[The evaluation of the nitrate content in river waters].

The present work gives the possibility to quality the waters of Maritsa, Iskŭr, Danube, Struma, Ogosta, Yantra and Tundzha rivers according to nitrate index, on the basis of monthly investigations for a period of 10 years, after the Unified national system. The rich content of the results and the sufficient period of time give grounds to calculate and draw the direction and rate of the tendency in the nitrate content changes, for this period, for the separate rivers--favourable for Struma, Iskŭr and Danube; with no changes for Ogosta and Yantra; pessimistic for Tundzha.