OPERATIONAL PROGRAMME SLOVENIA-HUNGARY 2007-2013
HYDROGEOCHEMICAL CONCEPTUAL MODEL within the framework of project Screening of the geothermal utilization, evaluation of the thermal groundwater bodies and preparation of the joint aquifer management plan in the Mura-Zala basin
T-JAM
Contents 1. Introduction…………………………………………………………………………………...…….1 2. Overview of the main hydrogeochemical processes in the T-JAM area………………………...2 3. Major factors related to the origin of groundwater and dissolved gas………………………….3 3.1 Hydrogeochemical stratification of hydrogeological units in Slovenia…………………..4 3.2 Hydrogeochemical properties of Zalaegerszeg area in Hungary……………………....…..5 3.3 Changes in thermal water chemical composition due to aquifer exploitation………..……6 3.4 Use of geothermometers for reservoir temperature prediction in Slovenia………………..7 4. Interpretation of the T-JAM project hydrogeochemical data …………………………………..9 4.1 Field work planning and field sampling execution…………………………………...……9 4.2 Interpretation of the chemical and isotope data……………………………………….….11 4.2.1 Correlation of hydrostratigraphic units based on geochemistry……………..…11 4.2.2 General chemical composition of groundwater………………………………..19 4.2.3 General chemical composition of sampled groundwater………………………22 4.2.4 Isotope composition of sampled groundwater…………………………………24 4.2.5 Observed changes in some investigated wells in Slovenia…………………….28 4.2.6 Dissolved and separated gas composition of sampled groundwater…………...29 4.2.7 Noble gas composition of sampled groundwater………………………………31 5. Summary: Identification of transboundary aquifers by hydrogeochemical research…….….34 6. References………………………………………………………………………………………….36
1. Introduction As part of the development of the conceptual hydrogeochemical model for the T-JAM research area, the processes and their impacts should be studied in accordance with the main geochemical processes described previously by Stuyfzand (1998) and T贸th (1999) (Fig.1).
Fig. 1 The main hydrogechemical characteristics along flow paths in different flow systems (T贸th, 1999)
The hydrogeochemical modelling is mainly a descriptive type of model interpretation, but also forms part of the hydrogeological transport modelling and where there is sufficient data, one dimensional water-rock interaction models can be defined along specific flow paths. In this case different processes can be modeled and saturation indices can also be calculated which can show those zones where oversaturation can be expected. This information can be very useful for the thermal and drinking water stakeholders, as scaling (precipitation of different minerals) on pipes caused by oversaturation may make extraction more difficult. The hydrogeochemical model can give useful information on understanding of the flow systems and can provide useful input to, or they can provide an independent control for the geothermal model through the temperature calculations based on geothermal elements. To estimate the subsurface reservoir temperatures, the effectiveness of different chemical geothermometers (chalcedony and quartz (Fournier, 1973 and 1977), Na/K (Giggenbach, 1988), Na-K-Ca (Fournier and Truesdell, 1973), Na-K-Ca-Mg (Fournier and Potter, 1979), K2/Mg (Giggenbach, 1988), Na/Li and Mg/Li (Kharaka and Mariner, 1989) may be evaluated. 1
The aims of our geochemical modelling are the delineation of joint transboundary aquifers based on thermal water geochemical properties, the delineation of groundwater flow direction in the Mura-Zala basin, the determination of possible origin of groundwater and dissolved gas and indication of possible mixing zones etc. This interpretation will help calibrate a hydraulic numerical model.
2. Overview of the main hydrogeochemical processes in the TJAM area The higher salt content of the Badenian Sea transgression flushed out the less mineralized “fresh� groundwater from both the karst and porous sediments, and also from the fractured aquifers in some Paleozoic metamorphic and Mesozoic carbonate rocks (1). The lower salt content of the Sarmatian Sea could be preserved only in the sediments deposited under the areas covered by the sea (2), except for the closed bays where hyper saline waters were characteristic. In the closed bays these waters could displace (3) the waters with a lower density. The precipitation infiltrated through the coral-reefs and basal-conglomerates of the Badenian-Sarmatian time, and was preserved (4) only in those zones where neither the young groundwater (5), recharged through continuous precipitation infiltration, nor the Pannonian Lake with its slightly higher density (6) did not flush it out. In the deeper parts of the Pannonian Lake, where fine grained sediments were deposited, the low salt content waters were captured in the pores of the sediments. As these sediments had a very low permeability, overpressure zones could have been developed under the weight of the younger sediments. The groundwater could have and still can escape towards the lower potential levels of the upper hydrostratigraphic units, transporting higher salt content groundwater. It can also be supposed that the groundwater flow and transport also occurred towards those parts of the basement where the gravitational flow systems were connected hydraulically in a near horizontal way. In these cases a modified groundwater composition can be expected during the transport processes, due to the water-rock interactions in the low permeability zones (7). Intermediate and regional groundwater flow systems evolved in the low salt content (brackish) lake sediments and in the younger, fresh water (lake) and fluvial sediments. Their flow paths were defined by the topographic and climatic boundary conditions of that time. In many cases they flushed out the previous pore waters. The hydrogeochemical characteristics of the gravitational flow systems were defined by the infiltration processes (e.g. evaporation and the interaction between the soil and the infiltrating precipitation), which were further modified along the flow paths due to the water-rock interactions and mixing (5a, 5b, 5c, etc.). The composition of the groundwater at the end of the flow paths (8) was and still is modified as in these areas they mix with groundwater with an oxidative redox state, and they are exposed to evaporation-evapotranspiration of the shallow groundwater. In addition to the above mentioned processes, geothermal effects (9), organic matter evolution and degradation (10) and autochtonous gas mixing from the deep basement (11) together with their wide spectrum of water-rock interaction must also be taken into consideration. The evaporites can also modify the groundwater composition (12). Due to convective heat flow the mixing areas and the intensity of mixing is modified. The water-rock interactions, including ion exchange, dissolution, and precipitation are different from the original processes in response to the temperature changes. These processes can be 2
connected to the effects of regional heat transport during the basin development, or to local effects around cooling volcanic rocks. The discharge of the reactive CO 2 gas from the deep basement may have a significant effect on the rock and water composition. Both the organic and inorganic compounds go through changes during the evolution of organic matter. The groundwater composition of the different flow systems (driven by gravitational or by density differences) can be significantly modified where they flow through evaporites which have high solubility. The evaluation of natural phenomena can, by analogy, provide useful guidance in predicting anthropogenic effects, including the sustainable use of geothermal energy.
