Published in March 2005 Australian Longwall Magazine

For the past six years, the small Kurri Kurri Landcare group has been treating acid and chemical flows from abandoned coal mines in the area. The deep red colour of the water comes from iron-rich acid. The metal-rich water is formed by the chemical reaction between water and rocks containing sulphur-bearing minerals, particularly iron pyrites. The acid run-off dissolves heavy metals such as iron, aluminium, manganese, fluoride, and nickel.

The water also hosts acidophiles, microbes that thrive in acidic environments, like sulphuric pools, where pH values range from one to five.

These organisms eat the sulphur and excrete acid that then leaches the other heavy metals from the spoil and soil.

Untreated, this material is lethal to small aquatic animals which live in the creek system and absorb excess nutrients. Killing them facilitates the growth of blue green algae. Water-breathing fish suffocate when their habitat falls below 3.6pH as excess aluminium coats their gills.

In the immediate Kurri Kurri vicinity, there are at least ten “acid mines” seeping chemically charged water into local streams and waterways.

Another estimated 400 sites in New South Wales face the same problem.

An old coal mine site can produce around 0.2–0.5 litres per second of seepage, or about one megalitre per month. Of the ten sites in the Kurri Kurri area, four are old underground workings with AMD exiting the mine. The remaining six sites are coal spoil piles which continually leach acid. These sites are perpetual pollution machines.

To combat the problem, Kurri Kurri Landcare has developed periodic, static and continuous lime dosing plants, along with a mobile plant that can treat up to 200,000 litres per hour. The water clean-up operation is funded by local industries and Hunter Water. Landcare's Water Quality Initiative is the operational guide.

Acid seeps (or springs) are trapped from mines or coal spoil piles and treated with hydrated lime. At the old Greta site, a continuous plant collects acid in the drain beside the New England Highway and automatically aerates hydrated lime doses, mixing and flocculating the acid in two poly 5000-litre tanks. The effluent is then pumped onto degraded land where grass and shrub growth is accelerated by the water, gypsum and inert rust residue.

“Each week at Neath, the worst site in our area, we pump the collected acid -- up to 200,000 litres -- down a 200m mixing drain to a stilling pond,” says Kurri Kurri Landcare Group president Col Maybury, a retired industrial instrument technician.

“Using high flow petrol-powered pumps into the mixing box of lime feeders, we use variable speed electric drills and wood augers to feed the hydrated lime out of plastic garbage bins.

“The liquid turns from purple at 1.8pH, to claret at 2.7, yellow at 4.5, light green at 6-7, and dark green at 8.5, then to blue at 9. The colour changes allow us to easily control the neutralisation and to flocculate the contaminants, mostly iron. Our aim is 8.5pH control.”

Agitation and aeration of the water takes place in the turbulent flow down the drain. The green mix flocculates in swirls and loops in the stilling pond.

“The method developed is cheap and easy and costs just over 0.06c per litre, or 60c per kilolitre, excluding labour, in worst-case acidic loads,” Maybury said. The overflow is drinkable water, close to Australian and New Zealand Environment and Conservation Council drinking limits.

Once neutralised, the flocculated solution can be returned to the mine void, where it remains inert even in acid conditions, or it can be sprayed on vegetation.

The largest site Kurri Kurri Landcare operates, the Neath coal spoil pile, contains around one million tonnes of spoil with around 30% recoverable coal.

“Since commencing treatment at Neath three months ago, we have noticed a slowing of the rate of flow and of the total acidity and iron content,” Maybury says. “We believe this is because we are diminishing the residual liquid load in the pile, also diminishing the environment suitable for the acidophiles feeding on the sulphur in the pile.”

Maybury says fish and minute aquatic animals are starting to return to previously dead waterways.

“We would be pleased to demonstrate our methods to any interested party or correspond with those contemplating action to protect sensitive waterways.”