Hydrogeochemical and isotopic characterization of groundwater at Žitný Island (SW Slovakia)

Hydrogeochemical and isotope studies of groundwater have been carried out in the past with the aim to better understand its origin, formation, dynamics, climatic impacts, its vulnerability and protection against anthropogenic impacts in the world (Gonfiantini et al. 1999; Kendall & McDonnell 1999; Aggarwal et al. 2006a, b), and specifically in Central Europe (Rank et al. 1995; Deák 2003; Povinec et al. 2006; Schiavo et al. 2009). Recently, new geostatistical tools have been developed to integrate isotope data into a relational database covering also hydrogeology and hydrochemistry of groundwater. Using geographical information system, it has been possible to create temporal-spatial isotope maps of groundwater (Bowen et al. 2005; Aggarwal & Araguás-Araguás 2006).

Isotope data together with hydrographic data have been used for better characterization of specific groundwater regions, for studying groundwater ages, infiltration areas, recharging characteristics of groundwater reservoirs, impact of climatic changes and a danger of groundwater contamination (Vitvar et al. 2007; Ockenden et al. 2014; Szczucinska 2014; de la Torre et al. 2015). These have been important studies for the protection and sustainable exploitation of groundwater from the long-term perspective.

Although several isotope hydrology studies were carried out in Central Europe (Rank et al. 1995, 1998, 2009; Deák 2003; Vitvar et al. 2007; Miljević et al. 2008), including Slovakia (Malík et al. 1995; Michalko 1999; Franko et al. 2005, 2008; Povinec et al. 2010), information on temporal and spatial groundwater variations, and specifically isotope depth profiles, have been missing. Development of an isotope groundwater database for Central Europe is underway which will identify regions with limited data sets, where new sampling campaigns and isotope analysis will be carried out. It is believed that with recently developed geostatistical tools the evaluation, assessment and management of groundwater resources in the region will be improved.

In this paper we present a first attempt to apply geostatistical tools in studying spatial and vertical distribution of isotopes in the groundwater of Žitný ostrov (Rye Island) in South-West Slovakia. The territory of Žitný Island is of great economic significance as it represents the largest reservoir of groundwater in Central Europe (about 10¹⁰m³, which represents potential ∼18 m³ s⁻¹). In 1987 the territory of Žitný Island was declared as the national protected water resources territory of Slovakia. The groundwater sources at Žitný Island are delivering drinking water to Bratislava as well as to many other places in South-West Slovakia. Žitný Island is also, because of its location, good soil and climatic conditions, the most important agricultural region of Slovakia. There is also located the largest Slovak water power plant (Gabčíkovo, established in 1992), which is producing 720 MW of electricity. The Gabčíkovo water system considerably influences the hydrology of the region, as well as the Danube River shipping conditions. The Gabčíkovo plant with the reservoir, and the inlet and outlet canals has a positive impact on regional groundwater conditions. Owing to the back water effect of the reservoir, the level of groundwater in the region of Bratislava has increased by about 2 m, with an important positive impact on all ecosystems in the region.

The territory of Žitný Island is located in the Danube basin with the core of the Gabčíkovo depression, bordered at the North-West by Small Carpathians, and from the South-East by a system of faults. There are three strata in the Quaternary filling: (i) bottom strata which is formed mostly by fine-grained sand gravel with frequently occurring clayey or silt sands up to clays with 10–350 m thickness; (ii) middle strata (the Danube gravel formation) from middle up to course-grained sandy gravels with sporadic intermediate layers of fine sediments up to a thickness of 160 m in the center of the depression; and (iii) top strata (bottom-land facies), mostly fine-grain Holocene sediments from 0.5 to 3 m thickness. The Quaternary sub-base is made of ruman, dak and pont sediments, mostly of gray up to gray-green weakly calcinated mica clays and dust with varying admixture of sand. The territory of Žitný Island is formed by terraces of fluvial sediments – clayey sands, sands, gravels, sand gravels and residual sands – and by bottom lands of fluvial sediments – sandy clay, clays, clayey sands and clayey gravels (Maglay et al. 2009). Quaternary sediments of Žitný Island may be allocated to lower, middle and upper Pleistocene and Holocene. The Pleistocene sediments have a thickness from 10 to 20 m south of Bratislava, 8–12 m at Komárno, and in the center of the depression at Gabčíkovo it is about 160 m. The sediments are mostly made of coarse gravels, sand gravels and sands without fine fractions, which indicate a dominance of stream-bed facies over bottom-land sediments. The Holocene sediments are formed by a diluvium enclosure of river bottom lands with admixture of gravel, and recent and fossil soils.

