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Samooczyszczanie wód w procesie tworzenia form deltowych

Rzętała M. A., Jaguś A., Rzętała M.

Rocznik Ochrona Środowiska

2013

Tom 15, cz. 3

2510--2525

The article concerns the self-purification of running waters in areas where they flow into water bodies. At such places, delta forms (alluvial fans, deltas) are created that are built up of the material accumulated by the watercourse. The study was conducted in the Upper Silesia region in southern Poland and concerned delta forms created in seven water bodies (Dzierżno Duże, Dzierżno Małe, Pogoria I, Pogoria III, Przeczyce, Kozłowa Góra, Pławniowice). The choice of the Upper Silesia region as the study area was dictated by the specific process whereby delta forms are created. In this region, which is among the most urbanised and industrialised in Poland, which results in most polluted surface waters. Running waters are heavily burdened with wastewater, by-products from processing minerals (particularly coal dust), technological water, sewage sludge, fertilisers, etc. The deposition of pollutants in river mouths leads to the rapid creation of delta forms with areas ranging from several hundred square metres to around 1 sq. km. The largest delta form has been created at the place where the Kłodnica River enters the Dzierżno Duże reservoir. During low water its area exceeds 1 sq. km and its thickness exceeds ten metres in some places. The overall volume of the Kłodnica River delta is estimated to be slightly over 2 million cu. m. Owing to the deposition that is taking place, running waters are subject to self-purification, particularly with respect to the solid particles they carry. This is a mechanical purification process. Apart from mineral substances, a significant amount of organic matter transported by the river is also deposited. The composition of delta sediments only partly reflects the characteristics of the substrate present in the catchments of the rivers that form the deltas in question, and the deposits often exhibit anthropogenic features. Delta sediments are a material that includes a large amount of organic matter, nutritional substances (mainly phosphorus compounds) and trace elements, including heavy metals. Concentrations of many elements exceed the levels characteristic of the natural geochemical background of sedimentary rocks. Therefore pollutants are immobilised, at least temporarily, during the process of creating the delta form, which protects the quality of the water in the water bodies. This purification effect is amplified by the vegetation overgrowing the deltas, which is dominated by common reed. This vegetation serves as a biological filter for waters that filter through it or remain stagnant, removing many pollutants by way of biological assimilation. Apart from their significance for water self-purification, deltas are important from the point of view of reducing the silting up of water bodies. Delta forms perform many functions in the natural environment as well. These include increasing the retention capacity of the surrounding area, shaping new habitats and increasing local biodiversity.