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A. Świeca

The river waters of the area studied largely come from underground supply. According to Z. Michalczyk (1986), the underground supply of the Łabunka river amounts to 72% at the gauging station in Krzak.

DIFFERENCES IN CHEMISM OF THE RIVER WATERS

On average about 6.5 million m³ of municipal and industrial wastewater coming largely from Zamość (63.1 thousand inhabitants in 1991) were discharged annually into surface waters in the Łabuńka river catchment up-stream of the gauging station at Krzak in the years 1989-1992. This constituted 18% of the mean annual river discharge of those years. Only the Topornica river waters were not polluted with wastewater; a smali amount of wastewater (0.2% of the mean annual discharge) got into the Czarny Potok river waters; as much as 99.8% of wastewater was discharged directly into the Łabuńka river, of which 98.4% came from the Company for Water Supply and Treatment in Zamość. The degree of sewage purification was unsatisfactory; 98.1% was treated only mechanically, i.e. by removing largely insoluble impurities. Only 0.9% of wastewater, was treated in a biological-mechanical system better reducing impurities; 1.0% was carried away in crude form.

The results of the studies allowed the author to follow chemism changes in Łabuńka river waters and the degree of pollution at the distance from the Łabuńki section (0.1 km of its course) to Krzak (4.2 km), i.e. along a distance of 25.9 km (75.2% of the river length), as well as for its two tributaries (Topornica and Czarny Potok). Histograms of the distribution of physico-chemical indices (Fig. 2), characteristic values (Table 1 and 2) and selected oxygen and biogenic indices of river waters (Table 3) allowed me to determine not only the differentiation of the studied catchment with regard to its bedrock lithology, but also the changes caused by economic actions.

On the basis of the results it can be seen that surface waters in the Łabuńka river catchment are characterized by a relatively high concentration of solutes. The mean annual solute concentration, however, varies considerably from 322 to 473 mg dm^-3 (Table 1). "Pure" waters, i.e. unpolluted with municipal and industrial wastewater, show a differentiation controlled by lithology of the bedrock. Higher indices for solute concentration (419-443 mg dm^-3) were found in waters drained off from bogs. According to the results of analyses in river sections at Łabuńki and Sitno (Table 1) the waters can be ąualified as hard and weak-alkaline; they are distinguished by the highest concentration of Ca²⁺ ions, and relatively high Mg²⁺ and SO ²“ concentrations (Table 2). A lower solute concentration (322-337 mg dm^-3) was recorded in catchments with a loess cover. The waters in the sections at Zarzecze, Topornica and Zdanów showed much lower hardness (Table 1) characteristic for medium hard ones; they also belong to the weak-alkaline group but with slightly higher indices;