When the Spill Line rings at Environment Canada in Whitehorse, Environmental Inspector Stephen Arrell is ready to deal with all sorts of human-made problems, from a rusty barrel in the bush to an overturned tanker pouring its contents into a stream.

But sometimes the apparent problem is no problem at all. Sometimes what looks like a spill of hazardous substances is actually a natural phenomenon, says Arrell.

Recently he received a call about a rainbow-like sheen on a pond. The caller thought it might be oil leaking from rusty barrels in the vicinity. When Arrell checked out the pond, he found a thin layer of naturally-occurring organic material floating on the water's surface.

The organic layer is a fairly common sight throughout the summer on standing water in the Yukon, says Arrell. It looks like oil, but the sheen is more glassy, more mirror-like, than an oil slick and there's no distinctive smell of oil. It also behaves differently when it's disturbed.

"If you throw a rock into a pond with a surface layer of oil, the layer will break apart and then flow back together again," he says. "If the surface layer is organic, it breaks apart and stays apart in broken, angular pieces."

Later in the summer, there's another natural phenomenon that often sparks calls to the Spill Line. If conditions are right, a form of algae that grows in Yukon waters multiplies rapidly and forms sheets that look remarkably like toilet paper. The sheets catch on branches or get tangled in fishing nets, prompting calls to the Spill Line about a possible sewage leak.

Under a microscope, the resemblance to soggy toilet paper disappears, says Arrell. Magnification reveals the cellular structure of the algae.

Although the algae bloom occurs naturally, the staff at Environment Canada don't mind when people report it.

"It's a good indication of nutrients in the water," Arrell explains. Occasionally an unusually abundant algae bloom in an abnormal location will be the tip-off that extra nutrients are reaching the water body, possibly because of run-off from agricultural chemicals or sewage.

There are several other common natural phenomena that people often mistake for chemical or fuel contamination, Arrell says.

In June, when pine trees release their pollen, the pollen settles like dust on the surface of ponds and lakes. Often, the wind blows it across the surface and concentrates it in small bays, where it forms greenish-yellow scum that looks unnatural to some people.

Occasionally, a red scum appears. When it was reported a few years ago, says Arrell, Environment Canada personnel examined samples under a microscope and found that the reddish powder forming the scum was pollen from a fungus that grows on spruce trees. It appears less regularly than the yellowish pine pollen because the fungus only flourishes and releases spores when its host trees are under some kind of stress.

A more common phenomenon is the dirty-white foam that appears fairly often on fast-moving rivers or lakes that have been churned up by wind. The foam is formed by fine organic matter that mixes with water and is aerated by wave action or a fast current, Arrell says.

"It blows into bays and piles up," he says. "It can get quite thick. I've seen it a foot deep."

The foam is harmless, as are most of the natural phenomena mistaken for spills. Nevertheless, people should look closely at any unusual substance, Arrell says, and report it to the Spill Line if they have any concerns. The sheen on a pond might be organic, but it might also be oil leaking from a long-abandoned, rusted-out oil drum. Leaking drums of oil and other chemicals have been discovered in fairly remote locations in the Yukon in the past, and might be again, he says.

"Oil can turn up in a lot of places that you wouldn't think it should be," says Arrell. It's better to report anything that looks odd, even if it turns out to be harmless pollen, rather than take the chance of missing something truly dangerous, he says.

To report a spill in the Yukon, call the Yukon Spill Line at (867) 667-7244. For more information about hazardous substances, contact the Environmental Protection branch of Environment Canada in Whitehorse.