Quick answer: The EPA drinking water limit for arsenic is 10 parts per billion (ppb), also written as 10 µg/L or 0.010 mg/L. Arsenic has no taste, smell or color, so a lab test is the only way to know if your well has it. Boiling does not remove arsenic. If your water is over the limit, the usual fixes are a reverse osmosis system at the kitchen sink or a whole-house adsorptive media filter, often with an oxidation step first.
Arsenic is one of the most common naturally occurring contaminants in private wells, and it is easy to miss. Penn State Extension points out that many well owners assume their water is fine because it looks and tastes good. With arsenic, that tells you nothing.
What the numbers on your report mean
Labs report arsenic in different units. These are all the same limit:
| Unit on the report | EPA limit (MCL) |
|---|---|
| Parts per billion (ppb) | 10 |
| Micrograms per liter (µg/L) | 10 |
| Milligrams per liter (mg/L) or ppm | 0.010 |
The EPA set this Maximum Contaminant Level in 2001, according to the USGS. Public water systems must meet it. Private wells are not regulated, so it is up to you to test and treat.
Some state health departments also use higher thresholds for urgent action. The Massachusetts Department of Public Health advises that people stop drinking the water right away if arsenic is above 200 ppb. Connecticut’s Department of Public Health asks well owners with levels above 500 ppb to contact the department for guidance.
Health effects of arsenic
Arsenic is a known cause of cancer in people. The main risk comes from drinking water with elevated arsenic over many years, not from a single glass.
According to state health agencies:
- Cancer: Long-term exposure raises the risk of skin, bladder, lung, liver, kidney and prostate cancer (Connecticut DPH and Massachusetts DPH).
- Skin changes: Thickening of the skin and corn-like growths, often on the palms and soles (Massachusetts DPH).
- Heart and blood vessels: Cardiovascular problems are linked to long-term exposure (Connecticut DPH).
- Other chronic disease: Dartmouth’s Arsenic and You program notes that low-level exposure may raise the risk of heart disease and diabetes later in life.
- High short-term doses: Nausea, vomiting, numbness or burning in the hands and feet, and lower red and white blood cell counts (Massachusetts DPH).
Children and other vulnerable groups are a particular concern, according to Penn State Extension. If your family has been drinking water over the limit, mention it to your doctor.
Where arsenic is common
Arsenic comes mostly from natural rock and sediment that the groundwater flows through. Mining and past pesticide use can add to it in some places.
According to the USGS:
- About 7% of domestic wells sampled nationally had arsenic above 10 µg/L.
- Nearly half of sampled wells had at least some detectable arsenic (1 µg/L or more).
- About 2.1 million people in the U.S. get their water from private wells with elevated arsenic.
Levels vary a lot by region. The USGS lists these as areas of concern:
| Region | What the USGS found |
|---|---|
| Southwest | Highest concern; about 16% of drinking water wells sampled exceeded the limit |
| Glacial aquifer system (northern U.S., including New England and the upper Midwest) | Elevated arsenic common in some areas |
| Piedmont and Blue Ridge (eastern U.S.) | Elevated arsenic in some wells |
| Mississippi Embayment and Texas Coastal Uplands | Elevated arsenic in some wells |
Even in lower-risk states, individual wells can be high. Penn State Extension reports that nearly 8% of groundwater samples in Pennsylvania had arsenic at or above 10 ppb. Neighbors’ results are a useful hint, but two wells a few hundred feet apart can differ. Test your own.
How to test for arsenic
- Use a state-certified lab. Ask for an arsenic test specifically; it is not always part of a basic bacteria and nitrate package. Massachusetts DPH estimates the test costs about $15 to $30.
- Collect the sample from a tap before any treatment, such as an outdoor spigot or the kitchen cold tap if you have no filter, following the lab’s instructions.
- Consider a speciation test if you are over the limit. Labs can separate arsenic III from arsenic V for an extra fee. This matters a lot when choosing treatment (see below).
- Test other chemistry at the same time. Iron, manganese, pH and sulfate can affect how well arsenic filters work, so a treatment dealer will want these numbers too.
For a full list of what to test for, see our well water testing guide.
How often to retest: Connecticut DPH recommends testing every well for arsenic at least once. If any arsenic is detected, retest every 5 years, since levels can change over time.
