Well water treatment by test result: which technology fixes what, and what doesn't

Most pages about well water treatment open with a system and work backward to your water. The agencies that deal with private wells do it the other way around. The CDC's guidance on treating well water says "No single treatment type will protect against all problems" and tells you to "first check to make sure the system removes the type of germ or chemical you are concerned about." The Minnesota Department of Health (MDH) goes a step further: if your water is contaminated, "first try to remove the source(s) of contamination or replace the contaminated water supply with a safer supply," and treat only if that is not possible.

This guide follows that order: find your lab result in the tables below, then read the primer for the technology its row names. Federal limits quoted here (EPA's MCLs) are written for public water systems. They don't legally apply to a private well; Penn State says such standards "only provide guidelines" for one.

Before you buy a well water filter: test first

Whether it is sold as a well water filter, a softener or a whole-house system, the order is the same. Vermont's Health Department puts it plainly: "Before deciding on a water treatment system for your home, it is important to complete all the recommended drinking water testing." Its own example shows why. A water softener "can be used to treat hardness, iron and radium, but it will not remove arsenic." A seller who sees orange stains and sells a softener has solved the stain and left the arsenic.

Three things in a lab report decide the technology.

• What the contaminant is. Wisconsin's device table (DHS P-03494) lists reverse osmosis for nitrate and arsenic, carbon for VOCs, and a softener for manganese.
• How much there is. Penn State Extension says iron and manganese can be removed "depending on both the form and concentration of the metals." Hydrogen sulfide is the same: carbon below about 1 mg/L, chlorination for the highest levels.
• The water around it. Vermont notes that "Pretreatment is sometimes necessary," usually for hardness, iron or manganese, because they "can make the treatment system less effective." pH, hardness, iron and cloudiness change which system works and in what order.

Confirm the result first (Wisconsin DHS says to retest high nitrate; Penn State says you "may want to submit a second sample" after a positive bacteria test), and get pH, hardness, iron and manganese on the same report. Our which-tests tool builds that list, the testing guide covers sampling, and the lab finder lists state-certified labs. If you already have a report, read your results first.

Wisconsin's DNR: "well owners should be wary of sales claims." MDH says "No single treatment unit can remove all contaminants in water," and you "may need to combine several treatment units into a treatment system."

Kitchen tap or whole house: let the exposure decide

MDH defines the two types: point-of-use (POU) units "treat water at one faucet or one location," and point-of-entry (POE) units "are installed on the water line as it enters the home" and treat all of it. Wisconsin DHS says POU devices are best when a substance poses a risk "when it is used for drinking and preparing foods," and POE devices are best when it poses a risk "when it is used for bathing, showering, and running the dishwasher and washing machine."

• Whole house is needed for gases and solvents. New Hampshire's environmental agency says "Radon is volatile and must be treated at point-of-entry or whole-house." EPA says point-of-use devices "are not effective in reducing the risk from breathing radon released into the air from all water used in the home." For VOCs, Connecticut's health department says shower and bath exposure through skin and air "can equal or surpass the amount of exposure from water ingestion."
• Whole house for recurring bacteria and for stains. Penn State describes continuous disinfection as a system "that continuously treats all of the water entering the home." For iron and manganese staining it says the metals "must be removed from all water entering the home."
• Often the kitchen tap for nitrate, arsenic, lead, PFAS and uranium. Virginia Cooperative Extension says for nitrate "only water to be used for drinking and cooking needs to be treated." Washington's health department says arsenic "is not absorbed very well through the skin." EPA's lead filter guide covers faucet, refrigerator and pitcher filters. MDH says for PFAS that point-of-use treatment "tends to be more economical." For uranium, Maine CDC says a kitchen-sink system "is good if you are worried about drinking and cooking," and a whole-house system is one "you may want" if there are young children in the house.

One trap runs both ways. MDH notes that a softener "may not be connected to cold water plumbing or kitchen faucet plumbing," so a whole-house unit may not reach the tap you drink from. And a kitchen unit does nothing for the shower.

The master table: health contaminants

Each row names its source. "Look for" gives certification wording only where an official page or listing states it. The linked explainer has the action levels.

The master table: nuisance results and water chemistry

These numbers matter twice: for the stain or smell, and for the pretreatment the health rows need. Our smell, taste and stains guide matches symptoms to tests.

