"Your water looks, smells and tastes fine. So why do you need to test it?" That's how Maine CDC opens its arsenic flyer, and it's the right question. EPA describes arsenic as "odorless and tasteless," and the Vermont Department of Health puts it plainly: "Testing is the only way to know if arsenic is in your drinking water." The number to know is 10 micrograms per liter (10 µg/L, the same as 10 parts per billion), the federal maximum contaminant level (MCL) for public water systems. Private wells aren't covered by that rule. Two other things matter as much and most pages skip them: arsenic comes in two chemical forms that respond very differently to treatment, and state health departments don't agree on how often to retest or where to draw the line.
Got a result above 10 µg/L? Vermont, Minnesota, Wisconsin and Washington all point to another drinking-water source or treatment at that level, and Minnesota and Wisconsin say to retest to confirm first. Maine CDC suggests bottled water for drinking in the meantime. Boiling won't help: Virginia's health department says it "will not remove arsenic." Bathing and washing dishes are generally fine (details below). If you're pregnant or have young children, ask your doctor or health department what applies to you.
What level of arsenic in water is dangerous?
Arsenic above 10 µg/L (0.010 mg/L, or 10 ppb) is over EPA's limit for public drinking water, but no level is risk-free: EPA's health goal is zero, and Minnesota's health department says years of water below the standard "can still increase your risk of cancer." Washington says to stop drinking water above 50 ppb immediately.
Three things sit behind that answer. The 10 µg/L limit is a legal line for public water systems, not a safety line; Washington's health department notes that EPA weighed "the cost and difficulty of removing arsenic" in setting it, and it doesn't bind a private well (details in the 10 µg/L section). The main risk is long-term: ATSDR, the CDC's toxic-substances agency, links long exposure to skin changes and cancer, and lists nausea, vomiting and other effects at higher levels (see symptoms). And the states set their own action points, from Vermont's "consider treatment" starting at 0.001 mg/L (1 µg/L) to Washington's stop-now line above 50 ppb; the decision table lists them by band.
What arsenic is, and how it gets into a well
EPA calls arsenic "a semi-metal element" that can enter drinking water "from natural deposits in the earth or from agricultural and industrial practices." In private wells, nature dominates. When the USGS and CDC estimated how many people drink high-arsenic well water, the USGS said "nearly all" of the arsenic in the groundwater it used "is likely from natural sources," coming "primarily from rocks and minerals through which the water flows."
The form in your water isn't the form in your fish. The Minnesota Department of Health (MDH) says inorganic arsenic "is the type found in contaminated drinking water, and is the most harmful type," while organic arsenic, common in fish and shellfish, "is less harmful to health."
Two chemistries, opposite triggers
Rock alone doesn't explain why one well has 2 µg/L and the next has 40. Water chemistry decides how much arsenic comes loose. The USGS national study of domestic wells (Circular 1332) names sulfide minerals and geothermal deposits as sources and describes processes that run in opposite directions:
- Low oxygen. Arsenic can be released from iron oxides, "which is favored by low dissolved oxygen concentrations and high pH, and is common in glacial aquifers and some crystalline-rock aquifers."
- Oxygen arriving. "Oxidation of sulfide minerals in some sandstone aquifers and mined areas" can also raise arsenic, as can evaporation in some desert basin aquifers of the Southwest and geothermal water in parts of the West.
Northeast Wisconsin shows the second process. The Wisconsin DNR says arsenic there "is tied up in sulfide minerals," and that scientists believe it is being released in the Outagamie, Winnebago and Brown County area "at least partly because people are now using more water than ever before." Lower water tables "allowed oxygen to get into the bedrock aquifers, creating chemical reactions that release arsenic into the water."
So arsenic can rise where oxygen is scarce, and elsewhere where oxygen shows up. MDH warns that "Arsenic levels can vary between wells, even within a small area." Your neighbor's result is a hint, not a substitute for your own test.