3. Major factors related to the origin of groundwater and dissolved gas Different origins of groundwater were previously established for the Mura-Zala basin (Žlebnik, 1979; Pezdič, 1991, 2003) which have been supported by the results of the T-JAM project. The youngest groundwater type is represented by young meteoric water with recharge stored in the Quaternary gravel, the Pontian - Pliocene Ptuj-Grad (Jelen et al. 2006, 2011) and probably the upper parts of the Pannonian - Pontian Mura Formation in Slovenia, plus in the Holocene and Quaternary sediments in Hungary. Older meteoric water with poor recharge has an age ranging from several hundred years to more than 7000 years in Radenci (Pezdič, 1991) and is stored in the Pannonian-Pontian Mura and the Pannonian Lendava Formation. Due to a strong reductive environment it contains CO 2 , H 2 S, CH 4 gases as well as geogenic iron, arsenic, manganese and ammonium. Similar, older meteoric water can be found in the Zagyva and Újfalú Formations and other Upper Pannonian sediments. The age of this groundwater is 10000-30000 years. The oldest water is probably represented by stagnant Tertiary dilluted brines in isolated aquifers of the Lendava and Špilje&Haloze Formations in eastern Slovenia. These oil brines are thermomineral and contain significant amount of methane and other hydrocarbons. Overpressured layers of the Lower Pannonian in Hungary can be considered similar to stagnant Tertiary diluted brines in isolated aquifers and aquicludes. Groundwater stored in the Slovenian Mesozoic carbonates probably dates to the same period but it was infiltrated during the subsidence of Tertiary marine sediments (Kralj, 2007). In some areas these aquifers are recharged through permeable faults, whereas in others they are hydraulically isolated from their surroundings. Except for isolated units like those around Sárvár, where high TDS concentrations occur, groundwater stored in the Hungarian Mesozoic carbonates has a recharge of meteoric origin and was recharged in the last 40000 years. H 2 S gas and sulphate ions can be connected to the presence of sulphate or sulphide minerals in host rocks. Some evaporites (sulphates) were identified in dolomite basement rocks in Strukovci, Dankovci and Ljutomer boreholes. In contrast, the sulphides (pyrite, marcasite) are very common in less permeable Tertiary mudstones and marls, and are therefore the most probable source of sulphur. Large quantities of dissolved CO 2 occur along the Raba fault zone. It has been identified in wells in Benedikt, in Ščavnica valley, Radenci, Radgona (A), Korovci, Strukovci, Pečarovci and Nuskova. One of the hypotheses is that the gas is still being produced (Pezdič et al., 1995) by the reaction between dolomite, quartz and clay minerals in Tertiary rocks at temperatures of 80-160°C. The same author (Pezdič, 1991) also believes that some gas originates from sulphate reduction as a byproduct of organic matter saturation. In contrast, its 3
occurrence is assumed to be connected to the degassing of metamorphic rocks in the Radgona-Vaš tectonic half-trench in the Raba fault zone (Kralj & Kralj, 1998; Lapanje, 2007). This is indicated also by a new geothermal well in Benedikt, which produces large amount of CO 2 , from aquifers identified in fractures and fissured Paleozoic metamorphic rocks. Therefore we believe that gas origin is related to deeper processes accompanying carbonate lenses metamorphism or even of mantle origin. Methane, is a result of thermogenic organic matter maturation (kerogene formation) under 'oil window' conditions (Pezdič, 1999).
3.1
Hydrogeochemical stratification of hydrogeological units in Slovenia
Vertical stratification of aquifers is reflected in the hydrogeochemical zonation of groundwater (Žlebnik, 1978; Pezdič, 1991; Kralj & Kralj, 2000b; Kralj, 2004; Lapanje, 2007). The process is governed by pressure difference in regional and local scale hydraulic fields which force groundwater migration. This can be controlled by natural processes such as changes in oxidation-reduction conditions, temperature changes, leakage, mixing in fault zones, CO 2 induced reactions or by artificial processes such as water extraction from wells. Interpretation of the data from 75 wells compiled in the expert T-JAM database confirmed the vertical stratification of aquifers in NE Slovenia (Table 1). This is additionally verified by the almost linear sodium-chloride relationship (452 samples) and stable isotope analyses. The trend is not so obvious with TDS composition (328 samples), since dissolved CO 2 can locally induce a strong dissolution of minerals. However, the spatial distribution of TDS shows that higher mineralization is strongly related to a higher stratigraphic age of the aquifers, especially in the Goričko hills area (Lapanje et al. 2009). Table 1: Number of hydrogeochemical water types classified by mean screened depth (75 wells) Ca-Mg-HCO3, Na-HCO3-Cl, Depth (m) Ca-HCO3 Na-HCO3 Na-Cl Ca-Na-HCO3 Na-HCO3-SO4 0-100 6 17 1 100-500 5 6 2 500-1000 2 11 4 1000-1500 13 4 1500-2000 3 1
The Plio-Quaternary gravel holds fresh water of Ca-HCO 3 type (precipitation). Some variations seen as Ca-Mg-HCO 3 and Ca-Na-HCO 3 type can be caused by longer retention time and better geochemical equilibration with dolomite or magnesium silicates or by ion exchange processes. The shallow Pliocene fresh water aquifers often contain Ca-Mg-HCO 3 water. In the Goričko hills this reduced water has significantly higher values of iron, manganese and arsenic. This is a big problem when using it as a source of drinking water, as values often exceed legally allowed and recommended amounts. Where reduced water naturally outflows towards shallower grounds and/or mixes with oxidized water, iron hydroxides (limonite, goethite) precipitate. These brown, up to one meter thick crusts are typically found at the contact of the Plio-Quaternary and Quaternary gravel, near Dobrovnik, Beltinci (Hrašica) and in Krapje gravel pit near Ljutomer. Lithostratigraphically the youngest geothermal aquifer in the Ptuj-Grad Formation contains water of Na-HCO 3 type. Its composition is governed by an exchange of calcium with sodium ions on clay minerals. The most exploited geothermal aquifer in the Mura Formation 4
contains water of Na-HCO 3 type and mineralization up to 1,2 g/l, with no or minor quantities of CO 2 and water temperature up to 60°C. However, in Radenci a high CO 2 gas content also induces other reactions therefore numerous water types occur. The local mineral composition of water-bearing sediments can also cause enrichment in sulphate ions. The Lendava Formation probably contains water with mineralization up to 20 g/l, but no well captures solely this aquifer. Thermomineral water from the Špilje&Haloze Formation is of Na-Cl and Na-HCO 3 -Cl type. Mineralization varies between 10 up to 20 g/l, while in isolated aquifers methane and/or dissolved CO 2 can be present. Thermomineral water in the Mesozoic carbonates and the Paleozoic metamorphic rocks has Na-HCO 3 character. Due to numerous faults and fractures it may contain dissolved gas (CO 2 , methane, H 2 S), while due to local anhydrite dissolution it can exhibit sulphate enrichment (Pezdič, 1991; Kralj & Kralj, 1998). Changes with depth are evident also in groundwater isotope composition. The Quaternary aquifers (wells Veščica, Rankovci and Lipovci) reflect the lowland recharge area of the Mura gravel. The Plio-Quaternary aquifer can be distinguished from this one, having more negative oxygen and deuterium values. This effect can be explained by a topographically higher recharge area, or mixing with older groundwater which contains less heavy oxygen. Water from the Pliocene aquifer in the Goričko hills (wells Mač-1a, Čep-1, VP-1, VK-2, Krp-1v, Grad-1) does not contain tritium isotope due to a longer retention time. Samples from the Pontian aquifers (wells ČM-2v, Rog-3) indicate old waters infiltrated during the last Ice Age (Lapanje et al., 2009). A similar interpretation was done for mineral waters in Radenci (Pezdič, 1991).
3.2 Hydrogeochemical properties of Zalaegerszeg area in Hungary One of the main thermal water extraction regions in the Hungarian project area is at Zalaegerszeg and its surroundings. The upper 500 meters of this area were surveyed by MÁFI (Tóth et al., 2006) and the main outcomes were the following. The nitrate concentration shows a variable distribution with high (expected) and low concentrations next to each other in shallow groundwater. This fact does not mean that the level of pollution is lower inside the settlement, but reductive environments can be found in different hydrogeological units. The concentration decreases significantly with depth, except for the hilly part of the city, where the confined aquifers still contain nitrate. In the deeper aquifers, below 50 meters, nitrate could still be detected sporadically, but the status of these wells should be checked. The relatively high ammonium concentrations measured in the shallow groundwater decrease slowly with depth. It is hard to differentiate between anthropogenic effects and natural occurrence. In general the ammonium has much lower concentrations in this area (including the deeper part of the Upper Pannonian sediments) than in the Great Hungarian Plain. Chloride is a very good pollution indicator. Its concentration in the shallow groundwater is significantly higher than in the confined groundwaters which are characterized by very low chloride concentrations. Due to significant differences in the chloride concentrations between the different flow systems of the whole project area, the chloride data can be used as a very good calibration tool or independent validation tool for the hydrogeological models.