Technology primers: what each one does and where it fails

Activated carbon (granular or block)

Wisconsin DHS: contaminants are "adsorbed (stuck) onto a carbon filter," and results depend on the contaminant, "the filter design, and the time the water touches the filter." Connecticut uses granular activated carbon for gasoline, heating oil and chlorinated solvents; EPA lists it for PFAS; it also handles taste, odor and low hydrogen sulfide. Carbon is not a lead fix by default: Penn State says granular activated carbon filters "are not recommended for copper and lead removal," while Wisconsin lists carbon devices certified to NSF/ANSI 53 for lead. The certification makes the difference. Upkeep is hard to predict. Vermont says some large filters last years and "some small filters may last for only weeks." MDH warns a neglected filter can grow bacteria or "become saturated and begin to release contaminants." Connecticut's VOC systems run two canisters in series, sampled before, between and after them, so "breakthrough" in the first canister can be caught.

Reverse osmosis (RO)

RO is the one technology on several health rows at once, so it has its own section below: reverse osmosis for well water.

Anion exchange

Vermont: typically used for nitrate, and can also lower uranium, sulfate and fluoride; after an oxidation step it also handles arsenic. It is usually installed whole house. Its failure mode is the one to know. Penn State explains that the resin prefers sulfate, so "If the resin becomes saturated, it releases the nitrates in place of sulfates, resulting in an increased nitrate concentration in the 'treated' water." Maine says a failed arsenic system can release "all of the arsenic on the resin" at once. Vermont adds that anion exchange lowers pH, so metal plumbing needs a neutralizer after it, and that iron and turbidity "may clog" the unit. It "will not lower the level of radium."

Cation exchange (water softening)

A softener swaps calcium and magnesium for sodium or potassium. Vermont says it can also remove radium, iron and manganese; Wisconsin's device table lists it, certified to NSF/ANSI 44, for manganese. Penn State limits it for iron to water with pH above 6.7, hardness of 3 to 20 grains per gallon and dissolved iron under 5 mg/L, and says raw water must not meet "air or chlorine before entering the softener." It is "not recommended for lead removal" (Penn State), will not lower uranium (Vermont), will not remove arsenic (Vermont; Wisconsin DNR: softeners "do not adequately remove arsenic"), is "not likely to remove PFAS" (MDH) and does not remove sulfate (MDH). For iron or manganese, Vermont suggests considering "non-salt treatment technologies such as an oxidation and filtration system."

Oxidation and filtration (iron, manganese, hydrogen sulfide)

An oxidant (air, chlorine or permanganate) turns dissolved metal into particles a filter catches. Penn State recommends manganese greensand for 3 to 10 mg/L combined iron and manganese and "oxidation followed by filtration" above 10 mg/L. Its birm filters need dissolved oxygen and pH of at least 6.8 for iron and 7.5 for manganese. New Hampshire says air is enough "For iron only," but manganese needs a strong pre-oxidant such as permanganate or chlorine. Penn State warns that the permanganate used to regenerate greensand "is toxic and must be handled and stored carefully." Vermont asks for NSF/ANSI 42 or 53 certification.

Aeration

Aeration "uses a fan to reduce radon in water" and vents the air outdoors (Vermont). EPA rates it at 95 to 99 percent for radon. Connecticut lists VOCs including MTBE, and Penn State uses it for methane and for hydrogen sulfide up to about 2 mg/L. EPA says aeration "requires annual cleaning." Connecticut says "Bacterial slime will grow in aerators and storage tanks," which may need chlorination, and wants the vent "above the eave of the roof." Wisconsin DNR says a radon aerator must be designed against fouling by iron, iron bacteria or hardness. MDH notes added oxygen "can be corrosive," a concern with copper or lead pipes.

UV and continuous chlorination (bacteria)

Both are whole-house fixes for bacteria that return after the well is repaired and shocked. Penn State says UV adds no chemical, needs water "completely clear and free from any suspended sediment or turbidity," often has a sediment filter ahead of it, gives no residual in the pipes, and needs a new bulb "about once a year." It is not recommended above 1,000 total coliform or 100 fecal coliform colonies per 100 mL. Ohio's private water system rule (OAC 3701-28-15, effective January 1, 2020) says the water must be pre-treated to the UV maker's water quality requirements, "including, but not limited to, hardness, iron, manganese, TDS, and turbidity," with the UV unit installed after any softener or iron or manganese filter; Ohio's older 2011 fact sheet put the line at dissolved iron of 0.3 ppm or more with hardness of 6 grains per gallon or more. Chlorination aims for 0.3 to 0.5 mg/L residual chlorine and about 30 minutes of contact time, often in a larger holding tank (Penn State). Iron, hydrogen sulfide and organic matter use up chlorine, and the systems "require significant maintenance." MDH adds that chlorination may form by-products.

