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 in water has no color, taste or odor.
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.
The current MCL for arsenic is 0.010 mg/l as of 01/23/06.
Information about arsenic in water can be found on the US Environmental Protection Agency website. Arsenic in drinking water