‘Impact being assessed’ after large minewater release at Giant

Up to 40,000 litres of “arsenic-impacted water” spilled at Giant Mine earlier this month when a valve at a pumphouse malfunctioned.

Officials leading remediation of the highly toxic former gold mine say they think all the water seeped into a gravel pad and will descend back into the underground pool it came from. None of the water was seen reaching the surrounding environment, they said.

However, the federally led project team said the impact of the spill was still being assessed and steps would be taken to stop such an accident happening again.

“While this is a large volume,” a federal spokesperson said, “the spilled minewater has infiltrated the gravel pad and was not observed to runoff to the nearby wetland.”

A spill report suggests a wetland is located about 45 metres away from the spill location.

Inspectors notified of the spill have asked the Giant team to take water samples and set out “corrective actions” to be taken.

Once a major gold mine that formed the beating heart of Yellowknife’s economy, Giant Mine is now a $4-billion federal remediation project because huge quantities of toxic arsenic trioxide, a mining byproduct, lie buried beneath it.

Not all arsenic issues at the site are related purely to those stores of arsenic trioxide. Arsenic is also naturally occurring in the area and, because almost all forms of arsenic are toxic to at least some degree, the Giant team says “all water from the underground mine pool is assumed to contain potentially harmful concentrations of arsenic” – and should be handled with appropriate precautions.

In this instance, an air bleed valve on a pump system at a pumphouse malfunctioned early on September 3. The automated pump switched on some time after 4:30am and ran for about four hours before someone discovered it was inadvertently spilling arsenic-impacted water.

“As the pump system is automatic and the incident occurred unsupervised, the exact volume of mine water lost is not known. However, volumes are estimated to be between 30,000 L and 40,000 L,” states a spill report filed by Parsons, the contractor managing the remediation site.

Why the valve malfunctioned is not yet clear.

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The pumphouse sits on top of a gravel pad. The project team says the spilled water appears to have gone back through the pad, which means it should eventually end up back in the same underground pool from which water is normally gradually pumped for treatment. (The pumphouse exists to bring water up from the mine’s underground to the surface so it can be treated.)

“The project team is conducting an investigation and will ensure, should the inspections determine further steps are required to address this spill, that these are carried out to the satisfaction of the inspector,” a spokesperson stated.

“The project team will submit a 30-day follow-up report, which will include the results from the investigation and any changes to procedures to prevent a similar incident from occurring in the future, to the regulators.”