Arsenic in Drinking Water

What Is Arsenic?

Arsenic is a naturally occurring element found in rocks, soil, and minerals throughout the Earth’s crust. Inorganic arsenic — the form most commonly found in drinking water — is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC), meaning there is sufficient evidence that it causes cancer in humans.

Unlike many drinking water contaminants that originate from industrial accidents or deliberate discharge, arsenic is often present in water simply because the local geology contains arsenic-bearing minerals. The US Geological Survey (USGS) has found detectable arsenic levels in approximately 43% of groundwater wells tested nationwide, underscoring how pervasive natural arsenic contamination is across the country.

Critically, arsenic is odorless, tasteless, and colorless in water. There is no sensory warning that arsenic is present. You cannot detect it by taste, smell, or appearance — only laboratory testing can confirm whether your water contains arsenic and at what concentration.

How Arsenic Gets Into Drinking Water

Natural Geology

Arsenic enters drinking water primarily through natural geological processes. As groundwater flows through rock formations containing arsenopyrite, iron oxides, and other arsenic-bearing minerals, the element dissolves into the water over time. This process is accelerated by specific chemical conditions: low oxygen environments, high pH, the presence of competing ions like phosphate, and the microbial reduction of iron oxides that release bound arsenic into solution.

Groundwater sources are far more likely to contain arsenic than surface water because groundwater is in prolonged contact with arsenic-bearing rock. Private wells are particularly vulnerable, as they are not subject to EPA monitoring requirements and owners may go years without testing. The USGS estimates that roughly 2.1 million people in the US rely on domestic wells with arsenic concentrations above the EPA limit of 10 parts per billion (ppb).

Human Activities

Human activities also contribute to arsenic contamination. Mining operations expose arsenic-bearing ore and generate acid mine drainage that can contaminate nearby water sources. Coal ash disposal — both in ponds and landfills — has leached arsenic into groundwater at numerous sites across the country. Decades of agricultural pesticide use, particularly lead arsenate applied to orchards, left arsenic residues in soil that can migrate to groundwater. Wood preserved with chromated copper arsenate (CCA) — a treatment common before 2004 — is another source.

Geographic Hotspots

Arsenic is not evenly distributed. Certain regions have consistently elevated levels driven by underlying geology:

Region	Typical Levels	Primary Source
Southwest (AZ, NV, NM)	10—100+ ppb	Volcanic geology, arid climate concentration
New England (NH, ME)	5—50 ppb	Metamorphic bedrock
Upper Midwest (MN, WI)	5—30 ppb	Glacial deposits, iron oxide dissolution
Great Plains (NE, SD)	5—20 ppb	Ogallala Aquifer sediments
Pacific Northwest (OR, WA)	5—25 ppb	Volcanic and geothermal sources

In these regions, arsenic concentrations in untreated groundwater can reach 50 to 100 ppb or higher — well above the federal drinking water limit. Residents relying on private wells in these areas face the greatest risk because, unlike public water systems, private wells are not federally monitored or regulated.

Health Effects

Chronic exposure to arsenic in drinking water poses serious health risks, even at relatively low concentrations. The health literature on arsenic is among the most extensive for any drinking water contaminant, drawing on large epidemiological studies in Bangladesh, Taiwan, Chile, and the United States.

Cancer

Cancer risk is the primary concern driving arsenic regulation. Long-term exposure to inorganic arsenic has been definitively linked to increased rates of bladder cancer, lung cancer, and skin cancer. Studies consistently show a dose-response relationship: the higher the arsenic exposure and the longer the duration, the greater the cancer risk. Bladder cancer risk has been observed at concentrations as low as 10 ppb in some populations, which is why the Maximum Contaminant Level Goal (MCLG) is set at zero — there is no known safe threshold for a carcinogen.

