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Plumbing Products Register

Contaminant: Arsenic (As+5)

Arsenic (chemical symbol As) is a naturally occurring contaminant found in many ground waters.  The current maximum contaminant level for arsenic is 10 micrograms per liter (?g/l) [micrograms per liter are equivalent to parts per billion (ppb)].   Arsenic has been detected frequently in wells in Brown, Outagamie and Brown counties, and sporadically elsewhere in Wisconsin.  For more detailed information of the occurrence arsenic in Wisconsin groundwater consult:  For a printable brochure that can be shared with family and friends consult: .

Arsenic generally occurs in two forms, pentavalent arsenic [also known as As (V), As (+5) or arsenate] and trivalent arsenic [also known as As (III), As (+3) or arsenite].  In its pure, solid form, arsenic is a silver-gray to white brittle metal.  Arsenic has no odor and is almost tasteless; thus the presence of arsenic in ground water can only be detected by a chemical analytical test.  On a public water supply, this information is provided by the water utility; on a private well, the responsibility is the owners.  Contact your local health department for advice on reputable labs in your area.  In natural groundwater, arsenic may exist as pentavalent arsenic, trivalent arsenic or both.  Particulate arsenic may also be present, especially in the presence of precipitating iron. Although all three of the aforementioned forms of arsenic are potentially harmful to human health, trivalent arsenic is considered most harmful.  For more detailed information on the toxic effects of arsenic consult:

People are exposed to arsenic primarily through the water they drink and foods eaten that are made with arsenic laden water.  Seafood contains naturally high levels of arsenic.  However, the arsenic in seafood is a relativity non-toxic, organically bound form sometimes referred to as “fish arsenic”.  Fish arsenic is rapidly excreted from the body.  Calcium supplements made with oyster shells may also contain high concentrations of arsenic.  The burning of materials that contain arsenic (e.g. treated lumber) in a wood burning stove or fireplace may expose people to dangerous levels of arsenic via inhalation.  For more detailed information on the risks and symptoms associated with arsenic exposure consult:

Trivalent arsenic is more difficult to remove from water that either pentavalent or particulate arsenic.  Trivalent arsenic can be converted to pentavalent arsenic in the presence of an effective oxidant such as free chlorine [e.g. sodium hypochlorite (NaHClO), calcium hypochlorite (CaHClO)], potassium permanganate (KMnO4), ozone (O3/OH-1), and hydrogen peroxide (H2O2).  Note that combined forms of chlorine (e.g. chloramines) are not sufficient to cause the conversion.  In Wisconsin, it’s vitally important that such oxidants not be added directly to a well.  This is because chemical oxidants added directly to a well may serve to further increase the arsenic concentration of the well water, and can cause a safe well to become unsafe in terms of arsenic concentration.  When treating water for arsenic in Wisconsin, the addition of any chemical oxidants must occur outside of the well system, downstream of the pressure tank.   The pre-oxidation step is not necessary if, by way of reputable and repeated testing, it’s determined that the arsenic present is already in the pentavalent form.  The pre-oxidation step is also not required if the water treatment device selected is approved for the reduction of pentavalent and trivalent forms of arsenic.  To differentiate between the forms of arsenic that may be present, a “total arsenic” test is not sufficient; to obtain this information you must request that the arsenic present be speciated.

More information about arsenic and its toxicity can be accessed via the U.S. Environmental Protection Agency website:

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