Arsenic in Drinking Water

Data source: U.S. EPA, CDC, IARC

What Is Arsenic in Water?

Arsenic is a naturally occurring metalloid element found in Earth's crust. It exists in both organic and inorganic forms; inorganic arsenic (the form regulated in drinking water) is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC). It is colorless and odorless in water and cannot be detected by taste.

Health Effects

Long-term exposure to arsenic in drinking water is linked to multiple serious health outcomes:

• Cancer — skin, bladder, lung, and kidney cancers; the evidence base is among the strongest for any drinking water contaminant
• Skin damage — keratosis (hardening), hyperpigmentation, depigmentation on palms and soles
• Circulatory problems — peripheral vascular disease, blackfoot disease (in severely affected regions)
• Diabetes — associations with type 2 diabetes at low chronic exposure levels
• Neurological effects — peripheral neuropathy, cognitive effects with chronic high exposure
• Developmental effects — fetal exposure is linked to adverse birth outcomes and childhood health impacts

Health effects from arsenic are primarily associated with long-term, chronic exposure rather than short-term events.

How Arsenic Gets Into Water

Arsenic enters drinking water through two primary routes:

• Natural geological sources — weathering of arsenic-bearing minerals releases arsenic into groundwater; this is the dominant pathway for most contamination in the U.S. The western states (Nevada, Arizona, California, Montana, Idaho) and parts of New England and the Midwest have naturally elevated arsenic in bedrock aquifers
• Mining and industrial discharge — historic and active mining operations, smelters, and certain industrial processes release arsenic-containing waste
• Agricultural use — arsenic-containing pesticides used historically (particularly in orchards) have contaminated soils and groundwater in some areas

Private wells drawing from bedrock aquifers in high-risk geology are at greatest risk and are not subject to federal monitoring.

EPA Standards

The EPA Arsenic Rule (effective January 2006) set the current MCL at 0.010 mg/L (10 ppb). This was reduced from the previous standard of 50 ppb, which had been in place since 1975.

The EPA Maximum Contaminant Level Goal (MCLG) — the level at which no known health risk exists — is technically zero, reflecting arsenic's classification as a carcinogen. The 10 ppb MCL represents a balance between health protection and technical/economic feasibility.

Approximately 3,000 community water systems serve water with arsenic levels above 10 ppb, according to EPA data.

How to Remove Arsenic From Drinking Water

• Reverse osmosis (RO) — removes 90–99% of inorganic arsenic; NSF 58 certified point-of-use systems are the most practical residential solution; effective for both arsenite (As III) and arsenate (As V)
• Iron-based adsorption media (e.g., iron oxide, GFO) — highly effective for arsenate (As V); used in whole-house and point-of-entry systems; NSF 53 certified
• Activated alumina — removes arsenate effectively; NSF 53 certified for arsenic reduction
• Distillation — effective but slow and energy-intensive
• Anion exchange resin — effective for arsenate in some configurations

Note: standard carbon block filters are not effective for arsenic removal. Verify NSF 53 or NSF 58 certification specifically listing arsenic.

Arsenic in U.S. Water Systems: What the Data Shows

ZipCheckup aggregates CCR data and EPA enforcement records to track arsenic contamination across the country:

• 15 water systems serving 406 ZIP codes reported arsenic detections in their most recent Consumer Confidence Reports
• 2,371 EPA enforcement actions for arsenic have been recorded across 460 ZIP codes — making it one of the most frequently violated contaminant standards
• In total, arsenic-related data affects 866 ZIP codes in our database

The high violation count reflects both the stringent 10 ppb MCL and the prevalence of naturally occurring arsenic in groundwater, particularly in western states.

Is Arsenic a Problem in Your Area?

Arsenic exceeds the 10 ppb MCL most frequently in the western United States — particularly Nevada, Arizona, New Mexico, Idaho, and parts of California and Montana — as well as in parts of Maine, New Hampshire, and Wisconsin. Private wells in these regions are at elevated risk.

ZipCheckup surfaces EPA SDWIS arsenic violation data for community water systems by ZIP code.