Heavy metal contamination of an arid river environment: Gruben River, Namibia

The dispersal of metalliferous pollution from several spoil heaps produced during 20th century copper mining in the Gruben River valley, Namibia is examined. The Gruben River flows through an extremely arid environment with an average rainfall of ∼25 mm p.a. The potential for physical and chemical remobilisation of Cu, Zn and Ni is assessed by examining the spatial and temporal distribution of metal-contaminated sediment deposited within the confines of the channel. The relationships between metal content, grain size, geomorphic environment and the downstream distribution of metals are discussed. The phase-specific heavy metal concentrations of sediments, collected as part of the downstream sampling programme, are also examined using progressively more aggressive sequential acid extractions. In addition, metal concentrations are compared with Dutch guidelines for soil contamination to ascertain the extent of environmental risk. Total metal concentrations show that the Gruben valley is highly contaminated, particularly with respect to Cu and Ni concentrations, which exceed Dutch target values for Cu (36 ppm) in 94.7% and Ni (35 ppm) in 90.5% of samples, respectively. Zn concentrations are much lower with only 6.3% of the samples exceeding the target value (140 ppm). As might be expected, the metal-sediment concentrations of Cu are the most highly elevated, with a peak value of 10,500 ppm being recorded from material collected from suspended sediment transported during a minor flow event that occurred in March 1999. Lower energy and fine-grained sedimentary environments are shown to be clearly associated with higher metal concentrations. Sequential extractions of metals show that of the three principal elements considered in this study, only a negligible proportion of Cu (0.41%) is held in the exchangeable phase. Ni and Zn were below detection limits. Although the total metal concentrations in the Gruben River valley sediments are extremely high and are in themselves a concern, the small percentage of metals held in the exchangeable phase and the low potential for remobilisation under the arid conditions would suggest that they pose only a minor risk to the environment.