Dioxins and 21 Other Persistent Organic Pollutants in 61 Sediment Samples from the Orange-Senqu River Basin (Botswana, Lesotho, Namibia, South Africa)

The Orange-Senqu River Basin covers four southern African states that are also Parties to the Stockholm Convention on Persistent Organic Pollutants (SCPOP). The basin’s main tributary, the Vaal River drains a large industrial region before confluence with the Orange-Senqu River (OSR), flowing past mining, rural and agricultural areas discharging into the Atlantic Ocean. We collected 61 sediment samples in 2009 across the basin and analysed for all POPs listed in the SCPOP in 2010. Geographically and in terms of range of compounds, this is the largest such survey in Africa. Nine sites in industrial regions had quantifiable concentrations of dichlorodiphenyltrichloroethane’s (ΣDDT; max: 9 ng/g dry mass (dm). Lindane was quantified at three sites on the upper OSR; no other organochlorine pesticides were quantifiable. Higher concentrations of polybrominated diphenyl ethers (ΣPBDE; max: 15 ng/g dm) were quantified in the Vaal River at its confluence with the Orange-Senqu (non-industrial) compared with industrial areas. Polychlorinated biphenyls concentrations were high in industrial mining areas (max ΣPCB; 1053 ng/kg dm). Most sediment samples from industrial regions had quantifiable concentrations of polychlorinated dioxins and furans (max ΣPCDD/F 83 ng/kg dm). POPs found to be above international sediment quality guidelines were lindane (three sites), p,p’-dichlorodiphenyldichloroethylene (DDE) (one site), and ΣDDT (four sites). Future studies should focus on PCBs, lindane, and DDTs at selected sites, while POPs added to the SCPOP since 2010 would need a wider survey, including studies on residues in biota. The present study provides a valuable baseline and assessment for Botswana, Lesotho, Namibia, and South Africa sharing an international catchment against which subsequent interventions and effectiveness evaluations should be measured.