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Stefan-Andrei Szobotka, National Institute of Marine Geology and Geo-ecology (Romania)
Valentin Boian, University of Bucharest, Faculty of Geology and Geophysics (Romania)
Gheorghe-Viorel Ungureanu, University of Bucharest, Faculty of Geology and Geophysics (Romania)
Ion Stanescu , National Institute of Marine Geology and Geo-ecology (Romania)
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Babeni reservoir, built in 1976, is situated on the Olt River, downstream the city of Ramnicu Valcea - Romania. Besides Olt River sediments, sand and finer sediments are transported and settled into the reservoir by two minor rivers: Bistrita and Topolog. Both these tributaries have built up deltas prograding into the reservoir. The environmental state of the entire area is heavily impacted by the chemical industrial platform that is situated upstream.
Emerged surfaces cover today 13% from the total area of the reservoir as a direct consequence of river sediment accumulation and shallow water depths. About one third of these areas are found at the upstream end of the reservoir, the remaining corresponding to the two mentioned deltas. Both Bistrita and Topolog deltas are rapidly prograding to the center of the reservoir. The first one has a smaller volume but because of the shallower initial topography created an extended emerged area. Topolog delta, mostly submerged, is the most prominent newly formed sedimentary body.
The morphological analysis of the present day reservoir bottom elevation in comparison with the initial topographical surface has shown a sharp modification of the statistic distribution of areas cover by different elevation intervals. Before the dam building the distribution of these surfaces was Gaussian. After 30 years of sediment deposition the distribution has shifted towards a predominance of higher altitudes.
The newly deposited sediment thickness is 1-3 m on 41% from the reservoir surface, with an average of 2.5 m. These values correspond to sedimentation rates of 3-10 cm/year with an average of 8 cm/year. The total volume of sediment deposited in the reservoir is 22.834 millions cubic meters.
The bottom sediment grain size is controlled by sedimentation processes and by the position of sediment depocenters. Most of the sand fraction is sedimented at the upper end of the reservoir. Deltaic sediments deposited by Topolog and Bistrita tributaries are mostly silty. Clays tend to accumulate in the lower part of the reservoir, near the dam.
The Olt River fine sediments were heavily contaminated with mercury by the effluents from the chlor-alkali plant situated on the industrial platform. As a consequence bottom sediments show high contents of this metal, up to 8 mg/kg. Sediments transported by the two tributaries are clean and dilute the polluted ones. Hydraulical flushing is seen by engineers as the best option to evacuate downstream the reservoir sediments in order to counterbalance the water volume decrease and to increase the life span of the reservoir. This is not the solution in the case of Babeni. Flushing will cause contamination with mercury of the reservoirs situated downstream. Cleaning the bottom sediments is an expensive and dificult task. Much easier would be to stop the mercury spill through industrial effluents and let the clean sediments of Bistrita and Topolog Rivers to seal the contaminants.
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