalk limestone formation were conducted and hydrogeochemically characterized. In addition, a hydrogeochemical model was developed to quantify the potential reactions during HT-ATES focusing on calcite dissolution and precipitation. The main objectives of this study are: (1) to determine the origin of the water from the three wells targeting the Muschelkalk aquifer, (2) to understand changes in hydrochemistry after system operation, and (3) to evaluate the long-term sustainability of a potential HT-ATES system with increasing temperature. The target formation is encountered by several wells at about 525 m below the surface with an average thickness of 30 m. Two hydraulic lifting tests including physical, chemical, and microbial groundwater as well as gas monitoring were carried out. In addition, several downhole samples of formation fluid were collected from the aquifer at in situ pressure and temperature conditions. Fluid analysis of the saline formation water indicate a seawater origin within the Muschelkalk with subsequent evaporation and various water–rock interactions with anhydrite/gypsum, dolomite, and calcite. With a salinity of 130 g/L, dominated by Na–Cl, a slightly acidic pH between 6 and 7, and a low gas content of 3%, the formation water fits to other saline deep formation waters of the NGB. Gas concentrations and microbial communities like sulfate-reducing bacteria and methanogenic archaea in the produced water indicate several geochemical alterations and microbial processes like corrosion and the forming of biogenic methane. Geochemical simulations of calcite equilibrium over 10 HT-ATES cycles indicated a pronounced propensity for calcite precipitation up to 31 mg/kgw, within the heat exchanger. At the same time, these models predicted a significant potential for calcite dissolution, with rates up to 21 mg/kgw, in both the cold and hot reservoirs. The results from the carbonate aquifer characterized in this study can be transferred to other sites in the NGB affected by salt tectonics and have provided information on the microbiological-chemical processes to be expected during the initial use of old wells. [ABSTRACT FROM AUTHOR]

Highly fractured Mesozoic carbonate rocks are the main reservoir of many geothermal resources in northern Croatia, being of environmental, cultural, and economic value for the local and regional communities. The Daruvar thermal springs (temperatures < 50°C) represent the outflow area of an intermediate scale, tectonically controlled, hydrothermal system hosted in Triassic carbonate rocks. Several investigations have been conducted in the Daruvar area detailing the architecture of regional and local fracture networks and quantifying the hydrogeological parameters of the thermal aquifer. In this work, an integrated approach based on structural and hydrogeological investigations was employed to model the network of fractures in the reservoir and quantify its impact on the hydraulic properties. Structural investigations were conducted in the Batinjska Rijeka quarry, considered as an outcrop analogue of the thermal aquifer, employing both a classical field approach and the virtual quantitative analysis of a 3D digital outcrop model. Structural analysis of the digital outcrop model allowed identification of two sub-vertical systems of discontinuities, dipping to the NW and the WSW respectively, in accordance with the data collected through direct field measurements. The main geometric features of the discontinuity network and their statistical distributions were employed to construct discrete fracture network models at both the outcrop scale (approximately 100 m) and the aquifer scale in Daruvar (approximately 700 m). Calibration of the input parameters allowed modelling of porosity and permeability values that reproduce the field values assessed through pumping tests, well tests, and well logging. This work highlights the importance of integrating geological and hydrogeological investigations to obtain a more reliable reconstruction and quantification of the processes driving the fluid flow in fractured aquifers and affecting the spatial distribution of their hydraulic properties. [ABSTRACT FROM AUTHOR]

The Gran Sasso carbonate aquifer is the largest and most productive in the Apennines. Its hydrogeological structure has been studied since the middle of the last century for the springs' characterization for drinking purposes and for a motorway tunnel. Meanwhile, its hydrodynamic parametrization is less developed and has been limited to monitoring the discharge and chemical and isotopic parameters. Secondary porosity characterizes the aquifer, and an underlying impermeable marly complex represents the basal aquiclude. It might appear inappropriate to characterize the hydraulic properties via pumping tests, as their reliability has been proven in homogeneous and isotropic media. However, the high extent of the aquifer, the wells' location, the scarcity of information available and the lack of alternatives has forced the estimation of hydrodynamic parameters as in porous aquifers and the experimental testing of the aquifer, especially in maximum pumping conditions, for a possible exploitation increase. Since aquifer testing was performed during the normal well field's activities, it was not possible to perform typical tests. Therefore, the step-drawdown test was conducted by turning on an increasing number of wells over time and keeping the observation points fixed. As results, a mean hydraulic conductivity of 5 × 10−3 m/s and a mean transmissivity of 0.3 m2/s were established without interrupting the water supply; meanwhile, the influence radius and flow directions were also estimated. [ABSTRACT FROM AUTHOR]

