VAAL RIVER

VAAL RIVER: Hydrology and geohydrology

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HYDROLOGY

Upper Vaal Area (Upstream of the Vaal Barrage at Vereeninging): The largest proportion (46%) of the surface flow in the area is contributed by the Vaal River upstream of Vaal Dam, together with its main tributary the Klip River. The Wilge River and the Liebenbergsvlei River contribute 36%, with the remaining 18% originating from the tributaries downstream of Vaal Dam. There are increases in runoff due to impermeable surfaces in urbanised areas. No significant afforestation occurs in the water management area. Numerous farm dams have also been built in the area, which negatively impact on the inflow to Vaal Dam. The natural mean annual runoff (MAR) of the area is 2423 million m³. (Ref.1.)

Middel Vaal Area (between Rietspruit River and Bloemhof Dam): The natural mean annual runoff (MAR) of the area is 888 million m³. (Ref.2.)

Lower Vaal Area (between the Bloemhof Dam and the Orange River): According to DWAF (2003a), "As a result of the low rainfall, flat topography and sandy soils over much of the water management area, little usable surface runoff is generated in the area. The runoff which does occur, is highly variable and intermittent." (Ref.3)

GEOHYDROLOGY

Upper Vaal Area (Upstream of the Vaal Barrage at Vereeninging): An important feature with regard to the groundwater resources of the area is the large dolomitic aquifers which extend across the north-west. Much of the water in the Mooi River, which is known for its strong base flow, originates as springflow from these aquifers. Large quantities of water are also abstracted through pumping for urban use (such as by Rand Water) and for irrigation. As a result of the direct connections between the dolomitic aquifers and surface streams, increases in groundwater abstraction will result in corresponding decreases in surface flow. Dewatering of the dolomitic compartment can also result in the formation of sink holes. Extensive de-watering of dolomitic compartments for mining purposes, has taken place in the north-west of the water management area where gold ore underlies dolomitic formations. This resulted in temporary increases in surface flow while water tables were being lowered. Reductions in surface flow which may last several years, will be experienced when mine pumping is stopped and the compartments are allowed to fill again.

The remainder of the area is mainly underlain by fractured rock aquifers, which are well utilised for rural domestic water supplies and stock watering. Although of specific importance in some areas, only 3% of the total water requirements in the water management area are supplied from groundwater.

The quality of groundwater is generally of a very high standard. Due to chemical reaction when groundwater infiltrates into mine caverns, poor quality water often results which can cause serious pollution when water decants or seeps from such mines. (Ref.1.)

Middel Vaal Area (between Rietspruit River and Bloemhof Dam): Large dolomitic aquifers occur in the northern part of the water management area. These extend from Stilfontein in a northern direction and across the area in the vicinity of Ventersdorp. The aquifers, which occur in different compartments, also underlay large parts of the Upper Vaal, Middle Vaal, and Crocodile (West) and Marico water management areas. The remainder of the area is mostly underlain by fractured rock aquifers, which are well utilised for rural water supplies and with little undeveloped potential remaining. Groundwater is essentially used for mining, agriculture and domestic use. (Ref.2.)

Lower Vaal Area (between the Bloemhof Dam and the Orange River): Groundwater utilisation is of major importance in the area and constitutes the only source of water over much of the area. Groundwater is essentially used for mining, agriculture and domestic use. Large dolomitic aquifers occur in the uppermost reaches of the Harts River. (Ref.3.)

References

Ref 1. Department of Water Affairs and Forestry, South Africa. 2004. Upper Vaal Water Management Area: Internal Strategic Perspective. Prepared by PDNA, WRP Consulting Engineers (Pty) Ltd, WMB and Kwezi-V3 on behalf of the Directorate: National Water Resource Planning. DWAF Report No P WMA 08/000/00/0304.

Ref 2. Department of Water Affairs and Forestry, South Africa, Middle Vaal Water Management Area: Internal Strategic Perspective, 2004. Prepared by PDNA, WRP Consulting Engineers (Pty) Ltd, WMB and Kwezi-V3 on behalf of the Directorate: Water Resource Planning. DWAF Report No P WMA 09/000/00/0304.

Ref 3. Department of Water Affairs and Forestry, South Africa. 2004. Lower Vaal Water Management Area: Internal Strategic Perspective. Prepared by PDNA, WRP Consulting Engineers (Pty) Ltd, WMB and Kwezi-V3 on behalf of the Directorate: National Water Resource Planning. DWAF Report No P WMA 10/000/00/0304.

DWAF (2003a). Department of Water Affairs and Forestry, Report No. P WMA 10/000/00/0203, Lower Vaal Water Management Area: Overview of Water Resources Availability and Utilisation.

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