Arsenic in Well Water: Where It Comes From, How to Test, and How to Remove It

Arsenic in well water is usually a natural geological problem, not a sign that someone polluted your property. Groundwater can dissolve arsenic from certain rocks, minerals, and sediments as it moves underground. The resulting water may look crystal clear because arsenic has no reliable taste, odor, or color.

The U.S. Environmental Protection Agency’s maximum contaminant level for public drinking water is 10 parts per billion, usually written as 10 ppb or 10 micrograms per litre (µg/L). Health Canada uses the same value as its maximum acceptable concentration: 0.010 milligrams per litre, which is also 10 ppb.

Long-term consumption above that level is associated with an increased risk of cancer and other serious health effects. Very high exposure can cause illness over a shorter period, but the more common concern for a homeowner is drinking a smaller amount day after day for years without knowing it is present.

Private household wells are not regulated under the U.S. Safe Drinking Water Act. Canadian provinces and territories also place much of the responsibility for a private well’s water quality on its owner. Unless a property sale, well construction rule, or local program requires a test, no agency may be checking your water for you.

Arsenic reaches groundwater through rock and water chemistry

Arsenic is a naturally occurring element in the Earth’s crust. It can be present in volcanic rocks, metamorphic rocks that were changed by heat and pressure, some sedimentary formations, metal-bearing minerals, and sediments left behind by glaciers.

Being present in rock does not automatically mean arsenic will enter a well. It must first be released from the rock or from coatings on mineral grains. That release depends on the chemistry of the groundwater, including its pH, oxygen content, mineral content, and how long it has been in contact with the formation.

One common mechanism involves iron minerals. Arsenic can attach to iron-rich coatings underground and later be released when chemical conditions change. In other settings, alkaline water can cause arsenic to detach from mineral surfaces. Water moving through fractures in arsenic-bearing bedrock may also pick it up directly.

This is why replacing a pump, disinfecting a well, or installing an ordinary sediment filter does not address the underlying source. The arsenic is dissolved in the water, often as individual charged or uncharged chemical species far too small for a conventional particle filter to catch.

Most household cases are not caused by surface pollution

Industrial waste, mining, historical pesticide use, and other human activities can release arsenic. Those sources matter when a property is near a known contaminated site or when local authorities have identified a plume of polluted groundwater.

For most private wells with elevated arsenic, however, the source is geologic. The element was already in the underground materials, and local water chemistry allowed it to dissolve. A septic system, farm field, or cracked well cap may create other water-quality problems, but those are not the usual explanation for naturally elevated arsenic.

If arsenic appears along with solvents, petroleum compounds, pesticides, or unusual metals, a broader investigation may be warranted. A qualified environmental consultant or local health agency can help distinguish a natural occurrence from contamination associated with a specific property or activity.

Some regions have higher risk, but no map can clear an individual well

Arsenic occurs unevenly across North America. In the United States, elevated groundwater concentrations are found more often in parts of the Southwest, West, upper Midwest, and New England. Conditions vary within each region:

• New England: Certain metamorphic and igneous bedrock formations, particularly in parts of Maine and New Hampshire, are associated with higher arsenic occurrence.
• Upper Midwest: Arsenic can occur in glacial sediments and deeper groundwater in portions of states such as Minnesota, Wisconsin, Michigan, and the Dakotas.
• Southwest and West: Volcanic materials, basin sediments, alkaline groundwater, and iron-rich formations contribute to elevated levels in parts of Arizona, Nevada, New Mexico, California, and neighboring states.

Canadian areas of concern include parts of British Columbia and Nova Scotia, along with localized occurrences elsewhere. British Columbia has documented naturally elevated groundwater arsenic in areas including the Sunshine Coast, Gulf Islands, Bowen Island, and portions of the central interior. Nova Scotia’s bedrock geology creates enough variation that the province maintains arsenic-risk information for private well users.

These hot spots indicate where testing deserves particular attention. They do not prove that every well in the area is affected, and living outside a mapped zone does not prove that your water is free of arsenic.

Two neighboring wells can produce very different results

A neighbor’s test is useful evidence that arsenic occurs locally, but it cannot substitute for testing your own supply. Wells separated by one property line may draw water from different fractures, depths, or sediment layers.

