3. Čunovo Reservoir and surroundings

3.1. Čunovo reservoir

The Čunovo reservoir (approximately 4000 hectares in extent) which was created between Bratislava and Dobrohošť (as far as bypass canal) is a new biotope. Although the reservoir represents the creation of a new biotope, it harbours conditions that are typical for river and flood-plain ecotopes, such as a moderately- to fast-flowing main channel, permanent moderately-flowing deep and shallow river branches, and flood-plain through-flowing lakes with variable flow velocity (Fig. 3.1).

The reservoir consists of an upstream part from Bratislava down to the Čunovo weir, and downstream part from the Čunovo weir down to the by-pass canal. The retention time of water flowing through the reservoir is short, from 1 to 6 days.

The upstream part of the reservoir begins at Bratislava, where it is still the original rapidly flowing Danube (at average discharges of 2000 m³/s 1.2 m/s), with water levels during a low discharge period higher than before, and similar water levels during higher discharges. This character of the river, continues down to ČThe upstream part of the reservoir begins at Bratislava, where it is still the original rapidly flowing Danube (at average discharges of 2000 m³/s ? 1.2 m/s), with water levels during a low discharge period higher than before, and similar water levels during higher discharges. This character of the river, continues down to Čunovo weir, where the flow velocity has an average discharge of 0.3 m/s. In this stretch sedimentation of bed-load and riverbed substrate is gradually changed from the original gravel one at Bratislava to one of more sandy gravel andFig 3.2).

The upstream part of reservoir is connected to the old river branch at Rusovce, through excavated pits, and by a 2 km long bay having several islands upstream from Kalinkovo village. It has a variable water depth and a high biomass productivity (Fig. 3.3). The area is also suitable for many limnic and some eurytopic rheofhile species. Rheophile species find appropriate living conditions along the path of the former river cannel. In addition, species typical for river branches may be expected in adjacent, shallower areas having moderate to slow flow conditions. No temperature and oxygen stratification occurs. A good food base and sprawling places exist. The area is suitable for nesting waterfowl (significantly reduced water level fluctuation compared to pre-dam conditions, especially during the nesting period, suitable gravel banks) (Fig. 3.4).

In general, the water quality is good, with better visibility, which improve the living conditions for many species that hunt on sight. The settlement of aquatic vegetation creates friendly living conditions for a wide range of species that are confined to shallow river water. All this together, especially the aquatic flora, contributes a great deal to self purification processes in the reservoir, which did not exist in this part of the previous heavy fortified Danube.

In the lower part of the reservoir, downstream from the Čunovo weir, the reservoir is divided into a main navigation canal with large flow velocities and a large area of variable depth and of comparatively small flow velocities. This part of the reservoir is more suitable for limnic species. The diversity of water flow velocity, and thus also the sedimentation-erosion processes, is ensured by the hydraulic structures in the reservoir. Here the reservoir is hydraulically divided into a main navigation canal, with larger flow velocities, and a large area with variable depth and comparatively smaller, but still turbulent, flow velocities. Hydraulic structures are active mainly when the reservoir water flow is significant.

A combination of fast flowing and slowly flowing reservoir parts, various water depths, large shallow areas and areas with aquatic plants, increases the variability of aquatic living conditions. The reservoir combines substantially conditions previously present in the river branches and in the main river cannel.

3.2. Kalinkovo Waterworks

The waterworks at the village of Kalinkovo (Fig. 3.5) were set in operation in 1972. Ten wells are situated near the old Kalinkovo river arm. The first well, with a screen (filter) for the interval of 21 - 55 m beneath the surface was excluded from the water supply because its waters carried high levels of iron and manganese. Therefore, all the other wells were equipped with well screens at depths of 40 - 80 m, and some of them are still in use. The capacity of the system of wells is 850 l/s. At present less than 200 l/s is used. Ground water is exploited mainly experimentally with the goal to study chemical processes in ground water and the impact of the reservoir on the ground water quality. Wells were originally covered and protected by an earthwork from flooding. Under pre-dam conditions, floods sometimes covered the waterworks area and polluted the ground water and wells from the surface for some time. It had been proposed to move these wells to other places, more distant from the reservoir. At present, wells are protected from flooding by reservoir dikes.

