Evapotranspiration from mine-affected riparian sites along the Vaal River in central South Africa

Abstract

Long-term gold mining within the Witwatersrand Basin Goldfields has led to the creation of over 200 tailings storage facilities (TSFs) releasing acid mine drainage. Plumes of contaminated shallow groundwater occur downslope of these TSFs. One such area lies adjacent to the Vaal River east of the town of Orkney. Natural vegetation in the riparian soils comprises a mixture of grasslands, reeds and woodlands dominated by Vachellia karroo. The Mine Woodlands Project initiated by Witwatersrand University and AngloGold Ashanti is exploring the use of engineered woodlands for hydraulic control of plumes and immobilization of contaminants to protect the wider hydrological resource. To estimate the net increase in evapotranspiration (ET) resulting from various remedial woodlands, ET from existing vegetation types requires quantification. Growing season patterns of ET were measured and modelled in riparian grassland and two V. karroo woodland sites close to the Vaal River. Two independent estimates of annual ET from these woodland sites (674 and 787 mm) were similar to that from grassland (697 mm) and much less than the 1100–1200 mm reported from some forms of engineered evergreen woodlands established in the vicinity. We conclude that the Vachellia woodlands have low potential for hydraulic control of contaminant plumes.

Keywords:

• Witwatersrand Basin Goldfields
• acid mine drainage
• Vachellia karroo
• riparian sites
• evapotranspiration
• sap flow

Acknowledgements

Anglogold Ashanti are thanked for permission to work at the sites, their funding commitment over numerous years and their encouragement and field support. The South African Department of Trade and Industry and the National Research Foundation are also gratefully acknowledged for their support of the project. Invaluable help was received from Ben Oageng, Jacob Mashlangu and Lucky Sibambo. Soils descriptions were provided by Bruce McLeroth (Red Earth). The South African Weather Services are gratefully acknowledged for supplying climatic data. Michelle Dye kindly assisted in designing Figure 1.