Rare & Emerging Contaminants

A note on the numbers below

Where a federal limit exists and is settled, we give it. Where a rule is still being written — perchlorate is — we say so plainly rather than quote a number that may change, because a confidently-wrong figure on a page like this would be worse than none. Some states set stricter limits than the federal government; if you are in one, your state’s number is the one that protects you.

Radionuclides — radioactive contaminants from bedrock

These occur naturally where water moves through certain rock, especially granite, and they are mainly a private-well concern. They are genuinely hazardous, the federal limits are settled, and this is squarely a test-first, often call-a-professional category. The EPA’s public-health goal for every one of them is zero; the enforceable limits below are what’s allowed in public systems, balanced against treatment cost.

Uranium

A naturally occurring heavy metal that is both chemically toxic to the kidneys and radioactive. The federal limit is 30 micrograms per liter. It is removed by reverse osmosis, distillation, anion-exchange resin, and certain adsorptive media — the same dissolved-metal toolkit as arsenic, and like arsenic the right choice depends on your water’s chemistry. Confirm it with a lab test, then size treatment to the result. Not a guess-and-buy contaminant.

Radium (226 + 228)

A radioactive metal, regulated as the combined total of its two isotopes, with a federal limit of 5 picocuries per liter. A water softener (cation exchange) removes radium incidentally along with hardness, and reverse osmosis removes it as well — but radium loads onto softener resin and concentrates in the backwash, which raises a real disposal question best discussed with your state’s drinking-water program. Treat a positive result as a reason to get professional guidance, not just a cartridge.

Gross alpha radioactivity

Not a single contaminant but a screening measure — the total alpha-particle radioactivity in the water, with a federal limit of 15 picocuries per liter (excluding radon and uranium). A high gross-alpha result is a flag to test specifically for radium and uranium to find the source. It is a “find out what’s actually there” number, not something you treat directly.

Radon (in water)

A radioactive gas. Most radon risk is the air in your home, not your water — but water from a private well can carry dissolved radon that releases into the air when you shower or run the tap. There is no finalized federal limit for radon in water; the EPA proposed one decades ago and never finalized it, so this is a test-and-mitigate situation guided by state advice. Removal is by aeration (the same logic as hydrogen sulfide — it’s a dissolved gas, so you strip it out, you don’t filter it) or by a granular activated carbon bed sized for radon. Because radon is also an air-and-lung issue, this is a test-first, talk-to-a-professional contaminant, not a DIY filter purchase.

Perchlorate — the rocket-fuel chemical

An industrial chemical from rocket propellant, munitions, fireworks, and flares, found mainly in water near military, aerospace, and defense sites. It interferes with the thyroid’s ability to take up iodine, which is why the concern centers on pregnant women, infants, and children. There is no final federal limit yet. The EPA published a proposed rule on January 6, 2026, setting a health-based goal (MCLG) of 20 micrograms per liter and co-proposing three possible enforceable limits — 20, 40, or 80 micrograms per liter. Which of the three lands is genuinely undecided, so we are not going to print one of them as though it were the number. Public comment closed in March 2026, and a court settlement requires the EPA to sign a final rule by May 21, 2027. Two states are well ahead of the federal government: Massachusetts and California already enforce stricter limits (2 and 6 micrograms per liter), so if you live there, your state’s standard is the one that applies. Removal is by anion-exchange resin (it’s a charged ion, caught the way nitrate is) or reverse osmosis. If you’re near a likely source, this is a test-first contaminant.

Chromium-6 — hexavalent chromium

The chromium made notorious by the Hinkley, California case — a known carcinogen when ingested, from both natural deposits and industrial pollution. Federal regulation covers total chromium at 100 micrograms per liter but sets no separate limit for the more dangerous hexavalent form. California is the only state that has set one, and it took three tries: adopted in 2014, thrown out by a court in 2017 over an inadequate cost analysis, and re-adopted at 10 micrograms per liter in April 2024, effective that October, with the smallest water systems given until 2028 to comply. California also holds total chromium to 50 micrograms per liter, half the federal figure. Removal is by reverse osmosis, and by anion-exchange or specialized reduction-and-adsorption media, but the chemistry is finicky — the form and the water’s pH both matter — which makes this a confirm-by-lab-then-match-treatment contaminant rather than an off-the-shelf fix. If your test flags it, get the treatment matched to your specific water.

Other regulated metals — the rest of the panel

A full laboratory test reports a longer list of metals than the ones with their own pages here. Most people never see a detection. When one does show up, the removal answer is usually the same family of tools that handles arsenic and uranium — reverse osmosis, distillation, ion exchange, or a purpose-matched adsorptive medium — because these are all dissolved metals that mechanical filters and carbon do not catch. The federal limits below are settled. As with everything on this page: confirm a detection with a certified lab before you treat, and for the most toxic of these, get professional guidance rather than guessing at a cartridge.

Cadmium

A toxic metal, sometimes from corrosion of old galvanized plumbing or from industrial contamination. The federal limit is 5 micrograms per liter. Removed by reverse osmosis, distillation, or ion exchange.