Several factors can produce different results:

• Well depth: A shallow sand-and-gravel well and a deeper bedrock well may tap separate water-bearing zones.
• Fracture location: One drilled well may intersect an arsenic-bearing fracture while the next misses it.
• Groundwater chemistry: Differences in oxygen, pH, and dissolved minerals affect whether arsenic remains attached to rock or moves into the water.
• Well construction: Casing depth and the portions of the formation open to the well influence which water enters.
• Pumping patterns: Heavy use, drought, or changes in groundwater levels can alter where a well draws its water.

Arsenic concentrations can also change over time. They are often more stable than bacteria results, but a major drought, well deepening, rehabilitation work, prolonged disuse, or a change in pumping conditions can justify another test.

The main health concern is repeated ingestion over many years

Arsenic’s health effects depend on the dose, the duration of exposure, the chemical form, and individual susceptibility. Finding arsenic in a well does not mean everyone who drank the water will become ill. It does mean that exposure should be reduced and that the result deserves a measured response.

Short-term exposure requires a much higher dose

Acute arsenic poisoning can cause nausea, vomiting, abdominal pain, diarrhea, weakness, and neurological or cardiovascular problems. These effects are generally associated with much higher doses than the low or moderate concentrations typically discovered during routine private-well testing.

If a laboratory reports an exceptionally high result, someone may have swallowed a concentrated arsenic product, or household members have unexplained symptoms after using the water, contact a physician or poison-control service promptly. Do not wait for a treatment company to evaluate a possible medical emergency.

Long-term exposure can remain unnoticed

Chronic exposure means consuming arsenic repeatedly over months or years. A person may have no obvious symptoms while exposure continues. Long-term ingestion of inorganic arsenic has been associated with skin changes, circulatory effects, cardiovascular disease, developmental effects, and increased risks of bladder, lung, and skin cancers. Research has also identified associations with other health conditions, although the strength of evidence differs among outcomes.

Children, pregnant people, and anyone who consumes a large amount of water relative to body size deserve particular care. Infant formula mixed with well water can be a major exposure route because water makes up most of the prepared formula.

What the 10 ppb limit means for a private well

The EPA’s 10 ppb maximum contaminant level is an enforceable standard for regulated public water systems. Health Canada’s maximum acceptable concentration is also 10 ppb and is used by Canadian jurisdictions in their drinking-water programs.

For a private well, 10 ppb is a practical health benchmark rather than evidence that an authority has approved the well. In the United States, the federal public-water rule generally does not regulate a well serving an individual household. Requirements for testing, disclosure, and treatment vary by state, county, province, territory, and municipality.

The number should not be interpreted as a switch between “harmless” at 9 ppb and “dangerous” at 11 ppb. Risk changes with concentration and exposure over time. The EPA’s health goal for arsenic is zero, while the enforceable 10 ppb limit also reflects what public systems can measure and treat. Health Canada advises keeping arsenic as low as reasonably achievable.

If your result is near 10 ppb, discuss confirmation testing and exposure reduction with your health department, public-health unit, or environmental health officer. Laboratory uncertainty and natural variation matter when a result sits close to the guideline.

Testing must specifically include arsenic

A “standard” well test does not have one universal meaning. Some basic panels cover only total coliform bacteria, E. coli, nitrate, and perhaps pH or hardness. Arsenic may be part of an expanded metals or chemical panel, but it is frequently absent unless the customer requests it.

Ask the laboratory whether the analysis includes total arsenic and what reporting limit it uses. The reporting limit should be comfortably below 10 ppb so that the result can distinguish a low concentration from one near the drinking-water benchmark.

Use an accredited drinking-water laboratory

For a decision involving health or treatment, laboratory analysis is essential. In the United States, use a laboratory certified by the appropriate state drinking-water program for arsenic. In Canada, use a laboratory accredited for drinking-water arsenic through the relevant provincial system or a recognized national accreditation program.

Home arsenic kits may be marketed as screening tools, but color interpretation, sample chemistry, and low detection limits can make them unreliable around the 10 ppb threshold. They should not determine whether water is safe to drink or whether an installed treatment system is working.

Arsenic-only laboratory analysis commonly costs about $25 to $50 in either U.S. or Canadian markets, although shipping, collection services, local fees, detection limits, and regional laboratory availability can push the price higher. A broader metals or general-chemistry panel costs more but often provides information needed to choose treatment.

Collect the right sample for the question you are asking

Obtain the sample bottle and instructions from the laboratory. Do not substitute a household jar. The lab may provide a preserved bottle, and its directions govern whether you should flush the tap, remove an aerator, or keep the sample cold.