The two forms: arsenite, As(III), and arsenate, As(V)
This decides whether a treatment system works. Dissolved arsenic "generally exists in either the +3 (tri-) or +5 (penta-) valence state, depending on local oxidation-reduction conditions," according to an EPA training module on the arsenic rule (EPA, Arsenic Mitigation Strategies). The +3 form is arsenite, the +5 form arsenate.
| Property | Arsenite, As(III) | Arsenate, As(V) |
|---|---|---|
| Where it dominates (EPA) | "Under anaerobic conditions," which EPA notes "are common in deep groundwater sources" | "Under aerobic conditions" (water with oxygen) |
| Charge at normal water pH (EPA) | None. In the "pH range of natural water, which typically is between 6 and 9, arsenite occurs as an uncharged particle" | Negative: "negative (-1 or -2) at natural pH levels" |
| Treatment (EPA) | "this form is not easily removed" | Can be "removed with greater efficiency" |
| Federal rules | The best available technologies for public systems are "BATs for Arsenic V. Pre-oxidation may be required to convert Arsenic III to Arsenic V" (40 CFR 141.62) | |
Most adsorption media and anion-exchange resin work by grabbing charged particles. Arsenate carries a charge and is caught easily; uncharged arsenite is, in EPA's words, "not easily removed." Iron-oxide media is the exception: Maine and Maryland say it removes both forms (see the treatment table). EPA's module calls conversion to arsenate "a critical element of any arsenic treatment process."
Which form you have depends on place. Maine CDC calls the +5 form "the kind of arsenic encountered most of the time in Maine groundwater." In Maryland's Coastal Plain, the state environment department reported in 2008 that preliminary research found "Arsenic III occurring at higher concentrations than Arsenic V." There's no national default.
Where arsenic is common in the U.S.
The USGS sampled about 2,100 private domestic wells in 48 states between 1991 and 2004. Arsenic was measured in 1,774 of them and was above the 10 µg/L MCL "in 6.8 percent of wells nationwide," one of the most frequent problems the survey found:
Show the numbers
| Item | Value | Note |
|---|---|---|
| Strontium | 7.3 % of wells above benchmark | HBSL 4,000 µg/L; 488 wells |
| Arsenic | 6.8 % of wells above benchmark | MCL 10 µg/L; 1,774 wells |
| Manganese | 5.2 % of wells above benchmark | HBSL 300 µg/L; 2,159 wells |
| Nitrate | 4.4 % of wells above benchmark | MCL 10 mg/L as N; 2,132 wells |
| Radon (higher proposed MCL) | 4.4 % of wells above benchmark | 4,000 pCi/L; 1,958 wells (65% exceeded the lower proposed 300 pCi/L) |
| Uranium | 1.7 % of wells above benchmark | MCL 30 µg/L; 1,725 wells |
| Boron | 1.3 % of wells above benchmark | HBSL 1,000 µg/L; 535 wells |
| Fluoride | 1.2 % of wells above benchmark | MCL 4 mg/L; 2,157 wells |
USGS sampling of private domestic wells, 1991-2004 (Circular 1332, table 2). Benchmarks are EPA MCLs, proposed MCLs or USGS health-based screening levels; arsenic's is the 10 µg/L MCL. Number of wells sampled differs by contaminant. Source: USGS Circular 1332, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991-2004: Overview of Major Findings (DeSimone, Hamilton, Gilliom, 2009), table 2, accessed 2026-11-01.
The average hides big regional differences. The report found arsenic above the MCL "in more than 10 percent of wells in several aquifer types, including crystalline-rock aquifers in New England; basin-fill aquifers in California, Nevada, Arizona, New Mexico, Texas, and Nebraska; and the Snake River basaltic-rock aquifers in Idaho." Earlier national studies, it adds, "showed elevated concentrations in the West, Midwest, and Northeast and the lowest concentrations in the Southeast."
A later USGS model went from wells to people. Ayotte and colleagues used arsenic results from 20,450 domestic wells to estimate the odds of arsenic above 10 µg/L across the lower 48 states and combined them with census data. The USGS data release says "Approximately 44.1 M people in the U.S. use water from domestic wells," and estimates 2.1 million of them use wells with predicted arsenic above 10 µg/L (95% confidence interval 1.5 to 2.9 million). The authors call it a tool for "broad screening," not a replacement for local data. The USGS lists Washington, Maine, Wisconsin, Ohio and Virginia, all states this site covers, among "some of the locations where it’s estimated the most people may have high-levels of arsenic."
