A naturally occurring element that forms a number of poisonous compounds. Organic arsenic is less harmful than inorganic. Inorganic arsenic compounds are mainly used to preserve wood, insecticides, pesticides and weed killers. Pentavalent arsenic is generally the most common form found in surface waters while the trivalent form is found mainly in ground waters and deep lake sediments. The concentration of arsenic in drinking water should not exceed 0.007 mg/L.