US Arsenic Risk Map (By Region)

Based on the U.S. Geological Survey National Water-Quality Assessment (NAWQA) program, which has analyzed arsenic in tens of thousands of groundwater samples nationwide, here is how the country breaks down. Risk reflects the percentage of wells in each region that exceed the EPA MCL of 10 ppb. Local geology can make any single well an outlier in either direction.

Why Arsenic in Groundwater Is Geographic

Arsenic in well water almost never comes from pollution. It is a naturally occurring element found in the earth's crust, and it dissolves into groundwater through weathering of the bedrock and sediment that water passes through.

Three geological settings produce most of the high-arsenic groundwater in the United States:

1. Granite and metamorphic bedrock in New England (especially New Hampshire and Maine). The bedrock contains arsenic-bearing minerals that release into fracture-flow groundwater.
2. Volcanic rock and ash deposits in the Southwest, Pacific Northwest, and parts of California. Arsenic leaches from volcanic glass and is concentrated by evaporation in arid climates.
3. Glacial sediments and shale aquifers in the Upper Midwest and Northern Great Plains. Arsenic is bound to iron and sulfide minerals in the sediment and is released under reducing (low-oxygen) groundwater conditions.

Because the contamination is geological, it does not respect property lines. A well drilled into one fracture system may be clean while the neighbor's well two hundred feet away pulls from a different fracture and tests at 25 ppb. State and county risk maps are useful for understanding the general pattern, but the only number that matters is the one from your specific well.

For more on how arsenic gets into groundwater and the difference between As(III) and As(V), see our guide on how to read your well water test results.

Check Your State's Arsenic Risk

Select your state to see the typical arsenic risk profile, the state's regulatory limit, and the recommended next step. This is general guidance based on USGS and state health department data. Your specific well can vary.

Region-by-Region Breakdown

New England (High Risk)

New Hampshire and Maine have some of the highest documented arsenic concentrations in private well water in the United States. The cause is the granite and metamorphic bedrock that underlies most of the region. Arsenic-bearing minerals like arsenopyrite and pyrite weather slowly into fracture-flow groundwater, and because so many homes in NH and ME are on private bedrock wells, the population exposed is large.

The New Hampshire Department of Environmental Services has documented towns where over 20% of private wells exceed the EPA MCL of 10 ppb. The state adopted a stricter 5 ppb MCL for public water systems in 2021. Maine's CDC estimates roughly 1 in 10 private wells statewide exceed 10 ppb, with parts of central and southern Maine substantially higher.

Eastern Massachusetts, Rhode Island, Connecticut, and Vermont share the same bedrock geology in pockets. Risk is well-by-well: two homes on the same street can have very different arsenic levels.

Upper Midwest (Moderate-to-High)

Michigan's Lower Peninsula, especially the "Thumb" region (Tuscola, Huron, Sanilac, and Lapeer counties), has the highest arsenic exposure in the Midwest. The Marshall Sandstone aquifer that supplies wells in the Thumb contains arsenic-rich glacial sediments that release into groundwater under reducing conditions.

Wisconsin's Fox River Valley (Outagamie, Brown, Winnebago counties) has well-documented arsenic problems in private wells drilled into the St. Peter and Wonewoc sandstone aquifers. The Wisconsin DNR has special well construction requirements in known arsenic zones.

Minnesota tracks arsenic in private wells through the Department of Health. Roughly 10-12% of new wells statewide test above 10 ppb, with higher rates in the central and west-central parts of the state.

Southwest (High)

Arizona, Nevada, and New Mexico have some of the highest arsenic concentrations in groundwater in the US. The drivers are different from New England: volcanic geology releases arsenic into aquifers, and the arid climate concentrates it through evaporation. Irrigation return flows can also re-mobilize arsenic from sediments. Parts of the Central Valley and San Joaquin Valley in California have similar issues.

Northern Great Plains (Moderate-to-High)

South Dakota, North Dakota, and parts of Montana and Nebraska have elevated arsenic in shallow aquifers. The Pierre Shale and other sedimentary formations release arsenic into groundwater. Many private wells in these areas exceed 10 ppb.

Pacific Northwest (Moderate)

Eastern Oregon, eastern Washington, and the Snake River Plain in Idaho have elevated arsenic from volcanic geology. The Washington State Department of Health publishes specific guidance on private well arsenic and notes that arsenic levels are higher than 10 ppb in many wells across the state.