Water pollution with potentially toxic elements (PTEs) becomes a major threat to the validity of that water for drinking and agriculture and hence human life. The current study aims to evaluate the carbonate aquifer groundwater PTE content, sources, and impact on water quality. To achieve this goal, drilling cutoff of two wells and 30 groundwater samples were collected and chemically analyzed. The groundwater has TDS values ranged from 271.2 to 971.8 mg/l. Freshwater recharge process led to the continuous freshening of the aquifer and enhanced the forward ion exchange reactions. Carbonate and evaporate mineral dissolution/precipitation processes are the main controller of groundwater chemistry. Cd and As represented the most hazardous elements in these rocks followed by Pb. The presence of high concentrations of PTEs (As, Cd, Cr, and Pb) in water-bearing carbonate rocks has led to detritus of water quality and its unsuitability for drinking owing to the occurrence of these harmful elements in water. Even though the high concentrations of PTEs in the studied water, these elements occurred in low mobility and toxicity species: Fe(OH)3, Mn2+, (HAsO4)−2, Cd2+, Cr2O3, Cu2O, CuO, and PbCO3. This reflects the importance of studying element species rather than total concentration. Even though the unsuitability of this water for drinking owing to high concentrations of some PTEs, all of the samples were found to be suitable for irrigation. [ABSTRACT FROM AUTHOR]

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The coastal and carbonate Mean Sea Level Aquifer (MSLA) of Malta is characterised by high anisotropy and heterogeneity, which together make evaluating the aquifer system parameters a challenging task. In this paper, we present an approach for the determination of the hydrogeological parameters of this coastal aquifer based on tidal-induced groundwater fluctuations that can be applied in other similar contexts. This work presents an analysis of data undertaken on monitoring boreholes located in the Malta MSLA exhibiting tidal-induced groundwater fluctuations. This allowed us to determine the values of three main hydrogeological parameters: hydraulic diffusivity, transmissivity and hydraulic conductivity. These will subsequently be used as an input for groundwater flow and reactive transport modelling purposes. In this study, a methodology based on the fast Fourier transform (FFT) is proposed to improve the applicability of the Jacob–Ferris method to the observed groundwater level and sea level fluctuations. The FFT reproduced signals allowed us to isolate the component induced by sea tides, thus eliminating short- and long-term variations of the water table induced by other disruptive factors. Results showed high variability of hydrogeological parameters within a short distance, reflecting the high anisotropy and heterogeneity of the aquifer system. The transmissivity values derived from the Jacob–Ferris method are complemented with results derived from the pumping tests with the aim of estimating the spatial distribution of the aquifer transmissivity for the study area. The spatial variability of transmissivity values is analysed by means of geostatistics tools for estimating uncertainty, correlation and variation in space through the use of semi-variograms. [ABSTRACT FROM AUTHOR]

On atoll islands, the fresh water lens (FWL) constitutes, for the island population, a very important fresh water resource for various usages. Its shape and thickness highly depend on the underground characteristics of several rock formations which constitute the underground of the island: upper Holocene sediments and lower Pleistocene limestone rocks separated by the Holocene Pleistocene Unconformity. In this study, several very simple investigation methods were applied on the Pacific atoll of Rangiroa in order to characterize the aquifer hydrodynamics and their impact on the FWL. A water budget of the aquifer was proposed, including the deep infiltration to the FWL. Pumping tests and tidal analysis demonstrated the confined character of the aquifer and its main hydrodynamic properties (storage and hydraulic conductivity). The role of the reef-flat plate on the hydrogeology of the atoll was highlighted. Its impermeable nature contributes to reduce the deep infiltration to the aquifer. It also contributes to the confined flow regime of the aquifer, inducing high and fast water level fluctuations due to tide forces, and consequently contributes to increase the thickness of the saline mixing zone. Both phenomena contribute to reduce the thickness of the FWL, which is only 2 m-thick in that atoll. [ABSTRACT FROM AUTHOR]

