Effects of Emission Reductions on Lake Water Quality in Western Québec  (1999)

Indication of a change in water quality between 1982 and 1996

The following figure presents the evolution of sulphate concentrations with reference to the distance separating the lakes from Rouyn-Noranda for 1982 and 1996. This figure enables us to see that the sulphate gradient, where maximum values are observed near Rouyn-Noranda, has clearly receded since 1982.  The sulphate concentration gradient for 1996 is now less steep, which emphasizes the significant decrease in sulphate concentration that occurred near the emission source of sulphur dioxyde. The mean sulphate concentration has dropped from 7.4 to 5.1 mg/l since 1982 with specific decreases ranging from 2 to 9 mg/l. This decline in sulphate near Rouyn-Noranda was also correlated with a similar drop in sulphate concentrations in precipitation and for other Québec lakes.

Change in sulphate concentrations that occurred
between 1982 and 1996 for lakes in the Rouyn-Noranda area.

The surface water analyses for 1982 and 1996 also show that change is not only limited to sulphate. Non-parametric statistical tests (such as the Wilcoxon paired test) have enabled us to detect a significant decrease in conductivity, calcium, magnesium, potassium, manganese and dissolved aluminum values. Significant increases were also observed for alkalinity, true colour, tanins, nitrate and iron. 

No significant change has been reported for pH, which represents a prime indicator when looking for a potential improvement in acidic conditions. However, the study of another set of ten clearwater walleye lakes, which were mostly acidic during the 1991 survey (4.9-5.7: mean 5.42 units), shows that lake pH conditions have indeed improved over the past few years. Mean pH in these lakes has increased by 0.44 since 1991. These lakes now exhibit pHs ranging between 5.52 and 6.24. Such an improvement now makes fish restocking possible.

The reason why mean pH of the 64 lakes has not changed much since 1982 could be explained by the fact that several of them may have been partly acidic before the onset of human-induced acidification, or because some substances may have slowed down or impaired acidification recovery. Other factors such as damages too great to ensure a successful recovery, acidic deposition still too high, or a long response time before recovery, are all possible reasons that could partly explain the lack of change. However, examination of relative concentrations of major anions with the help of a lake classification nomogram, and the careful study of physico-chemical indicators such as bicarbonate/sulphate ratio, sulphate/basic cations ratio, and alkalinity/basic cations ratio, all clearly show that there has been a significant improvement in surface water quality conditions for 80% of the sampled lakes. Even in the absence of a clear change in pH, all other changes point toward what could be expected following a decrease in sulphate inputs and a drop in the chemical alteration rate induced by acidic deposition.  

Is lake acidification receding?

Results show that pH has remained fairly stable since 1982, but several other water quality variables indicate an actual improvement trend in the acidity conditions of lakes. The walleye lakes do show that there is an acidification recovery going on for clearwater lakes that were known to be acidified in the past. Scientific literature indicates that acidic lake recovery (e.g. pH increase) is a gradual process that can take many years before becoming visible. In the worst cases where the basic cation pool is depleted, this recovery may take even longer to be achieved, if ever.

Recent studies also indicate that acidic lakes may only recover after acidic deposition reaches a value lower than the lake critical load. In fact, a large number of lakes in southern Québec, including those of the Abitibi-Témiscamingue area, are characterized by sulphate critical loads as low as 8 to 12 kg/ha/yr, while actual deposition still range between 15 to 30 kg/ha/yr, depending on the location. This means that actual deposition is still too high to allow recovery. We will need to wait until all emission reductions are completed in both Canada and the United States in order to evaluate the true benefits. One thing is certain, only a given fraction of the affected lake population will recover following these emission reductions because the initial goal was to protect moderately sensitive ecosystems. This goal was not intended for highly sensitive lakes, which are present in large numbers in Québec.

The nitrate effect

The international community began to consider the role played by nitrates (NO₃^-) and ammonia (NH₄⁺) in surface water acidification in the early 90s. Before then, the role of nitrate in water acidification was considered minimal. Recent studies show that may not be the case, that nitrates may even increase acidification further over the next 25 to 75 years.  According to these studies, nitrate concentrations are generally low until soils in the watershed become saturated with nitrogen. From that moment, the soils may leach out nitrates to the surface waters and hence provide an acidification agent as potent as sulphate. This could increase the acid load in surface waters of many sectors of the province. One of the advance signs of such soil nitrogen saturation is the significant increase in NO_x concentrations during fall and winter.

Even a small increase is to be taken seriously. Where the lakes from this study are concerned, statistical tests have not been able to detect a significant change in nitrate concentrations between 1982 and 1996, although a near significant change was detected with the Sign test at the 0.06 confidence level. The following figure shows that nitrate concentrations have increased by more than 200% in the immediate vicinity and northeast of Rouyn-Noranda, even though nitrate deposition has not changed in recent years and there is no local source of nitrogen. It is thus possible that these increases may be advance warnings that nitrogen saturation is beginning to affect lakes that were previously highly impacted by acidic deposition. It is too soon to conclude from these advance signs what role nitrates will have in surface water acidification, but these certainly imply that we must follow future trends in this variable more clearly.

 
Click to enlarge

Percentage of change in nitrite-nitrate concentrations between 1982 and 1996 for lakes from the Rouyn-Noranda area (values below 100% indicate a decrease in concentration while those above 100% imply a concentration increase).

Conclusion

The results of the 1982, 1991 and 1996 lake surveys show that SO₂ emission reductions have a significant impact on the water quality of these lakes. Statistical tests and other environmental indicators prove that a majority of lakes, especially the clearwater lakes that were acidic during the 1982 and 1991 surveys, are now recovering from acidification. However, acidity levels have not changed much for lakes with coloured waters. Although we can take comfort from the knowledge that our actions to reduce emissions have led to improvements in the acidity conditions of the lakes, the slow rate of recovery of acidic lakes should nonetheless convince us to take a hard alttitude toward any possible loosening of upcoming actions. There is no doubt, therefore, that environmental monitoring of the lakes is still needed to insure that the upcoming reductions will be sufficient to help the damaged water resources recover.