Mercury

A toxic metal, rare in drinking water but serious where it appears. The federal limit is 2 micrograms per liter — and it is written specifically for inorganic mercury, which is the form that turns up in groundwater. Removed by reverse osmosis, distillation, or activated carbon specifically certified for mercury — ordinary carbon is not enough, so read the certification. A confirmed detection is worth taking seriously with a certified retest.

Selenium

An element that is essential to health in trace amounts and toxic in excess — naturally occurring in some Western groundwater. The federal limit is 50 micrograms per liter. Removed by reverse osmosis, or by anion-exchange and adsorptive media.

Thallium

A rare and highly toxic metal, usually tied to industrial or mining contamination. The federal limit is 2 micrograms per liter. Removed by reverse osmosis or specialized adsorptive media. This is a test-first, get-professional-help contaminant — a confirmed thallium result is not a DIY cartridge decision.

Barium

A naturally occurring metal in some groundwater. The federal limit is 2 milligrams per liter — notably higher than the trace metals above, because barium is less acutely toxic at low levels. Removed by reverse osmosis or, incidentally, by a water softener, since cation exchange takes barium out along with hardness.

Antimony

A trace metal, occasionally from industrial sources or from certain plumbing and solder. The federal limit is 6 micrograms per liter. Removed by reverse osmosis.

Beryllium

A rare metal associated with industrial contamination. The federal limit is 4 micrograms per liter. Removed by reverse osmosis or adsorptive media. Rare enough that a detection warrants a confirming lab test before anything else.

Nickel

A metal that can leach from plumbing and fixtures. Nickel has a regulatory history rather than a number: an earlier federal limit was remanded back to the EPA and never replaced, so there is no federal MCL to hold your result against. Where your state sets one, that is the standard that applies to you. Removed by reverse osmosis or ion exchange.

Cyanide

Usually associated with industrial discharge. The federal limit is 0.2 milligrams per liter, measured as free cyanide. A detection is an industrial-contamination flag and warrants professional follow-up rather than a cartridge decision — which treatment is appropriate depends on the form the cyanide is in, and that is a question for a lab and a specialist. If your test flags it, get advice before you buy anything.

Nuisance ions — real on a test, rarely a health emergency

A full panel also reports several dissolved ions that affect taste, corrosivity, or diet but are not, at ordinary levels, a toxicity concern. They are grouped here so a detection doesn’t send anyone into a panic — most of these carry secondary (aesthetic) standards, not health limits.

Sulfate

Naturally occurring; at high levels it has a laxative effect and a bitter taste. It carries a secondary (aesthetic) standard of 250 milligrams per liter rather than a health-based limit. Removed by reverse osmosis, distillation, or anion exchange.

Chloride and sodium

Both are common, both matter mainly for taste, corrosivity, and — for sodium — people on sodium-restricted diets. Chloride carries a secondary standard of 250 milligrams per liter. Sodium is not federally limited but is worth knowing if a doctor has told you to watch it, and a salt-based softener adds some. Both are removed by reverse osmosis or distillation.

Bromate

Itself a disinfection byproduct — it forms when source water containing bromide is treated with ozone. The federal limit is 10 micrograms per liter. Prevention is largely a treatment-plant matter; at the tap, reverse osmosis reduces it. See also the chlorine, chloramine, and THMs profile for the broader disinfection-byproducts family.

Aluminum, lithium, strontium, and other trace elements

Occasionally detected, rarely at levels of concern in typical supplies. Aluminum carries a secondary (aesthetic) standard; the others are generally unregulated at the federal level for drinking water. Reverse osmosis and distillation remove dissolved trace elements broadly, so the catch-all answer applies.

Other emerging contaminants

These come up occasionally, mostly from industrial contamination or fuel, and most have no settled federal drinking-water limit yet — they are “emerging” precisely because the science and regulation are still catching up. The honest posture for all of them is the same: if a lab flags it, that’s the signal to get specific advice, because the treatment depends on the exact contaminant and concentration.

1,4-Dioxane

An industrial solvent stabilizer that is hard to remove — it slips past standard carbon and is not well caught by ordinary filtration. Effective treatment is generally advanced oxidation, which is a professional-scale process, not a home cartridge. A positive result here is a “consult a specialist” outcome, full stop.

MTBE

A gasoline additive that gives water a turpentine-like taste and smell at very low levels, usually from leaking fuel tanks. Activated carbon reduces it, and aeration helps because it’s volatile, but the taste threshold is so low that confirming the source (and stopping it) matters as much as filtering. Test to confirm, and treat the source, not just the symptom.

PCBs

Polychlorinated biphenyls — persistent industrial compounds, banned decades ago but still lingering in the environment. Activated carbon and reverse osmosis reduce them, but a confirmed detection is health-serious and warrants a certified retest and professional guidance rather than a default filter purchase. PCBs appear alongside the pesticides on a full lab panel; the complete list is on the pesticides profile’s reference list.

Don’t see yours even here? Your water is worth a real answer. Start with a test — a certified lab result tells you exactly what you’re dealing with, and from there the right treatment (or the right professional) follows. We would rather send you to a lab or a specialist than sell you a filter that doesn’t fit your water. Test your water first.