## GROUND WATER REGIME CHANGES IN THE AREA INFLUENCED BY CONSTRUCTION OF GABCIKOVO HYDROPOWER STRUCTURES

#### Lubomir BANSKY - Katarina MAZARIOVA

GROUND WATER Consulting Ltd., P.O.B. 6, 840 00 Bratislava 4, SLOVAKIA

CONCLUSIONS Long(term changes in ground water levels during the last 30 years and changes caused by the construction and operation of Gabcikovo hydro(power structures are represented on maps. The area of direct impact of hydro(power structures on ground water is defined. Ground water levels have increased after damming the Danube and recovered to nearly the stay which was known in 1960's.

#### INTRODUCTION

Ground water level has been measured at more than 600 observation wells in the broad region of the Danubian Lowland between Bratislava and Komarno since long ago. Part of the area has been influenced by the construction of Gabcikovo hydroelectric power structures since damming of the Danube in October 1992. Based on a series of ground water level data, in the last thirty years before damming the Danube and the two years after damming the Danube, long(term changes in ground water level regime have been evaluated.

The network of observation wells was established and measured by the Slovak Hydrometeorological Institute (SHMU). Weekly measurement is carried out manually, or continuously using limnigraphs or other automatic recorders. The location of SHMU observation wells is documented on map in Fig. 1. This map also documents the density of observation points in the area of interest.

#### METHODS OF PRESENTATION

Several methods to present ground water level data are known. The most common is the well hydrograph which plots the ground water level fluctuation on single wells in time. This is the first step to characterise the time dependent ground water level development. Several hydrographs placed on one page show the ground water regime changes in various places, in our case according to distance to the Danube. Also drawn in the figures is a linear regression line to show the time dependent drop of average ground water level in a long-term pre-dam period (See Fig 2a,b, c).

The canal network, agricultural drainage facilities and melioration have a stabilising effect on the ground water level changes and fluctuation. A long(term decrease of the ground water level on the both Danube sides is evident over a large part of the territory. Such development is also visible at wells chosen in the middle part of the area in the profile at the town Dunajska Streda, Fig 2c.

#### PRE-DAM CONDITIONS

The ground water level represented by the linear regression line (plotted in selected hydrographs) for a specific date is considered as a reference ground water level. The reference ground water levels for the years 1962 and 1992 were used for drawing the ground water level contour maps (ground water level equipotential lines), Fig. 3. Ground water level changes between 1962 and 1992 expressed as the difference between these two contour sets is shown in Fig. 4. From these maps a considerable decrease in the ground water level, which had occurred in the last 30 years (before putting the Gabcikovo part of hydropower system into the operation(, is evident mainly in the upper part of the Danubian Basin downstream of Bratislava.

These changes in ground water levels were at various places, of various origin. While some were caused by river bed sinking, others resulted from water pumping at hydraulic blanket at the Slovnaft refinery, urbanisation of Petrzalka, works at Gabcikovo, works in connection with construction of the Nagymaros part of hydropower project, agricultural melioration, changes in irrigation and drainage canal systems, and irrigation patterns, exploitation of ground water for municipal water supply, etc. This decrease of ground water level in the largest part of the area was irreversible and for a long time had already negatively influenced the natural conditions, mainly agriculture, forestry and ground water resources.

#### CHANGES AFTER PUTTING THE GABCIKOVO PART OF THE SYSTEM INTO OPERATION

To estimate the impact of putting the Gabcikovo part of the Gabcikovo - Nagymaros system of hydro(power structures into operation it is not sufficient only to compare water levels before and after damming the Danube, but it is necessary to calculate what would be the ground water level without the damming, and to compare those with measured values. The impact of the damming of the Danube (putting the Gabcikovo part into operation using the temporary solution known as Variant C) can be seen in details in Fig. 5. [2, 3]. On wells (numbering according to the Ground Water Consulting database) situated near the reservoir, Fig. 5 e.g. well No. 1998, an increase of ground water level was observed, while on wells situated near the by(pass channel, wells No. 1983 and 1977, a temporary decrease of ground water level was registered. This lasted until May, 1993, when the recharge system of water supply for the left side river branches was put into operation. The immediate impact on the water supply of the river arms in the floodplain area can be seen at well No. 1977 Fig. 5, which is situated between the Danube and by-pass (power) channel. A Similar situation is at well No. 1983, situated behind the by-pass channel.

To illustrate the general situation in ground water levels, map of average ground water level for interval from July 1993 to July 1994 have been plotted together with reference water level contours characterising the pre-dam conditions in 1992, Fig 6. Differences of these ground water levels, shown in Fig. 7, represent the impact of putting the Gabcikovo part of the project into operation. The major impact is the general increase of ground water levels in the upper part of the Zitny ostrov island area as well as on the right side of the Danube in the Slovak territory. Another limited impact is the decrease of the ground water level close to the Gabcikovo tail-race canal. A decrease of the ground water level is also seen in the area close to the Danube of the floodplain area, which is a result of the drainage effect of the old riverbed. A decrease in the northern part of the area is due to the dry years 1992, 1993 and 1994.

Measured changes in the ground water level in the main part of the floodplain area and in the whole Danubian Lowland region confirm the positive impact on the upper part of the area and an important positive role of the water supply for the Danube left side floodplain. It confirms that after completion of the water supply of the floodplain area close to the Gabcikovo tail-race canal and the small area at Dobrohost between the by-pass canal and the Danube, there will also be an positive change of previous development. Shallow underwater weirs in the old river bed of the Danube could also improve the situation along the Danube in the floodplain. In general, measured ground water levels have recovered to state, similar to those known 20 - 30 years ago, at the respective part of the territory. At the downstream confluence of the tailrace canal and the old Danube, no significant changes were measured.

Using a surface elevation map and ground water level maps for 1962, 1992, 1993/94 years, maps reflecting depths of ground water levels were constructed, Fig. 8, 9, 10. From the maps the areas with depths of ground water levels less than 0.5 m (areas with soil water logging possibilities) could be recognised. Another extreme situation is at places where the depth to ground water level is larger than 8 m. It can be seen, that the depth to ground water level is only exceptionally less than 0.5 m and this occurs only at the floodplain area where it is wanted and at the lower part of area of the Zitny ostrov island, which is not directly influenced by the Gabcikovo hydropower system of constructions.

In comparing the three maps, there is no doubt, that the ground water level have increased after damming the Danube and supplying the river branch system with water, to nearly the states which were known in the 1960's. This improved situation in comparison with pre-dam conditions is mainly near the reservoir downwards Bratislava and Petrzalka and generally in the upper part of the area.

The map defining the direct visible impact of the Gabcikovo part of the hydropower structures on the ground water level is based on previous maps and an interpretation of ground water level fluctuation was constructed and is presented in Fig. 11. In general this direct effect is positive and can be optimised by various technical means, e.g. mainly with shallow underwater weirs in the old Danube at some places, regulation of seepage canals, water supply and flooding the inundation area, etc.

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