Heat pumps combined with vertical borehole heat exchangers utilize the ground as a heat source or sink for the purpose of heating or cooling. The proper dimensioning of the borehole heat exchanger ...requires understanding of the thermophysical properties of the ground surrounding boreholes.
Presented research was carried out in two geologically different regions with varying thermal parameters relevant for the utilization of ground-coupled heat pumps: Pannonian Basin System and the Dinarides. Eight different sites were selected based on characteristic geological settings and climate in order to include inland, seaside and mountainous regions. In boreholes, 100 and 130 m deep, double U-pipe borehole heat exchangers were installed.
The improved method for evaluation of the ground thermal conductivity and thermal resistance of a borehole heat exchanger known as distributed thermal response test was used. This method employs optic fiber cables for the measurement of temperature profiles along the borehole heat exchangers. Measurements conducted at different locations show great variability of thermal properties and undisturbed ground temperatures. Obtained results indicate that the average thermal conductivity of sediments in the Pannonian part varies between 1.62 and 2.10 W/(m K), compared to Dinaridic carbonate and evaporite rocks' values between 2.01 and 3.19 W/(m K).
•In situ ground thermal properties determined by distributed thermal response tests.•Undisturbed ground temperature profiles measured.•Vertical distribution of shallow geothermal properties at eight locations obtained.•Ground thermal properties of two geological regions are compared.•More favorable underground thermal parameters are found in Dinaridic region.
Current situation and trends in geothermal water utilization in Croatia have been analyzed based on data gathering and observations in a period of 16 years with special attention devoted to their use ...for tourism purposes. Early 2014 geothermal water was utilized at 26 locations (15 springs and 11 deep boreholes) for 10 different purposes. The traditional modalities of use – recreation and balneotherapy – remain dominant. The fluctuations in the number of sites that utilized geothermal water were low and a result of temporary closure of facilities due to the necessary modernization or termination of their work because of non-profitability. The changes in geothermal water utilization during the considered period indicate stagnation in the development of this resource. Geothermal waters contain an energy potential significant for heating and cooling sector on a national scale, and for electricity generation and cascade utilization on local scale. Those could be energy self-sufficient tourism facilities with low environmental footprint, enriched by the attractions that can be provided with innovative geothermal utilization schemes. Given the untapped geothermal potential and the anticipated growth in demand for health and spa tourism, increased use of geothermal waters for this purpose can be expected. In the interest of safeguarding this valuable resource, it is important to plan sustainable utilization during each phase of geothermal site development cycle and fully implement joint management principles to transboundary geothermal aquifers by including neighbouring countries.
Hydrogeological and thermogeological properties of the shallow subsurface in the Dinaric karst area of Croatia were investigated in the context of its utilization for ground- and water-source heat ...pumps (GSHPs and WSHPs). The research encompassed four 100 m deep boreholes with GSHP installations in both coastal and inland Dinaric karst (different limestones and evaporitic rocks at one location), and a set of six exploratory boreholes, abstraction and reinjection wells for WSHP heating and cooling using seawater on the coast (fractured and karstified limestones). It was determined that rock thermal conductivities are favourable for GSHP utilization, but dependent on the wider rock mass characteristics which are hard to predict (size of karst voids and their saturation status). In addition, wells with high enough yield and stabile seawater or groundwater temperatures for WSHP utilization can be designed in appropriate structural settings (tensional fractures and fracture set intersections). Advantages and disadvantages of the utilized methodology have been pointed out, as well as methods which should prove useful in the future, especially if larger systems are planned. Hydrogeological, geotechnical, and thermal risks expected during the drilling, installation, and operational phases have also been identified. Presented case studies have given the insight into heat pump installation options and conditions in Croatian part of the Dinarides, but can be useful to other researchers and engineers both in the Dinarides and in similar karst regions.
Topusko is the second warmest natural thermal water spring area in Croatia, located at the southwest edge of the Pannonian Basin System. Due to favourable geothermal properties, these waters have ...been used for heating and health and recreational tourism since the 1980s. Thermal springs with temperatures up to 50 °C are the final part of an intermediate-scale hydrothermal system. However, systematic research on the Topusko spring area has not been conducted to lay the foundation for sustainable resource utilisation. Multidisciplinary research including the hydrogeochemical characterisation of naturally emerging thermal water, an electrical resistivity tomography (ERT) investigation conducted to reconstruct the subsurface geology, and hydrogeological parametrisation of the geothermal aquifer was carried out to refine the existing local conceptual model. The results show Ca-HCO3 facies of Topusko thermal waters, which get heated in a Mesozoic carbonate aquifer. The water equilibrium temperature in the geothermal aquifer is estimated to be 78 °C based on the SiO2-quartz geothermometer. The fault damage zone, which enables the upwelling of thermal water, was identified by ERT investigations. The transmissivity values of the aquifer derived from the results of step-drawdown tests range from 1.8 × 10−2 to 2.3 × 10−2 m2/s. Further multidisciplinary research is necessary to improve the existing conceptual model of the Topusko hydrothermal system.