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The total hardness also reflects the effect of settlements. Its values decrease with depth, but vary across a wide range, which may represent the differences between the different parts of the flow systems. The lower values are found closer to the surface at the discharge parts of the flow system. Since the total hardness measurements were widely used and come from reliable analytical methods in Hungary, even in the past, the interpretation of these data are useful in understanding the regional and intermediate flow system.
3.3 Changes in thermal water chemical composition due to aquifer exploitation Thermal water is widely used both in the Slovenian and in the Hungarian project area mainly for balneological purposes. The geothermal system in the Mura-Zala basin produces thermomineral water at depths, while in shallower areas the water is cooler but can still be mineralized. Mineral water in Radenci is used as drinking (bottled) natural mineral water with a high CO 2 content. Fresh water from the shallower Tertiary and Quaternary aquifers is used mainly for drinking water supply but also for irrigation and as industrial water. Low mineralized thermal water in the Ptuj-Grad and Mura Formations and the Paleozoic metamorphic rocks is used mainly for balneology and swimming, although sometimes also for district heating. Water from the Lendava and Ĺ pilje&Haloze Formations as well as from the Slovenian Mesozoic carbonates is not widely used at present, since it contains much CO 2 and methane and has very high scaling potential. It is used in some heat exchangers for space heating and for balneological purposes.
Fig. 2 Identified changes in thermal wells in the Slovenian part of the T-JAM project area (Rman et al. 2008)
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Changes in thermal and mineral water chemical composition in Slovenia have been recorded in the area of Murska Sobota (Kralj & Kralj, 2000a; Kralj, 2001). Overexploitation is seen in composition, water level and temperature fluctuation due to limited recharge of the productive aquifers. It should be noted that the inspected wells each capture water from two different aquifer formations. Changes have also been noticed in Radenci, where oxygen and sulphate isotopes were used for investigation (Pezdič, 2003). Here, older meteoric water flows towards the area of intense mineral water production. Other locations have in general not yet been evaluated in such details to be able to say that chemical composition is changing due to exploitation. However, other changes are noticed by borehole managers, such as water level decrease, temperature change etc (Fig. 2). No significant changes in temperature or chemical composition could be identified in the Hungarian thermal water exploitation wells. However, significant extraction of thermal water around Héviz, in the Eastern part of the T-JAM project area, caused changes in the flow paths leading to changes in the temperature of the extracted groundwater (Tóth et al., 2009).
3.4 Use of geothermometers for reservoir temperature prediction in Slovenia Cation and silica geothermometers have already been applied by Veselič (1980), Pezdič (1991) and Lapanje (2006) who pointed out they must be used with caution. Veselič has evaluated 37 samples and noticed that calculated silica and Na/K/Ca temperatures depend on the aquifer’s lithology. Later, a detailed study was done on 70 thermal boreholes and springs throughout Slovenia (Rman, 2009). Waters were first classified by hydrogeochemical and D'Amore (D'Amore et al., 1983) type. After this, all possible geothermometers were calculated for each group (Marini, 2001; White, 1970) and their accuracy checked. Thermal water samples were taken from different carbonate and clastic aquifers throughout Slovenia. A significant distinction between beta (carbonate aquifers) and other groups (clastic rocks, carbonate intercalations, mixing with marine water) could be made based on water types (Table 2). Table 2 Classification of samples from 70 representative thermal water wells and springs D’Amore No. Samples Hydrogeochemical No. Samples Aquifer lithology type type Beta 36 Ca-Mg-HCO 3 32 Limestone, dolomite 4 Limestone, dolomite Ca-Mg-HCO 3 -SO 4 Beta-delta 4 Na-Ca-(Mg)-HCO 3 4 Clastic rocks, dolomite intercalations Delta 22 Na-HCO 3 -(SO 4 ) 17 Clastic rocks, carbonate intercalations 5 Clastic rocks Na-HCO 3 -Cl Gamma 8 Na-HCO 3 -(SO 4 ) 1 Clastic rocks Na-HCO 3 -Cl 2 Clastic rocks Na-(Ca)-Cl 5 Clastic or carbonate rocks, marine water
Calculations (Table 3) showed that the chalcedony and amorphous silica geothermometers are not suitable for use on Slovenian groundwater, since the calculated temperatures are below measured values. Silica geothermometers differ only slightly between each other and give quite good results for predicting reservoir temperatures. The exception is the beta-delta group, where too high temperatures are calculated. The lowest expected temperatures are calculated by non-ionized dissolved silica, which is not useful for carbonate aquifers. It was concluded that for 7
reservoir temperature prediction in Slovenia quartz geothermometers are the most appropriate, with the exception for waters of beta-delta type. Ion geothermometers are useful only for an indirect estimation of aquifer lithology and not for reservoir temperature estimation in Slovenia. The Na/K, Ca/Mg and Na/K/Ca-Mg geothermometers show too high and geologically less probable temperatures. In contrast, K/Mg gives too low temperatures for beta water type, possibly for beta-delta type and slightly too high for delta and gamma water types. As already noticed by Veselič (1980), the Na/K/Ca geothermometer shows too low temperatures for beta type and too high for others. This research showed that the ratio between different calculated temperatures can be used to graphically distinguish between waters from aquifers with different lithology (carbonates versus clastic rocks) using numerous geothermometers: quartz, Na/K/Ca, silica, Na/K, K/Mg, Ca/Mg and Na/K/Ca-Mg) and their various combinations. Table 3 Calculated reservoir temperatures based on 70 representative thermal wells and springs Calculated reservoir temperature (°C) Geothermometer / D'Amore type β β-δ δ γ Measured temperature at WHD, spring 32±10 30±7 53±15 67±39 Quartz -Fournier, 1973 41±21 91±28 87±26 92±13 Quartz -Buntebarth, 1980 40±14 82±32 78±30 82±14 Quartz -Fournier & Potter, 1982 39±24 91±28 87±26 93±13 Chalcedony -Fournier, 1973 9±21 60±31 56±28 62±14 Chalcedony -Arnorsson et al., 1983 13±20 62±29 58±26 64±13 Amorphous silica -Fournier, 1973 -64±17 -23±24 -27±23 -22±11 Non-ionized silica (aq) -Arnorsson, 2000 24±24 76±29 72±27 78±14 Na/K -Truesdell, 1975 402±152 186±78 105±73 91±34 Na/K -Fournier, 1979 364±89 214±63 146±61 135±30 Na/K -Arnorsson et al., 1983 389±131 193±74 115±70 102±33 Na/K -Giggenbach et al., 1983,1988 364±78 229±59 165±58 155±29 Na/K/Ca -Fournier & Truesdell, 1973 4±13 63±40 182±104 224±95 Na/K/Ca-Mg -Fournier & Potter, 1978 117±18 73±36 144±72 160±115 Ca/Mg -Kharaka & Mariner, 1989 150±14 158±28 141±50 133±17 K/Mg -Giggenbach et al., 1983, 1988 21±8 45±19 94±39 110±33 Na/Li -Kharaka & Mariner, 1989 239±67 198±135 118±63 93±50 Mg/Li -Kharaka & Mariner, 1989 4±14 38±57 81±51 74±43
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4. Interpretation of the T-JAM project hydrogeochemical data 4.1 Field work planning and field sampling execution One objective of field sampling was to obtain supplementary hydrogeochemical information on the research areas. The data created was used for calibration of the joint hydrogeological flow and transport model and will help in the preparation of transboundary water resources management. The field sampling in Hungary and Slovenia and the respective laboratory analysis of samples was the responsibility of the Geological Institute of Hungary (MÁFI), while organizational arrangements were covered also by Geological Survey of Slovenia (GeoZS). For groundwater sampling we did not need permissions from the authorities but we obtained permissions from the well owners. As a first step, 61 thermal water wells were identified on the Hungarian side of the project area. Regarding thermal water, we used 25º Celsius as the search criteria instead of the 30º Celsius limit value usually applicable in Hungary. In the Slovenian project area 75 thermal water wells were identified based on the criteria of 20º Celsius. From the well data collected, 24 wells in total on both sides were chosen for sampling, primarily based on their spatial distribution, the wells’ activity and relationship to the main (transboundary) exploited aquifer in the T-JAM project region.
