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A. Świeca

much morę (Table 3). The Łabunka river waters at Krzak contained three times less oxygen and seven times morę organie substance; the values recorded most often did not meet the standards. In the Łabunka river waters at Krzak, the mean concentration of ammonium nitrogen (6.30 mg dm^-3) was 7 times higher than that characterizing the hydrochemical background, and that of phosphates (3.11 mg dm'³) even 12 times higher.

The oxygen and biogenic indices and ion concentrations of chloride, magnesium, sulphate, sodium and potassium account for the distinctly different character of the Łabunka river waters at Krzak, affected by waste-water.

THE CONTRIBUTION OF WASTEWATER TO THE RUNOFF OF SOLUTES FROM THE ŁABUŃKA RIVER CATCHMENT

An attempt was madę to determine the content of solutes from wastewater in a way similar to that presented by H. Maruszczak (1990). Calculations were performed for the gauging station section at Krzak from data for the

years 1990-1991.

It was assumed that the difference in total solute concentrations of waters polluted with wastewater down-stream of Zamość and unpolluted up-stream of this city is an indirect measure of the wastewater content in the outflow of solutes. In the catchment 416 km² in size up-stream of the gauging station at Krzak, the mean annual index of specific discharge was 2.4 dm³ s^-1 km^-2 in the years 1990—1991. The mean solute concentration of Łabuńka river waters polluted with wastewater was 474 mg dm^-3; the solute yield from this catchment was thus 35.9 tons km^-2 year^-1. About 15,000 tons of solutes flowed away through the gauging station at Krzak. The mean solute concentration of waters unpolluted with wastewater in 1990—1991, determined from the data for the Łabuńka river up-stream of Zamość and its tributaries, was 361 mg dm^-3. Accepting such a solute concentration as natural, representing the hydrochemical background for the whole Łabuńka river catchment, we can calculate the solute yield at 271 km^-2 year^-1 for the gauging station at Krzak, and the volume index of solutes at 11,400 tons. The difference in the volume of solutes from the catchment polluted with wastewater and estimated for the hydrochemical background is an indirect measure of the transport of solutes from wastewater. In the years 1990-1991 this value was 3600 tons, which constituted 24% of the total outflow of solutes.

For comparison, the results of similar calculations madę for other catchments can be given. For the Uherka river catchment (98.5 km²) polluted with wastewater from Chełm, a city of similar size (67,000 inhabitants in 1990), the content of solutes supplied with sewage was estimated at 18.3% from data for 1989-1990 (H. Maruszczak et al. 1993). The wastewater from Lublin, a much bigger city than Zamość (304,000 inhabitants in 1980, and 351,000 in 1990), constituted a considerably greater part of the total outflow of solutes in the Bystrzyca river