On the basis of statistical evaluation of the hydrographic data from 812 boreholes, the Žitný Island territory has been divided into four regions with the following hydraulics parameters: (i) a right-riverside of the Danube – a high value of median flow capacity (2.03 × 10⁻²m² s⁻¹), and median filtration coefficient (4.06 × 10⁻³ m s⁻¹), and an average value of specific strength (12.6 L s⁻¹); (ii) an upper part – a lower value of median flow capacity (5.01 × 10⁻² m² s⁻¹), and median filtration coefficient (1.00 × 10⁻² m s⁻¹), and an average value of specific strength (28.0 L s⁻¹); (iii) a middle part – a highest value of median flow capacity (1.55 × 10⁻¹m²s⁻¹), and median filtration coefficient (3.1 × 10⁻² m s⁻¹), and an average value of specific strength (93.25 L s⁻¹); and (iv) a lower part – a lowest value of the median flow capacity (7.95 × 10⁻³ m² s⁻¹), and median filtration coefficient (1.59 × 10⁻³ m s⁻¹), and an average value of specific strength (5.4 L s⁻¹).

A general trend in the flow of groundwater is mostly following the main rivers in the region (Danube, Small Danube and Váh). The Danube River during all its water levels in Žitný Island feeds groundwater in the region. Precipitation is influencing the groundwater regime of Žitný Island indirectly via elevated flow rates in rivers (as will be discussed later using isotope data). Increasing river flow rates have also been increasing the groundwater level, with different delays depending on the distance from the river. The groundwater regime of Žitný Island is thus mainly determined by interactions between Danube water (and other surface waters in the region) and groundwater in the region.

The sampling sites were identical with groundwater sources regularly monitored by the Slovak Hydrometeorological Institute in Žitný Island (Figure 2). The sampling strategies were determined by development of a relational isotope database of groundwater of Slovakia (which will cover hydrogeological, hydrochemical and isotope data), determination of the catchments areas of groundwater at Žitný Island and contribution to the protection of groundwater against contamination from surface waters (e.g. from the Gabčíkovo water system), from agricultural fertilizers and industrial products (e.g. oil products from the Slovnaft refinery located in Bratislava). Two sampling campaigns were carried out, one in November 2008 and the second one in June 2009. Altogether 38 boreholes were visited. Groundwater samples were taken from different horizons. A description of the sampling wells is presented in Table 1.

The sampling of water from boreholes was carried out in such a way that inflows were isolated from their overlying and/or underlying strata. All pipes of each borehole are cemented above perforation, so the wells are technically protected from inflows of waters into the borehole from its sealed part. This, however, cannot prevent mixing of waters during their flow in aquifers. Such cases can occur especially in discharge areas, when waters of deep flow may be influenced by a shallow groundwater.

During groundwater sampling in situ measurements of basic physical and chemical parameters (groundwater temperature, air temperature, pH, electrical conductivity, oxidation-reduction potential, concentration of dissolved oxygen and oxygen saturation) were carried out as well. Water samples for radiocarbon analysis (∼50 L) were collected directly from the source. Bicarbonates were extracted as soon as possible by precipitation with barium chloride. Produced BaCO₃ was stored in polyethylene containers and transported to the laboratory.

