Arsenic (As) is not easily dissolved in water. Therefore, if it is found in a water supply, it usually comes from mining or metallurgical operations or from runoff from agricultural areas where materials containing arsenic were used as industrial poisons. Arsenic and phosphate easily substitute for one another chemically, therefore commercial grade phosphate can have some arsenic in it. Arsenic is highly toxic and has been classified by the US EPA as a carcinogen, which was derived from toxicity considerations rather than carcinogenicity. The current Maximum Contaminent Level (MCL) for arsenic is 0.010 mg/l as of 01/23/06.