This paper concerns a stepwise modelling procedure for groundwater flow simulation in a folded and faulted, multilayer carbonate aquifer, which constitutes a source of good quality water for human consumption in the Apennine Range in Central Italy. A perennial river acts as the main natural drain for groundwater while sustaining valuable water-related ecosystems. The spatial distribution of recharge was estimated using the Thornthwaite–Mather method on 60 years of climate data. The system was conceptualized as three main aquifers separated by two locally discontinuous aquitards. Three numerical models were implemented by gradually adding complexity to the model grid: single layer (2D), three layers (quasi-3D) and five layers (fully 3D), using an equivalent porous medium approach, in order to find the best solution with a parsimonious model setting. To overcome dry-cell problems in the fully 3D model, the Newton–Raphson formulation for MODFLOW-2005 was invoked. The calibration results show that a fully 3D model was required to match the observed distribution of aquifer outflow to the river baseflow. The numerical model demonstrated the major impact of folded and faulted geological structures on controlling the flow dynamics in terms of flow direction, water heads and the spatial distribution of the outflows to the river and springs. [ABSTRACT FROM AUTHOR]

The objectives of this study are to characterize the input signal of a shallow carbonate aquifer in northeastern France and to clarify the recharge and groundwater mineralization processes, by analysing the chemical and isotopic (δ18O, δ2H) composition of rain and groundwater. The groundwater isotopic signature was very close to the calculated local meteoric water line and showed a narrow range, which is explained by the buffer effect of the matrix-dominated chalk aquifer. Nevertheless, intensive fracture networks can exist at local scale and lead to rapid aquifer response to rainfall, as attested by observed large variations in groundwater isotopic and chemical signatures at one specific site. Main factors controlling the groundwater mineralization are highlighted, including the input signal, water-rock interactions and human activities. Moreover, groundwater isotopic signatures are related to groundwater ages and historical climate conditions, showing that groundwater isotopic signal could also be used as a record of climate changes. [ABSTRACT FROM AUTHOR]

The Molasse Basin in Southern Germany is part of the North Alpine Foreland Basin and hosts the largest accumulation of deep geothermal production fields in Central Europe. Despite the vast development of geothermal energy utilization projects especially in the Munich metropolitan region, the evolution of and control factors on the natural geothermal field, more specifically the time-varying recharge and discharge governing groundwater and heat flow, are still debated. Within the Upper Jurassic (Malm) carbonate aquifer as the main geothermal reservoir in the Molasse Basin, temperature anomalies such as the Wasserburg Trough anomaly to the east of Munich and their underlying fluid and heat transport processes are yet poorly understood. We delineate the two end members of thermal–hydraulic regimes in the Molasse Basin by calculating two contrasting permeability scenarios of the heterogeneously karstified Malm carbonate aquifer along a model section through the Wasserburg Trough anomaly by means of two-dimensional numerical thermal-hydraulic modelling. We test the sensitivity of the thermal-hydraulic regime with regard to paleoclimate by computing the two Malm permeability scenarios both with a constant surface temperature of 9 °C and with the impact of paleo-temperature changes during the last 130 ka including the Würm Glaciation. Accordingly, we consider the hydraulic and thermal effects of periglacial conditions like permafrost formation and the impact of the numerous glacial advances onto the Molasse Basin. Thermal-hydraulic modelling reveals the effect of recurrent glacial periods on the subsurface targets of geothermal interest, which is minor compared to the effect of permeability-related, continuous gravity-driven groundwater flow as a major heat transport mechanism. [ABSTRACT FROM AUTHOR]

The Tazouta mini basin, located 30 km SE of Sefrou city, corresponds to a collapsed zone between two segments of the North Middle Atlas Fault (NMAF). Lithological formations, faulting and fracturing, play an important role in aquifers structuring, especially by the presence of several springs and wells intended for drinking water and irrigation purposes. The aim of this study is to describe hydrogeological and tectonic characteristics of the Tazouta basin, and to demonstrate the importance of faulting in surficial groundwater circulations. The genesis of this basin is linked to a polyphased tectonics from Miocene to Quaternary; tectonic events have influenced groundwater flows. Analysis of structures affecting the carbonate aquifer, structural sections, and the piezometric map produced from water points capturing the aquifer allowed to determine the aquifer geometry and limits as well as groundwater circulation pattern. The water flows along the fault planes with various directions and overlapping in time and space. Indeed, groundwater flows are from the southwest to the northeast and from the south to the north. These flows are perpendicular to the normal fault planes, generally towards collapsed compartments. Its emergence in water points (springs) is ensured by the influence of the lithological nature of the field and the tectonic structures. Spatial distribution of groundwater pH and electrical conductivity is showing important anomalies along the tectonic accidents where the Triassic evaporitic claystones outcrop. [ABSTRACT FROM AUTHOR]

