The 28,000 litres of diesel that spewed into the St. Lawrence River last week — and led to a municipal drinking water advisory in Longueuil, Que. — is just the latest in a long list of oil spills into the river. 

There were 334 spills involving ships in the St. Lawrence River between February 2002 and November 2012, according to federal documents obtained by Radio-Canada. 

The documents also show the limits of the system used by the federal and provincial governments to track the extent of spills and their potential environmental impacts. 

Most of the cases involved diesel, but the documents indicate fuel oil, heavy oil and lubricating oil also leaked into the river.

The amounts varied:

  • Half of the spills were for an amount less than 10 litres.
  • One-quarter were for between 10 and 50,000 litres.
  • One-quarter were of an "unknown quantity."

A ship near Trois-Rivières, for example, leaked an unknown amount of diesel into the water in December 2014.

Neither the provincial nor federal governments could say how much of the 22,000 litres of diesel on the ship went into the water.

Michel Plamondon, a spokesman for the Canadian Coast Guard, said "10,000 litres of pure hydrocarbons were recovered," but said it's impossible to know how much additional oil leaked into the water.

That's a major problem according to Steven Guilbeault, the co-founder and senior director of the environmental group Équiterre.

"We have no information, so is it tens of thousands of litres that end up in the Saint Lawrence River and all of a sudden we’re wondering why beluga whales are doing so bad and the species is declining? We should know. We should have better information," Guilbeault said, adding that much of the onus falls on the companies to report the extent of spills.

The numbers detailed in the documents don't include spills stemming from a source other than a ship, such as the generator leak at the water filtration plant in Longueuil.

Transport Canada said it could not comment on the numbers Radio-Canada obtained, but a spokeswoman told CBC News that reporting spills is a complex process. She said there are many factors that can influence how a spill is reported  such as where the ship is located when it begins to leak — which in turn can affect which government body is responsible for the cleanup.