Arsenic in Well Water by State: Where Is It Most Common?
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Arsenic in well water comes out of the rock around the well, so the state you live in changes your odds of it. The map shows the U.S. Geological Survey's estimate for every state of the share of private well users over the EPA limit of 10 parts per billion (ppb). Your state's odds still do not tell you what is in your own well.
Quick Answer
Arsenic in well water comes mostly from the rock, and the USGS estimates about 2.1 million Americans on private wells drink water over the EPA limit of 10 ppb. Public water has to meet that limit. Private wells are not regulated, so testing is up to you.
- Highest share of well users. Maine 18%, New Hampshire 14%, Nevada 14%, Idaho 11%, New Mexico 10%, Ohio 10%. See which states have the most arsenic.
- Most people. Michigan, about 193,000 well users over 10 ppb, then Ohio and Indiana. See your region.
- Stricter states. New Jersey and New Hampshire set 5 ppb for public water. See state arsenic limits.
- Your well is its own answer. High arsenic turns up in low-odds states too, and only a lab test gives your number. See why your well can differ.
What's in This Guide
Which states have the most arsenic?
Based on USGS data, the states with the highest share of private well users over 10 ppb are Maine, New Hampshire, Nevada, Idaho, New Mexico and Ohio. That comes from a 2017 USGS study that built a model from arsenic tests on 20,450 private wells and estimated the odds for every county in the lower 48 states. Nationally it put 4.8% of well users over 10 ppb.
"Most arsenic" can mean two things. Maine has the highest share, about 1 well user in 6. Michigan has the most people, because so many more homes there are on wells: about 193,000, then Ohio at about 189,000 and Indiana at about 151,000. North Carolina and California come next by people, at a lower share. Alaska and Hawaii were not part of the study.
Arsenic in well water by region
Each region gets its arsenic from different rock, and the USGS named where the people with high-arsenic wells are concentrated. Six of the ten counties with the most are in New England. The others are in Ohio, North Carolina, California and Idaho.
Maine
Maine has the highest share in the country: the USGS estimates 18% of its well users, about 102,000 people, drink water over 10 ppb. An earlier USGS study of 11,111 Maine wells tested at the state lab from 2005 to 2009 found about 10% over 10 ppb, and more than 25% over in 44 towns; Maine CDC also says 1 in 10 wells in Maine has too much arsenic. The two numbers count different things: one estimates people, the other counts the wells owners chose to test. That study found three large high-arsenic areas: the southern coast, central Kennebec County, and Ellsworth in Hancock County with the towns around it. By number of people, Penobscot, Kennebec and York counties lead.
New Hampshire
New Hampshire's environmental agency estimates 25 to 30% of private wells have arsenic at 5 ppb or more, and about 116,000 well users drink water over 5 ppb. Nearly half the state is on private wells. The USGS puts 14% of well users over 10 ppb. Rockingham and Hillsborough counties in the southeast are second and sixth in the country by number of people on high-arsenic wells. Since July 1, 2021, public water in New Hampshire has to meet 5 ppb.
Other New England states
Connecticut is at 6.0%, Massachusetts 5.7% and Vermont 5.3% of well users over 10 ppb, by the USGS estimate. Worcester County, Massachusetts and Tolland County, Connecticut are both in the top ten counties in the country by people. Rhode Island is low, at 1.3%.
Michigan
More people drink high-arsenic well water in Michigan than in any other state: about 193,000, or 7.2% of well users, by the USGS estimate. Michigan's own estimate, 230,000, falls inside the USGS range. Tuscola County in the Thumb leads the state, then Clinton, Livingston and Oakland. A regional study the USGS cites ties well arsenic in Michigan, Ohio, Indiana, Illinois and Wisconsin broadly to the old sedimentary bedrock under them.
The rest of the Midwest
Ohio is at 10% and about 189,000 people, second only to Michigan, and Wayne County, Ohio has more people on high-arsenic wells than any other county in the country. Indiana is at 9.1% (Kosciusko and Allen counties lead), Minnesota 7.1%, Iowa 6.0%, Illinois 5.9% and Wisconsin 4.4%. Minnesota's and Illinois's own estimates run higher than the USGS model. The USGS lists the glacial aquifers across the northern states among the places it finds arsenic.
New Jersey
New Jersey's Private Well Testing Act makes the seller or buyer of a home on a well test it before closing, and landlords test every five years, so the state has a large record: 51,028 wells were tested from 2002 to 2007 alone. Arsenic has been on that test statewide since 2018. In the twelve northern counties, 8.3% of wells tested have been over the state's 5 ppb limit and 2.8% over 10 ppb. The Piedmont region, which takes in counties such as Bergen, Morris, Somerset, Hunterdon and Mercer, is worst, at 17.3% over 5 ppb. The USGS puts the whole state at 4.2% of well users over 10 ppb.