State figures, where they exist:
- Maine. Maine CDC: "1 in 10 wells in Maine has too much arsenic," and "There are wells high in arsenic in all parts of Maine." See certified labs in Maine.
- Minnesota. MDH: "Approximately 10 percent of private drinking water wells in Minnesota have arsenic levels higher than 10 µg/L." Some reach 350 µg/L, "however most results are below 50 µg/L."
- Washington. The Washington State Department of Health says drinking water there "typically contains less than 3 parts of arsenic per billion," but "levels from 10 ppb to 33,000 ppb have been found in some wells." See certified labs in Washington.
Health effects: what EPA and ATSDR say
Arsenic is mainly a long-term risk. EPA's health-effects note for the standard reads: "Skin damage or problems with circulatory systems, and may have increased risk of getting cancer." EPA's health goal (MCLG) is zero. The CDC's toxic-substances agency (ATSDR) is more specific:
- Cancer. "Several studies have shown that ingestion of inorganic arsenic can increase the risk of skin cancer and cancer in the liver, bladder, and lungs." DHHS and EPA have determined that inorganic arsenic "is a known human carcinogen."
- Children and pregnancy. "There is some evidence that long-term exposure to arsenic in children may result in lower IQ scores," and some evidence it "can injure pregnant women or their unborn babies, although the studies are not definitive."
EPA's rule page also links arsenic to cancers of the kidney, nasal passages and prostate. MDH adds that long-term low-level exposure "is associated with diabetes" and that effects "can develop after many years." Washington's health department explains why you can't use how you feel to rule arsenic in or out: "Most health problems from long-term arsenic exposure are common illnesses that affect many people and have several possible causes besides arsenic." If you've been drinking water above 10 µg/L, MDH says to tell your medical provider.
Symptoms of arsenic in water
You can't see, smell or taste arsenic (EPA calls it "odorless and tasteless"), so a lab test, not a symptom check, is how you find it. What the federal health agencies describe depends on dose and time. The signs below are ATSDR's (ToxFAQs for Arsenic, based on its 2007 toxicological profile) and EPA's. They describe what exposure can do, not a way to diagnose yourself.
| Exposure | What the agency lists | Source |
|---|---|---|
| Low levels, swallowed for a long time | "A darkening of the skin and the appearance of small 'corns' or 'warts' on the palms, soles, and torso" | ATSDR |
| Long-term, at levels above the standard | "Skin damage or problems with circulatory systems, and may have increased risk of getting cancer" | EPA, drinking water regulations table |
| Very high levels | "Ingesting very high levels of arsenic can result in death" | ATSDR |
| Lower levels than that (ATSDR gives no number) | "Nausea and vomiting, decreased production of red and white blood cells, abnormal heart rhythm, damage to blood vessels, and a sensation of 'pins and needles' in hands and feet" | ATSDR |
| Skin contact with inorganic arsenic | "May cause redness and swelling" | ATSDR |
ATSDR's summary puts the two ends side by side: "At high levels, inorganic arsenic can cause death. Exposure to lower levels for a long time can cause a discoloration of the skin and the appearance of small corns or warts." For a household well, the long-term row is the one that matters, and ATSDR's advice for people in high-arsenic areas is simple: "use cleaner sources of water."
Medical tests answer a narrow question
ATSDR says tests can measure arsenic "in your blood, urine, hair, and fingernails." Urine "is the most reliable test for arsenic exposure within the last few days," while hair and fingernails "can measure exposure to high levels of arsenic over the past 6-12 months." Such tests show whether you've had above-average exposure, but they "cannot predict whether the arsenic levels in your body will affect your health." Whether you need a test, and what a result means for you, is a question for your doctor; if you're worried about symptoms, bring your well report to the appointment. The water test is the one you order yourself.
The 10 µg/L limit, and why it's not the whole story
EPA's chemical contaminant rules page: "In 2001, EPA adopted a lower standard for arsenic in drinking water," and "The new arsenic standard of 10 parts per billion (ppb) replaces the old standard of 50 ppb." It applies to community and non-transient non-community water systems and became effective "for the purpose of compliance on January 23, 2006" (40 CFR 141.6(j)).
