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Changes in chemism of the Łabuńka river..

catchment. Calculations madę by H. Maruszczak (1990) from the data of 1975-1980 for waters of the Bystrzyca (a basin of 1025 km²) up-stream and down-stream of Lublin showed that about 29% of the total solute outflow came from municipal and industrial wastewater. Corresponding data for the years 1981—1990 showed advancing degradation of the Bystrzyca river waters down--stream of Lublin; on average as much as 41% of the total outflow of solutes came from wastewater in that decade (H. Maruszczak et al. 1993).

The component coming from wastewater includes natural substances drawn from the lithosphere with groundwaters exploited for municipal and industrial needs, and "artificial" substances connected with economic actions. In the Łabuńka river catchment, water demands for municipal and economic needs are wholly satisfied by exploitation of Upper Cretaceous groundwaters. Due to that, the above component can be estimated from the volume indices of water carried away in the form of wastewater and from the average solute concentration of tap waters. In the years 1990—1991, the mean annual water volume discharged as wastewater into the Łabuńka river was 6.25 million m³, and the average solute concentration of tap waters was 380 mg dm^-3 in the 80’s. Thus about 2400 tons of dissolved substances came annually from groundwaters exploited by waterworks. The mean annual amount of solutes supplied by wastewater (3600 tons) reduced by the amount drawn from the lithosphere by means of wells (2400 tons) gives an "artificial" component (1200 tons). In the Łabuńka river catchment this magnitude constituted 8% of the total solute yields in 1990-1991. For the Bystrzyca river basin an analogous component was determined by H. Maruszczak (1990) at 12%, from data for the years 1975-1980.

The estimated index of the amount of municipal-industrial wastewater from Zamość is appropriate for the last years. Pollution of the Łabuńka river waters, particularly with sewage from Zamość, has changed with the development of this city. This can be illustrated by measurement data from 1959 published by K. Czyż et al. (1963); the number of inhabitants of Zamość was 29,000 in that year, and 63,100 in 1991. In 1959, the mean solute concentration of the Łabuńka waters in the yillage of Krzak (354 mg dm^-3), polluted with wastewater from Zamość, was on the level of today^ hydro-chemical background. Also on that level was the concentration of oxygen (9.2 mg dm^-3), ammonium nitrogen (0.80 mg dm^-3) and phosphates (0.22 mg dm^-3). However, the concentration of chlorides (14 mg dm^-3) and sulphates (18 mg dm^-3) was much lower than that characterizing the present hydrochemical background.

During the last few decades a big increase of symptoms of surface water degradation has occurred in the Łabuńka river catchment. This has been most distinctly documented by indices of chloride and sulphate ion concentrations, oxygen conditions and the pollution of waters with organie and biogenic compounds.