Isotopic tracing of groundwater at Žitný ostrov (SW Slovakia)

Geostatistical analysis of experimental isotope data has been carried out with the aim to study spatial variations in the distribution of water isotopes and radiocarbon in groundwater of Žitný ostrov (Rye Island), which is the largest reservoir (about 10 Gm) of groundwater in the Central Europe. Subsurface water profiles showed enriched δO levels at around 20 m water depth and depleted values below 30 m, which are similar to those observed in the Danube River. The core of the subsurface C profile represents contemporary groundwater with C values above 80 pMC.


Introduction
Stable and radioactive isotopes have been extensively used as environmental tracers during the last decades to study the water cycle, to better understand the origin, dynamics and interconnections of the different elements of the hydrologic cycle. It has been possible to study the present day distribution of water isotopes in the atmosphere, in the rain water, river water, groundwater, and then trace past isotopic compositions affecting many processes, such as atmospheric circulation, rain and snow formation, groundwater formation, ecology and paleoclimatology [1]. Radioactive and stable isotopes have been used to address key aspects of the water cycle, e.g. the origin, dynamics and interconnections of the different elements of the water cycle [2,3]. Fortunately with the development of the IAEA's Global Network of Isotopes in Precipitation (GNIP) database it has been possible to use isotopes in hydrological, ecological and climate studies, as input functions have been available for many areas of the world [4]. Many isotope data have been collected and several isotope databases have been developed. The GNIP database (www.GNIP.IAEA.org) has provided key data for the application of isotopes in hydrology, but also in climatic and ecological studies. Recently this monitoring activity has been enlarged to isotopes in the total water cycle, and the new isotope database (Isotope Hydrology Information System (ISOHIS), www.ISOHIS.IAEA.org), also covering groundwater data, together with the GNIP database enables to study dynamics and spatial characteristics of groundwater.
It is possible to trace the origin and pathways of different water masses on the bases of the developed isotopic maps, covering temporal and spatial distribution of hydrochemical and isotope data. Recently new geostatistical tools have been developed to integrate isotope data into a relational database covering also hydrogeology and hydrochemistry, which using GIS would be possible to visualize, and in this way to create temporal-spatial isotope maps of groundwater [5,6]. Such an integrated attempt will gather new information on temporal and spatial variability of groundwater, on its dynamics, on anthropogenic and climatic impacts, and on its vulnerability. From the existing and new δ 2 H -δ 18 O data obtained in the framework of the project, as well as from other data on isotopic composition and trace elements in groundwater, using results from Slovak institutions it will be possible to produce isotope maps of groundwater of Slovakia, and after their analysis to gather new information on groundwater.
Stable isotopes [7] and radiocarbon [8][9][10] have been applied in a few groundwater studies in Slovakia, however, there has not been done yet an integrated research, which would cover in full complexity all Slovakia ( Fig. 1), not to speak that such a research should also cover the Central European countries. Some previous isotope hydrology work in Slovakia, e.g. on mineral and thermal waters [8] contributed to understanding of origin of these waters, however, temporal and spatial information has been missing, which could better characterize specific groundwater localities, groundwater ages, infiltration areas, recharging characteristics of groundwater reservoirs, a danger of their contamination, climatic changes, etc., all important facts for the protection and correct exploitation of groundwater from the longterm perspective.
In this paper we report results on the spatial radiocarbon and stable isotope ( 18 O and 13 C) variability of groundwater found at theŽitný ostrov, SW Slovakia.

Hydrogeology background
TheŽitný ostrov with the area of 1200 km 2 covers the territory of the Danube Plane from Bratislava at NW to Komárno at SE (Fig. 1). It is bordered on the north by the river Small Danube, and on the east by the river Váh. The territory of theŽitný ostrov is of great economical significance as it represents the largest reservoir of groundwater in the Central Europe (about 10 10 m 3 , what represents potential ∼ 18 m 3 s −1 ). In 1987 the territory of theŽitný ostrov was declared as the National protected water resources territory of Slovakia. There are several groundwater sources situated at the territory of theŽitný ostrov, which are delivering drinking water to Bratislava as well as to many other places of the south western Slovakia. TheŽitný ostrov territory has also important a social value with several protected regional areas. TheŽitný ostrov is also because of its location and good soil and climatic conditions the most important agricultural region of Slovakia. There is also located the most important Slovak water power plant, called Gabčíkovo (established in 1992), which is producing 720 MW of electricity, and considerable influenced the Danube river shipping conditions. Due to the back water effect, the level of groundwater in the region of Bratislava has increased by about 2 m, what have had important positive impact on all ecosystems in the region. From the geomorphology point of view, theŽitný ostrov belongs to the Danube Plain. The territory represents a flat terrain with 136-129 m above sea level. The average precipitation during 1951-1980 was in Bratislava 580 mm. The average evaporation from the soil surface at theŽitný ostrov for the time interval between 1961 and 1990 was 450-500 mm. A total potential evaporation was between 700 and 800 mm.
The Danube River during all its water levels in theŽitný ostrov feeds groundwater in the region. A general trend in the flow of groundwater is mostly following the main rivers in the region (Danube, Malý Dunaj and Váh). Precipitation is influencing groundwater regime especially during summer, in connection with elevated flow rates in rivers, and also by increasing the groundwater level (with different delay depending on the distance from the river). The described sites are shown in Fig. 2.