Experimental ground water quality monitoring is stressed mainly because of the fact that water wells were situated near to the previously existed river arm, and experience showed, that this river arm had negatively influenced the first shallow well. At present, these water wells are situated close to the reservoir, just behind the seepage canals. The course of changes in the water quality from well S-4, which was selected because it is situated in the middle of the well system and is now closest to the reservoir, is not significant, which means that the measured changes do not exceed previously measured values. The filling of the reservoir with water did not have a significant negative influence on water quality, and this in spite of the vicinity of the reservoir (Fig. 3.7). The vicinity of the wells to the reservoir, negative experience with the shallow well, and generally negative experience at the observation wells situated close to the river branches were the reason of the construction of special ground water monitoring system of wells in the framework of the PHARE project [4]. This is described, for example, in the publication of the Faculty of Natural sciences in 1995 [3] (Fig. 3.6). The goal of the ongoing research is to study water quality processes and to propose new well field for waterworks at Kalinkovo, situated a larger distance from the reservoir. Such a solution was proposed despite the fact that the presence of chemicals indicating water pollution was not detected. According to the micro-biological and biological criteria of the Slovak Technical Standards, ground water from the wells is permanently suitable for drinking water purposes without treatment. The only signs of the changes were seasonal fluctuations and a decrease in the content of nitrates. A decrease of nitrates can lead firstly to an increase of the content of manganese and later also to an increase in the content of iron, which is usually treated by aeration of water in the waterworks or by in situ treatment directly in the aquifer.

A typical component of ground water in European countries, including Slovakia, is iron and manganese. Iron and manganese are not pollutants but are part of the geological composition of an aquifer. Whether iron and manganese occurs dissolved in the ground water depends on the oxygen condition in the water and the content of organic carbon in the water and in an aquifer. The common occurrence of iron and manganese dissolved in the ground water is well known from Hungary, in eastern Slovakia, the eastern part of the Žitný ostrov area - east of the city of Gabčíkovo, and in some places also downstream from Bratislava, mostly in places where the water wells are situated close to the river branches. A typical example is the area of the large well field at Rusovce, where in situ ground water treatment facilities for iron and manganese were installed. A similar situation is found, for example, at Vac in Hungary. Another typical examples are the Kalinkovo shallow well and observation wells at Dobrohošť etc. In Europe the best known and simplest treatment of ground water containing iron and manganese that is used for municipal water supply is water aeration and sedimentation of iron and manganese oxides.

3.3. Šamorín Waterworks and Hydraulic structures in the reservoir

The waterworks at Šamorín (Fig. 3.8, 3.9) is situated opposite the down stream part of the reservoir. The waterworks well system was set into operation in 1975, with six wells being equipped with screens (filters) at depths of 45 - 90 m, with the combined discharge of as much as 900 l/s. Three additional wells (as eventual compensation for the closing of the waterworks at Kalinkovo) were added just before filling the reservoir with water, which increased the discharged capacity of the whole waterworks to 1200 l/s. At present approximately half of this capacity is used.

The ground water quality, according to systematic observations since 1975, corresponds to the requirements of drinking water in all parameters, in accordance with the Slovak Technical Standards. The ground water quality is constant, without any significant changes (Fig.  3.10).

Measures to protect the quantitative and qualitative recharge of ground water with water from reservoir, included the construction of hydraulic guide structures in the downstream part of reservoir. Diversity of water velocity, and thus also of sedimentation- erosion processes is ensured by hydraulic structures (see aerial photography, Fig. 3.1). They are quite well active if the reservoir water flow is significant.

