Testing Guides › Arsenic

Arsenic in Well Water

Arsenic is a naturally occurring element in many rock formations across the United States. It has no taste or odor, and long-term exposure above the EPA limit of 0.010 mg/L is associated with increased risk of bladder, lung, and skin cancers.

Where Arsenic Comes From in Well Water

Unlike most well water contaminants, arsenic is rarely caused by human activity. It occurs naturally in many types of bedrock—particularly granite, certain sedimentary formations, and geothermal areas. When groundwater dissolves minerals from these rock formations, arsenic leaches into the water supply.

The U.S. regions with the highest naturally occurring arsenic in groundwater include:

  • New England (Maine, New Hampshire, Vermont, Massachusetts, Connecticut, Rhode Island) — granite bedrock with naturally elevated arsenic
  • Upper Midwest (Michigan, Wisconsin, Minnesota, Iowa) — glacial aquifer sediments
  • Southwest (Arizona, New Mexico, Nevada, parts of Texas) — volcanic and sedimentary geology
  • Pacific Northwest (parts of Oregon, Washington, Idaho) — volcanic terrain
  • Parts of the Southeast — certain coastal plain sediments

Arsenic levels can vary significantly even between neighboring wells depending on which rock formations they draw from. There is no way to predict whether your specific well has elevated arsenic without testing.

Health Effects of Long-Term Arsenic Exposure

The EPA MCL for arsenic in drinking water is 0.010 mg/L (10 parts per billion). This standard is based on long-term health risk rather than acute toxicity. At typical concentrations found in private wells, arsenic does not cause immediate illness.

Long-term exposure above 10 ppb is associated with:

  • Increased risk of bladder, lung, and skin cancers (the strongest evidence links)
  • Cardiovascular disease and peripheral vascular disease
  • Type 2 diabetes
  • Skin changes (keratosis, hyper/hypopigmentation) at high chronic exposures
  • Neurological effects in children at elevated exposures

The risk is cumulative and related to total lifetime dose. A result of 15 ppb does not mean you are in immediate danger, but it does mean reducing exposure should be a priority.

Treatment Options for Arsenic

Point-of-Use Reverse Osmosis

A reverse osmosis system installed at the kitchen sink is the most practical treatment for arsenic in most households. NSF/ANSI 58-certified RO systems reduce arsenic by 85–99%. This treats the water you drink and cook with, which is where most exposure occurs.

Adsorptive Media Filters

Specialized filter media including iron-based adsorbents (ferric hydroxide, titanium dioxide) are highly effective for arsenic removal and can be configured for whole-house point-of-entry treatment. These are more expensive than RO but treat all water in the home. Arsenic capacity depends on water chemistry—higher iron or phosphate can reduce effectiveness.

What Doesn't Work for Arsenic

  • Boiling — does not remove arsenic; may concentrate it slightly
  • Standard carbon (activated carbon) filters — no arsenic removal
  • Water softeners — no arsenic removal
  • UV disinfection — removes bacteria, not arsenic

Testing for Arsenic

Arsenic requires specific laboratory analysis—it is not included in a basic bacteria/nitrate annual panel. Request it specifically from your certified lab as part of a metals panel or as a standalone add-on test.

If you are in a region with known arsenic issues, test every 1–2 years. Arsenic levels can fluctuate seasonally as water table and redox conditions change.

My arsenic is 8 ppb — just under the limit. Should I still treat?▼
The EPA MCL of 10 ppb is a regulatory limit, not a bright line between safe and unsafe. The health risk is a continuum — lower is better. The EPA's own Maximum Contaminant Level Goal (MCLG) for arsenic is zero, meaning no level is completely without risk. At 8 ppb, you are below the regulatory limit, but long-term exposure still carries some risk. Whether to install treatment at that level is a personal risk decision. For households with children or planning pregnancy, erring on the side of treatment is reasonable.
My neighbor's well has high arsenic — does that mean mine does too?▼
Not necessarily. Arsenic levels can vary significantly between wells even on adjacent properties, depending on the depth, casing, and the specific rock formations the well penetrates. A neighbor's high result is a strong signal to test your own well, but it doesn't mean yours will be elevated. Test to know for certain.
Disclaimer Health effect information is based on EPA and IARC assessments. Treatment effectiveness varies by water chemistry and system design. Consult a certified water treatment professional for guidance on your specific situation.