Surface Water Acidity in Québec (1999)

Reduction of airborne pollutants

Reduction of airborne pollutants is the approach prescribed by most governments. Reduction of sulphur dioxyde (SO₂) and nitrogen oxyde (NO_x) is this only method that can ensure a possible acidification reversibility in the medium- or long-term. In North America, several control programs have been adopted and implemented since 1985. Québec wanted to reduce 55% of its SO₂ emissions by 1990, with reference to 1980, which was done and even improved upon. A large part of this reduction can be explained by the fact that Noranda Mines inc. has reduced its emissions by 70% in 1994. This reduction goal may reach 90% by the year 2000. Ontario has also been able to reduce more than 60% of its own SO₂ emissions by 1995. In the United States, the Clean Air Act makes provision for a 40% reduction in SO₂ emissions (in 2 phases of 20%: a first half by 1995, and the second half due by the year 2010), along with a 2 million tons reduction of nitrogen oxyde by the year 2000.

Probable effects of the decrease in airborne pollutants

Ongoing reductions of sulphur dioxyde and nitrogen oxyde will have a major effect on precipitation quality in Québec. In the case of sulphates, which is the main indicator of precipitation acidity, deposition for the years 1986-1990 were greater than 30 kilograms per hectare per year, and were higher than the target load of 20 kilograms per hectare for a large portion of the Québec territory. After 2010, when all control programs will have been fully implemented, most Québec areas should receive less than 20, or even less than 15 kilograms per hectare per year. Only in the Québec area that sulphate deposition will still be higher than the target load. The proximity of the Laurentian highlands, where the annual precipitation is highest in Québec, may explain this fact. Overall, emission reductions should bring a 30 to 50% decrease in acidic deposition in southern and southwestern Québec. Thus, southwestern Québec will benefit the most from the control programs. On the other hand, northeastern areas (Lake St.John and Côte-Nord) will benefit very little (0 to 20%) because acidic deposition from human origin are quite low in these parts. Although important, these emission reductions will not be enough to help recover the majority of acidified lakes.

Use of acidification models can also help to predict the impact of emission reductions and acidic deposition on the acidity of surface waters. According to these models, more than 77% of all acidic lakes in the Outaouais, Mauricie and Abitibi should see their pH climb over 5.5 following implementation of emission controls. Such a recovery should decrease the extent of acidic lake areas. Increases in pH should vary between 0.5 and 1.0 unit for most acidic lakes recently acidified. However, emission reductions will only have a minimal impact on lake recovery in the Côte-Nord area, because the acidity of acidic lakes is mostly coming from natural sources.

Will planned reductions be enough?

Ongoing emission reductions in Canada and the United States will help recover a large number of actual acidic lakes in the areas most impacted by acidic deposition. The recovery will not be complete though. A more stringent criteria will be needed to allow recovery of the remaining lakes still acidic despite the actual emission controls. Most scientists agree that an additional SO₂ reduction in the order of 50 to 75% will be needed to enable a more complete lake restoration. Such reductions would imply consideration of a sulphate target load coming close to 10 to 12 kilograms per hectare per year, which would ensure long-term protection of sensitive ecosystems.