Thawing ice and shifting soil could spell disaster for Dawson’s sewer and water systems.
The municipality’s permafrost is melting at an unknown rate because of urban activities and global warming, says Chris Burn a Carleton University geology professor.
Burn has been researching changes in Mayo’s permafrost for more than 20 summers.
Burn also understands Dawson’s geology and permafrost extremely well.
Warm air heats the ground and causes the temperature of the active permafrost layer to rise.
In Dawson City, the active layer is filled with natural groundwater that freezes in winter and melts in summer.
This freeze-thaw cycle causes the ground to heave during the fall and spring.
The active layer can be up to five metres thick in Dawson City and lies on top of deep permafrost that does not go through an annual freeze-thaw cycle.
And more snow than usual has fallen in Dawson over the past three winters.
The snow insulates the earth and prevents summer heat from escaping into the air, further warming the ground.
A continual warming of the active layer destabilizes the ground and any infrastructure it supports, explained Burn.
“In Dawson there is quite a lot of ice in the ground; it’s what we call ice lenses,” he said. “When those thaw out, we lose volume in the ground. There is nothing to hold the structure together.”
The municipality last measured the temperature of its active permafrost layer in 1979, when the current sewer system was installed.
The ground temperature was recorded at minus one degree Celsius.
That is considered very warm permafrost, said Burn.
“In town, there is lots of area that is melting at the surface and we know it hasn’t been melting for very long, so it’s probably been melting since the town was constructed,” he said.
Norm Carlson, Dawson’s public works manager, has been ensuring the town’s sewer and water systems operate properly for the last 25 years.
He understands the system intimately, and agrees with Burn.
“If the permafrost fails here, everything is going to snap. It will,” he said. “It just can’t take that kind of movement. Roads would melt, the whole town would sink.
“We would lose all our infrastructure wherever there is ice in the ground. It would be soup.”
Last month, a crew repaired a broken water line buried two metres deep on Fifth Avenue. The water line ruptured at a joint after a section of it sank 25 centimetres when the ground shifted.
The water ran in the ground at 70 pounds per square inch for a week before the crew could locate the source.
Fixing the problem cost taxpayers more than $20,000, said Carlson.
“Once you have a break in Dawson, it saturates all the ground around it,” he said.
“If that’s a permafrost area, it stays saturated and you have seriously weakened the infrastructure around the whole area.”
Dawson’s original wooden stave sewer system was replaced between 1979 and 1981 by a metal, inflexible system that did not stand up to pressure caused by the ground heaving and was subsequently crushed.
It was replaced in the north end — which has active permafrost — between 1990 and 1994.
The south end has yet to be replaced, and is in a state of disrepair, said Klondike MLA Peter Jenkins.
Carlson does not know for certain how well the plastic system will perform over a long time.
“With the pipe we’ve used, I’m fairly confident that the mains in the permafrost area will not collapse, which was a problem before. However, they will go out of grade. Things do melt and they will change,” said Carlson.
“It won’t break, but if they go out of grade you end up with pockets of water.”
The water lines are in worse shape. Heat was used to manually join 14-metre sections of plastic pipe when the water system was replaced throughout the 1980s and 1990s.
“There’s a lot of human elements in that type of connection, so, there is a lot of failure in that type of connection,” said Carlson.
“You are totally relying on the field. If it was a cold day, the operators maybe didn’t apply the correct pressure, maybe it was raining, who knows? So you end up with a joint that’s going to go.”
Each time a joint does crack, a city crew replaces it with a section of pipe that has a built in joint protector encased with wire.
This should ensure it is strong and flexible enough to withstand Dawson’s challenging soil conditions.
The changes come at a severe cost to taxpayers, however, because there is an almost uncountable number of joints in the town’s water system.
“There could be hundreds,” said Carlson. “It’s not documented. Any joint is prone to failure. Every joint is a potential problem.”
Carlson wants to better understand Dawson’s melting permafrost before the situation becomes unmanageable, he said.
“If global warming is here, there is going to be an impact on us that is pretty hard to avoid. That would be into the realm of scientists.”
So, he is bringing in Burn and installing a permafrost-monitoring system this summer.
A series of thermisters — ground probing thermometers — will measure the permafrost temperature for the first time since 1979.
Burn will bury up to five thermisters in and around town for three years to determine the permafrost temperature in town and in the Klondike Valley, in natural soil conditions.
Three years is “certainly” enough time to determine how quickly the permafrost is melting, said Burn.
However, he wants the program to last longer to collect and analyse some long-term data that can help the town with its future management plans.
“I hope the municipality will recognize the utility of the data it is getting and it will be the one that determines how long it wants to keep going.”
Carlson agrees and is enthusiastic about using science — not politics — to determine future management plans.
“This should have been done. It’s never been too late to start, so let’s start.”