HYDROCHEMISTRY OF THE UPPER ORANGE RIVER CATCHMENT

SUMMARY

A four year pre-impoundment study of the catchment of the Hendrik Verwoerd Dam was conducted. The Orange River was the major source of dissolved chemicals and suspended sediments, while the Caledon River contributed 24 and 30% respectively, and the Kraai River 9 and 8% respectively. The upper reaches of the Orange River catchment contributed mainly to the dissolved chemical and sediment loads, in contrast to the minimal contribution of the lower reaches. This was in spite of the relatively stable geological formations in the upper reaches and the erodable formations in the lower reaches. Dry-wet seasonal variations occurred in nutrient and sediment concentrations and loads, except for the Bell River where constant values were measured. The water was characterized by magnesium and bicarbonate dominance. Adsorption by the suspended sediments was dominated by calcium (potassium in the Bell River) and zinc among the trace elements. Due to its low salinity and sodium adsorption ratio, the water was suitable for irrigation. High magnesium levels made it, however, less attractive for irrigational purposes.

The phosphorus loading rate at the inflow of the Hendrik Verwoerd Dam was only 0,55 g P m⁻² a⁻¹, which was well below the 1,39 g P m⁻² a⁻¹ critical rate for eutrophic conditions, applying the Vollenweider Model. Therefore, this impoundment was classified as oligo-mesotrophic.

KEY WORDS:

• Orange River
• dissolved chemical loads
• sediment loads
• geochemistry
• irrigation potential
• nutrient adsorption
• trophic status