Cardiovascular and Metabolic Effects

Beyond cancer, arsenic disrupts multiple organ systems:

• Cardiovascular disease: Long-term arsenic exposure is associated with increased risk of heart disease, stroke, and peripheral artery disease. Arsenic damages blood vessel walls and promotes inflammation, contributing to atherosclerosis.
• Diabetes: Multiple studies have found associations between arsenic exposure and Type 2 diabetes, even at concentrations near the current MCL of 10 ppb. Arsenic appears to interfere with insulin signaling and glucose metabolism.
• Skin changes: Thickening and discoloration of the skin, known as arsenical keratoses, are a hallmark sign of chronic arsenic poisoning. These lesions typically appear on the palms and soles and may be a precursor to skin cancer.

Neurological and Reproductive Effects

• Neurological effects: Peripheral neuropathy — numbness and tingling in the hands and feet — is a recognized effect of chronic arsenic exposure. In children, prenatal and early childhood arsenic exposure is associated with lower scores on tests of intellectual function, reduced IQ, and impaired memory and attention.
• Reproductive effects: Arsenic crosses the placental barrier. Exposure during pregnancy has been associated with increased risk of low birth weight, preterm birth, and stillbirth in highly exposed populations.

Children’s Vulnerability

Children are particularly vulnerable to arsenic’s effects for several reasons. Their developing nervous systems are more susceptible to neurotoxic damage. They consume more water relative to body weight than adults. And early-life exposures can have lasting consequences that persist into adulthood. Pregnant women and families with young children should treat arsenic testing as a priority, especially if relying on a private well in a high-risk region.

EPA Regulation and Limits

The EPA’s current drinking water standard for arsenic is the Maximum Contaminant Level (MCL) of 10 parts per billion (ppb), or 0.010 mg/L. This standard took effect in January 2006, replacing the previous limit of 50 ppb that had been in place since 1942. The reduction from 50 ppb to 10 ppb was itself a major public health improvement, though it was debated extensively because of the treatment costs it imposed on small water systems.

Standard	Value	Notes
MCL (enforceable limit)	10 ppb	Applies to all public water systems
MCLG (health goal)	0 ppb	Set at zero because arsenic is a known carcinogen
Previous MCL (pre-2006)	50 ppb	In place from 1942 to 2006
WHO guideline	10 ppb	Matches current US standard
Proposed alternative (advocacy groups)	3 ppb	Based on updated cancer risk models

Public water systems must test for arsenic regularly — typically annually for groundwater systems and quarterly for those with seasonal variation. Systems exceeding the MCL must notify customers and take corrective action, which may include installing treatment, blending with lower-arsenic sources, or switching supply.

Some health organizations and researchers argue the current MCL should be lowered to 3 ppb based on updated cancer risk assessments that account for lifetime exposure at low doses. The EPA has not revised the standard since 2006, but monitoring continues and the science is actively reviewed.

How Widespread Is Arsenic?

Arsenic is one of the more common drinking water contaminants in the United States. The US Geological Survey’s National Water-Quality Assessment Program has found arsenic above 10 ppb in groundwater across all 50 states, though concentrations and affected populations vary substantially by region.

For public water systems, the EPA’s Safe Drinking Water Information System (SDWIS) tracks compliance with the 10 ppb MCL. While the majority of large public water systems meet the standard — often because they have the resources to install treatment — smaller systems serving rural communities, particularly in the Southwest and New England, have historically had more difficulty maintaining compliance.

The private well situation is more concerning. An estimated 2.1 million Americans rely on domestic wells with arsenic above 10 ppb, and the true number may be higher because millions of private well owners have never tested their water. Unlike public water customers who receive annual water quality reports, private well owners are entirely responsible for their own testing and treatment — and many are unaware of their arsenic exposure.

Regional patterns reflect underlying geology. The Southwest — particularly Arizona, Nevada, and New Mexico — has some of the highest groundwater arsenic levels in the country, driven by volcanic rock and arid conditions that concentrate dissolved minerals. New England’s granite and metamorphic bedrock releases arsenic into well water across New Hampshire, Maine, and parts of Massachusetts. The Upper Midwest’s glacial geology and the Great Plains’ Ogallala Aquifer also produce elevated concentrations in many areas. See the geographic hotspot table in the previous section for a regional summary.

