Gold tailings as a source of waterborne uranium contamination of streams - the Koekemoerspruit (Klerksdorp goldfield, South Africa) as a case study - part II of III: dynamics of groundwater-stream interactions

Abstract

Dissolved uranium (U) from the tailings deposits of various gold mines in South Africa has been found to migrate via seepage and groundwater into adjacent streams. The extent of the associated non-point pollution depends on the concentration of U in the groundwater as well as the volume and rate of groundwater seepage into the stream channel. Whilst the U concentration in groundwater is relatively constant and comparatively easy to determine, the same is not true for the flux of groundwater into the stream. In order to track the water exchange at the groundwater-stream interface, real-time in situ measurements by data-logger controlled probes for gauging heights and electrical conductivity (EC) were taken at 10 min intervals. As a result of a steep hydraulic gradient between water-saturated tailings deposits and the receiving watercourse, exfiltration (base-flow) of contaminated groundwater generally dominates. However, short-term inversions of the flow direction (infiltration of stream water into the groundwater) were also observed. These are attributed to an artificial flow regime of the Koekemoerspruit, which results from a pumping scheme that discharges groundwater from underground mine workings into the stream. Differences in pumping rates lead to pronounced diurnal fluctuations of gauging heights in the stream, which in turn cause even higher fluctuations of the associated groundwater table. The hydraulic mechanisms of the stream-groundwater interaction, as well as implications for the aqueous transport of U are discussed.


WaterSA Vol.30 (2) 2004: 227-232