Lenntech Water treatment & purification

Acids & alkalis in freshwater

Effects of changes in pH on freshwater ecosystems


Types of freshwater pollution

Toxicity response







Organic pollution



Unpolluted deposition (or rain), in balance with atmospheric carbon dioxide, has a pH of 5.6. Almost everywhere in the world the pH of rain is lower than this. The main pollutants responsible for acid deposition (or acid rain) are sulfur dioxide (SO2) and nitrogen oxides (NOx). Acid deposition influences mainly the pH of freshwater.

Nitrogen and sulfuric emissions come from natural and anthropogenic sources. Natural emissions include e.g. volcano emissions, lightning, and microbial processes. Power stations and industrial plants, like the mining and smelting of high-sulfur ores and the combustion of fossil fuels, emit the largest quantities of sulfur and nitrogen oxides and other acidic compounds. These compounds mix with water vapor at unusual proportions to cause acid deposition with a pH of 4.2 to 4.7. That is 10 or more times the acidity of natural deposition.

The acidification of freshwater in an area is dependent on the quantity of calcium carbonate (limestone) in the soil. Limestone can buffer (neutralize) the acidification of freshwater. The effects of acid deposition are much greater on lakes with little buffering capacity. Much of the damage to aquatic life in sensitive areas with this little buffering capacity is a result of ‘acid shock’. This is caused by the sudden runoff of large amounts of highly acidic water and aluminium ions into lakes and streams, when snow melts in the spring or after unusually heavy rains.

Effects on aquatic life
Most freshwater lakes, streams, and ponds have a natural pH in the range of 6 to 8. Acid deposition has many harmful ecological effects when the pH of most aquatic systems falls below 6 and especially below 5.
Here are some effects of increased acidity on aquatic systems:
- As the pH approaches 5, non-desirable species of plankton and mosses may begin to invade, and populations of fish such as smallmouth bass disappear.
- Below a pH of 5, fish populations begin to disappear, the bottom is covered with undecayed material, and mosses may dominate nearshore areas.
- Below a pH of 4.5, the water is essentially devoid of fish.
- Aluminium ions (Al3+) attached to minerals in nearby soil can be released into lakes, where they can kill many kinds of fish by stimulating excessive mucus formation. This asphyxiates the fish by clogging their gills. It can also cause chronic stress that may not kill individual fish, but leads to lower body weight and smaller size and makes fish less able to compete for food and habitat.
- The most serious chronic effect of increased acidity in surface waters appears to be interference with the fish’ reproductive cycle. Calcium levels in the female fish may be lowered to the point where she cannot produce eggs or the eggs fail to pass from the ovaries or if fertilized, the eggs and/or larvae develop abnormally (EPA, 1980).
See also metals in freshwater.

Extreme pH can kill adult fish and invertebrate life directly and can also damage developing juvenile fish. It will strip a fish of its slime coat and high pH level ‘chaps’ the skin of fish because of its alkalinity.
When the pH of freshwater becomes highly alkaline (e.g. 9.6), the effects on fish may include: death, damage to outer surfaces like gills, eyes, and skin and an inability to dispose of metabolic wastes. High pH may also increase the toxicity of other substances. For example, the toxicity of ammonia is ten times more severe at a pH of 8 than it is at pH 7. It is directly toxic to aquatic life when it appears in alkaline conditions. Low concentrations of ammonia are generally permitted for discharge.

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