Arsenic in Well Water

Arsenic is a naturally occurring element found in rocks, soil, and water throughout the world. It exists in different chemical forms. In drinking water, the forms of concern are inorganic arsenic, which is more toxic than organic forms. (ATSDR)

What makes arsenic challenging is that it gives no warning. Water with arsenic looks, tastes, and smells normal. The National Institute of Environmental Health Sciences notes that you cannot detect arsenic without testing. (NIEHS)

Arsenic is not an acute poison at the levels found in most drinking water. The concern is long-term exposure over months and years.

The EPA drinking water standard is set to protect against chronic exposure effects. (EPA)

Research links chronic arsenic exposure to increased risk of certain cancers and other health effects. The specifics depend on dose and duration. For private well owners, the practical takeaway is simple: if your arsenic level is elevated, the goal is reducing how much you consume over time.

This guide is informational and not medical advice. If you have concerns about health effects from arsenic exposure, consult a healthcare provider.

In most private well situations, arsenic comes from the geology, not from something you did or a nearby factory.

Natural Sources

Arsenic occurs naturally in certain rock types. As groundwater flows through these formations, it can dissolve arsenic into the water. The U.S. Geological Survey has documented arsenic in aquifers across many regions. (USGS)

Natural arsenic risk varies by:

• The type of bedrock or sediment near your well
• Groundwater chemistry and oxygen levels
• Well depth and which aquifer layer you tap

Two wells in the same town can have different arsenic levels. Your neighbor's results do not predict yours.

Human Sources

Arsenic can also enter water from human activities like historic pesticide use on orchards or industrial processes. In some areas, mining has contributed to elevated arsenic. (EPA)

However, for most private wells with arsenic, natural geology is the cause.

Multiple state health programs recommend testing private wells for arsenic every 3 to 5 years. (ME CDC)

Test sooner if:

• You have never tested for arsenic
• You are moving into a home with a private well
• You have a new well or significant well repairs
• Your area is known for arsenic issues
• You have installed treatment and need to verify it works

If you have never tested, that should be a priority. Some states explicitly recommend at least one arsenic test for all private wells. (MN DOH)

Arsenic testing is straightforward compared to bacteria testing. Samples are less time-sensitive and easier to handle.

Use a Certified Lab

Contact your state certification officer for a list of approved labs. (NIEHS)

What Labs Test For

Most labs report total arsenic. Some can do speciation (separating arsenic forms), which can help with treatment selection, but it is not required for initial screening.

Sample Collection

Follow the lab's instructions. They will provide the correct bottle. Unlike bacteria samples, arsenic samples are not as time-sensitive, but you still need to use the right container and any required preservatives.

Cost and Turnaround

An arsenic test typically costs $20 to $40 when included in a basic metals panel. Results usually come back within 1 to 2 weeks.

The EPA Standard

The EPA Maximum Contaminant Level (MCL) for arsenic in public water systems is: (EPA)

• 10 parts per billion (ppb)
• Same as 10 micrograms per liter (µg/L)
• Same as 0.010 milligrams per liter (mg/L)

Private wells are not regulated by federal rules, but this standard is widely used as a reference point.

What Your Number Means

• Below 5 ppb: Generally considered low. Continue periodic testing.
• 5 to 10 ppb: Approaching the limit. Consider retesting to confirm.
• Above 10 ppb: Exceeds the EPA standard. Consider treatment options.

If your result is close to 10 ppb, retesting helps confirm whether the level is persistent before investing in treatment.

If your arsenic exceeds 10 ppb, take practical steps to reduce exposure while you plan a longer-term solution.

Step 1: Confirm the Result

Consider retesting, especially if the result is borderline. Confirm persistence before investing in treatment.

Step 2: Reduce Exposure Now

Because arsenic concerns are about long-term ingestion, focus on water you consume directly:

• Drinking water
• Cooking water
• Beverages and ice

Using bottled water for these uses is a simple short-term step while you evaluate treatment options.

Step 3: Choose a Treatment System

Several treatment methods can reduce arsenic: (Dakota County)

• Reverse osmosis: Often installed under the kitchen sink. Effective for point-of-use treatment.
• Adsorptive media: Specialized filters designed to capture arsenic.
• Distillation: Effective but slower and less convenient for high volumes.
• Ion exchange: Can work depending on water chemistry.

Step 4: Verify Treatment Performance

After installing any treatment system, test the treated water to confirm it is working. Retest periodically because treatment performance can decline over time.

You cannot remove arsenic from your aquifer. But you can manage your exposure.

Track Your Results Over Time

Keep a record of every arsenic test. Levels can change, and having a history helps you make informed decisions.

Do Not Assume Based on Neighbors

Arsenic can be highly variable even within small areas. USGS research shows local "hot spots" are common. (NGWA)

Maintain Treatment Systems

Treatment is not "set and forget." Filters need replacement. Membranes wear out. Schedule regular maintenance and periodic retesting to verify continued performance.

Use a state-certified laboratory for arsenic testing. Your health department can recommend labs that serve your area.

Find well water testing providers in your state using our directory.