Fig. 3 Localities of hydrogeochemical field sampling sites in the T-JAM project area
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24 new samples were collected (Table 5, Fig. 3) and investigated by various analytical techniques in the framework of the T-JAM project. Beside the main components and trace element measurements, stable and radiogenic isotopes, dissolved and separated gas, noble gas and organic matter content were also analysed. Regarding the laboratory analyses several analyses were done by external laboratories. For this reason we posted a public tender and the selected laboratories are presented in table 4. Table 4 Selected laboratories and type of groundwater analysis performed Analysis Selected laboratory main components, trace elements 14C and δ13C in HCO 3 -, tritium δ34S in water (SO 4 2-) δ13C in hydrocarbon (CH 4 ) Noble gas δ D, δ18O TOC Phenol indice/Fenolindex, Phenoles (if indice of phenol >20ug/l) Acetate-propionate (if TOC >8), PAH (if TH 2 O>60oC and COD>2)
Laboratory of the Geological Institute of Hungary Hydrosys Kft. MTA Atommagkutató Intézete MTA Atommagkutató Intézete MTA Atommagkutató Intézete MTA Geokémiai Kutatóintézet Bálint Analitika Kft. Bálint Analitika Kft. Bálint Analitika Kft. 2F, S , I, Br Országos Közegészségügyi Intézet Dissolved gas, Separated gas Vízkutató Vízkémia Kft. Heavy hydrocarbon from dissolved gas, separated gas (if CH 4 is high) Vízkutató Vízkémia Kft. Radon Eötvös Loránd University Radium Ben Gurion University - Israel
Groundwater sampling was carried out exclusively by the Accredited Water Sampling Group (accredited by NAB) comprising of: Dr. Teodóra Szőcs, hydrogeologist, leader of the Accredited Water Sampling Group, Eszter Tihanyi Szép, assistant, quality control responsible, Éva Pálfi, assistant, member of the Accredited Water Sampling Group, Csaba Jerabek, assistant, member of the Accredited Water Sampling Group, Dr. Andrea Szűcs, hydrogeologist, member of the Accredited Water Sampling Group, Gabriella Katona, assistant, member of the Accredited Water Sampling Group, Dr. Nóra Gál, hydrogeologist, member of the Accredited Water Sampling Group. When sampling in Slovenia, mag. Andrej Lapanje, Tomislav Matoz and Nina Rman provided additional field support. Sampling was always preceded by close collaboration with the respective laboratory, as regards to sampling protocols and specifications of sample containers. The most timeconsuming job was the treatment of groundwater samples for radiocarbon dating. The carbonate content of 60-120 L groundwater had first to be precipitated and then sieved to recover the precipitate. Similarly, ca. 60 L groundwater was used for the field sample preparation for radium analysis. Samples needed to be passed through manganese fibers, then fibers had to be washed and weighted and the sample thus obtained had to be put into a sealable plastic bag and weighted again. Groundwater samples needed to be kept cool for general chemical analyses. For the analyses of the organic components, radium and gas sample preparations had to be done directly in the field and samples needed to be transferred to the laboratories quickly; otherwise the results would be inaccurate. This process had to be applied to all samples (Fig. 4, 5). 10
Fig. 4 and 5 Field sampling of water in Benedikt well Be-2 (Slo) –left and Lenti well B-33 (Hu) -right
Data generated by sampling and groundwater analysis is available in the T-JAM database located on the T-JAM project website (www.t-jam.eu). Table 5 A list of groundwater samples collected for the T-JAM project Well Sampling Sample No. Settlement Captured aquifers name date TJAM-101 Szentgotthárd B-44 11.5.2010 Somló and Tihany Formations and Pa1 TJAM-201 Lenti K-21 12.5.2010 Somló and Tihany Formations TJAM-301 Lenti K-23 12.1.2010 Somló and Tihany Formations TJAM-401 Lenti B-33 12.5.2010 Újfalu Formation TJAM-501 Zalaegerszeg K-193 13.5.2010 Újfalu Formation TJAM-601 Letenye K-59 18.5.2010 Újfalu Formation TJAM-701 Bázakerettye K-1 18.5.2010 Újfalu and Algyő Formation TJAM-801 Ormándlak K-27 19.5.2010 Zagyva Formation TJAM-901 Gutorfölde B-4 19.5.2010 Somló and Tihany Formations TJAM-1001 Pusztaszentlászló K-2 19.5.2010 Lajtai limestone Formation TJAM-2101 Gelse K-5 12.10.2010 Miocene and Algyő and Szolnoki Formations TJAM-2201 Zalakaros K-18 12.10.2010 Újfalu Formation TJAM-1101 Dobrovnik Do-3g 8.6.2010 Mura Formation TJAM-1201 Moravske Toplice Mt-7 8.6.2010 Mura Formation TJAM-1301 Moravske Toplice Mt-4 8.6.2010 Špilje and Haloze Formation TJAM-1401 Moravske Toplice Mt-8g 8.6.2010 Mura Formation TJAM-1501 Šalovci Čep-1/04 9.6.2010 Ptuj-Grad Formation TJAM-1601 Lendava Pt-74 16.6.2010 Mura Formation TJAM-1701 Banovci Ve-1 15.6.2010 Mura Formation TJAM-1801 Moravci v Slovenskih goricah Mo-2 15.6.2010 Mura and Lendava Formation TJAM-1901 Ptuj P-1 15.6.2010 Ptuj-Grad Formation TJAM-2001 Ptuj P-3 15.6.2010 Mura Formation TJAM-2301 Benedikt Be-2/04 28.10.2010 Paleozoic metamorphic rocks TJAM-2401 Prosenjakovci VP-1/00 28.10.2010 Ptuj-Grad Formation
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4.2 Interpretation of the chemical and isotope data The available archive data were collected in Hungary and Slovenia. The data source and its quality vary as they were collected by different organizations and analysed in different laboratories over the past few tens of years. Despite the fact that the analytical methods have improved a lot, according to the laboratories the results are still comparable. Following the data processing, outliers were omitted, but there may be still some non-representative data in the database used, as there was no possibility to check all the original archives. Where more than one data was available from a certain well either one representative data was chosen or a median value was calculated for different components. Data interpretation was made by using different numerical and graphical software, mostly MS Office Excel, AquaChem, Statistica and Grapher. 4.2.1 Correlation of hydrostratigraphic units based on geochemistry First a stratigraphic comparison was made in order to distinguish between waters from different formations. During data interpretation we managed to correlate some of the groundwater data from stratigraphically equivalent formations (Table 6), while in some cases this correlation was not possible, mostly due to the occurrence of locally isolated aquifers. Table 6 shows results of the Formation correlation, which was used in the data interpretation. Altogether samples from 533 wells were re-interpreted. Table 6 Hydrostratigraphic units based on Formation “correlation” used in hydrogeochemical interpretation Slovenia Hungary Formation Name Formation Age Formation Name Formation Age Hungarian Quaternary Quaternary Ptuj-Grad Formation Pontian, Pliocene Zagyva + Somló and Tihany Formations Upper Pannonian Mura Formation Pannonian, Pontian Újfalu Sandstone Formation Upper Pannonian Algyő Formation Lower Pannonian Lendava Formation Pannonian Szolnok Sandstone Formation Lower Pannonian Endröd Formation Lower Pannonian Kozárd Claylymarl Formation Sarmatian Szilágy Claylymarl Formation Badenian Tekeres, Budafa, Bekesi, Ligeterdo Formation Badenian-Karpatian Špilje&Haloze Formation Karpatian-Lower Pann. Lajta limestone Badenian Lower Miocene rocks and sediments Egg-Karpatian Volcanic rocks and sediments Eocene, Oligocene Mesozoic carbonates Mesozoic Mesozoic rocks Mesozoic Buki dolomite Devonian Paleozoic metamorphic rocks Paleozoic Paleozoic metamorphic rocks Paleozoic mixures of water from different Formations
Thermal waters from 70 different wells and 5 aquifer Formations were investigated in Slovenia: Ptuj-Grad (22 samples), Mura (19 samples), Lendava (7 samples), Špilje&Haloze (18 samples) Formations and carbonate (mostly dolomite) Mesozoic basement rocks (4 samples). For waters from the Slovenian Paleozoic metamorphic rocks only 2 samples were available, therefore statistics could not be calculated. Analytical data was sufficient to evaluate the general chemical composition of the Slovenian groundwater. More geological formations could be differentiated in Hungary, which made it possible to investigate the concentration distribution of various components in 11 hydrostratigraphic units. Those wells
12
which had data only on the investigated parameter were also included for Hungary therefore the number of samples is much higher there.