- It isn't a safety line. MDH notes that water "at levels lower than the EPA standard over many years can still increase your risk of cancer." Washington says EPA weighed "the cost and difficulty of removing arsenic" in setting 10, and that getting below 10 "is not low enough to completely eliminate" the risk.
- It doesn't bind your well. Washington: "there is no state-wide standard for arsenic in private wells," though a few counties have rules.
- Units vary. MDH: "One microgram per liter (µg/L) is the same as 1 part per billion (ppb)." Multiply mg/L by 1,000 to get µg/L.
| Report says (mg/L) | Same water in µg/L or ppb | Where it sits |
|---|---|---|
| 0.001 | 1 | Vermont's "consider treatment" starting point |
| 0.005 | 5 | Half the MCL; bottom of MDH's retest band for new wells |
| 0.010 | 10 | The federal MCL |
| 0.018 | 18 | Worked example below |
| 0.050 | 50 | The old federal limit; above it, Washington says stop using the water immediately for drinking and food preparation |
| 0.500 | 500 | Washington: above this, call the health department before bathing |
Why you can't taste or smell it
Maryland's environment department says arsenic "does not generally impart color, taste or smell to water." The amounts that matter are tiny: Washington compares 3 ppb to "adding one teaspoon of arsenic to an acre of water that is 4 feet deep." Stains aren't an arsenic sign either, but they can be a prompt. Wisconsin's DNR suggests a test "whenever a change in the color of your water or staining of fixtures is observed, or if water levels in your well have changed," and says that if pipes and fixtures are corroding, "there is a greater chance you have an arsenic problem."
How often to test: the states disagree
Arsenic isn't on the yearly list. The CDC recommends yearly tests for total coliform bacteria, nitrates, total dissolved solids and pH, and lists arsenic among chemicals to ask your health department about. Here's what seven of the ten states this site covers recommend (Ohio, Pennsylvania and Virginia aren't in the table yet):
| State | How often to test for arsenic | Action point the state gives |
|---|---|---|
| Connecticut | "At least once," "Ideally, repeat the test every five years" (DPH Publication 24, quoted by the CT Office of Legislative Research in 2019). New wells: since October 1, 2022 state law requires a lab test that includes arsenic (CGS § 19a-37). | DPH action level 10 µg/L |
| Maine | "every 3-5 years" (Maine CDC) | No separate state number: Maine cites EPA's 10 ppb MCL, and its checklist asks whether you've retested to confirm |
| Minnesota | "at least once." For a new well, "You may want to re-test" six months or more after drilling, "especially if the test results were between 5 and 20 µg/L." If tests find no arsenic, "further testing is not necessary" | Above 10 µg/L, confirmed by repeat sampling: alternate drinking source or treatment |
| Vermont | "every five years" | Above 0.010 mg/L: stop drinking it. Between 0.001 and 0.010 mg/L: "consider treatment" |
| Washington | "At least twice, while you own the well," in summer and winter (testing page); the arsenic page says late summer and early spring | No more than 10 ppb for drinking. 10 to 50: not long-term. Over 50: stop immediately |
| Wisconsin | "at least once every five years"; yearly if arsenic showed up before or you live in Outagamie, Winnebago or Brown county (Wisconsin DHS) | At or above 10 µg/L: retest, then switch drinking water and find a long-term fix |
| New York | "other contaminants every 3-5 years" (NY DOH; arsenic not named separately) | Not stated on that page |
Minnesota says that if testing finds no arsenic, further testing isn't necessary (though it suggests a new well may be worth retesting). Maine wants a repeat every three to five years, Vermont and Wisconsin every five, and Connecticut ideally every five. Washington wants two seasons. Levels do move: Connecticut notes "Levels can fluctuate over time," Wisconsin's DNR says concentrations "can change over time," and Minnesota is studying "how water sampling methods and timing impact arsenic test results."
Testing: total arsenic first, speciation when you treat
CDC says to "Use a state-certified laboratory to test your water." Search by state with the arsenic lab finder.
Expect the report to show total arsenic, both forms added together. Maryland's environment department: "In general, laboratories will test and report total arsenic unless you specify a particular method of arsenic speciation analysis." That total is what the decision table below works from.