Sampling location matters:

• To measure untreated well water: collect from a tap before water-treatment equipment, or place the system in bypass only if the laboratory or treatment professional directs you to do so safely.
• To check drinking-water exposure: collect from the faucet actually used for drinking and cooking.
• To verify treatment: collect paired samples before and after the arsenic system when possible. This shows both the incoming concentration and the treated result.

If the first result detects arsenic, particularly near or above 10 ppb, a second accredited-lab test can confirm the finding before you commit to major equipment. Continue using an alternate drinking-water source while confirming a result above the guideline.

Arsenic III and arsenic V behave differently during treatment

A total arsenic test tells you how much is present, but not its chemical form. Groundwater arsenic is commonly found as arsenic III, called arsenite, or arsenic V, called arsenate.

At the pH found in many wells, arsenic V carries an electrical charge that makes it easier for several treatment media and reverse-osmosis membranes to capture. Much of arsenic III is uncharged under the same conditions and can pass through equipment that performs well on arsenic V.

This difference is one reason a system advertised for “arsenic reduction” may not work adequately on every well. If treatment is being designed, ask whether the laboratory should perform arsenic speciation. Speciation separates arsenic III from arsenic V. It is more specialized and costs more than a total arsenic test, but it may prevent the purchase of an unsuitable system.

If arsenic III is present, treatment commonly includes an oxidation stage. Oxidation changes arsenic III into the more treatable arsenic V form. The oxidant and contact time must be selected around the home’s complete water chemistry; adding an oxidizer without a designed removal stage does not remove arsenic from the water.

Treatment should match the water, not just the arsenic number

The right treatment depends on how much arsenic is present, whether it is arsenic III or V, how much water must be treated, and what else is in the well. Iron, manganese, hardness, pH, silica, phosphate, sulfate, and total dissolved solids can affect performance or shorten media life.

Before accepting a proposal, provide the treatment professional with a current laboratory report covering arsenic and the supporting water chemistry they need. In the United States, use a properly licensed water-treatment professional where licensing applies. In Canada, look for the appropriate provincial licence or recognized certification and experience with arsenic-bearing well water.

Point-of-use reverse osmosis protects the water people consume

A point-of-use reverse-osmosis system is usually installed beneath the kitchen sink and supplies a dedicated drinking-water faucet. Water is forced through a membrane that rejects many dissolved contaminants, including arsenic under the conditions for which the unit was tested.

This is the most common residential choice when the objective is to treat drinking, cooking, ice-making, and infant-formula water without treating every gallon used by the house. It generally costs less than a whole-house system and produces a manageable amount of treated water.

Look for third-party certification under NSF/ANSI 58 with an explicit arsenic-reduction claim. Certification to the general reverse-osmosis standard does not mean every listed contaminant-reduction claim applies to that model. Confirm whether the claim covers pentavalent arsenic, another name for arsenic V, and whether pre-oxidation is required for your water.

Reverse osmosis sends some water to the drain while producing treated water. It also needs sediment and carbon prefilters, periodic membrane replacement, cleaning or sanitizing according to the manufacturer, and enough incoming pressure. High iron, manganese, hardness, or sediment may require pretreatment.

Whole-house adsorptive media treats water as it enters the home

Adsorption means dissolved arsenic attaches to the surface of a treatment material. Iron-based media and activated alumina are two common categories. The equipment is installed at the point of entry, where water enters the house, although smaller point-of-use cartridges also exist.

Whole-house adsorption can reduce arsenic at all fixtures. Its success depends heavily on pH and competing substances in the water. Phosphate, silica, sulfate, and other dissolved constituents can occupy treatment sites or reduce the amount of arsenic the media can hold.

The media does not last forever. Its replacement schedule should be based on treated-water testing and calculated capacity, not appearance or taste. Exhausted media may stop removing arsenic before the homeowner notices any change in the water.

Arsenic III often requires oxidation followed by removal

Where testing identifies substantial arsenic III, a treatment train may first add an oxidant and provide enough contact time to convert it to arsenic V. A following filter or adsorptive bed then removes the converted arsenic.

Depending on the complete water analysis, the system may also remove iron and manganese. That can be helpful because arsenic sometimes associates with iron particles after oxidation, but performance must be demonstrated with laboratory results. Oxidation alone changes arsenic’s form; it does not make the arsenic disappear.

Chemical feed equipment, contact tanks, backwashing filters, and media vessels require more space and oversight than an under-sink unit. The design must prevent underfeeding, overfeeding, clogged media, and untreated water reaching household taps.

