Arsenic, Iron, and Manganese: Common Well-Water Contaminants

Arsenic in well water

Arsenic is a naturally occurring element in many U.S. aquifers. USGS's national arsenic occurrence study identified elevated arsenic in private wells across the western U.S. (Nevada, parts of California, Idaho, Montana), the Upper Midwest (Wisconsin, Michigan), and parts of the Northeast (New Hampshire, Maine, Massachusetts). Approximately 2 percent of U.S. private wells exceed the 10 ppb federal MCL.

EPA's arsenic MCL is 10 micrograms per litre (10 ppb), reduced from 50 ppb in 2001. The MCLG is zero. Public water systems must meet 10 ppb at the customer tap; private well owners are not subject to federal regulation, but many state DOH agencies recommend testing well water for arsenic at well construction and periodically thereafter. New Jersey, New Hampshire, and California have set state arsenic notification levels at or below 5 ppb, more stringent than the federal MCL.

Arsenic exists in two oxidation states relevant to drinking water:

• Arsenic V (pentavalent, As[V]): The oxidised form, dominant in surface waters and oxygenated groundwater. Carries a stronger negative charge. Removable by reverse osmosis (NSF/ANSI 58 arsenic reduction claim) and by anion exchange.
• Arsenic III (trivalent, As[III]): The reduced form, common in deep wells and reducing aquifer environments. Carries a neutral charge. Not effectively removed by RO or anion exchange without prior oxidation. Free chlorine or potassium permanganate oxidises As(III) to As(V), which is then removable.
• Iron-based adsorption media: Granular ferric oxide and similar specialty media adsorb both As(V) and As(III). Used in some residential POE arsenic systems where oxidation is impractical.

Iron in well water

Iron is the most common metal complaint in private well water nationally. EPA classifies iron as a Secondary Drinking Water Standard at 0.3 mg/L; this is a Secondary MCL, not legally enforceable for utilities, set primarily for aesthetic concerns (taste, staining). Iron is not a federal health-MCL contaminant.

Iron exists in two forms relevant to home treatment:

• Ferrous iron (Fe2+, dissolved): The clear-water form, soluble in groundwater that is low in dissolved oxygen. The water emerges from the tap clear and develops orange-brown discolouration as the iron oxidises on contact with air. Difficult to detect at low concentrations without testing.
• Ferric iron (Fe3+, particulate): The oxidised form, visible as orange-brown particles or staining. Insoluble; settles out of water on standing.

Treatment for iron depends on concentration and form. Ferric iron is removable by sediment filtration alone if the particles are large enough. Ferrous iron requires oxidation followed by filtration. Three common configurations:

Oxidising filters (manganese greensand, birm): Specialty media catalyses the oxidation of dissolved ferrous iron to particulate ferric iron, which is then captured by the filter bed. Birm requires dissolved oxygen above 15 percent of saturation and pH above 6.8. Manganese greensand requires periodic regeneration with potassium permanganate but tolerates lower DO and pH.

Air injection / oxidation tanks: A continuous-injection air pump introduces oxygen ahead of a backwashing filter tank. The dissolved oxygen oxidises iron in the tank's upper headspace; the precipitated ferric iron is captured by the filter media below. This is the dominant approach for iron concentrations between 0.3 and 10 mg/L.

Chemical injection plus retention: Sodium hypochlorite or potassium permanganate is injected continuously into the raw water; a contact tank provides reaction time; a multi-stage filter removes the precipitated iron and the residual chemical. Used for very high iron concentrations (above 10 mg/L) or where iron is co-occurring with sulphide odour.

Manganese: similar to iron

Manganese behaves chemically similarly to iron and frequently co-occurs with it in private wells. EPA has a Secondary MCL of 0.05 mg/L for manganese (aesthetic, staining) and a Health Advisory of 0.3 mg/L (10-day) and 0.05 mg/L (lifetime, particularly for infants). Manganese causes black staining on plumbing, dishwasher interiors, and laundry, and at high concentrations is associated with neurological developmental concerns in infants.

Treatment is essentially the same as iron: oxidation followed by filtration. Manganese requires more vigorous oxidation than iron (potassium permanganate is more reliable than dissolved oxygen alone), and manganese removal benefits from elevated pH (above 8.0) when using greensand. Many residential iron filters are dual-rated for manganese reduction.

Treatment train order matters

On a well with multiple co-occurring contaminants, the order of stages in the train matters significantly. The standard order, raw water to treated water, is:

• Spin-down sediment (50 to 100 micron) - captures coarse particulates.
• Oxidation stage (air injection, chlorine injection, or specialty media) - converts dissolved iron and manganese to particulate forms.
• Iron / manganese filter (birm or manganese greensand) - captures precipitated iron and manganese.
• Depth sediment (5 micron) - polishes particulates that escaped the iron filter.
• Activated carbon - removes residual oxidant (chlorine if injected) and any organic contaminants.
• Softener (if hardness present) - replaces calcium and magnesium with sodium.
• UV disinfection - inactivates microbiological contamination, requires clear water from upstream stages.

A common mistake is installing a softener upstream of an iron filter; iron above 0.3 mg/L fouls softener resin and shortens its life dramatically. Softeners belong downstream of iron treatment, not upstream.

Related: Well water guide, RO for arsenic, Well water testing.