Speciation splits it into As(III) and As(V). Maryland says it "may be necessary to select the appropriate treatment technology." Ask the lab how to collect a speciation sample before you fill anything.
Add these at the same time:
- Iron and manganese. Maine CDC says they "can foul RO systems," and lists iron, manganese, sulfate, silica and organic carbon as things that reduce the effectiveness of iron-oxide filters. See manganese.
- Sulfate and pH. Sulfate competes with arsenic in anion exchange (Maine, Maryland), and higher pH cuts activated alumina's removal (Maryland).
- Uranium and nitrate. In the USGS domestic-well study (DeSimone 2009), two-thirds of the common mixtures (contaminants above one-tenth of their benchmarks, in 5 percent or more of wells) were made of two or more of "nitrate, arsenic, radon, and uranium, and to a lesser extent, molybdenum and manganese." See uranium, nitrate, or which tests do I need.
Sample an untreated tap to learn what's in the well. The guide to testing well water covers collection.
Reading your result: a decision table
Convert to µg/L first (mg/L × 1,000). Every threshold comes from the agency named. You can also check numbers with the results checker.
| Total arsenic (µg/L = ppb) | What sources say | Sensible next step |
|---|---|---|
| Not detected | MDH: "further testing is not necessary." Retesting is still recommended: every 3 to 5 years in Maine, every five years in Vermont and Wisconsin, ideally every five years in Connecticut. | Keep the report. Retest on your state's schedule, or sooner if the water or the water level changes. |
| 1 to under 5 | Vermont: between 0.001 and 0.010 mg/L, "consider treatment," because "even amounts below the drinking water standard could affect your health." | Decide whether to treat a drinking tap. Retest in another season. |
| 5 to under 10 | Same Vermont advice. MDH: for new wells in the 5 to 20 µg/L range, you "may want to re-test" six months or more after drilling. | Retest to see whether you're trending toward 10. If you treat, speciate first. |
| 10 to 50 | At or above the EPA MCL. Washington (above 10): not for drinking or food "over the long term." Wisconsin DHS ("Take action" at 10 or more) and MDH (above 10): confirm, then switch water or treat. Vermont: stop drinking it. | Use bottled or other tested water for drinking and cooking now. Retest with speciation and plan treatment. |
| Above 50 | Washington: "stop using it immediately for drinking and food preparation." | Switch drinking and cooking water today. Ask your health department about treatment. |
| Above 500 | Washington: bathing and cleaning are okay "unless it contains more than 500 ppb"; above that, call the health department. | Call before choosing a fix. |
Treatment: match the technology to the form
You can't remove natural arsenic at the source. Washington lists bottled water as a stopgap, a new well that "may or may not" have acceptable arsenic, and connecting to a nearby public supply. For treatment, the form of arsenic decides what works. The full comparison, with the tests to order first, the oxidation step, one-tap and whole-house options, competing ions, waste, costs and why systems fail, is in arsenic removal from well water. The short version from the state sources on this page:
- Form decides it. Reverse osmosis, activated alumina and anion exchange are rated for As(V). As(III) usually needs an oxidation step first, which Vermont says chlorine injection, specialized media or aeration can do. Maine rates iron-oxide media "effective for both trivalent and pentavalent arsenic removal."
- Some fixes don't work. Virginia: "Heating or boiling your water will not remove arsenic," and as water evaporates, concentrations "can actually increase slightly"; "chlorine (bleach) disinfection will not remove arsenic" on its own either. Vermont says a softener "will not remove arsenic," and Wisconsin's DNR says softeners, "carbon filters and sediment filters do not adequately remove arsenic."
- Read the certification claim. Vermont points to NSF/ANSI 58 for reverse osmosis and NSF/ANSI 53 for adsorptive and carbon-block systems. NSF's public listings word arsenic claims as "Arsenic (Pentavalent)" reduction up to a stated level, such as "<=50 ppb." If your water has As(III), or more arsenic than the claim covers, the certification doesn't cover your water.
Whole house or one tap?
For most arsenic problems one tap is enough, because the risk comes from swallowing the water. Washington says arsenic "is not absorbed very well through the skin and does not easily evaporate from water," so bathing or washing dishes "is unlikely to cause health problems." Wisconsin DHS says high-arsenic water is okay for bathing, brushing teeth and washing dishes. Thresholds differ: Washington's caution starts above 500 ppb, and an older Wisconsin DNR brochure (2017) drew the bathing line at 100 ppb. Check current advice if your level is very high.
