Elsevier

Journal of Hydrology

Volume 287, Issues 1–4, 25 February 2004, Pages 178-196
Journal of Hydrology

The significance of groundwater–stream interactions and fluctuating stream chemistry on waterborne uranium contamination of streams—a case study from a gold mining site in South Africa

https://doi.org/10.1016/j.jhydrol.2003.10.004Get rights and content

Abstract

Through seepage, dissolved uranium and other heavy metals migrate from tailings deposits of gold mines via groundwater into adjacent fluvial systems. The extent of associated stream contamination is determined, inter alia, by the retardation of dissolved contaminants along the pathway and the rate in which polluted groundwater enters the stream channel. Comparing several sediment-water systems of the aqueous pathway significantly higher immobilisation of U was found in (fast-flowing) surface water systems such as the stream than in (slow moving) alluvial groundwater of the floodplain. Mainly triggered by redox-initiated co-precipitation bottom sediments in streams act as geochemical barrier and long-term sink for U and other heavy metals from polluted groundwater. Real-time in situ measurements of hydraulic interactions between contaminated groundwater and streamwater suggest a highly dynamic water exchange between both water bodies, including daily inversions of the direction of flow in certain times of the year. This results in distinct diurnal differences of the associated stream contamination. The extent of subsequent downstream transport of U within the fluvial system is largely determined by pronounced diurnal oscillations of pH and redox potential in the stream, affecting U-speciation as well as adsorption and precipitation rates. In addition event-triggered fluctuations of both parameter impact on the fluvial transport of U.

Introduction

Uranium is a radioactive heavy metal with an average natural background concentration of 2–4 mg/kg (ppm). Auriferous sediments of the Witwatersrand basin (South Africa), do not only contain gold (that has been mined for more than 100 years), but also elevated concentrations of U—some 100 ppm in average, reaching up to 1000 ppm in places. Compared to ore with U-grades of 3000–60,000 ppm (0.3–6%) mined in Canada and Australia, this is regarded as low-grade ore (McLean, 1994, Cole, 1998). Therefore, in South Africa uranium was mainly produced as a by-product of gold, which subsidised the mining costs. Between 1952 (when the first regular U-recovery plant was commissioned) and 1991, a total of approximately 170,000 t of U3O8 was produced and sold (Ford, 1993, Wymer, 2001). After a peak in production in 1980 (some 7200 t-Venter, 2001), the uranium price on the world market, and subsequently uranium production in South Africa, steadily declined (Cole, 1998, Wendel, 1998). From 26 mines, which at one stage were feeding 18 uranium recovery plants, only three mines and four plants were left by 1995, producing about 1500 t U3O8 per year (Cole, 1998). Currently, less than 1000 t/a are produced by South Africa (Venter, 2001).

Owing to the much lower gold content in the ore compared to uranium (Au–U ratio ranges from about 1:10–1:100), large amounts of uranium are brought to the surface by gold mining operations. After milling and leaching, the remaining ore-material (‘tailings’) is deposited as a solid-water-mixture (‘slurry’) on so-called ‘slimes dams’. Since leaching with sulphuric acid (as most commonly used technology in South Africa) extracted some 90% of the original uranium content from the ore (Ford, 1993, Wendel, 1998), a tenfold increase of uranium concentrations is to be expected in tailings which are no longer leached for the radioactive metal. Currently about 6000 t of uranium are annually disposed onto slimes dams by gold mining activities in South Africa (Winde and de Villiers, 2002a, Winde and de Villiers, 2002b). Since 1886, when gold mining commenced in the Witwatersrand, about 6 billion t of tailings have been produced (Janisch, 1986, Robb and Robb, 1998a, Robb and Robb, 1998b, Wymer, 2001). Displaying an average uranium concentration of 100 ppm, slimes dams of gold mines contain higher U concentrations than tailings of many genuine U-mines. They currently expose about 600,000 t U3O8 to the biosphere (Winde and de Villiers, 2002a). With a surface area of some 400 km2 in the Witwatersrand basin alone this constitutes an environmental problem of extraordinary spatial dimensions (Robb and Robb, 1998a).

