May 30 (Reuters) - Canada has agreed to assess whether

naphthenic acids found in northern Alberta's oil sands tailings

ponds should be classed as toxic under federal law, a move that

could pave the way for stricter regulations, environmental group

Ecojustice said on Thursday.

Tailings - a mix of water, clay, sand and trace metals - are

a byproduct of extracting bitumen from mined oil sands and are

stored in huge engineered ponds, some of which have been

accumulating water since the 1960s.

The ponds have come under increased scrutiny since last

year, when it emerged wastewater had been leaking for months

from a tailings pond at Imperial Oil's ( IMO ) Kearl mine.

Ecojustice wrote to Canada's Environment Minister Steven

Guilbeault in March formally requesting that the government

study the risk that naphthenic acids from oil sands tailings

pose to human health and the environment.

"Both the request and the information being collected and

generated by the Government and others provide scientific

evidence on the potential toxicity of OSPW NAs (oil sands

process-affected water naphthenic acids) that warrants further

investigation," Guilbeault responded in a letter dated May 28

that was shared by Ecojustice.

Imperial and Canada's two other oil sands mining companies,

Suncor Energy ( SU ) and Canadian Natural Resources Ltd ( CNQ )

, did not immediately respond to requests for comment.

The chemical compound will be added to a plan prioritizing

which substances should be assessed under the Canadian

Environmental Protection Act, Guilbeault added. That plan will

be finalised by June 2025.

The request was submitted by Ecojustice on behalf of the

Athabasca Chipewyan First Nation, which lies downstream of the

oil sands, and environmental groups. The nearby Mikisew Cree

First Nation also submitted a similar request, Ecojustice said.

Naphthenic acids from tailings ponds have been found in

surface water and groundwater in the region, said Ecojustice

lawyer Bronwyn Roe.

"The harms that these chemicals cause to people and the

environment need to be better understood, and NAs need to be

regulated to prevent harmful exposure," Roe said in a news

release.

Scientific studies have shown that naphthenic acids from oil

sands tailings are toxic but the Canadian government has never

formally classed them as such, said Alienor Rougeot, climate and

energy program manager at Environmental Defence.

"If they are found to be toxic that would unlock a lot of

regulatory tools," Rougeot added.