Samples and analytical methods
The sampling sites were identical with groundwater sources regularly monitored by the Slovak Hydrometeorological Institute in the south-western Slovakia (Fig. 2). Sampling campaigns were carried out in November 2008 and in June 2009, visiting 38 boreholes. Groundwater samples were taken from deepest horizons. Description of the 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 prevented 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 (EC), oxidationreduction potential (Eh), concentration of dissolved oxygen, and oxygen saturation) were carried out as well. Water samples for radiocarbon analysis of ∼ 50 l volume were collected directly from the source. Bicarbonates were extracted as soon as possible by precipitation with barium chloride. Produced BaCO 3 was stored in polyethylene containers and transported to the laboratory. Simultaneously small volume water samples (1 l) were collected for analysis of tritium and stable isotopes. Table 1 describes the sampling sites as well as the shallow wells.
Laboratory analyses included: analysis of stable isotopes ( 18 O, 13 C), preparation of gas fillings and 14 C activity measurement. A few ml of carbon dioxide liberated from the BaCO 3 sample used for the determination of the isotopic ratio of 13 C/ 12 C. δ 13 C values are expressed relative to the VPDB standard (in ). 18 O/ 16 O isotopic ratio was analyzed directly in water samples [11], and the δ 18 O data are reported relative to VSMOW (in ). Relative uncertainties were below 0.2 (at 1 σ). For 14 C analysis carbon dioxide was released from barium carbonate by addition of H 3 PO 4 . Methane [12] synthesized from carbon dioxide was used as a filling gas of the low-level proportional counter [13]. Measuring time of samples was from forty to sixty hours. In addition to each water sample, samples of background and of radiocarbon standard (NIST Oxalic Acid) were also measured. 14 C results are expressed as percent modern carbon (pMC) relative to the NIST (National Institute of Standards and Technology, Gaithersburg, USA) 14 C standard. All 14 C data were corrected for δ 13 C. Relative uncertainties were below 10% (at 1 σ). Department of Nuclear Physics of the Faculty of Mathematics,   Physics and Informatics of the Comenius University in Bratislava has over 40 years of experience in radiocarbon measurements [14]. Quality management of all analyses has been assured by analysis of reference materials, and by participation in intercomparison exercises.

Results and discussion
The position of sampling sites and isotope data are presented in table 1 and 2, respectively. The spatial distribution of δ 18 O in surface and subsurface waters with longitude of theŽitný ostrov area is presented in Fig. 3.  system. The obtained data are in good agreement with isotope data measured the for Danube river system (values between −10.92 and −12.26 for Danube, and between −0.57 and −11.39 for Malý Dunaj). As expected, the Danube river system is the main source of shallow groundwater observed atŽitný ostrov. It is possible that surface and shallow subsurface waters showing enriched δ 18 O values may be due to land irrigation, which has been often used in this agriculturally heavily industrialized region. Fig. 4 also shows a similar profile for δ 13 C. Here we see enriched δ 13 O levels in surface waters, and a depleted layer at water depths around 40 m. From the subsurface radiocarbon profile with longitude also shown in Fig. 3 we can see a subsurface core of about 50 pMC at around 60 m water depth, while the surface samples up to 10 m water depth show 14 C values above 80 pMC, representing contemporary groundwater (Fig. 5). This has been a first attempt to construct isotope maps and to study surface and subsurface distribution of isotopes in groundwater of Slovakia. More groundwater samples from theŽitný ostrov area will be collected and analysed during 2010 and 2011 expeditions, which will help to improve the spatial density of isotope data, and thus contribute to better understanding of theŽitný ostrov groundwater system. We hope that this new research approach will improve the capability and efficiency in using isotopic tools for deeper evaluation, more rigorous assessment and more efficient management of water resources in the region.