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Hydrological conseąuences of human action..

The thickness of Quaternary formations in the Łęczna—Włodawa Lakę Region fluctuates from less than 1 to 80 m (Buraczyński 1983; Henkiel 1983). Cretaceous beds emerge on the surface in the form of gently-shaped elevations, which do not exceed 20 m. Where Cretaceous rock is exposed from overlying deposits, numerous karst dolines are found on its surface. The Łęczna-Włodawa Lakę Region and the neighbouring Dorohusk Depression differ from subregions of western Polesie in such karst forms as funnel-shaped and flat-bottomed dolines occurring densely in Cretaceous hills and denudation plains.

Fine-grained deposits — silty sands, muds and clays — play a large part in the structure of accumulation plains. Their shallow occurrence strongly reduces water infiltration into deeper layers and favours the formation of swamps and wetlands.

Within the lower accumulation plain, the groundwater table occurs just below or at the land surface. In the area of the higher accumulation plain precipitation waters easily percolate through the layer of sands and are intercepted by less permeable deposits at a depth of several metres. The table of these waters occurs mainly at a depth of 2-5 m. Groundwaters of both accumulation plains form one table accordant with the lakę levels. It also extends to the area of denudation plains and Cretaceous hills, where groundwater depths even exceed 10 m.

Thus in the whole Lakę Region the depth of the groundwater table fluctuates from zero to about 8-10 m. High permeability of sands facilitates considerable seasonal oscillations of the groundwater table, which is reflected by the varying rangę of wetland areas. A few decades ago many areas were still inaccessible during spring thaw and some villages were isolated.

Older Pleistocene deposits of various lithology and permeability have been preserved under muddy-sandy deposits on which waters of the first Quaternary horizon are intercepted. Waters occurring in the former formations are most often associated with water-bearing Cretaceous horizon.

Quaternary deposits cover a series of Upper Cretaceous rocks, the thickness of which is from 300 m in the Bug river region to 550 m on the western peripheries in the Tyśmienica river valley (Krassowska, Niemczycka 1984). Marły chalk of the Lower Maestrichtian occurs along the eastern Bug river region; in the remaining area rocks of the Upper Maestrichtian are found, largely chalk and marls with interbeddings of harder opokas.

In the top part, Upper Cretaceous rocks are shattered, forming a dense network of weathering fissures, whereas joint fissures occur deeper. Fissures conducting water easily are found only in opokas and harder marls. In soft marls and chalk the fissures are easily constricted and stopped, which makes groundwater movement difficult; its rates are only a few metres per 24 hrs. With increasing depth, the number of open fissures decreases; at a depth of 120-170 m Cretaceous rocks are practically impermeable (Różkowski, Rudzińska 1982; Conditions... 1989).

The zonę of free water exchange thus comprises Quaternary formations, locally-preserved residues of Tertiary formations of no hydrological