Water chemistry and quality of the White Nile at Khartoum

Abstract

Fortnightly measurements of physical and chemical variables were made on the White Nile
near Khartoum from May 2000 to May 2001. Variables measured were: temperature, pH, and
concentrations of dissolved oxygen, alkalinity, phosphate-phosphorus, nitrate-nitrogen, silica,
calcium, magnesium, sodium, potassium, iron, and oxidizable organic matter. The seasonal
variations of these factors in the White Nile are presented, and the interrelationships existing
between some of them and phytoplankton growth are discussed. Comparisons are made with
earlier studies on the same river and with a few tropical rivers. In the White Nile at
Khartoum, changes in nutrients are largely dependent upon the flood regime, partly derived
from the Ethiopian Plateau through the Sobat River and partly from utilization by seasonal
phytoplankton in the upstream Jebel Aulia reservoir. Nitrate, phosphate, silicon, and iron
increased considerably at Khartoum during the rainy season (July-September) when the
maximum concentrations of 388 µg NO3-N L -1, 306 µg PO4-P L-1, 12.5 mg Si L-1 and
0.52 mg Fe L-1 occurred therein. The concentrations of these nutrients, except possibly
nitrate, are not expected to limit phytoplankton growth in the White Nile at Khartoum
although appreciable depletion occurred during periods of maximum development of diatoms
and Cyanobacteria. Fall in silica concentrations, unlike falls in concentrations of nitrate,
phosphate and iron, was always followed by a rapid restoration of a higher level. The
seasonal variations of calcium and magnesium were irregular and without any obvious
seasonal pattern. The White Nile at Khartoum is far from being polluted by heavy metals; no
cadmium, lead, or nickel was detected in the surface waters.