- Point of use (one tap). RO, distillation or an adsorptive cartridge at the kitchen sink for drinking, cooking, coffee and formula. Virginia notes these methods are typically used "at only one faucet." The tradeoff is volume.
- Point of entry (whole house). Anion exchange or an adsorptive media tank treats every tap. Worth considering when people drink from several taps or the system must handle another problem too. It costs more, and Maine warns that if an anion-exchange unit fails, "all of the arsenic on the resin can be released at once," and that its treated water can be corrosive (more in the removal guide).
Retest after treatment, and keep retesting
Vermont: "Retest your drinking water for arsenic after any treatment system is installed." Washington: after installation and routine maintenance, "your water should be tested to ensure that the system is removing arsenic." MDH and Wisconsin's DNR say the same. A practical routine: sample the treated tap after installation, after each media or membrane change, and on your state's schedule, with a raw-water sample alongside so you know what the system faces.
Worked example: "Arsenic 0.018 mg/L"
Your report, from an untreated tap, shows Arsenic: 0.018 mg/L.
- Units. 0.018 × 1,000 = 18 µg/L (18 ppb), 1.8 times the MCL.
- What the states say. It's inside Washington's 10-to-50 band (don't "drink water containing these levels or use it for food preparation over the long term") and above the 10 µg/L action points of Vermont, Wisconsin, Minnesota and Connecticut. Wisconsin DHS and MDH say to confirm with a retest. Meanwhile, use bottled or other tested water for drinking and cooking. Showers and dishes are fine at this level by Washington's and Wisconsin's guidance.
- Retest, and ask for more. Request speciation, plus iron, manganese, sulfate and pH.
- If it's mostly As(V). A kitchen RO unit fits. At Maine's "usually about 95%" removal, 18 µg/L would drop to roughly 1 µg/L (18 × 0.05 = 0.9), near Vermont's 1 µg/L line. Only a test of the treated tap tells you what your unit actually delivers. Choose a unit whose NSF/ANSI 58 listing carries an Arsenic (Pentavalent) claim covering 18 ppb; a "<=50 ppb" claim does.
- If it's mostly As(III). RO alone isn't enough: Maryland rates its As(III) removal "variable," and Maine says to pre-oxidize. Options are an oxidation step before RO, or iron-oxide media, which Maine says handles both forms. Maine CDC, for one, gives well owners free technical help.
- Verify. Test the treated tap after installation, then raw and treated water on your state's schedule: every 3 to 5 years in Maine, every five in Vermont, and every year in Wisconsin (because arsenic has been found).
The reverse case: 0.0018 mg/L is 1.8 µg/L. That's well under the MCL, but inside Vermont's "consider treatment" band. Slip a decimal place and you'd think it was 18. Check whether the report says mg/L or µg/L before you compare it with 10.
Next step
If your well has never been tested for arsenic, order that test. Minnesota, Connecticut and Wisconsin ask for at least one, and Maine, Vermont and Wisconsin want repeats every few years. Ask for total arsenic, plus speciation if you know you'll treat. Find a state-certified lab with the arsenic lab finder, and keep every report.
Questions people ask
What is a safe level of arsenic in well water?
What are the symptoms of arsenic in well water?
How do I remove arsenic from well water?
Does boiling water remove arsenic?
Is it safe to shower in well water with arsenic?
How often should I test my well for arsenic?
Will a water softener or carbon filter remove arsenic?
