Arsenic
Arsenic occurs naturally in water or enters it through pollution. If a person drinks two liters (more than half a gallon) of tap water that exceeds the former Maximum Contaminant Level of 50 parts per billion (ppb) every day over a lifetime, there is a risk of cancer. EPA reduced this level to 10 parts per billion (ppb) in 2002 and drinking water systems had to comply with the new regulation starting in 2006. The National Research Council estimates that men and women who daily consume water containing 20 ppb of arsenic have about a 0.7% increased risk of developing bladder or lung cancer during their lifetime. According to a recent film funded by the US Superfund, "In Small Doses"., millions of private wells have unknown arsenic levels, and in some areas of the US, over 20 percent of wells may contain levels that are not safe.
Read more about this topic: Drinking Water Quality In The United States, Substances For Which There Are Federal Standards