Distillation

Distillers boil water and condense the steam, leaving contaminants behind. MDH lists nitrate, arsenic, lead, uranium, radium, sulfate, sodium and bacteria among what they remove; its list does not include VOCs, PFAS or radon. Penn State says a typical home unit "produces about a gallon of water every four to five hours" and "consumes significant energy." Vermont says they sit at or under the kitchen sink and asks for NSF/ANSI 62 certification.

Neutralizers (low pH)

Penn State says acid-neutralizing filters add calcium and raise pH, and "act to prevent lead from entering the water rather than removing it at the tap." Vermont names marble, calcium carbonate, soda ash and lime as common media, and says to test pH and alkalinity first. If lead shows up in a flushed sample, Vermont says neutralization "is not recommended," because the lead is coming from upstream.

Boiling? The CDC says "Boiling water kills germs in water" but "Boiling will not remove chemicals from water." MDH says boiling concentrates nitrate; the Virginia Department of Health says arsenic "can actually increase slightly."

Reverse osmosis for well water

MDH: "RO uses energy to push water through a membrane with tiny pores. The membrane stops many contaminants while allowing water to pass through." Its home-treatment guide (MDH, Home Water Treatment, revised March 29, 2024) lists RO as "designed to fully or partially remove" arsenic, bacteria, calcium, chloride, copper, fluoride, iron, lead, magnesium, manganese, nitrate, nitrite, other dissolved solids, pesticides and other synthetic organic compounds, PFAS, radium, selenium, sodium, sulfate, other metals, TCE, trihalomethanes, uranium, vinyl chloride, viruses and VOCs, plus color, taste and odor. Its stated pro: it "removes a wider variety and greater amount of contaminants than many other treatment options." Its cons: it "can create a lot of wastewater" and "may require pretreatment to prevent the membrane from getting clogged."

That list is why RO shows up so often on well reports, and the reported rates show it is not a 100 percent device:

Kitchen tap or whole house. RO usually sits at the kitchen tap. Penn State says units this size "can produce only a few gallons of treated water per day." MDH's cost estimates, based on 2017 quotes and 2018 research, are $300 to $1,500 to install a point-of-use unit and $100 to $200 every one to two years to maintain it, against $5,000 to $12,000 for point-of-entry RO and $250 to $500 every one to two years. Vermont says a neutralizer is "usually required" after whole-house RO.

Water use and upkeep. Vermont puts efficiency at "only about 25%," with 3 to 4 gallons wasted per gallon treated, and notes that "chlorine can damage some reverse osmosis membranes," which matters if you chlorinate for bacteria upstream. Virginia Extension gives membranes "an average lifetime of 3-5 years."

Bacteria. MDH lists bacteria and viruses among what RO can remove, but a kitchen unit protects one tap. A well with recurring coliform needs the well fixed and, if bacteria return, whole-house continuous disinfection (see the UV and chlorination primer above and the coliform page).

Certification and proof. Look for NSF/ANSI 58, and then for the specific reduction claim for your contaminant on the listing, as the master table shows for nitrate, arsenic and PFAS. Then test the treated tap: a lab result is the only proof of what your unit delivers.

Fix guides, by test result

Each of these goes deeper than the tables above for one result line, and each starts from the test:

• Arsenic: arsenic removal from well water, matching the filter to arsenic(III) or arsenic(V).
• Hardness, iron, or "do I need a softener?": water softener vs water filter, decided line by line from the report.
• Rotten-egg smell: how to remove sulfur smell from well water, source first, from the anode rod to aeration.
• Sand, silt or muddy water: sediment in well water, with filter micron ratings and the after-rain bacteria check.
• Copper or blue-green stains: copper in well water, from first-draw sampling to fixing corrosive water.
• Fluoride: fluoride in well water, including what removes it and what doesn't.

Putting units in order: the treatment train

When a report needs several units, order matters. Each step below comes from an agency or extension page.

