Acid Rain is the increased acid content in precipitation. This is almost always sulfuric and nitric acid, and is attributed heavily to human pollution of lakes, rivers, and groundwater with toxic spill-off from factories and other industrial activity.
Briefly, when chemical content enters the atmosphere or the water system, it can combine with water molecules to form acids, with raise the acidity of precipitation. High acid content can kill plants, erode land and rocks, and kill animals if there is too much in standing water. These chemicals are also released by lightning, which splits oxygen atoms, and volcanic eruptions, which release SO2 and NO2 directly into the atmosphere.
In the Boreal Shield, a biologically diverse ecozone in Canada, the natural rivers and lakes have been damaged by acid rain blowing up the jetstream from Canadian and American factories. Tree growth has been affected, as well as fish and marine wildlife in the area. Current studies show that some regrowth is occurring as ecological standards increase, but it is possible that many of the natural lakes and forests have been damaged beyond repair.