2949775643

38

H. Maruszczak, M. Wilgat

diagram s. These data only indirectly provide information about the potential content of municipal and industrial wastewater in river solute yield.

One can estimate the ąuantity of such wastewater comparing the data from our map of the solute yield from rural areas with the results of adeąuate calculations madę for the catchments including urban and industrial centres. Volume calculations of the total river solute yield from such catchments were madę using the simplified formula presented in the previous chapter. For calculations we chose twenty catchments of the Vistula river tributaries for which we could collect the results of measurements of the solute concentration and of discharges at gauging stations "closing" these catchments.

We managed to complete the aforementioned measurement data for the years 1976—1985 for almost all catchments. The results of calculations are presented in Table 1. Annual indices and mean values from ten years were calculated in two ways. Analysis of the results shows that the ten-year indices calculated by multiplication of mean solute concentration and mean discharge usually differ by about +8% to -2% from the sum of values calculated for particular months. Only in the case of strong pollution with minę wastewater in the Little Vistula river catchment (from its spring to the Przemsza river mouth) is this difference +15%. Of course, differences are bigger when such calculations are madę for particular years of the analysed period. The indices calculated for one year by multiplication of mean values differ from the monthly sums by about +15% to -10%; only in the case of the Little Vistula river catchment is it +34%.

Thus, the most radical differences were found in the Little Vistula river catchment. At the point closing this catchment, water is very strongly polluted with industrial wastewater. However, the level of pollution varied greatly here. In the years 1976-1980 it was significantly lower, but it morę than doubled in the years 1981—1985. Analysis of monthly indices of the solute concentration shows that in both periods it changed stepwise many times. These changes could be connected with building new mines in the Rybnik Coal Region. This assumption is supported by the fact that water of the Przemsza river — also very strongly polluted with minę wastewater — did not show greater changes in the solute concentration in the whole discussed period. The old mines in the Przemsza river catchment had a stabilized system of wastewater disposal. Therefore, two methods for calculations of annual indices of the solute yield from this catchment give only slightly different results (Table 1).

Therefore, for twenty selected catchments, calculations of the solute yield were madę in two ways. However, the results obtained in the simplified way are usually overstated. So, in the balance calculations presented in this paper we took into account the results obtained from summing up monthly values of solute yield.

Then the results of calculations of total solute yield from selected catchments, including municipal and industrial wastewater, had to be compared with the results typical only for rural areas. This necessitated