Irving Pulp and Paper's plan to launch a constitutional challenge to Canada's environmental regulations is expected to shine a bright light on water pollution standards for mills across the country.

The company, which has a mill at the Reversing Falls in Saint John, is charged under the Fisheries Act with 15 counts of dumping a harmful substance into the St. John River.

If convicted of all charges, the company will face a minimum fine of $3 million.

Irving's defence will include a direct Charter challenge against the way a water pollution test used nationwide for several decades is applied by Environment Canada in its pollution regulations.

Known as the acute lethality test, the procedure involves placing live fish in a tank of pure mill effluent to see if they survive.

The test has its critics, including Lynn McCarty, an Ontario based eco-toxicologist who has published papers about it.

McCarty is not involved in the Irving Pulp and Paper Ltd. case but has done consulting work in the past for parent company, J.D. Irving Ltd.

McCarty said the lethality test itself can be useful, but not enough information is gathered to know the actual impact effluent will have on fish once it's discharged into a river, harbour or other body of water.

In an interview, he referred to an old saying in the field of toxicology that everything is a poison — it is only a question of dose.

"Does the toxicity in the effluent mean that that's a deleterious substance when it's deposited into the body of water?" McCarty said. "And my answer would be, 'It depends.' 

"I can open a case of scotch, pour it into an aquarium, put a fish in there. He dies. Then, my God. Alcohol is toxic, so we shouldn't be exposed to it. But people drink it every day and manage to get by because they're not exposed as much as that fish was."

In its Charter challenge, Irving is asking the court to strike down sections of the Fisheries Act tied to the lethality test when it comes to pulp and paper mills.

That suggestion is alarming to Matthew Abbott of the Conservation Council of New Brunswick.

Irving Pulp Paper

Irving Pulp and Paper faces charges under the Fisheries Act of depositing a deleterious substance into the St. John River over a two-year period beginning in 2014. (CBC)

He said the legislation depends on the test precisely because it is difficult to prove harm to the environment after the contaminated substance has disappeared into it.

"This is a really important part of the Fisheries Act that has been applied to great effect to the benefit of all Canadians," Abbott said.

Les Burridge, a retired toxicologist who spent 35 years with the Department of Fisheries and Oceans, said he stands by the test although he acknowledges it's not perfect.

He spent much of his career working with the lethality test to measure the impact of pollutants and farm chemicals on aquatic life.

Evidence of the death of fish in a tank of effluent is not intended to directly link to fish deaths in the wild.

Polluted water, he said, can have have sub-lethal effects, including on fish reproduction or the elimination of the animal's food source.

"We would typically say that lethality tests are designed to identify a hazard," Burridge said. "And that then that piece of information, along with other things, are used to assess risk.

"[The fish] might not eat as well, they might not grow as well."

Court dates will be set Oct. 19.

The six-week trial is expected to unfold in mid to late 2018.