Mercury as a metal can be hazardous but the actual degree that mercury from power plants is affecting streams and water supplies is actually very low: this comes from the EPA:

"Mercury is a toxic, persistent pollutant that accumulates in the food chain. Mercury in the air is a global problem. While fossil fuel-fired power plants are the largest remaining source of human-generated mercury emissions in the United States, they contribute only a small amount (about 1 percent) of total annual mercury emissions worldwide. "

http://www.epa.gov/CAMR/factsheetfin.html