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Changes in chemism of the Łabuńka river..

they contain fewer Ca²⁺ions and are characterized by the lowest concen-trations of Cl"and SO ions (Table 2).

The results obtained for the upper Łabunka river in the section at Łabuńki, for the Topornica river in the sections at Zarzecze, Topornica and Zdanow, and for the Czarny Potok river at Sitno were the basis to determine mean indices for waters unpolluted with wastewater, being representative for the present hydrochemical background (Table 4). It should be noted that the sections close partial catchments (206 km² in all), in which surface waters are influenced by farming; 71.8% of the area is farmland (Table 5). On the basis of the results of studies in the years 1989-1991, waters recognized as hydrochemical background were characterized by an average solute concen-tration of 364 mg dm^-3; they were hard waters (6.22 mval dm'³) of weak-alkaline reaction (pH — 7.86). An analysis of the lowest values for dissolved oxygen in Czarny Potok and Topornica waters unpolluted by sewage (Table 3) indicates that they were not in critical situations as regards their oxygenation. This attests to their limited pollution with organie substances and relatively good reaeration. Mean oxygen content in waters of the hydrochemical background was about 9.0 mg dm^-3, and the BOD₅ index — 2.8 mg 0₂dm^-3; both oxygen concentration and the BOD₅ index were most often recorded in intervals specific for class I water ąuality (Table 3). Also relatively Iow were the concentrations of phosphates and ammonium nitrogen — 0.20 and 0.94 mg dm^-3, respectively.

TABLE 5. Land use in measurements	the Łabunka	river	catchment according		to the	author’s
Catchment	Tbtal		Arabie land	Meadows Forests		Others
Catchments unpolluted by wastewater*	km^2	205.8	128.0	19.7	49.6	8.5
	%	100.0	62.2	9.6	24.1	4.1
Łabunka river catchment	km^2	416.0	262.5	67.4	51.2	34.9
up-stream of the gauging station at Krzak **	%	100.0	63.1	16.2	12.3	8.4

’ Summarized values for the river catchments: Topornica up-stream of Żdanów, upper Łabuńka up-stream of Łabuńki village and Czarny Potok up-stream of Sitno.

Catchmer.t including the city area of Zamość with municipal- industrial wastewater sources.

In relation to the thus-determined indices characterizing the hydrochemical background of the catchment studied, the Łabunka river waters at Krzak show symptoms of considerable degradation. In the years 1989-1992, their solute concentration was higher by 29.9%, hardness higher by 12.4%, and pH lower by 4.7%. In waters polluted with wastewater a higher concentration of the analysed macrocomponents was also found (Table 4); being highest in the case of the following ions: Mg²⁺ (by 48.8%), Cl^- (by 118%) ar.d Na⁺ + K⁺ (by 217.6%). The oxygen and biogenic indices differed