What is an acidic lake?
Acidity can be expressed as a function of pH, which value can range from 0 to 14 units. On a strickly chemical standpoint, neutral water has a pH of 7. The water is acidic when the pH is lower than 7, while it is considered alkaline at a greater pH than 7. The pH scale is logarithmic, so that for each decrease of one unit of pH, acidity increases ten-fold.
For scientists who evaluate the effects of acid precipitation on surface waters, the term acid has a slighly different meaning. According to the scientific community, a lake becomes acidic when its pH reaches 5.5, below which value dammages to aquatic organisms become very important. At such a pH, a lake has almost used up all its capacity to neutralize incoming acidic inputs from the atmosphere or the watershed. On the other hand, a lake is considered in transition when the pH varies between 5.5 and 6. Such a lake is not yet acidic, but will become very rapidly with a small increase in acidic loadings. pH is generally very unstable in such lakes, and changes often during the year. For most scientists, a lake is non-acidic when its pH is greater than 6 units. Lakes exposed to low acidic deposition generally exhibit such pHs.
The fact that a lake is acidic does not necessary imply that it was recently acidifed or that its level of acidity is originating from human sources only. Lake acidity can be natural, human-induced, or a combination of both. The natural acidity of a lake is generally linked to the presence of organic matter (brownwater lakes), or in rarer instances, to the presence of acidifying agents from geological sources (sulphur-rich rocks and minerals). Such lakes have probably been acid well before the onset of industrialisation. It is not the case for human-induced acidification. In this case, acidification has occurred sometime between the turn of this century and present days. This is the type of stressful acidification mainly responsible of dammages caused to aquatic organisms.
Contrary to the popular belief, dead lakes are very rare. There is always some kind of tolerant species that can survive in even the most stressful environments. It is not uncommun to observe biological organisms in lakes having a pH as low as 4 units. These organisms are often of the insect and bacteria types. The concept of dead lake is generally used to describe a lake where the entire fish population has disappeared.
We often refer to a lake as been acidic or not acidic. Numerous lakes are acidic all over the year. However, for many lakes where acidity remains low during summer, fall or winter times, the spring runoff can be highly stressful to aquatic life. The acidic substances accumulated in the snowpack during winter are washout in very little time at the beginning of the spring snowmelt. These acidic meltwaters can reduce greatly the pH of nearby streams (as low as 4 units). This episodic acidification may induce a serious stress to certain fish species, whose eggs come to hatch during this time of year.
Pollutant emission reductions still represent the best long-term way to eliminate the problem of acid rain. However, it is sometime necessary to rely on temporary solutions while waiting for these changes to take place. Liming represents one of these alternate mitigation methods.
Liming consists in pouring into the lake a product capable of neutralizing the acidity of lake waters. Finaly-crushed limestone is probably the most common product used in liming. Liming with crushed limestone ensures a gradual increase in water pH up to a level adequate for aquatic organisms. Other products can also be used: lime, hydrated lime, caustic soda, etc. However, these products are not recommended for liming natural habitats, because they are too agressive and generate a much too rapid increase in pH for the aquatic organisms. Continuous liming is also used for small streams, or for lakes that have a low water renewal time.
Liming only constitutes a temporary solution, and must be applied periodically in order to maintain an acceptable pH over the years. Liming does not represent an ideal solution to correct the acidity problem in Québec, because of the very high number of acidic lakes, of the cost involved for liming all these lakes, and the liming criteria that are not met for a majority of Québec lakes (not all lakes can be limed). In Québec, liming was experimentally tested by the Ministry of the Environment and Wildlife. Twenty or so lakes, mostly located in the Trois-Rivières area, are actually limed on a regular basis.
Two concepts are often used when evaluating the benefits of pollutant emission reductions: critical loads and target loads. A critical load represents the maximal deposition of a given pollutant (nitrate or sulphate) that can be applied to a lake or an ecosystem without causing long-term irreversible dammages to biological organisms. The critical load is determined on a scientific basis according to the lake characteristics and those of its watershed. On the other hand, the target load represents an environmental goal that is set by a government or an agency. In 1983, the Canadian Government decided to set a target load for annual wet sulphate deposition at 20 kilograms per hectare, that was supposed to ensure long-term protection of moderately sensitive lake ecosystems. However, recent studies show that this target load is still too high to adequately protect more sensitive lake ecosystems, such as those found in Québec. According to studies from the Québec Ministry of the Environment and Wildlife, and Environment Canada, acidic deposition should not be greater than 10 to 12 kilograms per hectare per year in order to maintain pHs greater than 6 in Canadian Shield lakes.
According to scientists, surface water acidification should be reversible. The experience achieved with emission controls in Sudbury and Coniston, Ontario, shows that emission reduction did indeed improve the surface water quality of neighbouring lakes. Field experiments in Ontario and Norway confirm these results. Acidification recovery was also observed in clearwater lakes near Rouyn-Noranda, where a mean 0.5 increase in pH was measured between 1991 and 1996. This improvement correlates well with the decrease in wet sulphate deposition (more then 30% decrease since 1985). However, pH of the more colored acidic lakes did not change much over this period, even though there are signs of improvement for other water quality variables. The slow recovery of these colored lakes may be explained by several factors: acidic deposition still too high, time of recovery longer than initially thought, presence of chemical substances such as organic acids slowing down the recovery time, etc. Although chemical recovery seems possible, even if a bit long in certain cases, the re-establishment of the original fish populations may only be partial. It would also take much more time to achieve. Certain fish species will probably be lost forever because of the complexity of the colonization process taking place in a natural habitat. Thus for fish, the biological reversibility may even require restocking fish populations in previously acidic lakes.