Arsenic:


Source: Enters environment from natural processes, industrial activities, pesticides, and industrial waste, smelting of copper, lead, and zinc ore. Causes acute and chronic toxicity, liver and kidney damage; decreases blood hemoglobin.
Arsenic is a semi-metal element in the periodic table. It is odorless and tasteless. It enters drinking water supplies from natural deposits in the earth or from agricultural and industrial practices. Non-cancer effects can include thickening and discoloration of the skin, stomach pain, nausea, vomiting; diarrhea; numbness in hands and feet; partial paralysis; and blindness. Arsenic has been linked to cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate. EPA has set the arsenic standard for drinking water at .010 parts per million (10 parts per billion) to protect consumers served by public water systems from the effects of long-term, chronic exposure to arsenic. Drinking water which meets the EPA standard is associated with little to none of this risk and should be considered safe with respect to arsenic.



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