THE ORIGIN OF STRONTIUM AND THE STRONTIUM ISOTOPE BUDGET OF THE RIVERNILE

The river Nile has been used as an example to study the evolution of t he Sr content and the Sr isotope ratios in rivers in dependence on roc k types of the catchment areas, the mixing of the waters from tributar ies, the levelling out in a reservoir and evaporation losses. The Blue Nile, Sobat and Atbara rivers drain the Ethiopian Highlands, which co nsist partially of oceanic crustal rocks. Hence, the Sr content of the se rivers is comparatively high and the Sr isotope ratio exhibits an o ld oceanic crustal signature. The tributaries of the White Nile, in co ntrast, drain mainly supracrustal metamorphic rocks with higher Sr iso tope ratios. The seasonal change of the Sr contents in the Blue Nile a nd Atbari waters can be modelled using a factor deduced from published - experimental data for Sr contents in river waters at high and low s tage conditions. It is demonstrated that the Aswan High Dam reservoir levels out seasonal fluctuations of the Sr contents and Sr isotope rat ios. The rate of evaporation loss along the river was determined to be 0.038% per km. Contrary to the situation in highly industrialized urb an areas, there is no measurable effect on the Sr isotope ratio from w aste waters of Cairo.