The underground mines in Cape Breton aren't producing coal any more, but they are churning out about six billion litres of waste water a year.
Before it spills out and causes environmental damage, the contaminated water is treated and cleaned at four mine water remediation sites in the Cape Breton Regional Municipality, N.S.
One of the facilities was recently expanded to more than double its capacity. The Neville Street facility in Reserve Mines can now treat 19,000 litres of mine water per minute. The $6.1-million expansion began in 2014 and was completed this fall.
Joseph MacPhee, the regional manager for mine water for Public Services and Procurement Canada, said the water comes from a network of 10 abandoned mines in the area.
"The mines naturally flood, just from groundwater and surface water that makes its way into old openings from illegal workings or from natural conduits, down into the ground," said MacPhee.
"The natural geology around the coal field is pyritic slate, so as the water flows through and it absorbs the sulphides in the pyritic slate, it becomes acidic and it dissolves the metals that are in that rock."
He said pH levels have to be adjusted and metals removed to make sure fish habitat isn't destroyed when the water is discharged into the environment.
The 14-hectare site at Neville Street includes four settling ponds and two engineered wetlands.
As it's pumped into the facility, the water is sent over a structure of steep metal stairs to aerate it. Then it's treated with caustic soda and fed into a stepped series of settling ponds, each a metre lower than the one before.
In the final treatment phase before the water is released to Cadegan Brook, it flows through engineered wetlands of reeds and bullrushes to catch any of the metals that didn't get removed in the settling ponds.
"Basically they're a vegetative filter, so any iron or metals that get precipitated out of the water but haven't settled, get caught in the last filtration net," said MacPhee.
The water is treated to Canadian Council of Ministers of the Environment freshwater aquatic guidelines for fish habitat.
The site is monitored daily and the water coming from the facility is tested twice a month for metals; the pH is tested weekly.
The entire mine field is tested once a year.