The water in Baker Creek outside of Yellowknife is being tested for contaminants after meltwater from Giant Mine accidentally spilled into the creek.
Over the weekend, a small amount of surface water broke through a clay barrier set up last year to contain water on site.
"The material that entered was from a constructed berm, so we aren't anticipating high levels of metals or arsenic, but we are going to confirm that with the results," said Jane Amphlett, operations manager with the Giant Mine clean-up project.
Amphlett said the team caught the problem very quickly because Baker Creek is monitored so closely.
The project has been working for years to restore the creek so it can once again support fish populations.
The meltwater has now been diverted underground, where it's being pumped out and treated.
Test results on the water are expected Wednesday.