Fig. 6 TDS content distribution in groundwater in five Slovenian Formations
Fig. 7 TDS content distribution in groundwater in eleven Hungarian Formations
13
Fig. 8 Cl- concentration distribution of groundwater in five Slovenian Formations
Fig.9 Cl- concentration distribution of groundwater in eleven Hungarian Formations
14
Fig. 10 HCO 3 - concentration distribution of groundwater in five Slovenian Formations
Fig. 11 HCO 3 - concentration distribution of groundwater in eleven Hungarian Formations
15
Fig. 12 Na+ concentration distribution of groundwater in five Slovenian Formations
Fig. 13 Na+ concentration distribution of groundwater in eleven Hungarian Formations
16
Fig. 14 Cation ratio distribution of groundwater in five Slovenian Formations; (Ca2+ + Mg2+)meq/(Na+ + K+)meq
Fig. 15 Cation ratio distribution of groundwater in eleven Hungarian Formations; (Ca2+ + Mg2+)meq/(Na+ + K+)meq
17
Fig. 16 NH 4 + concentration distribution of groundwater in five Slovenian Formations
Fig.17 NH 4 + concentration distribution of groundwater in eleven Hungarian Formations
18
The detailed Box&Whisker diagrams (Fig. 6 – 17) show that the Slovenian Pliocene and the Hungarian Quaternary to Upper Pannonian Formations are comparable as all samples contain low mineralized water with a high cation ratio. The Mura and Újfalu Formations are also comparable containing more mineralized groundwater with a low cation ratio. The same applies to more mineralized groundwater stored in the Lendava and Szolnok Formations. All these mentioned formations represent potential transboundary geothermal aquifers since the chemical composition of groundwater is comparable and transboundary flow is hydrogeologically possible. In contrast numerous Hungarian Miocene formations contain very limited or isolated aquifers with very high mineralization. It is apparent that the Mesozoic aquifers investigated in this project are not comparable since the Hungarian aquifers clearly contain fresh water with a higher cation ration than reported for the diluted brines in the Slovenian aquifers. 4.2.2 General chemical composition of groundwater The total dissolved solid (TDS) content of thermal water was investigated versus depth distribution, and then the general composition of groundwater was evaluated according to the hydrostratigraphic units (Table 7). Table 7 Number of data based on formations used for further hydrogeochemical interpretation Formation No. of data Quaternary HU 4 Ptuj-Grad Formation SLO 32 Zagyva+Somló+Tihany Formations HU 95 Mura/Újfalu Formation 40 Algyő Formation HU 5 Lendava/Szolnok Sandstone Formation 39 Špilje&Haloze (SLO)+ M4-7 Formation (HU) 74 Lajta limestone HU 17 Lower Miocene rocks and sediments HU 12 Volcanic rocks and sediments HU 11 Mesozoic carbonates Slo 4 Mesozoic rocks HU 60 Buki dolomite HU 2 Paleozoic metamorphic rocks 12 mixures of water from different Formations 126
The distribution of total dissolved solid content versus depth was investigated in order to see the general vertical TDS picture in the research area (Fig. 18). It can be seen that the dissolved solid content increases with depth, with the highest values between -1500 to -2000 m a.s.l., below which lower TDS contents can again be measured. Hungarian groundwater samples contain higher TDS values than the Slovenian in the -1300 to -3500 m a.s.l. depth interval, which can be attributed to a longer flow path on the Hungarian side. However, in some cases contamination during the drilling could also lead to higher TDS values, so very high (above 30000 mg/l) values should be evaluated with caution. In the 250 to -500 m a.s.l. depth interval the TDS values of the Slovenian groundwater samples are much higher than the Hungarian, which can be due to the outcropping Pre-Pannonian rocks on the Slovenian side. The TDS values of the samples collected in the framework of the T-JAM project fall into the general TDS distribution trend.
19
Fig. 18 TDS distribution versus average captured aquifer depth
The Piper diagrams (Fig. 19, 20) were drawn revealing important chemical differences between waters from different aquifers. In the Quaternary and Pliocene aquifers the Ca-MgHCO 3 water type prevails. The Zagyva, Somló&Tihany, and the lower part of the Ptuj-Grad Formation show a developing trend of cation (calcium-sodium) exchange characteristics due to the longer retention time of groundwater. The water type changes from Ca-Mg-HCO 3 to Na-HCO 3 in deeper levels. The Mura and Újfalu Formations store alkaline Na-HCO 3 water, where cation exchange has mostly finished. Locally, this water is enriched in chloride or sulphate anions, mostly due to mixing. Water stored in the marly Algyő Formation is a rather isolated brine of Na-Cl type. In contrast, the Lendava and Szolnok Formations store water which is less isolated from its surroundings and is often mixed with other groundwater from Miocene aquifers therefore anions show a wide range. The Middle Miocene Formations, including Špilje&Haloze, store different waters depending on the burial depth. Where layers outcrop the infiltrating Ca-Mg-HCO 3 water is observed, while towards deeper parts the longer retention time, cation exchange, mixing, dissolved gas and other geochemical processes modify its composition, so Na-HCO 3 to Na-Cl types prevails. The Na-Cl brines are stored in the Lajta limestone, sometimes with higher calcium or bicarbonate ions. In the Lower Miocene aquifers an alkaline and brine character is found therefore Na-HCO 3 to NaCl water types prevail. The Eocene and Oligocene volcanic rocks store mineralized water of the Na-(Ca)-Cl-(HCO 3 ) character due to a diverse mineral composition. The Mesozoic carbonate aquifers in Slovenia mostly store diluted brines of Na-Cl type, while in Hungary slightly mineralized water with multiple ions combination of Ca-Mg-(Na)-HCO 3 -(Cl)-(SO 4 ) is observed. Brines of Na-Cl type are stored in the Buki dolomite, indicating isolated aquifers. The Paleozoic metamorphic rocks usually do not represent important geothermal 20
aquifers, but where they contain fractured carbonate (marble) lenses they may form an important aquifer as in the case of the Raba fault zone. The water here is alkaline to Na-Cl type and highly mineralized.
