Lead:


Source: Enters environment from industry, mining, plumbing, gasoline, coal, and as a water additive. Affects red blood cell chemistry; delays normal physical and mental development in babies and young children. Causes slight deficits in attention span, hearing, and learning in children. Can cause slight increase in blood pressure in some adults. Probable carcinogen.

References
http://www.dep.state.fl.us/water/drinkingwater/inorg_con.htm
http://water.usgs.gov/edu/groundwater-contaminants.html