- U.S. EPA, National Primary Drinking Water Regulations (arsenic MCLG 0, MCL 0.010 mg/L as of 01/23/06, health effects, sources) (last updated August 31, 2026) (accessed 2026-11-01)
- U.S. EPA, Chemical Contaminant Rules: Arsenic Rule background (2001 standard of 10 ppb replaces 50 ppb; odorless and tasteless; health effects) (last updated March 16, 2026) (accessed 2026-11-01)
- 40 CFR 141.62, Maximum contaminant levels for inorganic contaminants (arsenic MCL; best available technologies and small-system technologies, pre-oxidation footnote) (accessed 2026-11-01)
- 40 CFR 141.6(j), Effective dates (arsenic MCL effective for compliance January 23, 2006) (accessed 2026-11-01)
- U.S. EPA, Arsenic Mitigation Strategies (Arsenic Rule training module, 2002; arsenite vs arsenate chemistry, speciation, pre-oxidation) (accessed 2026-11-01)
- CDC/ATSDR, ToxFAQs for Arsenic (page last reviewed March 12, 2015; based on the 2007 Toxicological Profile) (accessed 2026-11-01)
- CDC, Guidelines for Testing Well Water (updated July 1, 2024) (accessed 2026-11-01)
- USGS Circular 1332, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991-2004: Overview of Major Findings (DeSimone, Hamilton, Gilliom, 2009) (accessed 2026-11-01)
- USGS Scientific Investigations Report 2008-5227, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991-2004 (DeSimone, 2009) (accessed 2026-11-01)
- USGS data release, Estimated county level domestic well population with arsenic greater than 10 micrograms per liter based on probability estimates for the conterminous U.S. (Ayotte, Medalie, Qi, August 10, 2017; DOI 10.5066/F7CN724V) (accessed 2026-11-01)
- USGS news release, Study Estimates about 2.1 Million People using Wells High in Arsenic (October 18, 2017) (accessed 2026-11-01)
- Minnesota Department of Health, Arsenic in Drinking Water (last updated November 18, 2024) (accessed 2026-11-01)
- Minnesota Department of Health, Home Water Treatment (test treated water after installation; certified units) (accessed 2026-11-01)
- Maine CDC, Private Well Water (testing frequency) (accessed 2026-11-01)
- Maine CDC Drinking Water Program, Arsenic in Drinking Water (treatment technologies, pros and cons, checklist) (accessed 2026-11-01)
- Maine CDC, Have you tested your well water for arsenic? (flyer, March 2020) (accessed 2026-11-01)
- Vermont Department of Health, Arsenic in Drinking Water (accessed 2026-11-01)
- Vermont Department of Health, How to Test Your Drinking Water (last updated October 30, 2026) (accessed 2026-11-01)
- Vermont Department of Health, How to Treat Your Drinking Water (accessed 2026-11-01)
- Washington State Department of Health, Arsenic and Your Private Well (accessed 2026-11-01)
- Washington State Department of Health, Testing Your Water (accessed 2026-11-01)
- Wisconsin Department of Health Services, Arsenic (last revised February 24, 2026) (accessed 2026-11-01)
- Wisconsin DNR, Arsenic in Drinking Water Wells, PUB-DG-062 (2017) (accessed 2026-11-01)
- Connecticut General Assembly, Office of Legislative Research, Regulation of Private Residential Wells, 2019-R-0246 (October 16, 2019; quotes DPH Publication No. 24, May 2019) (accessed 2026-11-01)
- New York State Department of Health, Private Wells (revised October 2026) (accessed 2026-11-01)
- Virginia Department of Health, It's YOUR Health: Arsenic in Private Wells (accessed 2026-11-01)
- Maryland Department of the Environment, Arsenic Water Treatment for Individual Wells in Maryland (February 2008; copy hosted by University of Maryland Extension) (accessed 2026-11-01)
- NSF, Certified Drinking Water Treatment Units: NSF/ANSI 58 listings (Arsenic (Pentavalent) Reduction claims) (accessed 2026-11-01)
- NSF, Certified Drinking Water Treatment Units: NSF/ANSI 53 listings (Arsenic (Pentavalent) Reduction claims) (accessed 2026-11-01)
- Connecticut General Statutes § 19a-37 (new private wells tested since Oct 1, 2022, incl. arsenic and uranium) (accessed 2026-11-01)
Find arsenic testing labs by state
Labs each state lists as certified for arsenic, with the ones that take homeowner samples first.
Got a bad result?
- Confirm the result: re-test with a state-certified lab (find one in the finder on this site) and ask how to collect the sample.
- Call your state or county health department and ask what is known about this contaminant in your area and what it recommends.
- If you consider treatment, ask for proof that the system is independently certified for your specific contaminant, and re-test the treated water afterwards.