Fig. 19 General groundwater composition; Piper-diagram, all samples from all Formations
Fig. 20 General groundwater composition; Piper-diagram showing 10 selected Formations
21
Groundwater samples taken in 2010 show all the typical and expected characteristics of their aquifers. From an interpretation of the Piper diagrams it can be noticed that groundwater evolves from freshwater to alkaline and brines according to the stratigraphic age of the aquifer. The main indicated geochemical processes are cation exchange, mixing, dissolved gas enhancing mineral dissolution etc. However, not all formations form part of the active flow paths. In some, mostly Miocene aquifers the groundwater is probably originally infiltrated brine which later re-equilibrated with its surroundings. 4.2.3 General chemical composition of sampled groundwater A strong correlation (R2=0.99) can be found between the TDS content and the bicarbonate and sodium concentrations (Fig. 21, 22). The lowest values are detected in the groundwater samples in the Ptuj-Grad Formation and the concentrations grow along the flow path. They coincide with stratigraphic age of aquifers from the youngest to the oldest; Ptuj, Zagyva, Mura and Ăšjfalu, Lendava and Szolnok Formations. Samples which represent mixtures of groundwater from different formations have concentrations in between. The highest concentrations deviating from the trend line are found in sampled well Mt-4 capturing the Ĺ pilje&Haloze Formation which contains a thermomineral water rich in gas and hydrocarbons. The sample collected from Benedict well (Be-2) also differs, possibly being positioned between the two end members, and representing a groundwater of the metamorphic basement with fissured carbonate lenses. Moreover, chloride-TDS and chloridesodium (Fig. 23) ratios do not show such a distinctive trend, and the Be-2 and Mt-4 samples show enrichment in sodium compared to the chloride trend line. As chloride is a conservative component this indicates that some additional geochemical processes are active in the aquifers under investigation.
Fig. 21 TDS versus Na+ distribution
22
Fig. 22 TDS versus HCO 3 - distribution
Fig. 23 Na+ versus Cl- distribution
Regarding the trace element concentrations, the groundwater sample from the Paleozoic basement (Be-2) and the groundwater sample from the Ĺ pilje&Haloze Formation (Mt-4) are very different to the other groundwater samples, a feature shown by their main components and isotope data. The B, Li, V, Rb, Sr, Cs and Tl concentrations are one to two times higher than in the rest of the samples. The lowest B, Li and Rb concentrations could be found in the relatively recently infiltrated (see below, isotope and noble gas data interpretation) water of 23
the Ptuj-Grad Formation (VP-1, Čep-1) and in the samples of the Zagyva and Somlo&Tihany Formations (K-21, K-23, K-27, B-4). These are also the wells with the lowest TOC content. The sample of thermomineral groundwater from the Špilje&Haloze Formation has a TOC content two orders higher than in the rest of the samples, and also has very high PAH (351 µg/l), phenolindex (1850 µg/l), and total phenols (62600 µg/l) values. 4.2.4 Isotope composition of sampled groundwater Our interpretation was based on the data of δ18O, δD, 14C, δ13C and tritium measurements. The isotope data shows some grouping of samples, as well as a slightly different sequence than shown by the TDS plots (Fig. 24). According to the δD and δ18O data, most of our samples are positioned on the meteoric water line, which shows water origin from precipitation. Mean groundwater residence time (or groundwater age) was calculated from radioactive decay of carbon-14. Where young water was likely to occur, tritium was also measured. The analysis results are available in the T-JAM project database on the web page www.t-jam.eu. As already mentioned, a grouping of samples is seen on various isotope plots. The youngest infiltrated water from the Holocene (δ18O>-10‰, 14C>70pmC) was identified in shallow wells Čep-1 and VP-1 in the Ptuj-Grad Formation, in the Goričko hills in Slovenia. Modern water in Čep-1 is tritium active and implies fresh (less than 50 years) recharge. The deeper screened VP-1 well is submodern, by carbon-14 measurements and it is possibly younger than a thousand years. Other samples show a strong paleoclimatic shift towards paleo-groundwater (Clark et al. 1997) (Fig. 25). The third and the deepest screened well in the Ptuj-Grad Formation, situated in the Ptuj region (P-1) is located 50 km SW of Čep-1 and VP-1, and indicates much older water. The next group (K-21, K-23, K-27, B-4) is shown on the δ18O-TDS, δ18O-chloride and δ18Oδ13C (Fig. 24, 26, 27) isotope plots and is represented by waters stored in the Somló&Tihany Formations near Lenti, which are in the upper part of the Újfalu Formation in Hungary. Captured groundwater here has a rather low mineralization and it is depleted in deuterium and oxygen-18. If the idea of the Somló&Tihany Formations being a part of an active groundwater flow system is applied, we may believe that low oxygen-18-deuterium values may be a result of a higher recharge area. This group shows more negative carbon-13 values compared to the other samples from the Újfalu Formation, while their carbon-14 content is low and its age is estimated to be more than 25.000 years. Based on the general spatial distribution, the Ptuj-Grad, Zagyva and Somló&Tihany Formations probably belong to an active regional flow system, which is recharged from the NW, mostly in the Goričko hills. Most of the samples represent groundwater from the Mura and Újfalu Formations which was infiltrated in the Pleistocene. Its estimated ages vary mainly between 20.000 and 30.000 years. Spatial distribution data indicates that the groundwater flow is very slow, but this is probably still a part of the active regional flow system. Near the state border (line Lenti-Lendava-Letenye) the isotope data on groundwater indicates an active recharge area in the past. However, the sample density network is still very poor therefore additional sampling and analyses will have to be made. The main outlier seen on different plots is Mt-4, the groundwater from the Miocene Špilje&Haloze formation. The different isotope composition could be the result of the water24
rock exchange at elevated temperatures helped by slow or no regional water flow in the aquifer, or it was recharged under strong evaporation conditions. The probability of the evaporation effect is supported by the very similar evaporation lines of Lake Balaton (δD=5.2* δ18O-13.8; Barna&Fórizs 2007) and Lake Kelemenszék (δD=5.5* δ18O-18.2; oral report Mádlné 2006). Dissolution of carbonate rocks is often enhanced by cation exchange, which was already indicated in the evaluation of the main cations. This water is assumed to be the oldest amongst the Slovenian samples, but due to possible various geochemical reactions further studies are required before this can be verified. Some other outliers may result from the isotope exchange with dissolved gases (CO 2 , methane) or different mixtures of waters and/or gases. Water from Be-2 is clearly of meteoric origin. It may be a mixture of old(er) and young(er) meteoric groundwater or due to CO 2 enrichment was capable of dissolving more minerals than others. The negative oxygen-18 values and the very low chloride concentrations (Fig. 26) around the state border (in the Lenti-Lendava area) suggest a recharge during a colder period of the Pleistocene, when practically no evaporation or evapotranspiration occurred. The age calculations based on carbon-14 measurements (Fig. 28) were done by two methods, with and without the application of a carbon-13 correction. The radiocarbon ages reported by the laboratory vary from fresh water up to 28.600 years, with much younger ages than expected for waters collected from wells B-33, K-193, K-2, K-1 and Pt-74. The carbon-13 values vary in across a wide range (Fig. 27) between about -21 ‰ and +2 ‰. The very negative carbon-13 values of the wells VP-1 and Čep-1 reflect a very similar value to the carbon-13 value of the soil CO 2 , which is in accordance with the very young infiltration and shallow aquifer practically without any water-rock interactions. The groundwaters collected from the wells B-33, K-193, K-2, K-1, Pt-74, Be-2 and Mt-4 reflect a very strong water-rock or water-rock-gas interaction, mainly an interaction with carbonates and CO 2 gas. In these cases the carbon-13 values are shifted significantly towards very positive values and can not be used for radiocarbon age corrections, since the DIC values of the originally infiltrated waters have been greatly altered. In the case of groundwater where relevant water-carbonate rock interaction is present or a CO 2 effect can be taken into account, the age calculations are more realistic with simple carbon-14 decay calculation, as it was generally proposed by Plummer et al. (2004): "Radiocarbon dating of dissolved inorganic carbon (DIC) in groundwater is based on the actual measured carbon-14 activity rather than the commonly reported normalized activity that has been modified (normalized) for assumed carbon-13 isotope fractionation from an assumed initial value of 25 ‰ to the measured carbon-13 of the sample. For example, carbon-13 of DIC can be more positive than -25 ‰ because of isotope dilution from dissolution of carbonate rocks that are enriched in carbon-13, and not because of in-vitro fractionation processes. When dating DIC in groundwater, the actual number of carbon-14 atoms in the DIC is needed to determine the time elapsed since the DIC of the modern reservoir (that is, the soil CO 2 derived from plants and air) and its carbon-14 atoms were recharged and isolated from air.” The very high bicarbonate concentrations in waters from the wells B-33, K-193, K-2, K-1, Pt-74, Be-2 and Mt-4 show that strong water-rock and/or water-rock-CO 2 interactions occurred along the flow paths. Excluding the wells Be-2 and Mt-4, where additional bicarbonate from the carbonate rocks and CO 2 contribution is even higher than at the other sites, there is a clear correlation (R2=0.69) between the carbon-13 values and the bicarbonate concentrations. The 25
more positive the carbonate-13 is, the higher are the bicarbonate concentrations are found in the groundwater.