1. Sediment first. Penn State says a sediment filter is "routinely installed ahead of the chlorinator" and "often installed ahead of the UV unit." Connecticut's carbon systems include one.
2. Oxidize before arsenic media. EPA says groundwater typically carries arsenite, so "pre-oxidation will probably be necessary." Chlorine and permanganate work "within one minute"; "Ultraviolet (UV) light, by itself, is also ineffective."
3. Treat iron, manganese and hardness before UV, RO and anion exchange. Ohio's rule names hardness, iron, manganese, TDS and turbidity for UV, Maine CDC iron and manganese for RO, and Vermont hardness for RO and iron for anion exchange.
4. Keep oxidants away from a softener. Penn State: no air or chlorine before the softener when it is removing iron.
5. Deal with chlorine residual. Penn State says after chlorination "An additional activated carbon filter may be necessary to remove the chlorine residual." Vermont says "chlorine can damage some reverse osmosis membranes, so pre-treatment may be needed."
6. Neutralize after whole-house RO or anion exchange. Vermont says neutralization "is usually required" after those systems.

How to read a certification claim

A certification mark on a box tells you less than it seems. EPA's lead filter guide describes the full claim: the certifier's mark, a statement of testing against a named standard, and the contaminant. Its example packaging text reads "Tested and Certified by (certification body) against NSF/ANSI Standards 42 and 53 for the claims specified on the Performance Data Sheet." So a claim has three parts.

• The standard number names the test family: 53 for health-effects filters, 58 for RO, 44 for softeners, 55 for UV, 62 for distillers; 42 covers claims such as particulate reduction (NSF listings, EPA, Vermont, Wisconsin).
• The claim names the contaminant. A device certified for one contaminant is not certified for the others. EPA says to look for "a claim of lead reduction" and, for PFAS, certification "for PFAS reduction."
• The data sheet and listing give the limits. EPA says the performance data sheet or the certifier's online directory shows the claim. NSF's arsenic listings carry a footnote: the product "is intended for use only on water supplies that have been shown to be at or below the referenced concentration and valency of arsenic."

MDH says certifiers "do not certify treatment units for all contaminants." And EPA says PFAS certification standards (as of April 2024) "do not yet indicate that a filter will remove PFAS down to the levels EPA has now set." Certification makes a device credible; only a test of your treated tap shows it works on your water.

After installation: re-test, maintain, keep records

Four agencies say to test the treated water. MDH: "After installing treatment, test the treated water to make sure the treatment is working." Wisconsin DHS, for devices connected to your plumbing: "test the treated water after it is installed to make sure the device is working." The CDC: test "before drinking or using it." Vermont adds that if you already did the full panel, "you can choose to test only for the contaminant the treatment system was installed for."

Some sources set a schedule. Connecticut tells owners of aeration systems to test both raw and treated water at a state-certified lab, then "annually or more frequently." Maine recommends re-testing radon "every 2 years after a mitigation system is installed." New Hampshire says whole-house carbon used for radon should be changed "every 1 to 2 years." The CDC's yearly baseline for coliform, nitrate, total dissolved solids and pH still applies to the raw well water.

Maintenance is not optional. Wisconsin DHS says neglected devices "can even make water quality worse," MDH says they can "make you sick," and EPA says not replacing a PFAS filter on schedule "can increase your risk of exposure." Our advice: keep each lab report with the date, the sampling point (raw or treated) and the last media or bulb change.

Worked example: nitrate, iron and coliform in one report

An illustrative report, not a real one: nitrate 14 mg/L as nitrogen, iron 1.2 mg/L, total coliform present. Nitrate is 1.4 times EPA's 10 mg/L. Iron is four times EPA's secondary level of 0.3 mg/L, a nuisance value rather than a health limit. The rows used are coliform and nitrate in the health table and iron in the nuisance table.

What to expect at step 7: if the RO unit removes 83 to 92 percent, as Penn State estimates, 14 mg/L would come out at roughly 1.1 to 2.4 mg/L. That is our arithmetic, and only the test confirms it. Whole-house anion exchange is the other nitrate route, but per Vermont it may need iron pretreatment and, with metal plumbing, a neutralizer, and per Penn State it can dump nitrate when the resin saturates. MDH also advises that "Even with home water treatment," babies under six months should not drink the water, in case the treatment fails. For more on each result, see nitrate, coliform bacteria and manganese, which often travels with iron.

Questions people ask