Friday, 23 May 2014

Atmospheric trace metals in the Bushveld Igneous Complex.

The Bushveld Igneous Complex is the world’s most productive area for platinum group metals, responsible for about 80% of global production, as well as having significant reserves of many other metals. It covers an area of 66 000 km² across the North West, Gauteng, Mpumalanga and Limpopo Provinces of South Africa, and is the most heavily industrialized area on the African continent. Despite this environmental monitoring has not historically been a priority in the area, there is no permanent monitoring system in place for many environmental indicators, and the impact of a large concentration of mines and related industrial sites on both human health and the wider environment is not fully understood.

In a paper published in the South African Journal of Science on 27 March 2014, a team of scientists led by Pieter van Zyl of the Unit for Environmental Sciences and Management at North-West University present the results of a 12 month study in which atmospheric levels of 26 metals were monitored at Marikana in North West Province over the period November 2008 until October 2009.  There are known to be over 30 metal mines and 11 industrial smelters within a 60 km radius of the test site making atmospheric trace metals a potential area of concern.

Geographical map of southern Africa and a satellite image indicating the location of the measurement site, as well as the extent of the western Bushveld Igneous Complex and the major point sources. The sampling site is also shown on a satellite image obtained from Google Earth. Van Zyl et al. (2014).

Marikana has a cool dry winter and a warm wet summer, a pattern that was reflected in variations in the levels of concentration of all metals in the atmosphere. Atmospheric metal concentrations were highest from April to September, peaking in June, and lowest in the rainy months of October through March.

Four of the metals tested for (palladium, mercury, thallium and uranium) remained below detection levels for the methods used for the entire year, while another six metals (calcium, cobalt, arsenic, cadmium, barium and gold) were only at detectable levels for 25% of the year.

The highest atmospheric trace metal concentration was for iron, averaging 2.5 μg/m³ (micrograms per cubic meter of air), with an annual variation of 0.42-19.01 μg/m³. The next highest concentration was magnesium, with an average level of 2.04 μg/m³, and a range of 0.01-13.6 μg/m³. Sodium, Boron and Aluminium were also found at fairly high levels with average annual concentrations of 1.41 μg/m³, 1.30 μg/m³ and 1.28 μg/m³ respectively.

Only thee metals tested for exceeded European Commission of Air Quality Standards limits during the test period; nickel, arsenic and cadmium. The European Commission recommends that nickel concentrations do not exceed 0.02 μg/m³, while the annual average at Marikana was 0.33 μg/m³. For arsenic the European Commission recommends 0.006 μg/m³, the average annual level at Marikana being 0.26 μg/m³, despite concentrations being below detectable levels for 75% of the year. Cadmium was also only at detectable levels for 25% of the year, yet nevertheless was present at an average annual level of 0.03 μg/m3, far above the European Commission’s recommended level of 0.005 μg/m³.

It was not possible to directly compare the chromium or vanadium levels to any system of regulation in use, as chromium levels need to be monitored only at certain oxidation states, and vanadium in daily concentrations, though neither of these metals appeared to be present at dangerous levels.

The only metal tested with South African regulations in place was lead (which was regulated due to the danger from car exhaust fumes rather than industry or mining), but the levels present at Marikana did not approach the levels proscribed by regulations (probably because South Africa, like most countries, has now banned lead as a fuel additive). South Africa also has plans for the regulation of mercury levels in its atmosphere, but levels present at Marikana did not appear to be a problem for this metal either.

See also…

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