Fig. 24 TDS versus δ18O distribution
Fig. 25 δ18O versus δD distribution
26
Fig. 26 δ18O versus Cl- distribution
Fig. 27 δ18O versus δ13C distribution
27
Fig. 28 δ18O versus 14C distribution
4.2.5 Observed changes in some investigated wells in Slovenia The figures shown above indicated that the chemical composition of some thermal water in Slovenia has changed during the years of exploitation. Water from Mt-4 has less sodium, chloride and TDS, much lower deuterium and more bicarbonate than 30 years ago. It is possible that deeper aquifers are slowly being depleted, so less mineralized water is extracted now. A deuterium shift towards more negative values can be influenced by a changed source of methane gas or other hydrocarbons, or by some other influence of organic material. Be-2 water has more TDS, bicarbonate and more positive oxygen-18 and deuterium in 2010 than 3 years ago (Kralj et al. 2009). Earlier samples may have been collected soon after the drilling, which was done only with water, so it is possible that a mixture of different waters was analysed at that time. However, it is also possible that water originally produced at the well showed an oxygen isotope exchange with CO 2 gas as it was equilibrated with the surroundings, while after years of exploitation the water is now less equilibrated and a more meteoric origin is indicated. Mt-7 water shows more positive deuterium value and slightly less sodium and chloride than in 1993. It is possible that here also deeper aquifers are becoming depleted in quantity. However, the deuterium shift is not explained yet. Mt-8 water is not comparable to any of old analyses as its TDS content and most of the ions show enrichment. As the investigated sample should contain water very similar to the well Mt-7 it is highly unusual that Mt-8 shows much older water than the Mt-7. It is worth mentioning that the well Mt-8 captures two different formations, Mura and Ĺ pilje&Haloze, by separate tubings. They are separated by a cementation plug. However, our analysis shows that it is possible that the cement plug or tubing are not impermeable, so some leackage of older and more mineralized water from the Ĺ pilje&Haloze Formation into the Mura Formation water takes place. In the well Do-3g well two aquifers in the Mura Formation were sampled but both are now opened for free flow. Analysis show that comparing to 2005 the upper aquifer still produces the most water, however, the occurrence of nitrate concentrations in thermal water is currently still unexplained. Well Pt-74 in Lendava also shows depletion in TDS and 28
most ions, except for bicarbonate. The most probable explanation is again depletion of stored water quantity in deeper water-bearing layers. Changes in water composition are observed also in two shallow wells VP-1 and Čep-1. Both capture a multilayered sandy-silty aquifer in the Ptuj-Grad Formation, which has probably a limited extent. Čep-1 is situated at a higher elevation and is shallower, while its chemical composition shows a trend of decreasing TDS and bicarbonate, and increasing cation concentrations. It is possible that here an inflow of fresh water is induced. In contrast, VP-1 shows an increase in TDS and all main ions concentrations by almost 40%, from which we conclude that it is possible that higher water extraction has led to leaching of water stored in the surrounding silty layers, which is being produced now. 4.2.6 Dissolved and separated gas composition of sampled groundwater Thermal water in the T-JAM project area has diverse gas content. The water/gas ratio is shown in the following figure (Fig. 29), revealing some wells with very high gas content. This is not directly connected to the aquifer formation but more often to the proximity of the well to permeable fractures, natural gas or oil fields and over-pressured areas. Where mainly CO 2 is degassing (example Be-2, Mt-4) carbonate scaling occurs and needs to be mitigated. Where methane and other hydrocarbons are degassing (example Pt-74, Ve-1), scaling is not an issue and degassing to air is sufficient to make the thermal water useable.
Fig. 29 Gas/water yield ratio
Thermal water samples show some differences in dissolved and separated gas content connected to the aquifer formations. The carbon dioxide is often the main dissolved gas (Fig. 30) in the Ptuj-Grad Formation water. In the Zagyva and Somló&-Tihany Formations mainly air is dissolved, showing prevalent nitrogen gas. The Mura and Újfalu Formations have local enrichments in carbon dioxide or methane, but mostly air is dissolved in the water. Similar enrichments are also observed in mixed water of different Miocene Formations. Deeper buried and older aquifers in the Špilje&Haloze Formation, Lajta limestone and Paleozoic
29
metamorphic rocks are strongly enriched in carbon dioxide. This trend is also shown on a plot showing separated gas composition without air (Fig. 31).
