Macroinvertebrates in the Tweelopiespruit, in the West Rand area, which was previously a sterile aquatic environment because of the severe impact of raw and untreated acid mine drainage (AMD), have returned for the first time in nearly three years, reports the Council for Scientific and Industrial Research (CSIR).
At the bottom end of the Tweelopiespruit, a little over 6 km downstream of a mining area, the CSIR’s Natural Resources and Environment Unit has been monitoring the flow and quality of the water.
“We have conducted surveys to assess the aquatic life at a number of stations in the Bloubankspruit river system every few months,” says CSIR senior research hydrogeologist Phil Hobbs, adding that the fact that life has returned to the severely affected Teelopiespruit tributary in this system shows that the aquatic environment is resilient in terms of its ability to recover from severe AMD impacts.
“The severity of the impact has lessened, owing to the upgraded and refurbished mine-water treatment plant that was commissioned by the Department of Water Affairs and its implementing agent, the Trans-Caledon Tunnel Authority, in June 2012. Since August 2012, we have seen a remarkable change in the quality of the water in river systems downstream from the plant,” he says.
Hobbs adds that the neutralised water released into the environment contains almost no raw mine water, which has enabled the ecosystem to start recovering.
“Recovery is gauged by the diversity of life in the river system. If only one species returns to the river, it is a good sign, but not as good as two or three species returning,” he explains.
Hobbs adds that, by way of example, the diversity and score achieved at this site were slightly better than those of a nearby stream that had been receiving municipal wastewater effluent for many years.
“That particular stream has thousands of bloodworms as the dominant organism because they thrive in those poor water quality conditions. However, a bloodworm is a species that ranks low on the scoresheet compared, for example, to dragonflies which were found at both sites,” he says, adding that the positive Tweelopiespruit results need to be confirmed with future surveys.
Hobbs adds that there is also a definite improvement in the in-stream habitat of the Tweelopiespruit.
“The iron oxide/hydroxide precipitation that formed a crust over the river bed has started to erode away, which has provided the habitat for life to return,” he says.
Meanwhile, Hobbs explains that the Witwatersrand Western basin started to decant eleven years ago and, during that time, the type of water released into the environment has varied from neutralised mine water mixed with raw mine water in various proportions, to an exclusively neutralised mine water.
“Depending on the mixture of the mine water being released into the aquatic system, the impacts are different. Raw mine water is lethal to the aquatic environment and kills all life in that environment, which is what happened to the Tweelopiespruit,” he says.
Hobbs adds that the precipitate that had subsequently formed and caused the vegetation in the streambed to die, was similar to the resi- due accumulating in the pipes that distribute mine water in the mine area.
“It is a function of the chemistry of the mine water that is transported and released, and can lead to severe blockages in pipelines,” he states.
Hobbs adds that in the first eight years after decanting started, a much greater percentage of water that was released down the system had been treated and neutralised. These circumstances changed in February 2010.
“For every four units of raw mine water being released, there was only one unit of treated mine water. This reversal lead to the aquatic life in the area being destroyed,” he says, adding that up to June 2012 the treatment plant was able to treat only about 12-million litres of mine water a day.
“The raw mine water requiring treatment increased to some 60-million litres per day which was too much for the treatment plant to handle. That is why there was so much raw mine water entering the environment,” explains Hobbs.
He adds that the volume of water needing treatment normally increases significantly during the rainy season, when more untreated raw mine water comes to surface.
“After the formation of the Inter-Ministerial Committee on AMD in 2010, a team of experts was tasked to assess the management of AMD on the Witwatersrand. This group compiled a report that recommended an immediate and short-term intervention, specifically for the West Rand basin where active decanting is happening.
“The recommended interventions suggested that, as much as possible, the mine water that was coming to surface be treated, neutralised and released into the environment,” Hobbs says.
Hobbs states that the treatment plant has been upgraded to treat more mine water more efficiently, making the high-density sludge treatment technology more efficient in its process. The current treatment capacity of the plant is in the order of 24-million litres per day.
“The raw mine water component in the mix has been removed completely – practically all of the water released into the environment currently is treated and neutralised,” he concludes.