Przemiany geosystemów zbiorników wodnych powstałych w nieckach osiadania na Wyżynie Katowickiej

Abstract

The group of water reservoirs filling subsidence depressions is unquestionably result of interaction between human and the environment. These reservoirs undoubtedly are an unintentional effect of human activity, carried out in the Katowice Upland. They originated, are formed and still will appear irrespective of human being. Spatial and temporal variability of analysed reservoir occurrence directly results from the intensity of subsiding process, and the complexity of geological-mining factors in the area described causes, that terrain deformations, induced by deep mining, appear during mineral resource exploitation and after its finishing. The tendency to close mines, observed not long before now, will influence on the rate reduction of reservoir origination only inconsiderably, because subsiding can be observed even after tens of years after output stopping. The process of reservoir forming will be finished not before the complete stabilisation of rock mass. Owing to these reasons the described water reservoirs are element of landscape, which is characterised by large dynamics of changes in their number and occupied area as well. Considering shape, the reservoirs in subsidence depressions usually refer to forming subsidence depression, therefore the outline of shoreline often assumes an oval shape. But in respect of the presence of numerous terrain barriers (e.g. railway and road embankments) and human interference in newly originating reservoirs, and especially objects, which function in the environment for a certain time, their appearance becomes notably modified. In the area of the Katowice Upland very small and small reservoirs predominate because their range is permanently limiting in an essential way. The decided majority of them possesses the unitary area, which does not exceed 10 ha, but only not numerous objects are characterised by larger size. In the case of their depth the situation is similar, these depths are small and usually shaped at the level of 2—3 meters, more rarely reaching greater values in the deepest places. Not great area and capacity of described water reservoirs influences the limitation of shore processes development considerably. There are, admittedly, changes of different kind in morphology of shore zone but originated landforms have decidedly more accumulation than abrasion character. Simultaneously, their number is small and little varied. Among natural factors modelling shores of these reservoirs the largest importance is played by wind waving, but then human activity causes the most visible changes in their morphology. In described water reservoirs flat shores predominate. They are almost usually overgrown by vegetation, and these shores have a natural character simultaneously. Large contribution is also typical for anthropogenic shores, which are shaped by human being. They are high shores. Within analysed water reservoirs in subsidence depressions natural high shores were not observed, they are also devoid of anthropogenic flat shores. Water reservoirs in subsidence depressions located in the Katowice Upland are characterised by very large pollution of bottom deposits by heavy metals. Among five analysed elements especially zinc, lead and cadmium exceeded many times the level of accepted geochemical background. It especially refers to reservoirs located in the terrain of natural-landscape complex „Żabie Doły” (“Frog pits”). Among them one should mention reservoir No. 6, in case of which maximum zinc content amounted to 5 404.43 mg/kg, whereas average lead concentration was shaped at the level 1 335.43 mg/kg, and cadmium was present in amount of 41.68 mg/kg. In some reservoirs overstandard copper content also pointed at the contamination of bottom deposits. This phenomenon also refers to many other artificial water reservoirs located in the area of Katowice Upland. Thereby, it confirms varied influence of human impact, which decides of pollution degree of bottom deposits. Bottom deposits prove ecological changes occurring in the neighbourhood of water reservoirs as geosystems. Functioning of water reservoirs in subsidence depressions in the Katowice Upland under conditions of varied anthropopression decides also of different physicochemical properties of stored waters. But for the majority of them significant pollution, especially by biogenic compounds (phosphates and nitrates), is observed. The last-mentioned compounds are responsible for eutrophication of these geosystems. In reservoirs No. 1 and 2 in the period 2003—2005 the presence of phosphates at the level above 1 mg PO4 3−/dm3 was stated many times, and in the case of reservoir No. 5 phosphate concentration exceeded 2 mg PO4 3−/dm3 with maximum amounting to 2.444 mg PO4 3−/dm3. In described water reservoirs very large nitrate amounts occurred, they often exceeded the level of 100 mg NO3−/dm3. These compounds in such amounts were especially typical for reservoirs No. 3, 4, 6 and 8, whereas absolute maximum values referred to reservoir No. 2 situated in Sosnowiec, in which sporadically present nitrates were observed at the level of 340 mg NO3−/dm3. Large amounts of bioelements in the described geosystems cause systematic growth in water fertility, intensifying biological production in a form of increase in algae biomass. Among disadvantageous signs of eutrophication in the described reservoirs are also numbered as follows: decrease in water transparency, overoxygenation of surface water layers at simultaneous lack of oxygen at the bottom and thereby threat to life of aerobic organisms, worsening of light conditions, distinct increase in water pH-reaction and intensive overgrowing of reservoirs. High degree of limnic water salinity and the presence of heavy metals betoken also degradation of the majority of characterised lake geosystems. Especially chlorides occurred in above-natural amounts, what peculiarly refers to reservoirs No.: 2, 3, 4, 6 and 8, in which their amount usually exceeded 200 mg Cl−/dm3. These compounds rather often reached the level of 1 000 mg Cl−/dm3, and absolute maximum referred to the reservoir No. 8, in which chlorides were measured in amount of 1 580 mg Cl−/dm3. In a case of sulphates the situation looks similarly, the largest amounts occurred in reservoir No. 5, with maximum amounting to 1 450 mg SO4 3−/dm3. That this why the mentioned parameters were usually numbered among the lowest quality classes of surface waters, and noted concentrations of heavy metals, such as zinc, exceeded the border values of parameters of water quality referring to good and higher than good ecological state of surface waters. But bad qualitative state of waters does not refer to all described reservoirs. Reservoirs, which undergo moderate influence of human impact, have waters of relatively good quality, although in these cases the periodical worsening in physicochemical properties also happens. Water reservoirs in subsidence depressions in an essential way modify the local water cycle. They undoubtedly are hydrological objects making new link, thanks to which the changes in direction of water migration in the area of Katowice Upland happen. On the one hand, subsiding processes appearing at the terrain surface cause calculable material wastes, but on the other hand — functioning of the described reservoirs, especially at degraded parts of the Katowice Upland, can influence the improvement of landscape aesthetics of these terrains. Water reservoirs, together with their nearest neighbourhood, fulfil also very important natural functions, making the habitat for existence and reproduction for many animal species and causing the increase in biological diversity. The more so because, this genetic group of anthropogenic water reservoirs possesses completely shaped connections with particular components of the natural environment since the beginning of bowl forming. Processes occurring here have a natural character. Reservoirs in subsidence depressions are treated as the mining waste and according to the idea of legal rules they should be reclaimed through the restoring former state. But numerous investigations carried out within this range prove, that in many well-founded cases the best effects are obtained by land reclamation in water direction. The final effect of such activities is total naturalisation of described reservoirs, which in the end fulfil the same function as natural lakes. In the Katowice Upland many reservoirs in subsidence depressions occur, which as left to themselves undergo spontaneous regeneration and currently they make a valuable element of this terrain landscape. Therefore one should tend to preserve such reservoirs and also to restore the following water-mud biotopes in post-exploitation terrains of the Katowice Upland.