Planning-level treatment costs

The following ranges are budgeting estimates, not quotes. Installed prices vary substantially by region, water chemistry, flow rate, equipment certification, plumbing layout, pretreatment needs, electrical work, and local labor. Canadian prices also move with equipment sourcing and exchange rates.

Do not compare proposals by equipment price alone. Ask what incoming arsenic concentration and water chemistry the design assumes, what treated concentration it is intended to achieve, which certification applies, how performance will be verified, and what annual filters, media, testing, electricity, salt, chemicals, and wastewater will cost.

Living with an elevated result while treatment is arranged

An arsenic result above 10 ppb calls for reducing ingestion, but it does not usually mean the home must be abandoned or that every use of the water must stop. The important distinction is between swallowing the water and using it for activities where very little enters the body.

Use a verified alternate source for anything consumed

Tested bottled water is a practical interim source for drinking and food preparation. Choose commercially packaged water from a regulated supplier, or use a refill station only if you can confirm that its water is treated and monitored for arsenic.

Use safe water for:

• Drinking and filling reusable bottles
• Making coffee, tea, juice, and other beverages
• Preparing infant formula
• Making ice
• Brushing teeth
• Washing or preparing foods that will absorb the water
• Giving drinking water to household pets

Do not rely on an ordinary refrigerator filter, carbon pitcher, faucet-mounted carbon cartridge, water softener, ultraviolet light, or well chlorination unless that exact treatment arrangement has been independently certified and verified for arsenic reduction. Most products in those categories address other contaminants and do not remove dissolved arsenic by themselves.

Showering and bathing are generally acceptable

Inorganic arsenic is poorly absorbed through intact skin, and it does not readily move from water into the air during an ordinary shower. Government health guidance therefore focuses on ingestion rather than bathing, handwashing, laundry, or household cleaning.

Showering and bathing are generally considered acceptable when arsenic is the only identified concern. This advice may change if the water also contains bacteria, volatile chemicals, or another contaminant with a different exposure route. Open wounds or a physician’s advice may also warrant individual precautions.

Cooking water deserves the same care as drinking water

Cooking does not destroy arsenic. Foods that absorb water can retain it, and evaporation can concentrate what remains. Use treated or bottled water for soups, stews, rice, oatmeal, beans, pasta, sauces, gelatin, and reconstituted foods.

Rinsing produce under untreated water is a much smaller exposure than drinking a glass, but using safe water for the final rinse avoids unnecessary ingestion. Do not assume that draining boiled food removes all the arsenic that entered during cooking.

Treatment is not proven until the treated water passes a laboratory test

Installation is only the beginning. Collect a treated-water sample after the system has been flushed and placed into normal operation according to the manufacturer’s directions. Have an accredited laboratory analyze it for arsenic before relying on the treated tap for drinking or cooking.

For a whole-house system, paired raw-water and treated-water samples provide the most useful baseline. For an under-sink system, test the dedicated faucet and clearly label any nearby untreated taps so household members and guests know which water to use.

A sensible monitoring plan usually includes:

1. An initial post-installation test to confirm that the selected equipment works on your water.
2. Additional testing during the first year at intervals recommended by the treatment professional, particularly when media life is uncertain.
3. Ongoing laboratory testing at least as often as required by the manufacturer, local health authority, or treatment design.
4. Immediate retesting after major service, media replacement, well deepening, pump changes, flooding, drought-related changes, or a noticeable change in water quality.

Keep the laboratory reports, equipment model numbers, filter and media dates, service records, and the treatment professional’s operating instructions together. Arsenic provides no sensory warning when a membrane fails or media becomes exhausted. Maintenance dates and laboratory results are the evidence that the household’s drinking water remains protected.

The central lesson is practical: arsenic in well water is manageable, but geology determines the source, laboratory testing reveals the risk, and verified treatment controls exposure. Clear-looking water and a newly installed filter are not proof. A current accredited-lab result is.

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Author: Well Drilling Guide Editorial Team

The Well Drilling Guide editorial team researches and writes our guides on water-well drilling, pumps, and well water for homeowners across the US and Canada. We translate current cost data, NGWA and state well-construction standards, and EPA and CDC guidance into clear, practical answers. What we stand for: honest, independent help. Every price and claim is researched and fact-checked against multiple 2026 sources — never guessed, never borrowed from a single advertiser. We answer to homeowners, not to the companies we write about, because our only goal is to help you make a confident, well-informed decision about your water.