Laboratory analyses included analysis of stable isotopes (¹⁸O, ¹³C), preparation of gas fillings for proportional counters and ¹⁴C activity measurements. A few mL of carbon dioxide liberated from the BaCO₃ sample was used for the determination of the isotopic ratio of ¹³C/¹²C. The δ¹³C values are expressed relative to the Vienna Pee Dee Belemnite standard (in ‰). ¹⁸O/¹⁶O isotopic ratio was analyzed directly in water samples. The δ¹⁸O data are reported relative to Vienna Standard Mean Ocean Water (in ‰). Relative uncertainties were below 0.2‰ (at 1 σ). Stable isotope analyses were carried out using mass spectrometers at the Dionýz Štúr Geological Institute in Bratislava.

The radiocarbon measurements were carried out in the Department of Nuclear Physics and Biophysics of the Faculty of Mathematics, Physics and Informatics of the Comenius University in Bratislava. For ¹⁴C analysis of groundwater samples, carbon dioxide was released from barium carbonate by addition of H₃PO₄. Methane (Povinec 1972) synthesized from carbon dioxide was used as a filling gas of the low-level proportional counter (Povinec 1978). Measuring time of methane samples was from 40 to 60 h. In addition to each water sample, samples of background and of radiocarbon standard (National Institute of Standards and Technology (NIST)) oxalic acid standard were also measured (Usačev et al. 1973). ¹⁴C results are expressed as percent modern carbon (pMC) relative to the NIST ¹⁴C standard. All ¹⁴C data were corrected for δ¹³C. Relative uncertainties of ¹⁴C measurements were below 10% (at 1 σ). Quality management of all analyses has been assured by analysis of reference materials, and by participation in intercomparison exercises.

The spatial isotope maps have been constructed using the Ocean Data View software, which reproduces well surface, as well as depth distributions of isotopes in the aquatic environment (Povinec et al. 2011). The present spatial resolution of the obtained maps is 5 km in the horizontal plane, and 10 m in the vertical plane.

The physical, chemical and isotopic characteristics of the collected groundwater samples are presented in Tables 2 and 3. The chemical composition of the groundwater of Žitný Island depends on the chemical composition of Danube River water (and also of Small Danube and Váh river waters), as well as on the length of infiltration of rivers waters into the groundwater. Although the groundwater samples were collected during summer and winter (however, at different sampling sites during different seasons), we did not observe a statistical difference in the stoichiometry of the samples collected during different sampling periods. The groundwater is characterized by potamogenic mineralization, and its chemical composition is influenced by anthropogenic contamination. The Quaternary environment, especially the alluvial sediments of the Danube, Small Danube and Váh rivers, has mostly silicate character (gravels, sand gravels and sands).

Under anthropogenically non-influenced conditions, groundwater is of Ca-Mg-HCO₃ type with TDS between 400 and 600 mg L⁻¹. In regions with anthropogenic influence, sulfites, chlorides and nitrates dominate among anions. Sodium and potassium dominate over calcium and magnesium in the cation group, and TDS is usually above 800 mg L⁻¹.

In the layer up to 25 m, groundwater is characterized by three chemical types: (i) a basic distinctly A₂ type (Ca-(Mg)-HCO₃) with average TDS of 426 mg L⁻¹, which is dominating in the fluvial zone of the Danube River; (ii) a basic-indistinctly A₂ type (Ca-(Mg)-HCO₃) with average TDS of 662 mg L⁻¹, typical for the central part of Žitný Island, in a continuous strip from Bratislava down to Kolárovo; and (iii) a mixed type with dominance of A₂ and S₂(SO₄) components with average TDS of 911 mg L⁻¹, which is bordered from the south by the line Bratislava – Komárno, and from the north by the Small Danube up to the Váh River.

In the layer below 25 m, groundwater is characterized by three chemical types: (i) a basic distinctly A₂ type (Ca-(Mg)-HCO₃) with average TDS of 381 mg L⁻¹, which is dominating on the territory of Žitný Island; (ii) a mixed type with dominance of A₂ and S₂(SO₄) components with average TDS of 680 mg L⁻¹, which is probably overlapping from the first layer; and (iii) a basic distinctly A₁ type (Na-HCO₃) with average TDS of 626 mg L⁻¹, which is found in the South-East part of Žitný Island.

