Danger below: Underground acid mine water levels on the Eastern basin pose a threat
One of the three plants designed to treat polluted water from old underground mines in the Witwatersrand’s three mining basins is still not up and running, five months after operations ground to a halt.
In April, the Mail & Guardian reported that all three deep-level abstraction pumps at the R1-billion Eastern Basin acid mine drainage plant in Springs were no longer working. By early February, the last pump motor, which was reportedly functioning at only 33%, had failed, temporarily shutting down the treatment plant.
The Trans Caledon Tunnel Authority (TCTA), the state-owned agency that runs the plant, previously said it would restart operations by early June, but this has not happened.
The Eastern Basin treatment plant is supposed to pump between 70 million and 100 million litres of acidic water every day to maintain the environmental critical level (ECL), which is 120m below ground level, to protect the dolomite and groundwater sources, before it is neutralised and discharged into the Vaal River system.
Rising tide
The plant’s closure means that a tide of toxic mine water is rising significantly underground, which could decant into the environment within months. Acid mine drainage is harmful to people, plants and animals because it is acidic and carries atoms that emit radiation and salts in hazardous concentrations.
The rising water level has already breached the environmental critical level. The risks of this include the rise of mine water into the overlying dolomitic aquifer; contamination of shallow groundwater resources required for agricultural use and human consumption; and increased seismic activity, which could have a moderate localised effect on property and infrastructure, according to Mariette Liefferink, of the Federation for a Sustainable Environment.
“The plant is currently not operational as delivery of parts [from its German supplier] was further delayed by almost two months from the original estimated date,” the TCTA said this week. “A wire to fix the first pump was eventually received last week, effectively moving the anticipated date of start of operations by a month or two.”
The entity is in discussions with Gold One, a neighbouring mine, to procure more motors to get the plant up and running.
A highly-placed source at the department of water and sanitation, who requested anonymity, said the operations and maintenance of the plant had been neglected, which had led to the current failures.
“A decant is one thing. But even at 80m, you have to start considering the risks to the groundwater, the dolomites etc, and at 60m, it becomes even more pertinent,” the source said.
“So, now that we’re in the 50m territory; there is clearly a risk of groundwater pollution before we can even start worrying about decant … My informed guess is that once the ECL is breached, I don’t believe we can take it back where it is supposed to be.
“You may just make nominal gains in terms of a 5m or 10m preservation, but it’s going to cost a huge sum of money, electricity, treatment chemicals to take this back to the 120m mark below surface or even the 80m mark.
“You’ve got to bear in mind that the plant is pumping the ocean because of the amount of ingress that goes in there, so even if you ramp this up at capacity you basically strain the system.”
‘Unreliable estimates’
The TCTA maintained that, “when all the pumps are available, the plant will be able to treat 110 million litres per day, which is more than the 70 to 80 million litres required to keep the water level constant”.
The current water levels are 54.66m below the surface, it said, and it was not aware of any effects, including seismicity or groundwater contamination. It said studies had shown that the acid mine drainage will start to decant when the level in the shaft reaches the “danger zone” of 25m below ground level, causing environmental degradation. The TCTA said that if operations remained stalled, this decant would occur by December.
But the 25m figure is based on “very unreliable estimates”, the source said. “This is from what is known in the historic records — decant could potentially even materialise from as low as 40m below surface.”
It would probably take three months before the plant could be brought up to capacity, if the one pump was brought in by July, followed by the two remaining pumps by September, said the source.
“When you install and switch the plant on, there’s no assurance you’ll be able to run that plant at full capacity at about 100 million litres a day, primarily because there is still a bottleneck on the reactor system and the clarifying system — that’s where it sort of chokes the whole system because of some mechanical issues that have not been resolved.”
‘Serious situation’
According to the Council for Geoscience, at the current rates of rise, discharge is not expected to occur in the next two to three months. “However, longer term, if the situation is allowed to persist, water discharge will eventually inevitably occur, especially in lower lying areas. Urban infrastructure is unlikely to be compromised on a substantial scale. Should discharge occur, groundwater quality may be compromised, although the degree of contamination cannot be predicted at the moment.”
The situation is serious, Liefferink said. “The precautionary measures and the risk-averse measures should have been applied … We’ve been credibly informed that any dewatering or rewatering can trigger seismicity in dolomitic areas, like the East Rand. What is happening is that the basin is being rewatered again because there is no pumping.”
The council said studies on fluid-induced seismicity that it and others had conducted in the East Rand have demonstrated an increase in seismicity following rapid fluctuations in water levels, “usually with a delay in the onset”.
“South African mines are not only deep but there are extensive regions of high stress in mined out areas, which are susceptible to collapse when exposed to changes in water pressure.”
The council said that large areas of the East Rand and Gauteng are underlain by dolomitic rock, which is prone to the development of sinkholes and subsidences. Other than dolomitic land, historically shallow undermined areas, which are not necessarily located on dolomitic land, are also susceptible to the formation of sinkholes and subsidences, especially in the first 100m below surface.
“The occurrence of dolomite-related sinkhole and subsidence formation is known to be exacerbated by the concentrated ingress of water and large-scale fluctuations in groundwater levels. For this reason, active regional monitoring of groundwater levels is routinely undertaken to ensure that seasonal (or artificially affected) groundwater levels are kept in check,” according to the council.
The source said it would take two years to fully understand the real effect from the inability to maintain the environmental critical level. “That dolomite has been dewatered for well over 50 years and it’s only recharge is from the terrestrial environment, meaning what’s come in from precipitation and obviously there’s some contribution from leaks. When dolomite is re-watered now with a mildly acidic solution, the first thing is it’s a very rapid recharge.”
Dolomite doesn’t like to be dry and then suddenly re-watered. “It’s forgiving if it’s a gradual rewatering … so the first thing we could potentially pick up is a dolomitic dissolution; with that you’re then going to have issues of subsidence and sinkholes,” said the source. “That is a very real risk considering the large tracts of dolomitic land in the area.”
Another big risk is groundwater pollution. The groundwater coming out of the dolomitic region is reasonably clean but, with the high levels of sulphates and iron in the acidic water, “that water then potentially goes out of drinking spec”, the source said. “For the farming community, who rely on it for animal and livestock watering, for irrigation and even for their domestic use in plots, that proves a big risk for them.”
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