The hydraulic conductivity of an unconfined carbonate aquifer at the uplifted atoll of Minami-Daito, Japan, was evaluated by a combination of cross-spectral analysis, analytical solution, and density-dependent groundwater modeling based on observed groundwater levels in 15 wells and at sea level. The island area was divided into 10 subregions based on island morphology and on inland propagation of ocean tides. The hydraulic conductivity was obtained for each subregion using analytical solutions based on phase lags of M2 constituents of ocean tides at each well by assuming two aquifer thicknesses (300 and 1,800 m) and two effective porosities (0.1 and 0.3). The density-dependent groundwater model evaluated the hydraulic conductivity of the subregions by reproducing observed groundwater levels. The hydraulic conductivity in the subregions was estimated as 3.46 × 10−3 to 6.35 × 10−2 m/s for aquifer thickness of 300 m and effective porosity of 0.1, and as 1.73 × 10−3 to 3.17 × 10−2 m/s for aquifer thickness of 1,800 m and the effective porosity of 0.3. It was higher in southern and northern areas, and higher in interior lowland than in the western and eastern areas. Fissures and dolomite distributions on the island control differences of the omnidirectional ocean tidal propagation and cause these differences in hydraulic conductivity. The method used for this study may also be applicable to other small islands that have few or no data for hydraulic conductivity. [ABSTRACT FROM AUTHOR]

The highest part of the Nera River basin (Central Italy) hosts significant water resources for drinking, hydroelectric, and aquaculture purposes. The river is fed by fractured large carbonate aquifers interconnected by Jurassic and Quaternary normal faults in an area characterized by high seismicity. The 30 October 2016, seismic sequence in Central Italy produced an abrupt increase in river discharge, which lasted for several months. The analysis of the recession curves well documented the processes occurring within the basal aquifer feeding the Nera River. In detail, a straight line has described the river discharge during the two years after the 2016 seismic sequence, indicating that a turbulent flow characterized the emptying process of the hydrogeological system. A permeability enhancement of the aquifer feeding the Nera River--due to cleaning of fractures and the co-seismic fracturing in the recharge area--coupled with an increase in groundwater flow velocity can explain this process. The most recent recession curves (2019 and 2020 periods) fit very well with the preseismic ones, indicating that after two years from the mainshock, the recession process recovered to the same pre-earthquake conditions (laminar flow). This behavior makes the hydrogeological system less vulnerable to prolonged droughts, the frequency and length of which are increasingly affecting the Apennine area of Central Italy. [ABSTRACT FROM AUTHOR]

Abstract Crystallization of carbonates is a key process affecting the operation of geothermal facilities and aquifer heat storage systems. The crystals formed in an aquifer heat storage test in the Upper Jurassic carbonate aquifer were investigated at injection temperatures of $$65\,^{\circ }\hbox {C}$$ 65 ∘ C to $$110\,^{\circ }\hbox {C}$$ 110 ∘ C , with varying $$\hbox {CO}_{2}$$ CO 2 partial pressures, and varying Mg/Ca ratios. Water samples were directly filtrated, and analyzed by SEM/EDX. Complementary autoclave experiments were run. In the autoclave experiments with tap water, aragonite crystals dominated at all temperatures (45–110 $$\,^{\circ }\hbox {C}$$ ∘ C ). In the autoclave experiments with ultra-pure water, calcite crystals dominated at the same temperatures. In the field test, mainly calcite crystals were found up to temperatures of $$90\,^{\circ }\hbox {C}$$ 90 ∘ C . Only at very high temperatures of $$110\,^{\circ }\hbox {C}$$ 110 ∘ C aragonite crystallization prevailed. $$\hbox {CO}_{2}$$ CO 2 partial pressure varied especially between injection and production stages. Mg/Ca ratio varied through all stages, and depended on the dissolution of the rock matrix. Together with the autoclave experiments, this study suggest that temperature and Mg/Ca ratio had no influence on the crystallization, and only supersaturation affected the $$\hbox {CaO}_{3}$$ CaO 3 polymorphs. We further assume that we produced initially injected crystals back during the following production stage. That results in the assumption that existing particles can maintain an equilibrium in the dispersion, and reduce precipitation on surfaces like pipes and heat exchangers.

