An ongoing probe by the Department of Environment, Forestry and Fisheries (DEFF) to determine the cause of the high levels of air pollution reported in Mpumalanga and Gauteng during the week of February 11 to 17 has determined that sulphur dioxide (SO₂) levels were in compliance with ambient air-quality standards across all stations during the week, but that levels of hydrogen sulphide (H₂S) had been elevated at times during the week.

The probe followed reports of persistent, unpleasant smells in the western parts of Mpumalanga and eastern Gauteng during that week. The DEFF said the sulphurous stench was most likely a combination of elevated levels of SO₂ and H₂S.

Extreme peaks of H₂S of 234 part per billion (ppb) were noted at Lebohang monitoring station at 11:00 on February 12 and 220 ppb was recorded at 11:00 on February 16 at the Springs monitoring station.

In Pretoria, H₂S measurements at the South African Weather Service’s station in Irene, Centurion, showed that H₂S exceeded the World Health Organisation (WHO) recommendation on February 12, and from February 15 to 17, with the highest peak of 61 ppb recorded at 01:00 on February 16.

"The measurements show that there were two typical periods during the day when the levels of H₂S were most severe: at about 01:00 and 11:00 in the mornings. Such peaks are typical of transported air with pollution from high level sources, such as tall stacks," the DEFF says in a February 28 statement.

"The WHO recommends that H₂S concentrations should not exceed 5.02 ppb within a 30-minute averaging period in order to avoid substantial complaints about odour annoyance among the exposed population," the DEFF statement points out.

The DEFF statement notes that, "at this stage, the DEFF and Gert Sibanda municipality, in Mpumalanga, cannot say definitively what the major source/s of the air pollution was. The DEFF and the municipality suspect that the stench experienced over parts of Gauteng may have been the result of the cumulative impact from a number of sources in the surrounding areas."

An inspection of the Sasol Secunda plant to check on operations was conducted on Thursday, February 18, as it was believed to be the possible source of the sulphur stench experienced in parts of Gauteng and Mpumalanga. The inspection, however, found that the release of SO₂ and H₂S from the Secunda plant was within the approved limits of the Atmospheric Emission Licence (AEL) issued to it by the Gert Sibanda municipality in April 2019.

There was also a low-pressure system in the north of the Mozambique Channel that gave rise to a rather uncommon circulation pattern over the region. These conditions created prevailing south-easterly winds that allowed for transportation of air pollution from the Mpumalanga industrial region into Gauteng, particularly over the cities of Ekurhuleni, Johannesburg and Pretoria, the DEFF says.

It has, therefore, been recommended that other sources that could have contributed to the odour experienced in Gauteng also be investigated. These include power plants and wastewater works that also emit hydrogen sulphide.

The findings of the probe will be made available once it is completed, the DEFF says.