Mid-Atlantic and the Southeast (Lower Risk Overall)

Pennsylvania, Maryland, Delaware, Virginia, the Carolinas, Georgia, and Florida show generally lower arsenic in groundwater compared to the high-risk regions, though local hotspots exist. NY's southern tier (Steuben, Allegany, Cattaraugus counties) has documented elevated arsenic. Some of the Carolina Slate Belt in NC has localized issues. Across the Mid-Atlantic, private wells should still be tested for arsenic, but it is rarely the headline issue. Acidic water (low pH) and iron tend to be the more common findings here.

Full State-by-State Reference Table

Below is the general arsenic risk profile for each of the 50 states based on USGS NAWQA data and state health department records. Risk reflects what is typical across the state. Local geology varies, and the only way to know your specific well is to test it.

Risk categories are general statewide profiles drawn from USGS NAWQA, state health department reports, and peer-reviewed literature. Even in "lower risk" states, individual wells can exceed 10 ppb. Even in "high risk" states, many wells test below 10 ppb. The only number that matters is the one from your well.

State Regulations Stricter Than EPA

The federal EPA Maximum Contaminant Level for arsenic in drinking water is 10 ppb (10 micrograms per liter). This applies to public water systems, not private wells. The 10 ppb standard was set in 2001, lowered from the previous 50 ppb limit. The EPA Maximum Contaminant Level Goal (MCLG) for arsenic is zero, reflecting that arsenic is a known human carcinogen.

A small number of states have set stricter limits:

The reason NJ and NH set 5 ppb instead of 10 ppb is that the National Research Council's review of the science estimated meaningful cancer risk at concentrations between 5 and 10 ppb, especially with long-term exposure. The EPA originally proposed 5 ppb as well, but settled on 10 ppb at the federal level for cost-feasibility reasons. If you want to be conservative regardless of where you live, treat your water if your test comes back above 5 ppb.

What to Do If You Live in a High-Risk State

If your state shows up as High or Moderate-to-High in the table above, the path is the same regardless of which state it is:

Step 1: Test your specific well

Risk maps and statewide statistics are useful for understanding the general pattern. They cannot tell you your number. Two homes on the same street, drawing from the same aquifer, can have arsenic readings 10x apart depending on the specific fracture pattern at each wellhead.

The right test for arsenic is a certified-lab test, not a DIY kit. Arsenic at parts-per-billion levels requires lab precision (ICP-MS or equivalent). Strip kits and home test kits cannot reliably detect arsenic in the 5-10 ppb range, which is exactly the range where the answer to "do I need to treat?" is decided.

Our Well Water Test Kit is $199. It uses NELAC and ELAP certified labs, includes 53 contaminants (arsenic, plus lead, copper, nitrate, bacteria, hardness, pH, iron, and the rest of the panel a well owner needs), and ships with prepaid return shipping. Aidan reviews the results before any treatment recommendation. There is no obligation to buy a system after the test.

For the arsenic-specific testing walkthrough (sample collection, lab selection, what the report should include), see how to test for arsenic in well water. For broader context, see our general how to test well water guide and how to read well water test results.

Step 2: Treat the water if your result is above the action level

Use these thresholds as a starting point:

Step 3: Choose the right treatment system

For whole-house arsenic treatment, the most reliable approach is arsenic-selective ion exchange resin. This media is engineered to bind arsenic preferentially out of the water and is effective on both forms of inorganic arsenic typically found in well water (As(III) and As(V) after pre-oxidation). MAW carries two configurations:

• Whole House Arsenic Removal System (10 GPM Metered) at $3,895. Backwashing system with a Fleck control valve. Higher flow rate and longer media life. Recommended for most family homes.
• Whole House Arsenic Filter (Non-Backwashing, No Electricity) at $2,895. No drain line, no electricity required. Same WQA Gold Seal arsenic-selective resin. Good for off-grid installations or where a drain line is not practical.

Both systems are listed in the full arsenic filter collection. For a side-by-side comparison and a recommendation on which configuration fits your home, see our guide to the best arsenic water filter for well water. For installation, operating costs, and total cost of ownership, see the full arsenic water filter cost breakdown.

For a single drinking-water tap (not whole-house), an under-sink reverse osmosis system can also remove arsenic. Whole-house treatment is appropriate when you want protection at every fixture, including for cooking and bathing. RO is sufficient if your only concern is drinking and cooking water and your levels are not extremely high.

Frequently Asked Questions

Written by Aidan Walsh, owner of Mid Atlantic Water. 32+ years installing well water treatment systems across Pennsylvania, Maryland, Delaware, New Jersey, Virginia, and ship-to-site installations across the United States. Article reviewed April 2026.