Fig. 30 Dissolved gas composition
Fig. 31 Comparison of separated gas without air
30
4.2.7 Noble gas composition of sampled groundwater Groundwater samples have been taken so that dissolved noble gas concentrations and isotope ratios could be determined. The results of noble gas measurements can be seen in Table 8. Helium covers a wide range from 5·10-8 to 6·10-6 ccSTP/g indicating the presence of terrigenic helium (ccSTP: cubic cm at standard temperature {0°C} and pressure {1 atm}). Neon concentrations are between 4·10-8 and 4·10-7 ccSTP/g. Since the solubility equilibrium constant for neon is about 1.7–2.2·10-7 ccSTP/g, neon concentrations in the range of·10-8 ccSTP/g indicate subsurface degassing, mainly due to gas stripping of methane. Argon concentrations below 2.5·10-4 ccSTP/g confirm this degassing effect. The Neon isotope ratio for K-193 (20Ne/22Ne=10.21) suggests diffusive degassing. Radiogenic argon from the decay of 40K can be seen in three samples where the 40Ar/36Ar ratio is significantly higher than 300 (K-193, Mt-8g and P-3). Table 8 Noble gas concentrations and isotope ratios of TJAM water samples
B44 K-21 K-23 K-193 K-27 B-4 Do-3g Mt-8g Čep-1 P-1 P-3 VP-1
He (ccSTP/g) 2.16E-06 9.84E-07 3.36E-07 9.19E-07 2.22E-07 3.02E-07 1.78E-07 5.57E-07 5.35E-08 2.00E-06 6.65E-06 9.80E-08
Ne (ccSTP/g) 1.88E-07 3.55E-07 3.19E-07 5.03E-08 1.86E-07 2.21E-07 6.63E-08 3.96E-08 2.07E-07 2.82E-07 3.78E-07 3.65E-07
Ar (ccSTP/g) 4.79E-04 4.78E-04 2.49E-04 3.66E-04 4.12E-04 2.36E-04 1.54E-04 3.36E-04 4.50E-04 5.11E-04 4.34E-04
3
He/4He
R/R a *
20
40
5.31E-07 3.61E-07 3.92E-07 6.33E-07 3.24E-07 3.40E-07 3.88E-07 1.15E-06 1.82E-06 1.35E-07 6.82E-08 1.30E-06
0.383 0.261 0.284 0.457 0.234 0.246 0.280 0.828 1.314 0.097 0.049 0.942
9.95 9.75 9.80 10.21 9.81 9.74 9.83 9.85 9.78 9.76 9.74 9.77
295.0 297.3 2890.9 296.7 296.6 301.3 357.9 301.8 299.4 560.8 294.2
*:R/R a refers to the ratio of 3He/4He isotope ratio in the sample (R) and air (R a )
Ne/22Ne
Ar/36Ar
Figure 32 shows a detailed evaluation of the data. Green squares represent the neon and argon concentrations of those water samples which are in solubility equilibrium with the ambient air. These concentrations have been calculated for a recharge elevation of 200 m a.s.l. Excess air can be usually seen in natural groundwater. Blue lines represent unfractionated excess air in addition to the equilibrium components. Red dots represent argon and neon concentrations excluding the radiogenic argon component, while blue dots represent argon concentrations including the radiogenic argon component. Infiltration temperatures can be read from Fig.32, extrapolating the measured neon-argon concentrations parallel to the blue excess air lines to the solubility components. It can be seen that groundwater of Čep-1 and VP-1 infiltrated the surface at around 15°C, while other waters (K-21, K-23, B-4 and P-1) at 6–7°C. A few samples have suffered subsurface degassing. Reduced argon and neon concentrations of samples K-193, Do-3g, Mt-8g and P-3 show that these values are lower than the solubility equilibrium component, as if some gas would have been lost. Sample K-27 has also a degassed pattern. The methane content of these waters is relatively high compared to shallow groundwaters. Additionally, three samples (K-193, Mt-8g, P-3) contain radiogenic 40Ar shown by the elevated 40Ar/36Ar ratios. In Figure 33 the helium isotope ratio is plotted against the helium concentration. Samples (Čep-1, VP-1) containing less than 1·10-7 ccSTP/g have the highest 3He/4He isotope ratio. It 31
is due to the tritiogenic 3He content. These two waters seem to be very young, and they recharged in a warm period.
Fig. 32 Argon versus neon concentrations of TJAM water samples
Fig. 33 Helium isotope ratio versus helium concentration (R and R a : 3He/4He isotope atio in the sample and air)
Significant amounts of helium can be found in the other samples (Fig. 34). The higher helium values occur together with lower isotope ratios. The excess helium has a terrigenic origin that can include either a crustal or a mantle component, or both. However, if we assume only a 32
crustal helium component (having R/R a to be 0.002; R and R a : 3He/4He isotope ratio in the sample and air) the helium isotope ratios should be lower in most cases. It can be concluded that samples B44, K-193 and Mt-8g contain a significant component of mantle derived helium (R/R a =8).
Fig. 34 Neon versus helium concentrations of TJAM water samples
From noble gases analysis it can be concluded that water from the Ptuj-Grad Formation in the Goričko hills (VP-1, Čep-1) is young and infiltrated in warm period. Their relatively recent infiltration (between few tens and few hundreds of years) is supported also by their very high 14 C values (>70% pmC) and their relatively positive d18O values (above -10‰). The water from the Ptuj-Grad Formation in Ptuj (P-1) and the Somló&Tihany Formations in Lenti (K21, K-23, B-4) infiltrated during the cold period. The temperature of infiltration can't be estimated for the degassed samples, but based on the stable isotope and carbon-14 data, it can be said that the rest of the collected samples also infiltrated during a cold period. Samples from the Mura and Újfalu Formations (K-193, Mt-8g, Do-3g, P-3) show strong diffusive subsurface degassing and except of Do-3g they show a radioactive decay of potassium-40. Samples B-44, K-193 and Mt-8g contain helium of mantle origin.
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5. Summary: Identification of transboundary aquifers by hydrogeochemical research The work carried out within the framework of T-JAM project and presented here shows that a uniform hydrogeochemical methodology can be applied for identification and evaluation of the potential transboundary geothermal aquifers. This is the first time that such a methodology has been successfully applied. An interpretation of the archived hydrogeochemical data and the newly sampled thermal and cold waters in the T-JAM project area from the NE Slovenia and SW Hungary leads to the following conclusions: - Transboundary geothermal aquifers between Slovenia and Hungary are shown to exist. The Ptuj-Grad Formation groundwater in Slovenia is similar to the groundwater of the Zagyva and Somló&Tihany Formations in Hungary. Groundwater from the Mura Formation in Slovenia is similar to groundwater from the Újfalu Formation in Hungary. Besides, the same is observed with the Lendava Formation groundwaters in Slovenia and the Szolnok Formation groundwaters in Hungary. - A regional groundwater flow is hydrogeologically possible and has been confirmed by hydrogeochemistry in the transboundary geothermal aquifers identified in the TJAM project area. - The Ptuj-Grad, Zagyva and Somló&Tihany Formations probably form an active regional groundwater flow system, which recharges in the north lying Goričko hills in Slovenia. The flow direction is assumed to be from Slovenia to Hungary. This groundwater has a low TDS content and a high cation ratio. Carbon dioxide is often the main dissolved gas in the Ptuj-Grad Formation water, while in the Zagyva and Somló&-Tihany Formations mainly nitrogen gas is present. - The Mura and Újfalu Formations are also a part of the active regional groundwater flow system, probably hydraulically separated from the shallower one. This groundwater has higher TDS values, but they have lower cation ratio compared to the previous group. They are enriched locally in carbon dioxide or methane, but mostly air is dissolved in the groundwater. Samples show strong diffusive subsurface degassing. - The Lendava and Szolnok Formations contain similar groundwater which is probably not a part of the active regional groundwater flow system. Water probably infiltrated in the same period but is now more or less stagnant and isolated from the surroundings. Due to this fact they have a high TDS content. - The Lower and Middle Miocene Formations aquifers are most likely of a very limited extent or isolated from their surroundings, and therefore a transboundary flow is less possible. They have a high TDS content. - The Mesozoic aquifers investigated in Hungary and Slovenia on the T-JAM project area are not comparable. Very scarce data is available from the Mesozoic aquifers in Hungary which could possibly represent an eastern continuation of the Raba fault zone carbonates from Slovenia. Consequently, it has not yet been possible to investigate the transboundary flow in these aquifers. The Mesozoic carbonate aquifers in Slovenia mostly store diluted brines, while in Hungary slightly mineralized water with a combination of multiple ions is observed. - In the case of strong water-rock or water-rock-gas interactions, mainly due to interaction with carbonates and/or CO 2 , the carbon-13 values are shifted significantly 34
towards very positive values and cannot be used for radiocarbon age corrections, since the DIC values of the originally infiltrated waters have been greatly altered. However, in spite of only 24 new groundwater samples being collected, analysed and interpreted, the spatial distribution of the all available data is still scarce. The T-JAM project provides new information on the understanding of the transboundary geothermal aquifers and their hydrogeochemical characteristics. We suggest that similar researches should be conducted in the near future in order to confirm or reject our hypotheses.
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