Water supply of the islands is a global challenge, especially in the countries which have highly indented coastlines with numerous islands. The island of Mljet in Croatia was investigated due to its ...unique source of water supply: desalination of water from brackish lakes—blatinas—fed by groundwater and connected to the sea by karst conduits. Water sampling and chemical analyses were performed during hydrological minimum and maximum with regard to groundwater levels in 2005/2006 and minimum in 2016. A total of 13 samples were analysed within the study: 10 samples were taken from blatinas, 1 from pit well, and 2 from borehole wells. All waters sampled from the lakes are of Na-Cl type. The seawater percentage in the lakes used to extract feed water for desalination plants, calculated by conservative mixing approach, is relatively low (0.7-9.8%) and varies in correlation with hydrological seasons. Low proportion of seawater is an essential factor of cost minimisation in desalination by the installed reverse osmosis (RO) plants. Daily monitoring of total dissolved solids in the feed water was introduced in May 2016, and its results were analysed in the context of precipitation—a sole source of island aquifer recharge. Maximum concentrations were observed during September and interpreted to be caused by a combination of natural and anthropogenic pressure during the summer tourist season. Minimum concentrations were expected after the rainy season in the cold part of the year but were observed in June instead. Due to a short observation period and untypical distribution of precipitation in the same time interval, the data can only be considered indicative. An unusual pattern of sulphate anion concentrations, which cannot be attributed solely to fresh- and seawater mixing, was observed in one of the blatinas, but its origin could not be determined based on available data. Taking into account all the presented data on groundwater quality, climate change predictions, the connection of water supply system to the mainland and problems with the effluent treatment, it is clear that the main future challenge will be the creation of an island-wide sustainable water management plan followed by continuous monitoring and research.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Thermal waters in Topusko (Croatia), with temperatures of up to 65 °C, have been used for heating, health, and recreational tourism for the past fifty years. Hydrogeochemical monitoring can provide ...insights into deeper geological processes and indicate system changes from baseline levels. It helps to identify potential anthropogenic impacts, as well as natural changes. Hydrogeochemical, geothermometrical, and environmental isotope studies of thermal waters in Topusko were conducted to improve the existing conceptual model of the Topusko hydrothermal system (THS), providing a baseline for continuous monitoring of the thermal resource. 2-year thermal springs and precipitation monitoring took place from March 2021 until March 2023. Major anions and cations, stable and radioactive isotopes (i.e.
18
O,
2
H, SO
4
2−
,
3
H and
14
C) and geothermometers were used to assess the origin of thermal waters in Topusko and their interaction with thermal aquifer. The results indicate the meteoric origin of thermal water, which was recharged in colder climatic conditions around the late Pleistocene–Early Holocene. Thermal water was last in contact with the atmosphere before approximately 9.5 kyr. Ca-HCO
3
hydrochemical facies suggests carbonate dissolution as the dominant process driving the solute content. Geothermometrical results indicate an equilibrium temperature in the reservoir of 90 °C.
The United States Bureau of Reclamation (USBR) formula was derived from a table presenting values of hydraulic conductivity as a function of grain size,
K
=
f
(
d
20
). The original table was ...empirically designed as a sequence of variation of different permeability coefficients of deposits and was intended for the design of earth dams, for the purpose of assessing leakage where percolation tests are not available. The USBR formula has since been used for predicting the hydraulic conductivity of water-bearing uniform sand deposits but systematically derives values of hydraulic conductivity several times lower than realistic values for materials. In this article, the optimal analytical formulation of the series of original data for
K
=
f
(
d
20
) from Justin et al. (1945) is presented. Additionally, through calibration using results of hydrogeological research in Croatia, Germany, China and Nigeria, a formula (named USCRO) for predicting the permeability of sediments over a wide range of uniformity and
d
20
grain size was derived. The validity of this function for expressing permeability and the utilization of relative nondimensional coefficients is examined through a graphical correlation of the permeability of uniform and especially well-graded materials. Samples of poorly graded sand (63) and well-graded sandy gravel (131) were included in the calibration procedure. Data for mechanical analyses were taken from published articles. The numerical correlation of the USCRO formula for uniform sand samples resulted in a Pearson correlation coefficient of
R
2
= 0.902; for the well-graded sandy gravel,
R
2
= 0.838. Justin JD, Hinds J, Creager WP (1945) Engineering for Dams (Vol III), John Wiley & Sons.