How WaterVerge Tracks Arsenic

WaterVerge pulls arsenic monitoring data from the EPA’s Safe Drinking Water Information System (SDWIS), the authoritative federal database for public water system compliance. Every public water system in the United States must submit arsenic test results to SDWIS — typically annually for groundwater systems or quarterly for those with more complex source water profiles.

WaterVerge displays these results at the city and water-system level, showing both the most recent values and historical trends so you can see whether arsenic levels have been stable, declining, or rising over time. The platform flags any MCL violations and tracks whether the water system has taken the required corrective action following a violation. For private well users, WaterVerge cannot provide individual well data — that requires direct testing — but the site identifies counties and regions with elevated groundwater arsenic risk to help well owners understand whether they should prioritize testing.

How to Remove Arsenic

If your water contains arsenic above 10 ppb — or if you want to reduce any detectable level given the MCLG of zero — several treatment technologies are effective. The right choice depends on whether you want to treat all water entering your home (point-of-entry) or only water used for drinking and cooking (point-of-use).

Method	Removal Rate	Certification	Best For
Reverse osmosis (RO)	90—95%	NSF/ANSI 58	Under-sink drinking water
Adsorptive media (iron-based)	85—95%	NSF/ANSI 53	Under-sink or whole-house
Distillation	98%+	NSF/ANSI 10	Countertop, small volume
Standard activated carbon	Not effective	N/A	Not recommended for arsenic

Reverse osmosis (RO) is the most widely used point-of-use option. A quality RO system removes 90—95% or more of arsenic from drinking water by forcing water through a semipermeable membrane. These systems are typically installed under the kitchen sink and treat water used for drinking and cooking. Look for systems certified to NSF/ANSI Standard 58.

Adsorptive media filters use iron-based media (such as iron oxide or ferric hydroxide), activated alumina, or titanium dioxide to bind arsenic as water passes through. These filters can work at point-of-use or point-of-entry scale. One important consideration: arsenic in groundwater often exists as arsenic III (arsenite), which is harder to remove than arsenic V (arsenate). Some adsorptive media systems require pre-oxidation — adding a small amount of chlorine or using an oxidizing filter — to convert arsenic III before the adsorption step. Check the product’s testing conditions to ensure it performs at the arsenic speciation in your water.

Distillation removes arsenic by boiling water and condensing the steam. Since arsenic does not evaporate with water, it stays behind in the boiling chamber. Countertop distillers can achieve greater than 98% removal and are effective for small volumes.

Standard activated carbon filters — including pitcher filters, refrigerator filters, and basic faucet-mount filters — are not effective at removing arsenic. This is a common and potentially dangerous misconception. Carbon filtration works well for chlorine, some organic compounds, and taste and odor issues, but it does not remove inorganic arsenic at meaningful levels. If arsenic is your concern, you need reverse osmosis, certified adsorptive media, or distillation.

Always look for treatment products certified to the relevant NSF/ANSI standard for arsenic reduction, and replace cartridges and membranes on the manufacturer’s schedule. A filter that has exceeded its capacity may no longer remove arsenic effectively, and there is no sensory warning when a filter stops working.

Private well owners with arsenic above the MCL should also consult with a state-certified water treatment professional, as whole-house treatment may be appropriate depending on water usage patterns and household size.

Related contaminants that share groundwater pathways with arsenic include chromium-6 and nitrate. If your well has elevated arsenic, testing for these contaminants as well is advisable. See also our guide on well water testing for a comprehensive overview of what private well owners should test for and how often.

Check Your City

Arsenic contamination varies significantly by location, water source, and the treatment infrastructure of your water system. Use WaterVerge’s city search to look up the latest arsenic test results for your public water system. You can see current detected levels, historical trends going back years, any MCL violations on record, and how your city compares to the federal limit. If your home uses a private well, contact your local health department or a state-certified laboratory to have your water tested — private wells are not included in federal monitoring data, and testing is the only way to know your actual exposure.