Turbidity: What is it and why does it affect our drinking water?

For more than a month this spring, Yellowknife was under a boil water advisory that was blamed on turbidity.

Bottled water sales skyrocketed, fountain pop dispensers were off limits at fast food restaurants and children went without slushies.

But what is turbidity and why does it affect our water quality?

Turbidity is measured by the amount of light that can pass through water. The more sediment, the less light passes through.

The problem is that bacteria and heavy metals attach themselves to sediment. That means that a cup of murky water is more likely to contain more bacteria and metals than a cup of clear water.

"It doesn't necessarily imply that (bacteria) are there but with more turbidity in the water, the water becomes more difficult to treat," says Steve Kokelj, a scientist with the N.W.T. Geological Survey.

Factors that affect turbidity are the type of sediment we have, water levels and our landscape.

Water levels

This spring, the Yellowknife region had some of its lowest water levels on record.

"The summer that we had last year was one of the driest on record," says Kokelj.

"During hot summers we have lots of evaporation from our lakes. We have huge surfaces of exposed water so you end up with lower water levels. So there's less water going into the lake and less evaporation."

Low water means the muddy shore is exposed. When waves lap up against it, that mud mixes with the water creating murky water, which scientists describe as water with a high turbidity.

Landscape

About 10,000 years ago, Yellowknife, like most of Canada, was covered by massive glaciers. Those massive sheets of ice weighed down the earth's surface. As those glaciers retreated, meltwater and sediment began to pool, creating large lakes. The Yellowknife area was covered by what's known as glacial Lake McConnell.

As the glaciers continued to retreat, the land began rising making the size of the glacial lakes smaller.

Sediment

But what doesn't always get smaller is the amount of sediment. That stays and pools in the low-lying areas at the bottom of those shrinking lakes. That means Great Slave Lake and its tributaries have a lot of fine sediment in them.

Fine sediment can be an annoyance when it comes to drinking water.

"The particles are very small and they tend to stay in suspension in the water column for a very long time," says Kokelj.

​That means when water and fine sediment mix together, like they did this spring near Yellowknife, the water stays highly turbid for weeks.

Future advisories

So what does this mean for future springs with low water? If we were to experience low water again next year, Kokelj says we probably wouldn't see the high turbidity that we saw this year.

That's because that muddy shore that was exposed by low water this year will grow vegetation through the next few months. That means next year, when waves lap up against it, the vegetation will make the sediment more stable and less likely to mix with the water.

Also, Yellowknife's boil water advisory ended last week when the new water treatment plant came online. It has a filtration system that removes sediment before water is treated, so turbidity should no longer cause problems with the drinking water.

And there should be an unlimited supply of slushies.