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47

Structure of the solute yield in the Vistula..

it decreased rather than increased, because the downstream tributaries of the Vistula river drained weakly-industrialized areas. So, to calculate the balance for the 30’s we assumed that the mean index of solute concentration in the Vistula river mouth was 240 mg dm^-3. In the years 1976-1985 this index was 440 mg dm^-3 on average, i.e. higher by 83%. Assuming that the mean discharge of the Vistula river in the 30’s was like that of many years that we accepted in our work, we calculated the total solute yield in 1934 at 35.5 tkm^-2y^-1. In it, Chemical denudation probably constituted about 27-28 t km^-2y^-1, the component from precipitation — 2-3 t km^_2y^_1, and all other components — about 5 t km^-2y^{1 (3)}. The component from precipitation was then almost entirely of natural origin, and other components were partially also natural (from groundwaters pumped by man). Therefore, "synthetic" substances probably constituted 3-4 t km^_2y^_1, i.e. about 10% of the total solute yield.

The estimated balance of the total solute yield for the 30’s presented in this paper allows us to suggest that the Chemical denudation of the lithosphere was at that time lower than now. The index for the present Chemical denudation is about 10% higher. This difference may resultfrom insufficient data for deductive determination of the indices for 1934. However, it seems ąuite logical that it is the conseąuence of "acid rain", which have resulted in a considerable increase in the aggressiveness of waters infiltrating the lithosphere.

FINAŁ REMARKS

1.    The structure of river solute yield from rural areas can be used as a numerical index of the natural characteristics of geosystems typieal for the particular landscape belts in the Vistula river basin. It is differentiated in relation to hydroclimatic conditions, geological structure and relief.

2.    At present the indices for the total solute yield in the Vistula river basin are not a good measure of the intensity of Chemical denudation of the lithosphere, even in typical rural areas. In a large part of this basin, in morę industrialized areas, these indices are rather a measure of degradation and pollution of the environment.

3.    As a morę direct measure of environmental pollution we can assume that part of the total solute yield which consists of "synthetic" substances.

n

The component coming from precipitation can be estimated on the basis of the results of analyses from the 60’s (Chojnacki 1967). They reveal that the total solute concentration of precipitation in unpolluted rural areas

o

was about 15 mg dm . Accepting this value as average for the whole basin of the Vistula river in the 30’s, we

_n _ i

calculate that this component amounts to 2.21 km y . The percentage of the component coming from municipal and industrial wastewater can also be estimated only indirectly on the basis of very scarce data illustrating the

_    n_

content of Cl and SO in river waters in the Vistula river basin, compiled by M. Stangenberg(1958). The content of Cl~ in unpolluted rivers in the 30’s was below 10 mg dm , and in the middle part of the Vistula river about

_0    n_    *)    q

15 mg dm . The contents of SO were below 20 mg dm and about 30 mg dm , respectively. Therefore, about

_ _t    2    2_ _t    o

7 mg Cl in 1 dm and about 15 mg SO in 1 dm can be attributed to pollution. Together with other components it constituted about 10% of the total solute yield, i.e. 3—4 t km^-2 y^-1. The component coming from agricultural Chemicals (largely soil liming) can be estimated at 1-2 t km^-2 y^-1.