Origin of atmospheric lead in Johannesburg, South Africa
Résumé
The city of Johannesburg in South Africa was founded in 1886 after thediscovery of the rich gold deposits of Witwatersrand Basin. Most of the miningactivity stopped in the immediate vicinity of the city more than a decade ago, andaround 200 mine tailing dumps, some as tall as 50 m, are still exposed in themetropolis. Some of them are now reprocessed because of the considerableimprovements in the extractive metallurgy of gold. Although lead-free fuel isavailable in South Africa, leaded gasoline still holds 80% market share.Automotive exhaust gases are thus an important contributor to lead in theatmosphere. Many other sources may be involved, such as widely-used domesticcoal burning, dust from unpaved road in poor townships, and surface miningoperations. Consequently, in historically Black residential suburbs adjacent tomining areas, heavy metals associated to dust emissions from dumps and miningextraction can be added to those emitted by other human activities and mayenhance the breathable pollution risk level for populations living nearby.On the basis of elemental and lead isotopic analyses of gasoline, coals,mine dumps, and about thirty epiphytic lichen samples, we attempted to assessthe origin of lead in Johannesburg’s atmosphere. Isotopic compositions ofpotential sources are highly contrasted with 206Pb/207Pb ratios ranging from 1.06to about 2.9, and 208Pb/206Pb between 0.6 and 2.20. Such a wide range is veryrarely encountered because uranium-rich sources like mine dumps areinfrequently involved in the surface cycle of lead. In our case, this specificityconsiderably extends the discrimination power of the isotopic method, especiallywhen the isotopic information is combined with elemental concentrationsexpressed as La/Pb ratios. It appears that the normalization procedure of metalconcentrations to lanthanum is necessary because the sole absolute elementalconcentrations in lichens did not bring useful information on atmosphericdeposition. Lead associated with dusts transported by winds from the numerousmine sites is recorded in small amount in the immediate vicinity of the dumps,but becomes quickly insignificant just a few kilometres away. Lead associatedwith domestic coal burning fly ash could never be clearly identified probablybecause of its low contribution. Throughout the city, all of these sources areobliterated by leaded antiknock compounds which are added to the gasoline,constituting the main source of lead emissions in Johannesburg’s atmosphere.