Cause of boil water advisory in the Northwest Calgary determined

The City of Calgary, Water Services, has completed an extensive review and determined how sediment entered the distribution system that supplies clean drinking water to portions of some Northwest communities.

This sediment caused high turbidity (cloudiness) in the water and resulted in a precautionary boil water advisory being issued on June 7, 2012, to protect public health for those households impacted in the Northwest communities. The precautionary boil water advisory was completely lifted by June 12, 2012, as soon as testing confirmed that water quality had been restored.

According to Dan Limacher, Director, Water Services, “An extensive review has determined that an extreme and localized rainfall event flooded a part of the City’s water infrastructure. Specifically, an underground valve chamber was flooded which allowed sediment to enter both a feedermain and a new underground water storage reservoir.”

An unfortunate series of events resulted in a perfect storm including:

• An unforeseen and localized 1 in 100 year rain event ocurred during a period of persistent and prolonged rain.
• The intense and heavy rainfall occurred during a period when final site grading, paving and drainage work at the underground reservoir site had begun but could not be completed due to wet site conditions.  
• The intense rain caused significant soil erosion which plugged the stormwater and drainage systems that were in place.
• Once the drainage system was plugged, the water pooled in a low lying area adjacent to the valve chamber.
• Once the water reached the manhole access to the valve chamber, the water flooded the chamber and completely submerged the valve assembly present in the chamber.
• This submerged condition created water pressure conditions surrounding this valve assembly which triggered the valve to open and allowed sediment to enter the water distribution system.
• The section of the water infrastructure where sediment entered the system remains isolated and will only be returned to service once the final site grading, paving and drainage are completed. 

The review included the draining and inspection of the underground reservoir and inspection of other underground reservoir sites throughout the city and found that all sites have grading, paving and drainage systems in place to handle an extreme rainfall event of this magnitude.

“The City of Calgary takes great pride in protecting public health by providing clean and safe drinking water. In order to ensure that our water supply is protected from future extreme rain events, new underground reservoir sites will only be brought into service once the final grading, paving and drainage work is complete,” says Limacher.