3.4. Kopáč island and Biskupické rameno river arm

The nature reserve of Kopáč Island, on the left side of the Danube, surrounded by the Biskupické rameno arm, is an area covered by a flood-plain forest (Ulmeto-Fraxineto carpineum) which at some places turns into a forest steppe biotopes with xerotermofilous zoocoenoses. The area was not flooded before the Danube damming. The ground water level was dropped severely before the filling of the Čunovo reservoir. Before 1993, the drying process was visible especially in the tree stratum (Fig. 3.11, 3.12). After 1992, after filling the Čunovo reservoir; the ground water level raised and the Biskupické rameno arm and terrain depressions were filled with water (Fig. 3.13), leading to the regeneration of the most humid willow-poplar forest. Numerous natural reseeding of poplar, up to this time rare in this area, have been observed. Lowering of leaf losses and an increase in thickness increments in poplars in comparison to previous conditions were measured. A comparison of the qualitative structures of terrestrial fauna, for example the Centipede taxocoenoses, before and after filling of the reservoir shoved a moderate shift from the xerothermophilous species to the mesohygrophilous ones. In general it means humidity increase. After the rising of the ground water level, the water stayed also in the gravel pits and other terrain depressions, which became a hatching site for frogs. The recorded species reproduced there, finished their development up to metamorphosis and migrated into the surrounding vegetation. The amphibian taxocoenosis is stabilised. All monitoring data indicate that the continuing retardation of aridization of this area and improvement of conditions for hygrophilous and mesophilous species (Fig. 3.14). This means that the hydrological and soil moisture conditions prevailing downstream from Bratislava some 30 years ago are being re-established.

3.5. Water intake structures of irrigation water supply system

Seepage canals on both sides of the reservoir and on each side of the by-pass canal were designed to channel excess seepage water from the reservoir, to regulate by the reservoir evoked rise of ground water level, and to ensure ground water level fluctuation. The water level can be regulated by up to 2 m in amplitude.

Water from seepage canals is also used to supply water to the irrigation canals in all of Žitný ostrov. The system of irrigation canals and irrigation facilities were been constructed as a response to the long term pre-dam ground water lowering and long term climatic changes, mainly decrees of precipitation and increase of temperatures. There are several intake structures for water supply of irrigation canals (Fig. 3.18).

3.6. Area Right side of the Danube

A lowering of ground water level occurred on the right-side of the Danube, during the long-lasting pre-dam lowering of the river bed of the Danube. The ground water flow from the inland agglomeration of Petržalka and the Austrian territory supported the transport of contaminants towards the wells of waterworks at Rusovce-Ostrovné lúčky-Mokraď and other local municipal wells. The rise of the water level in the Čunovo Reservoir constituted a radical change in the ground water level and flow (Figs. 1.16, 1.19, Chapter 8). The ground water level in the area of water works and also the right side of the Danube River branches rose approximately 2 4 m. The rise of the ground water level has had a positive impact on the discharge of the waterworks wells, on the inundation area of Rusovecké ). The ground water level in the area of water works and also the right side of the Danube River branches rose approximately 2 ? 4 m. The rise of the ground water level has had a positive impact on the discharge of the waterworks wells, on the inundation area of Rusovecké ostrovy and also on agriculture. At present, the prevailing direction of ground water flow is from the Danube towards the water supply well field at Rusovce, and further inland. Since the total dissolved solids in the ground water were originally very high (asFigs. 3.15, 3.16, 3.17). Concerning micro-biological parameters, the water level raised in the Danube has not caused any extraordinary changes in the wells. Concentration of all metal micro-elements is far under the standard limits for drinking water. Therefore, a very positive change with respect to ground water quality, linked with the increase of the proportion of the water infiltrated from the Danube, has been observed. New conditions constitute, from both quantitative and qualitative points of view, an unambiguous profit.

The Rusovce-Ostrovné lúčky-Mokraď waterworks, located between villages of Rusovce and Čunovo parallel to the Čunovo reservoir, utilise ground water recharge from the reservoir on places of the previous Danube riverbed. The system consists of 23 wells located about 120 m from the seepage canal, and 500 ? 600 m from the reservoir. The distance between the individual wells is 100 m. The capacity of the whole waterworks, after setting the hydroelectric power structures of Gabčíkovo step into operation, equals 2480 l/s.