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Ash and debris from wildfires threaten the water supply – and climate change means it's only going to get worse, writes Leyland Cecco

Weeks after a wildfire engulfed Fort McMurray, heavy rains washed ash and burnt debris from tributaries into the Athabasca River. As the slurried water flowed past scorched banks toward the city's treatment plant, its workers moved quickly to blunt an unfolding crisis – shutting off the intake before the plant's filtration systems overloaded.

The rains added even more pressure to already fatigued staff who were working 20-hour shifts since the fire first hit just to keep the city's water treatment facilities running. More than that, the emergency spoke to concerns that scientists have about future forest fires, which are expected to increase in size and severity in the coming years, accelerated by climate change. Communities across Western Canada, like Fort McMurray, rely on water from rivers that flow down the mountains and pass through forests – sources that are at greater risk from intense forest fires.

In the aftermath of the wildfire at Fort McMurray, with an advanced water treatment facility that is only three years old, much of the population is under a boil-water advisory. Staff continue to have to put in long hours monitoring fluctuating water quality. Heavy sedimentation makes it difficult for water treatment systems to detect bacteria and other contaminants entering the reservoirs.

Provincial monitors are scouring water samples, but test results take at least a week. After the recent rain, they're still unsure what is in the water.

"While [the Athabasca] is a very large river and you'd expect things to be diluted, the burned area is right above the region where the drinking water plant is," says Uldis Silins, professor of forest hydrology at the University of Alberta.

An ariel shot of part of the forest consumed by the Fort McMurray fire.

An ariel shot of part of the forest consumed by the Fort McMurray fire.

Carrie Tait/The Globe and Mail

Dr. Silins is one of the 11 members of the Southern Rockies Watershed Project, a team that has spent years studying the effects of fires on water sources. Forests work as a natural filter for mountain-fed water, thanks to extensive gnarled root systems and porous soil. Members of the Southern Rockies team met with Alberta officials months before the Fort McMurray fire to highlight the risk to the city's drinking water supply. The water treatment system is as high-calibre as Calgary's, but the northern city is surrounded by tracts of boreal forest, making it highly vulnerable to wildfires.

After a wildfire, ground cover turns to ash and water becomes a turbid cocktail of contaminants. This is a natural process and the effects are usually short term and negligible on community water sources. But much larger, more intense fires hitting cities and their water treatment systems has experts troubled.

In 2013, the United Nations' Intergovernmental Panel on Climate Change recognized the Southern Rockies Watershed Project's pioneering work linking the threat of climate change with drinking water.

Studying the impacts of fires on water systems, often in remote and unsafe areas, costs millions of dollars and requires a concerted balance of resources and logistics, says Dr. Monica Emelko, an associate professor of civil engineering at the University of Waterloo and one of the principal investigators alongside Dr. Silins.

"Ten years ago, we couldn't get anybody to listen to this."

The communities most vulnerable to disruptions to their drinking water will be those whose water supply passes through the forest in an area of high fire risk, Dr. Emelko says.

"Two thirds of the people in Alberta … their water comes at least partially, if not fully, from forested areas," she says.

The mouth of the Horse River where it empties into the Athabasca River on June 10 after the first big rainfall event.

The mouth of the Horse River where it empties into the Athabasca River on June 10 after the first big rainfall event.

Sarah Hustins/AEMERA

Cities and towns in Western Canada haven't yet experienced large disruptions to drinking water quality after a wildfire – largely due to luck. Even Alberta's Slave Lake fire in 2011 occurred downstream from the community's water treatment facility. While it destroyed hundreds of properties, it left the treatment facility unscathed.

The biggest problems in water systems don't necessarily manifest in the days and weeks after a fire. It's often the months or even years afterward that highlight the vulnerability of water systems.

In the United States, large fires burning close to communities have provided much-needed data for researchers to better understand the link between wildfires and water quality. Shortly after the 2010 Fourmile Canyon fire in Colorado, researchers found a marginal increase of contaminants in the community's water sources. Researchers then recorded large spikes in contaminants as heavy rain months later pulled in residue from the fire. Researchers are still struggling to make sense of the impact it might have on community drinking water.

The Southern Rockies team has largely focused attention on the 2003 Lost Creek Fire in southern British Columbia. Among many red flags, they have recorded high levels of phosphorus in sediment in the water, which can lead to toxic algae blooms. But with so many variables, it can be difficult to predict the effects fire will have on drinking water quality.

Even with adequate infrastructure in place, when large fires inevitably hit, small communities' water plants are likely to be understaffed.

"We put in our heart and soul just to protect this city … I logged one hundred hours in the first five days," says Guy Jette, the Fort McMurray water plant's manager who has been in his job for 38 years.

"Some [staff] lost their homes and they were here on their next scheduled day without whining."

Cities such as Fort McMurray have the infrastructure to withstand extended shutdowns with their multiple reservoirs, but other towns don't have this capacity.

"All kinds of chemical and biological substances harmful to health could potentially slip through," says Dr. Gerhard Benade of Alberta Health Services and member of the Water Task Force overseeing Fort McMurray. "You're looking at infectious diseases, parasitic diseases and a variety of chemicals."

Drinking-water treatment has improved to quickly filter out known contaminants, especially the toxic material recently found in the soil and ash of Fort McMurray. Instead, it's likely to be the natural contaminants, such as phosphorus, that are the most challenging, says Mr. Jette.

Decades of water treatment infrastructure improvement mean systems are now refined enough to detect trace elements of harmful compounds, says Dr. Emelko. But the finely tuned systems are unable to cope with large fluctuations in water quality naturally occurring after a large fire.

Events like heavy rainfall after a large fire overwhelm the system and force it into lockup.

Mr. Jette recognizes the potential troubles aren't yet over. After the Fort McMurray fire, Dr. Emelko and the provincial government recommended the plant purchase a zeta meter. The device tracks and adjusts chemical levels in real time, and will help the staff to closely monitor water quality over the coming months. It also makes the system more resilient to big swings in water quality.

"In a place like Canada, where we're so water rich, it has been relatively easy to find a new source of high quality water," says Dr. Emelko.

"With increased pressures on water related to industry, development, climate change, those days are now gone."