Vertical distribution of δ¹⁸O in groundwater with latitude and longitude of Žitný Island is also presented in Figure 4. While the bottom samples (below 40 m) are depleted in δ¹⁸O values, generally below −10.5‰, the sub-surface core observed at around 20 m water depth shows enriched δ¹⁸O values between −10.0 and −9.5‰. However, the sub-surface samples (up to 10 m water depth) on the north and north-east side of the island show again depleted δ¹⁸O values, close to the values observed for the Danube River system (from −12.4 to −10.2‰; Michalko et al. 2011).

The measured δ¹⁸O values in the Danube River (Bratislava) have been varying during the year showing maxima (up to −10.2‰) in winter and minima (down to −12.4‰) in June (Michalko et al. 2011). The June minimum (also accompanied by the largest river flow rates) should be associated with melting of Alpine snow. A δ¹⁸O in Alpine precipitation as low as −18‰ was observed by Pastorelli et al. (2001). The δ¹⁸O values in shallow wells situated close to the Danube River should be therefore following this temporal variation (with a time delay of a few years; Michalko et al. 2011).

On the contrary, the measured δ¹⁸O values in precipitation in south-west Slovakia varied from about −15‰ during winter to about −5‰ during summer (caused by evaporation losses of ¹⁶O), with average annual values for Bratislava of −8.83‰, and for Topoľníky (situated at the center of Žitný Island) of −9.35‰ (Holko et al. 2012). Unfortunately, we do not have δ¹⁸O sub-surface groundwater data available yet for the Žitný Island stations, which would be collected both during summer and winter seasons at the same stations at the same depth. The δ¹⁸O data presented in Table 2 do not show large differences between groundwater samples collected in summer (top horizons down to 25 m with average value of −10.54‰) and in winter (deeper horizons <25 m with average value of −10.74‰). However, for proper temporal variations studies we would need new δ¹⁸O results for top horizons in winter and bottom horizons in summer.

The obtained δ¹⁸O groundwater data are in reasonable agreement with isotope data measured for the Danube River system indicating therefore that the Danube River should be the main source of groundwater observed at Žitný Island. As we already mentioned, precipitation should be influencing the groundwater regime of Žitný Island only indirectly via elevated flow rates in rivers. We should get, however, a more-clear picture when summer/winter data are available for the same stations.

This has been a first attempt to construct isotope maps and to study spatial and vertical distribution of isotopes in groundwater of Slovakia. Groundwater samples collected during 2008–2009 in SW Slovakia, at the area called Žitný Island, improved the data density so that geostatistical methods of data treatment could be applied.

The chemical composition of groundwater of Quaternary sediments of Žitný Island depends on the chemical composition of Danube water, and also of Small Danube and Váh river waters, as well as on the length of infiltration of river waters into the groundwater. The groundwater is characterized by potamogenic mineralization, and its chemical composition is influenced by anthropogenic contamination.

The obtained results on spatial variability of ¹⁴C, δ¹³C and δ¹⁸O suggest large isotopic heterogeneity in the groundwater of Žitný Island. While the bottom samples (below 40 m) are depleted in δ¹⁸O values, generally below −10.5‰, the sub-surface core observed at around 20 m depth shows enriched δ¹⁸O values between −10.0 and −9.5‰. However, the sub-surface samples up to 10 m depth on the north and north-east of the island show again depleted δ¹⁸O values, close to the values observed for the Danube River system. In the ¹⁴C profile a sub-surface core of about 50 pMC at around 60 m depth at the south-east of the island was observed, in agreement with expectations due to the presence of Neogene clay sediments there. The sub-surface samples up to about 20 m depth show ¹⁴C values above 80 pMC, representing thus contemporary groundwater mostly supplied by the Danube River system.

The obtained groundwater isotope data are in good agreement with isotope data measured for Danube water indicating that the Danube River should be the main source of groundwater observed at Žitný Island. More groundwater samples from the Žitný Island area will be collected and analyzed during forthcoming expeditions, which will help to improve the spatial density of isotope data, as well as their seasonal characteristics, and thus contribute to better understanding the groundwater system of Žitný Island.