Science & Research
Slovak Water Management Enterprise

Changes of Water Quality on the Bottom of Water Reservoirs

The Hron river basin operates in the Slovak Ore Mountains three water reservoirs (WR). They were built in the rivers Slatina (Hrinova WR), Klenovska Rimava (Klenovec WR), and Ipel (Malinec WR).

The surface water, by being caught in the WR, changes its character and the running water is changed to still water. Such change is reflected on the surface of WR especially by increased eutrophization of water, higher saturation of water by oxygen, higher water temperature, and in spring months, also by higher pH.

Going deeper, the water temperature, pH, and number of organisms of bioseston decrease, and the volume of solved oxygen in water is reduced. The volume of solved O2 on the bottom of the WR is regularly changed during the year. The highest volume is in April, and the lowest volume, depending on the size of the reservoir, altitude, and the volume of water flowing to the reservoir, in the months of September – November.

The deficit of solved O2 is closely related to occurrence of ammonia ions on the bottom of the reservoir, which result from decomposition of most organic substances containing nitrogen. If the volume of solved O2 is sufficient, the final stage of decomposition of organic nitrogen substances are nitrates, and a small volume of ammonia ions. If, in some year, the volume of nitrates increases in respect of the previous year, the volume of ammonia ions is reduced in the same year, and vice versa.

Two maximums may be seen during a year also in respect of concentrations of iron. A difference is only that the autumn maximum is much higher than the spring one. The volume of iron on the bottom of the reservoir greatly depends on the volume of solved O2. The mutual relation between concentrations of iron and the volume of the solved O2 in water is remarkable.

The volume of iron on the bottom of the reservoir is increased mostly by that the trivalent iron hydroxide which sedimented, because of sufficient volume of oxygen, in precipitated form, in the bottom of the reservoir, is changed due to insufficient oxygen into soluble bivalent acid ferrous carbonate which colours the water near the bottom of the reservoir into reddish brown.

Therefore, mainly for the months of August – October it is typical that the water flowing from the reservoir is reddish brown. Several year monitoring revealed that the water is coloured if the concentration of iron in the water on the bottom of the WR is approx. between 0.300 and 0.400 mg.l-1.

Smaller maximum of iron concentrations in the water on the bottom of the WR may be seen also in the months of February – March, when the volume of solved O2 is reduced, and the volume of iron on the bottom of the WR is relatively increased. Also during this maximum in some cases, concentrations of iron may be several times higher, and the water has then reddish brown colour.

In winter, the volume of solved O2 in the reservoir is probably much affected by that the water in the reservoir is frozen. The ice created in the reservoir prevents waves of water, overoxidation thereof, and movement of water in vertical direction.

The inflowing water, at the outfall to the WR, creates layers, and from such layered water, then only heavier particles with more nutrients drop down deeper. The lighter parts from the fresh flowing water cannot usually reach the bottom of the reservoir, due to stagnation and layers of water during winter months. It is concentrated on the surface of the reservoir in close contact with the ice cover. The result of such distribution of water in the reservoir is, in some years, low unit conductivity on the surface of the reservoir and increased volume of nitrites, of consumption degree No. 2.

The most part of the year, in all three water reservoirs, the value of unit conductivity is balanced, and it fluctuates only in winter months. Clearly lowest unit conductivity is in the month of March.

An accompanying element of iron in the nature is manganese. In the WR, it represents approx. one half of the volume of iron. As compared with iron, the manganese compounds are worse precipitated, but better solved. In the individual months, manganese in the WR usually copies the increase and decrease of concentrations of iron.

The only exception is the WR in Malinec, where during the months of June – December, the volume of manganese exceeds the iron. This phenomenon is probably caused by the size of the reservoir, long and insufficient mixing of water in the reservoir, low volume of the solved O2 in the bottom of the reservoir, and relatively small inflow of water to the reservoir in summer months. In this period of the year, the estimated duration of exchange of water in the Malinec WR is the same as in the Hrinova WR or Klenovec WR.
In: Water Management Journal No. 12/2003, p. 9

Information and Contact:

Mgr. Bruno Brvenik et col.
Slovak Water Management Enterprise
Radnicna 8, 969 39 Banska Stiavnica
OZ Banska Bystrica
Tel: 00421 45 41429

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