The contamination of streams by adjacent slimes dams poses a severe risk to the health of people in informal settlements where polluted stream water is often consumed without appropriate treatment. In addition to this, long-term effects on cattle and crop farming and established drinking water supply schemes are also of concern as shown by a number of recently launched projects (Hearne and Bush, 1996, IWQS, 1999, Wade et al., 2000).

The majority of studies into mining-related stream pollution conducted so far focus either on point-discharges of mining effluents or on erosion of tailing particles from slimes dams as main sources of stream contamination. Based on the assumption that negative annual water balances in gold mining areas largely prevent the generation of seepage (Funke, 1990), the transport of dissolved uranium from tailings deposits was long neglected. This assumption, however, is contradicted by a number of more recent studies which suggest that seepage, containing high concentrations of dissolved contaminants, migrates from tailing deposits into subjacent aquifers and finally enters adjacent streams (Hearne and Bush, 1996, Winde, 2001a, Winde, 2001b). While off-site pollution by eroded slime-particles can be prevented relatively easily and cost effectively (e.g. by covering slimes dams with vegetation), the same is not true for the aqueous pathway as experiences with rehabilitation of uranium mining sites in the USA (Robinson, 1995) and in Germany (Winde, 2000, Winde, 2002) show.

While a range of models exist to describe the transport of dissolved contaminants in groundwater (Wang et al., 2002, Cermakova et al., 2002) the same is not true for the associated diffuse contamination of streams and the subsequent downstream transport. Sediment-water systems through which U passes on its way from the slimes dams to the stream are schematically displayed in Fig. 1. The shallow aquifer developed within the alluvial sediments of the floodplain associated with the stream is termed ‘alluvial aquifer’ (Seiler et al., 2002, Noseck et al., 2002) (Fig. 1).

This study mainly focuses on

  • Possible mechanisms which immobilise U along the aqueous pathway,

  • Hydraulic interactions between the stream and contaminated groundwater in and near the stream channel, and

  • Diurnal and event-related fluctuations of stream chemistry impacting on the rate and spatial extent of downstream transport of U.

Section snippets

Study area

The Koekemoerspruit, located some 10 km west of Orkney in the North West Province, South Africa, is a tributary of the Vaal River (Fig. 2).

The Koekemoerspruit is a non-perennial stream with a total catchment area of about 860 km2 (DWAF, 1999). With a mean annual precipitation (MAP) of some 600 mm, which mainly occurs as intensive convectional thunderstorms during summer (December–March), and a potential evapotranspiration of about 1700 mm/a, the watershed yields a mean annual runoff of 9 million m

Pathways of stream contamination

Several unlined slimes dams of the Buffelsfontein gold mine are located about 500 m east of the Koekemoerspruit. Deliberately placed on well-draining dolomite they contain tailings from gold production as well as from combined gold and uranium production, which display significantly elevated uranium levels compared to the regional background of about 2.5 ppm. With 127 ppm, gold tailings contain about ten times the U concentration of tailings that were leached for the radioactive element (13

Geochemical analysis

Sediment and soil samples were collected as mixed samples from four different spots of the upper 1–10 cm of the respective profile and the fraction <2 mm analysed for its aqua regia soluble content of heavy metals. Uranium (Unat) was determined by inductively coupled plasma optical emission spectrometry (ICP-OES), with a lower detection limit of 2 ppm. All other heavy metals were determined by mass spectroscopy (ICP-MS) in water and by atomic absorption spectroscopy (AAS) in solids. Uranium in

Uranium mobility along the aqueous pathway

The mobility of U was analysed for the sediment-water systems of the floodplain (groundwater), the concrete channel via which groundwater is discharged into the Koekemoerspruit and the Koekemoerspruit itself. The uranium concentrations in water and solid samples taken from different parts of the aqueous pathway are displayed in Fig. 4.

Conclusions

Chemical analyses of water and sediments samples from the Koekemoerspruit have established the significance of gold and uranium slimes dams as sources of waterborne stream contamination. Dissolved uranium and other heavy metals move along with seepage from tailings deposits into groundwater which finally seeps into the Koekemoerspruit. A rapid increase of the uranium concentration in tailings, resulting from the abandonment of uranium production by many mines, has significantly elevated the

Acknowledgements

The study was conducted as part of a comparative research project in Germany, South Africa, Namibia and Australia funded by the German Academy of Natural Scientists Leopoldina (BMBF-LPD 9801-17).

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