books.google.com - Most people associate fluoride with the practice of intentionally adding fluoride to public drinking water supplies for the prevention of tooth decay. However, fluoride can also enter public water systems from natural sources, including runoff from the weathering of fluoride-containing rocks and soils...http://books.google.com/books/about/Fluoride_in_Drinking_Water.html?id=nrzA2zrqGaMC&utm_source=gb-gplus-shareFluoride in Drinking Water
