Heavy Metal

Rock music aside, heavy metals include tin, lead, gold, platinum, selenium, mercury, cadmium, vanadium, iron, nickel, silver, and cobalt. Many of these are toxic in greater than trace amounts, but toxicity (usually expressed as the amount of a given substance needed to kill 50% of a population of organism, or LD50) varies with the tolerance limits for each organism. Many of these elements are contaminants of freestone rivers and emanate from abandoned mining operations. Other metals can also present problems for trout. For example, aluminum, a naturally occurring element found in trace amounts in most soils, is extremely toxic in low levels (100 parts per billion), especially when calcium is in short supply. Aluminum becomes a problem in rivers receiving acid water, as occurs in the spring throughout the North Eastern United States in the form of acid snow melt.

Links
  http://www.chemsoc.org/viselements/pages/pertable_j.htm
http://www.epa.gov/ebtpages/pollutants.html