Tap & Air

Private wells

Arsenic and nitrate in well water: who's at risk and how to remove them

The two most common health-related well contaminants. Where they show up, who is most at risk, why you can't boil them out, and which treatment works.

Arsenic and nitrate are the two contaminants most likely to put a private well over a health-based limit. Neither has a taste, color or smell, and the only way to know is a lab test. Our state pages show modeled levels for every county, built from USGS models via the CDC Well Water Index.

Arsenic

Arsenic comes from rock, not pollution, so it shows up in rural wells far from any industry. USGS estimates about 2,101,600 people in the lower 48 states use wells with arsenic above 10 micrograms per liter, the limit for public water systems. Long-term exposure raises the risk of several cancers, heart disease and diabetes, which is why the limit is set for a lifetime of drinking.

  • Test at least once, and again every few years or if the water changes. Levels can vary between neighboring wells drilled into the same rock.
  • Treatment: reverse osmosis at the kitchen tap is the usual choice, since arsenic is a drinking and cooking concern, not a bathing one. Look for a system certified to NSF/ANSI 58 for arsenic reduction.
  • Know the form. Arsenic exists in two forms. Reverse osmosis and adsorptive media remove arsenate (arsenic V) well but arsenite (arsenic III) poorly; an oxidizing pre-treatment converts one to the other. Ask the lab for speciation if your level is high.
  • Whole-house adsorptive media systems exist for very high levels or when you want every tap treated, at a much higher cost.

Nitrate

Nitrate comes from fertilizer, manure and septic systems, so it is highest in shallow wells in farm country and near older septic systems. The limit for public water is 10 mg/L (as nitrogen), set to protect babies under six months, whose blood can lose the ability to carry oxygen when formula is mixed with high-nitrate water.

  • Test every year, and before using well water for infant formula.
  • Do not boil it. Boiling concentrates nitrate rather than removing it.
  • Treatment: reverse osmosis, anion-exchange systems or distillation. Carbon filters and softeners do not remove nitrate.
  • Fix the source where you can: a well cap that seals, distance from septic fields and manure, and a casing that extends above ground.

One system for both

A single under-sink reverse osmosis system handles arsenic (in the right form), nitrate, uranium and lead for drinking water. Retest after installation to confirm it works on your water, and change the membrane and filters on schedule.