Water quality assessment of rivers in Mongolia.

A study was made on the water quality of the rivers in Mongolia over the years 1949-1990 for the three river basins in the country. In most of the 72 monitoring stations, the water quality is governed by geological and hydrological characteristics; bicarbonate and calcium are the most abundant major ions. No significant trends were observed between river discharges and total dissolved solids contents. The sediment yields (expressed in the run-off of total suspended solids, TSS) were always low, viz. between 0.02 and 20 x 10(3) kg TSS/km2 yr. In general, the rivers in Mongolia showed low pollution levels for organic pollution, phosphorus, nitrogen, etc., and no time trends were observed in this respect. This does not hold, however, for the northern Tul River, where especially high BOD and ammonium values were found. This can probably be ascribed to the presence of industries and larger cities, as well as agricultural practices. The rivers in Mongolia can generally be classified as "clean" or "of acceptable quality". In contrast, the Tuul River was shown to be "moderately polluted", with a deteriorating trend in water quality over recent years.