Arsenic

At a glance: Arsenic is a natural element that dissolves from rock into groundwater, and USGS estimates about 2.1 million Americans get drinking water from private wells above EPA’s 10 µg/L limit. Long-term exposure raises the risk of several cancers and other diseases. It has no taste, color or smell, so every well should be tested at least once. If yours is high, a certified reverse osmosis unit or adsorptive media filter at the kitchen tap is the usual fix.

Public water systems must meet the 10 µg/L limit. Private wells are the owner’s responsibility.

What it is and where it comes from

Arsenic occurs naturally in rocks and soil, and small amounts dissolve into groundwater, according to the Minnesota Department of Health. USGS says nearly all arsenic in US groundwater is likely from natural sources. EPA’s health table also lists runoff from orchards as a source.

It is patchy and hard to predict. USGS mapped the highest-risk areas for private wells across much of the West (Washington, Oregon, Nevada, California, Arizona, New Mexico), parts of the Northeast and Midwest (including Maine, New Hampshire, Massachusetts, New Jersey, Michigan, Wisconsin, Ohio, Indiana and Illinois), and some southeastern coastal states. Maps show where risk is higher, but only a test tells you about your own well, which is why MDH advises testing every private well.

Arsenic in groundwater comes in two forms. Arsenic III (trivalent) and arsenic V (pentavalent) behave differently in treatment, and MDH notes both occur in its groundwater.

Health and household effects

Arsenic is a health issue only. It causes no stains, taste or smell.

EPA’s health table lists skin damage, circulatory system problems and increased risk of cancer. MDH adds that long-term exposure to low levels is associated with diabetes, cancers of the bladder, lungs and liver, cardiovascular disease, and effects on children’s development, as well as skin lesions and discoloration. WHO lists arsenic among its 10 chemicals of major public health concern.

MDH notes that even levels below 10 µg/L carry some long-term cancer risk, which is why EPA’s health goal is zero.

Limits and what they mean

Standard	Level	What it means
EPA MCL	10 µg/L	Enforceable for public systems since January 2006
EPA health goal (MCLG)	Zero	No level is considered risk-free
WHO guideline	10 µg/L	Provisional international guideline
New Jersey standard	5 µg/L	Stricter state limit for public systems

A USGS study of domestic wells found arsenic above the health benchmark in roughly 5 to 7 percent of wells, alongside nitrate and manganese. A result of 6 or 8 µg/L is below EPA’s limit, but given MDH’s note on long-term risk it is worth discussing with your health department if you drink the water every day.

How to test

Use a state-certified lab. Wisconsin’s state lab charges $38 for arsenic. MDH recommends testing every private well at least once, and retesting a new well after six months or more if the first result falls between 5 and 20 µg/L. In New Jersey, the Private Well Testing Act requires arsenic testing when a home with a well is sold.

If the result is high, ask for arsenic speciation ($150 at Wisconsin’s lab), which reports arsenic III and arsenic V separately. That decides whether you need a pre-oxidation step. Our well testing guide covers labs and sampling.

Treatment options

Method	How well it works	Certification to look for	Cost range
Reverse osmosis, point of use	High for arsenic V	NSF/ANSI 58, arsenic (pentavalent)	$300 to $1,500
Adsorptive media (specialty media), point of use	High for arsenic V	NSF/ANSI 53, arsenic (pentavalent)	$300 to $700, plus $300 to $500 every 6 to 12 months
Adsorptive media, whole house	High for arsenic V	NSF/ANSI 53	$2,400 to $4,500, plus $700 to $900 a year
Chlorination as pre-oxidation	Converts arsenic III to V	None	$500 to $2,500
Iron filter or air injection	Low; only with high iron	None	Not an arsenic fix
Distillation	High	NSF/ANSI 62	$300 to $1,200

Cost ranges are Minnesota Department of Health estimates based on 2017 and 2018 quotes, low end self-installed and high end professionally installed.

Know which form you have. NSF certifications for arsenic cover arsenic V. MDH says pre-oxidation with chlorination, aeration or ozonation may be needed so treatment removes the type of arsenic in your water, and recommends working with a water treatment professional. MDH adds that sulfate can also affect which treatment works.

Point of use is usually enough. Most homes treat drinking and cooking water only. See whole-house filter vs reverse osmosis for the trade-offs, and use the filter finder to check which RO systems list an arsenic claim, not just an NSF/ANSI 58 certification.

When to call a professional

Call your county or state health department if a result is above 10 µg/L. MDH advises switching water sources or installing treatment when arsenic exceeds 10 µg/L on repeat sampling.

Work with a certified water treatment professional to choose between RO and adsorptive media, because arsenic form, pH, phosphate, sulfate and iron all affect performance. After installation, test the treated water, then retest at least once a year and whenever you change filters or media.