EDMONTON - A study published in a scientific journal says the Athabasca River that flows through Alberta's oilsands region has become increasingly contaminated over the last decade.

Hydrocarbon chemicals known to be toxic to fish and suspected of causing cancer in humans increased by 40 per cent in river sediments between 1999 and 2009, says the paper published Thursday in Environmental Science and Technology.

But the polycyclic aromatic hydrocarbons remain below levels considered harmful to humans.

Using data from government monitoring programs, study author Kevin Timoney found that PAH levels have been increasing by about .05 milligrams per kilogram of sediment every year. Alberta Environment says the average level sits at about 1.22 milligrams per kilogram. But Timoney's research finds it has increased to 1.72.

"There is a significant upward trend," said Timoney, an independent Alberta ecologist.

The provincial government has long said there's been no change in the levels in the Athabasca. Any apparent differences, it says, are the result of the river eroding greater or lesser amounts of bitumen from the region's naturally occurring oilsands outcrops.

Timoney points out that higher river flow doesn't necessarily deposit more chemicals -- in fact, the opposite is probable as faster rivers carry away more sediment. He also argues the government's model doesn't account for runoff over the nearby landscape.

He found that the more bitumen was mined, the more chemicals were likely to be found in the following year's sediments.

He also points to other recent studies linking deposits to smokestack emissions from oilsands facilities and to land-clearing for oilsands operations.

Timoney said there aren't enough data to determine whether the rate at which the chemicals are deposited is picking up or staying constant.

"When you have only 10 years of data it's really difficult to describe anything other than, 'OK, it's increasing over time."'

But he points out that the chemicals that now exist in the sediments are already at levels that can cause liver disease, reproductive impairment and stunted growth in fish.

"It's definitely at a level that should cause ecological concern."

Human exposure depends on lifestyle.

"If you had a person who was just buying food from the south, that's a very low exposure. But if you have someone who's eating moose and duck and wildlife from the delta, then that's a very different exposure level."

Timoney said his findings question recent conclusions from the Royal Society of Canada that there is "no credible evidence" that contamination from the oilsands is increasing human cancer risks. He said the society relied too heavily on the Alberta government's monitoring program, which has been criticized by several studies -- including the society's.

"There is very credible evidence," Timoney said.

"They are the only scientists who have said there's not a problem. You go through the literature and there's always concerns."

Timoney said his study adds urgency to the need for better environmental monitoring of the oilsands. The Alberta government has appointed a committee of scientific and industry representatives to propose a better approach.

The committee is to report in June.