A 1:50,000 hydrogeological map of the Pale di San Martino Mountains (Northern Italy) was created. The map presents the merge of various pre-existing data with new field data collected between the years 2014 and 2016. Through the use of symbols and specific colours, the map shows various groundwater-related data such as the hydrogeological complexes, the location and size of the main springs, the extension of the recharge areas, the hydrogeological boundaries, as well as information on groundwater usage. Given the absence of hydrogeological maps in the entire mountain range of the Dolomites, the approach followed in this study could be used as a guide for future representations in this alpine region. At the local scale, the map could serve as a conceptual base for future research involving groundwater and for water management planning.

The dissolution of carbon‐dioxide (CO2) in deep saline aquifers is an important trapping mechanism in carbon storage. This process is triggered by unstable high‐density CO2 front, which later promotes density‐driven mixing, hydrodynamic dispersion of CO2, and favors the long‐term sequestration. In many former studies, effects of hydrodynamic dispersion and multispecies geochemical reactions have been ignored. This work elaborates the impacts of these simplifications on the dynamics of convective mixing by numerical simulations. Geochemical effects were studied by the implementation of rock‐fluid and fluid‐fluid interactions for a typical carbonate aquifer. Results show that accounting for the hydrodynamic dispersion decreases the convection onset time and increases the CO2 dissolution flux, which is more significant in larger dispersivities and Rayleigh numbers. Results indicate that carbonate geochemical reactions intense the long‐term overall efficiency of the process, while decrease the total amount of sequestered carbon in the diffusion‐dominated period. Results also reinforce the importance of realistic geochemical representation and importance of spatial and temporal dependence of the reactions pathway, subsequent to the finger development for more detailed simulation of the CO2 storage process. Plain Language Summary: Geological carbon storage is one of the technologies envisaged to play a critical role in decarbonization and achieving the net‐zero carbon emission. It is estimated that 17% of the whole decarbonization by 2050 will be achieved by carbon storage. Based on a modeling study, we demonstrate the role of natural geochemistry of the subsurface is important in the dissolution of carbon in brine and its long‐term performance. Key Points: Effect of hydrodynamic dispersion is studied on the dynamics of CO2 convectionChemical reactions have nonmonotonic effects on carbon storage before and after convection onsetThe importance of realistic geochemical modeling and different reaction pathways subsequent to plume propagation is shown [ABSTRACT FROM AUTHOR]

This study offers a reinterpretation of archive aquifer tests, predominantly on the basis of recovery data, from an original datasheet of thermal water wells located in carbonate and sandstone aquifer units in the vicinity of Budapest, Hungary. The study compares the hydraulic conductivity (K) and specific storage (Ss) values derived in the first instance from an aquifer test evaluation. This included an initial application of the classical analytical Cooper and Jacob method. Subsequently, the visual two-zone (VTZ) numerical method was applied, then third, a more complex model, namely, WT software. It was found that the simple analytical solution is not able to represent the field conditions accurately, while in the course of the application of the VTZ model, it proved possible to alter the various hydraulic parameters within reasonable limits to fit the field data. In the case of the VTZ model, the researcher is required to calculate the accuracy of the fitted model separately, while with the WT model, this is automatic, the software seeks out the best fit. In addition to VTZ parameters, the WT model can efficiently incorporate data on up to 500 model layers, water level, and pressure. The optimization of the parameters may be achieved by automatic calibration, improving the accuracy of the numerical results. Recovery tests for 12 wells were numerically simulated to obtain values for vertical and horizontal hydraulic conductivity and specific storage for Triassic and Eocene fractured carbonate and the Upper-Miocene-Pliocene granular sandstone aquifer units. When an analytical solution is applied, only average values could be obtained. The conclusion reached was that the results of the analytical solution can be improved by the use of numerical methods. These methods are able to incorporate basic information on well design, aquifer material and the hydrogeological environment in the course of the evaluation. The revision of the archive recovery data using numerical methods may assist in the quest for better data for numerical flow and transport simulations without the need to perform new tests. In addition, the methods employed here can explain cases in which the original analytical interpretations proved unable to yield reliable data and predictions. [ABSTRACT FROM AUTHOR]