Pennsylvania to the Carolinas
Shares here run lower, but these states have a lot of well users, so the people add up. North Carolina is at 3.6% and about 120,000 people, the fourth most of any state, and Mecklenburg County is third in the country. Pennsylvania is at 2.4% and about 81,000 people, led by Crawford and Erie counties in the northwest. New York is 3.2%, Maryland 3.9%, Delaware 5.4% and Virginia 3.2%. The USGS lists the hard-rock aquifers of the Piedmont, Blue Ridge and Valley and Ridge among the places it finds arsenic.
The Southwest
Nevada is at 14%, New Mexico 10% and Arizona 7.8% of well users over 10 ppb. In the USGS's sampling of Southwest drinking-water wells, about 16% were over 10 ppb, more than twice the national rate. The arsenic comes from volcanic and granite rock, and long time underground, high pH, a dry climate and irrigation push it up. California is at 4.7% but about 116,000 people, led by Los Angeles, San Joaquin, Fresno and Merced counties. Texas is at 3.9%, led by Lubbock and Midland counties.
The Northwest
Idaho is at 11%, and Canyon County is eighth in the country by people on high-arsenic wells, with Twin Falls next in the state. Montana is at 8.2%, Wyoming 5.4%, Washington 5.2% and Oregon 4.3%.
The Plains states
The Dakotas have some of the lowest numbers of people on high-arsenic wells: South Dakota is at 1.6% of well users and North Dakota 4.1%. Nebraska is at 5.0%, Kansas 4.1%, Colorado 4.6%, Utah 4.2% and Oklahoma 1.4%.
The Southeast
The lowest shares in the country are here: Alabama 0.5%, Mississippi 0.6%, Arkansas 0.8%, Kentucky and Tennessee 1.0%, Louisiana 1.1%. Georgia, South Carolina and Florida run 2.3 to 2.7%. Low is not zero. The USGS lists the Mississippi Embayment and Mississippi River valley aquifers among the places it finds arsenic, and one of the Mississippi wells my customers tested came back at 10 ppb or more.
Arsenic odds for every state
This is the share of each state's private well users the USGS estimates drink water over 10 ppb, and the counties with the most people on high-arsenic wells. The study covered the lower 48 states.
| State | Well users over 10 ppb | Counties with the most people over 10 ppb |
|---|---|---|
| Alabama | 0.5% | Chambers and Baldwin |
| Alaska | Not in the USGS study | Not in the USGS study |
| Arizona | 7.8% | Maricopa and Yavapai |
| Arkansas | 0.8% | Greene and Craighead |
| California | 4.7% | Los Angeles and San Joaquin |
| Colorado | 4.6% | Alamosa and El Paso |
| Connecticut | 6.0% | Tolland and Fairfield |
| Delaware | 5.4% | Sussex and Kent |
| Florida | 2.7% | Palm Beach and Osceola |
| Georgia | 2.3% | Fulton and Coffee |
| Hawaii | Not in the USGS study | Not in the USGS study |
| Idaho | 11% | Canyon and Twin Falls |
| Illinois | 5.9% | Will and Kankakee |
| Indiana | 9.1% | Kosciusko and Allen |
| Iowa | 6.0% | Kossuth and Webster |
| Kansas | 4.1% | Reno and Sedgwick |
| Kentucky | 1.0% | Pike and Bullitt |
| Louisiana | 1.1% | St. Tammany and Ascension |
| Maine | 18% | Penobscot and Kennebec |
| Maryland | 3.9% | Wicomico and Baltimore |
| Massachusetts | 5.7% | Worcester and Middlesex |
| Michigan | 7.2% | Tuscola and Clinton |
| Minnesota | 7.1% | Blue Earth and St. Louis |
| Mississippi | 0.6% | Jackson and Harrison |
| Missouri | 1.2% | Greene and Lincoln |
| Montana | 8.2% | Missoula and Ravalli |
| Nebraska | 5.0% | Douglas and Sarpy |
| Nevada | 14% | Nye and Clark |
| New Hampshire | 14% | Rockingham and Hillsborough |
| New Jersey | 4.2% | Gloucester and Burlington |
| New Mexico | 10% | Valencia and Sandoval |
| New York | 3.2% | Suffolk and Steuben |
| North Carolina | 3.6% | Mecklenburg and Wake |
| North Dakota | 4.1% | Burleigh and Ward |
| Ohio | 10% | Wayne and Licking |
| Oklahoma | 1.4% | Texas and Beaver |
| Oregon | 4.3% | Malheur and Umatilla |
| Pennsylvania | 2.4% | Crawford and Erie |
| Rhode Island | 1.3% | Providence and Washington |
| South Carolina | 2.4% | York and Florence |
| South Dakota | 1.6% | Pennington and Meade |
| Tennessee | 1.0% | Hawkins and Sevier |
| Texas | 3.9% | Lubbock and Midland |
| Utah | 4.2% | Cache and San Juan |
| Vermont | 5.3% | Orange and Washington |
| Virginia | 3.2% | Fairfax and Spotsylvania |
| Washington | 5.2% | Yakima and Snohomish |
| West Virginia | 2.9% | Hampshire and Morgan |
| Wisconsin | 4.4% | Waukesha and Winnebago |
| Wyoming | 5.4% | Laramie and Fremont |
Source: USGS, Ayotte and others, 2017, with the USGS county data release. The USGS says to use these estimates with local data where it exists, not as a forecast for one well.