Arsenic III vs. arsenic V: why it matters
Arsenic in groundwater comes in two main forms:
| Form | Charge | How easy to remove |
|---|---|---|
| Arsenic III (arsenite) | No charge (neutral) | Harder to remove; passes through many treatment methods |
| Arsenic V (arsenate) | Negative charge | Easier to remove |
Because arsenic III has no charge, methods that rely on grabbing charged particles miss much of it. The standard solution is oxidation: adding an oxidizer such as chlorine before the filter converts arsenic III into arsenic V, which then filters out much more easily. Connecticut DPH notes that metal oxide filters work on both forms when pre-oxidation is used.
Treatment options
Here is how the common systems compare, based on guidance from Connecticut DPH and Massachusetts DPH:
| Method | Where installed | Works on | Notes |
|---|---|---|---|
| Reverse osmosis (RO) | Point of use (one tap) | Arsenic V: about 95–98% removal. Arsenic III: only about 60–65% | Wastes about 3–4 gallons for every gallon treated. See reverse osmosis for well water. |
| Adsorptive media (metal oxide such as iron-based media, or activated alumina) | Point of use or whole house | Both forms, with pre-oxidation for arsenic III | Media wears out and must be replaced. |
| Anion exchange | Whole house | Arsenic V only | Cannot remove arsenic III because it has no charge. |
| Oxidation + filtration | Whole house | Converts arsenic III to V before a filter | Often paired with media or used where iron is also present. |
What does not work: The Ohio Department of Health states that water softeners, activated carbon filters and sediment filters do not remove arsenic. A typical whole-house water filter sold for taste and odor will not solve an arsenic problem unless it is specifically designed and certified for arsenic.
Point of use or whole house? Dartmouth’s Arsenic and You program recommends a point-of-use system for levels up to about 250 ppb, since you mainly need safe water for drinking and cooking. At 250 ppb or higher, it recommends point-of-entry (whole-house) treatment. Whatever you choose, look for a system certified by NSF to reduce arsenic; Dartmouth advises choosing one that brings arsenic down to 5 ppb or lower.
Costs: Massachusetts DPH reports a median installation cost of about $1,200 for a point-of-use system, with about $343 a year in maintenance. Whole-house systems had a median installation cost of about $3,000, with about $550 a year in maintenance.
Will a new well fix it? Not necessarily. The Ohio Department of Health warns that because arsenic comes from the local geology, there is no guarantee a new well will have lower levels.
Why boiling does not help
The Ohio Department of Health and Dartmouth both state it plainly: boiling does not remove arsenic. Unlike bacteria, arsenic is not killed by heat. As water boils off, the arsenic stays behind, so the remaining water can become more concentrated. Use bottled water or treated water for drinking and cooking until you have a system in place.
Retesting after treatment
Arsenic treatment only works as long as it is maintained. Media gets used up, and RO membranes wear out.
- Test the treated water soon after installation to confirm it is below the limit.
- Dartmouth recommends testing treated water every few months at first, until you know how often your system needs maintenance, then once a year.
- Connecticut DPH recommends testing treated water at least annually.
- Keep records of every result and every filter change, so you can spot a system that is wearing out.
Frequently asked questions
Is it safe to shower in water with arsenic?
Generally yes, at typical well levels. Massachusetts DPH says arsenic is not absorbed through the skin and does not evaporate into the air, so bathing and washing are considered safe unless arsenic is above 500 ppb.
Can you taste or smell arsenic in well water?
No. Arsenic has no taste, odor or color, according to Penn State Extension. Clear, good-tasting water can still be above the limit.
Will a Brita or refrigerator filter remove arsenic?
Most standard carbon filters are not designed for arsenic, and the Ohio Department of Health notes that activated carbon is not effective. Only use a filter that is specifically certified to reduce arsenic.
Is arsenic below 10 ppb completely safe?
The 10 ppb limit is a regulatory standard, not a line between safe and dangerous. Dartmouth notes that even low-level exposure may raise long-term health risks, which is why it suggests treatment systems that reduce arsenic to 5 ppb or lower.
How long does it take to get arsenic test results?
It depends on the lab and on whether you ordered a speciation test. Ask the lab when you order the kit, and use bottled water in the meantime if you have reason to suspect a problem.
Related guides
- Well water testing: what to test for and how often
- Reverse osmosis for well water
- Buying a home with a well
Sources
- USGS – Arsenic and Drinking Water
- Connecticut Department of Public Health – Arsenic in Private Well Water
- Massachusetts Department of Public Health – Arsenic in Private Well Water FAQs
- Ohio Department of Health – Arsenic Treatment and Removal for Private Water System Wells
This guide is for general information. Have your water tested by a state-certified lab, and ask your doctor if you have been drinking water with arsenic above the limit.
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