Abstract
M.Sc.
The Senqu-Orange basin is the principal carrier of the mean annual runoff for the larger
portion of South Africa, encompassing also Lesotho. The transformation of this river
regime by inter-basin transfer from south-westerly flowing rivers to the northerly flowing
Senqu-Orange system of water as a resource may have adverse impacts on freshwater
quality of ecosystems.
This report presents a case study of the Lesotho Highlands Water Project (Phase 1),
investigating alterations of physical and chemical water quality caused by the three types
of large infrastructure: a dam (Mohale Dam); a dam and weir (Katse Dam and Matsoku
Weir); and a delivery tunnel (from ’Muela Dam to the Ash River Outfall). The aim of the
study was to determine significant differences, if any, of selected water quality variables in
water released or transported from the infrastructure, using water from an undisturbed
sampling site located on the Senqu River as a reference. Analyses are carried out of longterm
trends and variations of the following selected water quality variables: pH,
conductivity, total nitrogen, magnesium and sodium. Sampling station sites, operated by
South Africa Department of Water Affairs and Forestry from 1990 to 2003, are located at
the Malibamatso, Senqunyane, Matsoku, Senqu and Ash rivers. Time series graphs were
used for analysing long-term trends. ANOVA or the H-test and Scheffé multiple
comparison statistical test methods were used to determine significant differences.
Total suspended solids (TSS) and turbidity measurements, together with discharge rates,
were made available from a study by the Lesotho Highlands Development Authority from
January 1998 to September 1999, for two sites downstream of Matsoku Weir and Mohale
Dam. Short-term temporal water quality effects due to dam discharges were assessed.
pH values are uniform, and comply to target water quality range (TWQR) guidelines of
the Department of Water Affairs and Forestry. There are no long-term trends for total
nitrogen, conductivity, magnesium and sodium. There are records of intermittent episodes
of total nitrogen exceeding TWQR, caused possibly by anoxic conditions of the artificial
impoundments associated with the three types of large infrastructure. Total suspended
solids were affected by discharge rates - concentrations increased with increasing flow
ii
discharge of surface water. Turbidity values increased in association with enhanced TSS
concentrations, although this is not a causal relationship.
From statistical evaluations, significant variations in pH were observed between each test
site (dam, dam and weir, and delivery tunnel) and the reference site. The group means in
pH at each test site were lower than the reference site. Scheffé test results indicted
significant difference in total nitrogen between the dam, the delivery tunnel and the
reference site, but not for the dam and weir in relation to the reference site. Similarly, mean
concentrations of total nitrogen were lowest at delivery tunnel and the dam. Although,
significant differences were observed for conductivity, sodium and magnesium between
the dam and weir, and the reference location, such variations are attributable to the Senqu
River valley and not the infrastructure of the transfer scheme. Thus, results from the
assessment of significant differences in conductivity, sodium and magnesium at the dam
and delivery tunnel were unreliable and discarded.