The highest part of the Nera River basin (Central Italy) hosts significant water resources for drinking, hydroelectric, and aquaculture purposes. The river is fed by fractured large carbonate aquifers interconnected by Jurassic and Quaternary normal faults in an area characterized by high seismicity. The 30 October 2016, seismic sequence in Central Italy produced an abrupt increase in river discharge, which lasted for several months. The analysis of the recession curves well documented the processes occurring within the basal aquifer feeding the Nera River. In detail, a straight line has described the river discharge during the two years after the 2016 seismic sequence, indicating that a turbulent flow characterized the emptying process of the hydrogeological system. A permeability enhancement of the aquifer feeding the Nera River—due to cleaning of fractures and the co-seismic fracturing in the recharge area—coupled with an increase in groundwater flow velocity can explain this process. The most recent recession curves (2019 and 2020 periods) fit very well with the pre-seismic ones, indicating that after two years from the mainshock, the recession process recovered to the same pre-earthquake conditions (laminar flow). This behavior makes the hydrogeological system less vulnerable to prolonged droughts, the frequency and length of which are increasingly affecting the Apennine area of Central Italy.

This study aims to assess the climate change impact on water resources of the Essaouira Basin (Morocco). The rainfall index shows a significant interannual variability which is organized in an alternation of the excess period (1940–1956), two deficit periods (1956–1986 and 1998–2015) separated by a normal period (1986–1998). The statistical approach reveals the presence of a break in 1956 with a rainfall deficit of 14.45% and that the temperatures show a general upward trend with a warming of 2.3 °C for mean and minimum temperature and of 2.8 °C for maximum and this for the period 1987–2014. Flow rates show a slight upward trend that could be explained by a short and intense rainfall that characterizes our semi-arid environment. Concerning the piezometric approach, it shows a general descent in piezometric surface for the period of observation (reached 12 m in some wells). The downward evolution of piezometric surface due to the decrease of precipitation has resulted in the spatial and temporal degradation of the groundwater quality. A decrease in nitrate levels has been noted; this could be explained by the downward trend in precipitation by favoring the retention of nitrates in the superficial parts of the soil and by limiting their infiltration to the aquifer. [ABSTRACT FROM AUTHOR]

Crystallization of carbonates is a key process affecting the operation of geothermal facilities and aquifer heat storage systems. The crystals formed in an aquifer heat storage test in the Upper Jurassic carbonate aquifer were investigated at injection temperatures of 65 ∘ C to 110 ∘ C , with varying CO 2 partial pressures, and varying Mg/Ca ratios. Water samples were directly filtrated, and analyzed by SEM/EDX. Complementary autoclave experiments were run. In the autoclave experiments with tap water, aragonite crystals dominated at all temperatures (45–110 ∘ C ). In the autoclave experiments with ultra-pure water, calcite crystals dominated at the same temperatures. In the field test, mainly calcite crystals were found up to temperatures of 90 ∘ C . Only at very high temperatures of 110 ∘ C aragonite crystallization prevailed. CO 2 partial pressure varied especially between injection and production stages. Mg/Ca ratio varied through all stages, and depended on the dissolution of the rock matrix. Together with the autoclave experiments, this study suggest that temperature and Mg/Ca ratio had no influence on the crystallization, and only supersaturation affected the CaO 3 polymorphs. We further assume that we produced initially injected crystals back during the following production stage. That results in the assumption that existing particles can maintain an equilibrium in the dispersion, and reduce precipitation on surfaces like pipes and heat exchangers. [ABSTRACT FROM AUTHOR]

A 1:50,000 hydrogeological map of the Pale di San Martino Mountains (Northern Italy) was created. The map presents the merge of various pre-existing data with new field data collected between the years 2014 and 2016. Through the use of symbols and specific colours, the map shows various groundwater-related data such as the hydrogeological complexes, the location and size of the main springs, the extension of the recharge areas, the hydrogeological boundaries, as well as information on groundwater usage. Given the absence of hydrogeological maps in the entire mountain range of the Dolomites, the approach followed in this study could be used as a guide for future representations in this alpine region. At the local scale, the map could serve as a conceptual base for future research involving groundwater and for water management planning. [ABSTRACT FROM AUTHOR]

