Acid deposition

Acids (e.g., nitric and sulfuric) form as the result of burning fossil fuels and also emanate from various industrial sources (e.g., paper mills, steel mills). When volatilized into the air, they accumulate on dust particles in the atmosphere and eventually settle to the ground, often being brought down by rain. Buffering compounds in the ground neutralize acid-laden rainwater. Unfortunately, many soils in the industrialized regions of the world have been depleted of its buffering capacity. When the soil can no longer negate the acids deposited, leaching of heavy metals occurs (e.g., calcium, magnesium, molybdenum). Aluminum is the last metal to leach out, often creating toxic events in trout streams during the spring when acid snow melts. In northern and central Sweden and Norway, thousands of lakes have been sterilized because they lack an outlet, and the land around those bodies of water no longer has the capacity to neutrali-ze. The same process has evolved in a number of Adirondack lakes in northern New York State.

Links
  http://www.dnr.state.md.us/streams/acid/
http://www.hubbardbrook.org/education/Introduction/Intro13.htm