Crystalline rocks are generally characterized by negligible porosity and permeability in terms of groundwater exploitability. However, alteration processes can greatly increase their fracture ...permeability and induce formation of modest, but locally important aquifers. Therefore, subsurface characteristics of alteration zones are of major importance for hydrogeological evaluation of crystalline terrains. Alteration processes greatly affect rock total porosity and water content, causing contrasting electrical resistivity of rocks affected by varying degrees of weathering. This makes electrical resistivity tomography (ERT) a preferable geophysical method for the exploration of alteration zones in crystalline rocks. In our research, we used an integrated approach, combining the ERT method with monitoring of spring discharge and hydrochemistry to characterize metamorphic aquifers on slopes of the Medvednica Mountain (Croatia). Significant fracture flow aquifers are found to be formed in intensely fractured but not highly weathered rock masses (medium to high resistivity values), while highly weathered masses (low resistivity values) form local barriers for fracture flows. Subsurface structure of the alteration zone proved to be highly irregular, with sharp contacts between more and less weathered rocks. Decrease of permeability below the alteration zone keeps the water level near the surface and enables spring occurrence on the mountain slopes. Studied aquifers have relatively limited extent, resulting in typical capacity of major springs of a few l/s. More frequent but less productive springs are attributed to the draining of the shallow part of the alteration zone (mostly saprolite). Combination of the ERT method with spring monitoring proved to be very effective as a first and relatively inexpensive methodology for hydrogeological characterization of crystalline terrains, both in local and catchment scales.
•Validating the conceptual model of a hydrothermal system in fractured carbonates.•Parametrization of the units through reinterpreted data and new local measurements.•2D numerical simulations ...confuted the available conceptual model.•Revised conceptual model of the system physically validated by simulations.•Applicability of this approach for initial investigations in hydrothermal systems.
Carbonate rocks represent the most abundant geothermal aquifers worldwide, excluding active volcanic areas. The most important challenge is to accurately define the conceptual model of a regional hydrothermal system. The focus of modelling is generally on the outflow area of the system and its fluid flow dynamics, while the geological and hydrogeological characterisations of the recharge area and the estimation of the local infiltration rate are rough. Thermal water is usually characterised by long residence time, and classical hydrogeological balance can give misleading results. On the other hand, long-term numerical simulations could represent a useful tool to understand the regional and local fluid flow circulations and to estimate the hydrogeological features of the recharge area. Numerical modelling has to be supported by an accurate characterisation of the regional and local hydraulic and thermal properties of the reservoir. This methodology was applied for the first time in Croatia in the case study of Daruvar hydrothermal system. Its recharge area had been determined decades ago solely by geological mapping, as a wide zone in the mountainous hinterland of natural thermal springs. Mesozoic fractured carbonate rocks represent the geothermal aquifer, and the tectonic juxtaposition of these permeable formations with low permeable Neogene deposits enables the rising of thermal water (38–50 °C) up to the surface. 2D coupled flow and heat transport numerical simulations of the hydrothermal system were performed for the first time in this research using measured data on its temperatures, and hydraulic and thermal conductivities. However, their results did not support the existing conceptual model. Conceptual model of the system was therefore revised decreasing the extent of the possible infiltration area and considering the fracturing induced by local faults in the Daruvar subsurface. A new set of simulations was conducted using the proposed model. The obtained results reproduced the temperature of the Daruvar thermal springs and the regional fluid flow. Despite its limitations, the employed modelling approach was useful to perform a first simulation of the hydrothermal system dynamics, and it could be employed in similar systems to evaluate the consistency of different hypotheses on the available conceptual model and their impact on the temperature distributions. In addition, the modelling demonstrated that the recharge area is smaller than previously considered, which has implications on the protection of the whole hydrothermal system. The greatest danger to the system currently comes from the increasing anthropic impact in the recharge area by multiple active dolomite quarries in the region and the related seismic disturbances, which should be limited in the newly defined recharge area.
In the region of Hrvatsko zagorje there are a lot of spa localities. The study area is in the northern Pannonian part of Croatia which is characterized by a high geothermal gradient (0.049 °C/m) and ...surface heat flow (76 mW/m^sup 2^). Although there are a lot of thermal occurrences in the study area, only ten were taken into consideration because there was a lack of geochemical data at other locations. The thermal springs considered are: Harina Zlaka, Krapinske toplice, Tuheljske toplice, Stubicke toplice, Sutinske toplice, Semnicke toplice, Toplicica (Maðarevo), Toplicica (Gotalovec), Podevcevo, Vrazdinske toplice. A compilation of geochemical data from different sources together with our own measurements has been used in this study. The aim of this paper is to review the thermal waters' geochemical characteristics and demonstrate how these features can be used to discern their origin and the aquifer they equilibrated with. The geochemical characteristics of thermal waters of the study area suggest that the water is in equilibrium with dolomite which means that dolomite is the thermal aquifer in the study area.
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