Viruses and bacteria which are characterized by finite lives in the subsurface are rapidly transported via fractures and cavities in fractured and karst aquifers. Here, we demonstrate how the coupling of a robust outcrop characterization and hydrogeophysical borehole testing is essential for prediction of contaminant velocities and hence wellhead protection areas. To show this, we use the dolostones of the Permian Magnesian Limestone aquifer in NE England, where we incorporated such information in a groundwater flow and particle tracking model. Within this aquifer, flow in relatively narrow (mechanical aperture of ~ 10−1–1 mm) fractures is coupled with that in pipe cavities (~ 0.20-m diameter) following normal faults. Karstic cavities and narrow fractures are hydraulically very different. Thus, the solutional features are represented within the model by a pipe network (which accounts for turbulence) embedded within an equivalent porous medium representing Darcian flowing fractures. Incorporation of fault conduits in a groundwater model shows that they strongly influence particle tracking results. Despite this, away from faulted areas, the effective flow porosity of the equivalent porous medium remains a crucial parameter. Here, we recommend as most appropriate a relatively low value of effective porosity (of 2.8 × 10−4) based on borehole hydrogeophysical testing. This contrasts with earlier studies using particle tracking analyses on analogous carbonate aquifers, which used much higher values of effective porosity, typically ~ 102 times higher than our value, resulting in highly non-conservative estimates of aquifer vulnerability. Low values of effective flow porosities yield modelled flow velocities ranging from ~ 100 up to ~ 500 m/day in un-faulted areas. However, the high fracturing density and presence of karstic cavities yield modelled flow velocities up to ~ 9000 m/day in fault zones. The combination of such flow velocities along particle traces results in 400-day particle traces up to 8-km length, implying the need for large well protection areas and high aquifer vulnerability to slowly degrading contaminants. [ABSTRACT FROM AUTHOR]

Highlights • The characterisation of low temperature carbonate geothermal systems is discussed. • Thermodynamic uncertainties in geothermometrical calculations are addressed. • Limitations in mineral equilibria assumptions are discussed. • Calcite-dolomite equilibrium is evaluated in detail. Abstract Geothermometrical characterisation of low-temperature, carbonate-evaporitic geothermal systems is usually hampered by the lack of appropriate mineral equilibria to successfully use most of the classical geothermometers and/or by the thermodynamic uncertainties affecting some of the most probable mineral equilibria in low temperature conditions. This situation is further hindered if the thermal waters are additionally affected by secondary processes (e.g., CO 2 loss) during their ascent to surface. All these problems cluster together in the low-temperature Alhama-Jaraba thermal system, hosted in carbonate rocks, with spring temperatures about 30 °C and waters of Ca-Mg−HCO 3 /SO 4 type. This system, one of the largest naturally flowing (600 L/s) low temperature thermal systems in Europe, is used in this paper as a suitable frame to assess the problems in the application of chemical geothermometrical techniques (classical geothermometers and geothermometrical modelling) and to provide a methodology that could be used in this type of geothermal system or in potential CO 2 storage sites in similar aquifers. The results obtained have shown that the effects of the secondary processes can be avoided by selecting the samples unaffected by such processes and, therefore, representative of the conditions at depth, or by applying existing methodologies to reconstruct the original composition, as is usually done for medium to high temperature systems. The effective mineral equilibria at depth depend on the temperature, the residence time and the specific lithological/mineralogical characteristics of the system studied. In the present case, the mineral equilibria on which classical cation geothermometers are based have not been attained. The low proportion of evaporitic minerals in the hosting aquifer prevents the system from reaching anhydrite equilibrium, otherwise common in carbonate-evaporitic systems and necessary for the specific SO 4 -F geothermometer or the specially reliable quartz (or chalcedony) – anhydrite equilibrium in the geothermometrical modelling of these geothermal systems. Under these circumstances, the temperature estimation must rely on quartz (or chalcedony), clay minerals and, especially, calcite and dolomite. However, clay minerals and dolomite present important thermodynamic uncertainties related to possible variations in composition or crystallinity degree for clays and order/disorder degree for dolomite. To deal with these problems, a sensitivity analysis to the thermodynamic data for clay minerals has been carried out, comparing the results obtained when considering different solubility data. The uncertainties associated with dolomite have been addressed by reviewing the solubility data available for dolomites with different order degrees and performing specific calculations for the order degree of the dolomite in the aquifer. This approach can be used to find the most adequate dolomite thermodynamic data for the system under consideration, including medium-high temperature geothermal systems. Finally, the temperature estimation of the Alhama-Jaraba waters in the deep reservoir has been obtained from simultaneous equilibria of quartz, calcite, partially disordered dolomite and some aluminosilicate phases. The obtained value of 51 ± 14 °C is within the uncertainty range normally affecting this type of estimations and is coherent with independent estimations from geophysical data. [ABSTRACT FROM AUTHOR]