Why your well can differ from your state
Your well's arsenic depends on the rock it draws from, so a well in a low-odds state can still test high. New Hampshire's environmental agency puts it plainly: because the bedrock is so fractured, wells a short distance apart can have very different water. Depending on where you live and what you're surrounded by, contaminants can vary.
The tests my customers send in show it. Of 450 well tests from 43 states with an arsenic result, 16 came back at 10 ppb or more, and those 16 came from 13 different states, including Virginia, Maryland, Texas and Mississippi. A home buyer in Utah, a state the USGS puts at 4.2%, read me 0.0170 off the well's report: 17 ppb.
You cannot see, smell or taste arsenic, even at high levels, so the only way to know your number is a lab test.
State arsenic limits
The EPA limit is 10 ppb in every state, and New Jersey and New Hampshire hold public water to 5 ppb.
| Who sets it | Arsenic limit | Since | What it covers |
|---|---|---|---|
| EPA, every state | 10 ppb | Set in 2001, down from 50 ppb; in force January 23, 2006 | Public water systems |
| New Jersey | 5 ppb | January 23, 2006 | Public water, plus the well tests required when a home is sold or rented |
| New Hampshire | 5 ppb | July 1, 2021 | Public water systems |
| Private wells, any state | No federal limit | Not regulated | The owner decides when to test |
The EPA's goal for arsenic is zero. What a 3, a 7 or a 25 on your report means, and when to treat, is in what your arsenic level means.
What to do in any state
Test your own well with a certified lab, whatever your state's odds. The well water test kit we sell is $199 and reports 53 contaminants from one sample, arsenic included, so you get your pH, iron and hardness on the same report. Those decide which arsenic system fits your water.
One sample gives you your arsenic number
One certified lab test, mailed to you, run by an accredited lab, reports arsenic and 52 other contaminants. If you already have a lab report, skip this.
When the number comes back, read it against what your arsenic level means. If it is over 5 ppb, New Hampshire recommends a follow-up test for arsenic III before you pick a system, because that kind is much harder to remove; arsenic III vs arsenic V explains the test.
Text Aidan a photo of your report
Aidan will tell you what the arsenic number means for your house and whether anything needs treating.
8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.If your water needs treating for every tap, the whole-house arsenic system we sell is $4,795: an arsenic resin tank with a carbon tank after it. It and the replacement resin are on our arsenic filters page. I recommend having a plumber put it in: both tanks are just an inlet and an outlet, with no drain line and no electricity, and I'm a phone call away for your plumber. If you'd rather do it yourself, text me with any questions.
Frequently asked questions
How common is arsenic in well water?
In a national USGS study of the aquifers people drink from, nearly half the wells sampled had at least a trace of arsenic (1 ppb or more), and about 7% were over 10 ppb.
Why is there arsenic in our water?
Arsenic is a natural trace part of many rocks and sediments, and groundwater picks it up as it moves through them. In the Southwest it comes from volcanic and granite rock; in northern New Jersey researchers trace it to pyrite and other minerals in the bedrock. Old arsenic pesticides are not a significant source in groundwater, by the same New Jersey research.
How can you tell if your water is poisoned?
With arsenic, you can't by looking, smelling or tasting. It has no color, taste or odor even at high levels, so a certified lab test is the only way to know. A well can look and taste perfect and still be over 10 ppb.
Can arsenic be removed from a well?
Yes. A whole-house arsenic tank treats every tap, and a certified reverse osmosis unit treats one kitchen faucet. One form, arsenic III, is harder to remove and needs an extra treatment step first. The options are compared in what removes arsenic in well water.
Is Michigan drinking water safe?
Public water in Michigan has to meet the EPA's 10 ppb arsenic limit. Private wells are not regulated, and Michigan has more people on high-arsenic wells than any other state (see Michigan). If you are on a well in Michigan, test it.
Aidan Walsh
Co-owner & Residential Water Treatment Specialist, Mid Atlantic WaterAidan Walsh has spent 32 years in residential water treatment, the first stretch of it installing and servicing well systems before Mid Atlantic Water became an online supplier. Mid Atlantic Water is a family company; every recommendation on this site is based on what actually failed in the field and what fixed it. If you have a question this page did not answer, call 800-460-5810 or text him at 443-277-2204.
Aidan answers calls and texts 8 AM to 5 PM Eastern, every day.Number copied. Text it from your phone: 443-277-2204.