Does Reverse Osmosis Remove Nitrates, Arsenic and Lead?
The lab report came back with three lines highlighted in the margin: nitrate 12 mg/L, arsenic 14 ppb, lead 9 ppb. Under the kitchen sink sits a four-stage reverse osmosis unit you bought two years ago because the water tasted flat. The box said "removes up to 99% of contaminants." Nobody could tell you whether that covers these three numbers.
Start with the lab report, not the spec sheet
None of the three announce themselves. Nitrate is tasteless, arsenic is colorless and odorless, and lead at 15 ppb changes nothing you can see. The TDS meter is no help: 10 ppb of arsenic is 0.010 mg/L, invisible inside a reading of 150–400 mg/L. It tells you the membrane is alive, nothing about a single contaminant.
On a private well, testing is on you. The Safe Drinking Water Act doesn't cover private wells. CDC guidance is a nitrate test yearly and a broader panel — arsenic, lead, metals — every few years. A state-certified lab charges $20–100 per parameter, $100–300 for a well panel; EPA's Safe Drinking Water Hotline (800-426-4791) points you to one.
On city water, the report already exists. Your utility's annual Consumer Confidence Report lists nitrate and arsenic at the plant. Lead is the exception — it leaches from service lines, solder and brass fixtures in your own plumbing, so the number is specific to your address. Sample first-draw, after 6+ hours with no water used.
If arsenic shows up, ask for speciation. A standard result gives total arsenic; speciation splits it into As(V) and As(III). Low-oxygen water — deep wells, water carrying iron or sulfide — often skews toward As(III). It costs $50–150 extra and changes what you buy.
What RO actually does to each of the three
A thin-film composite membrane rejects dissolved ions by charge and size, with pores near 0.0001 micron. Lead is a heavy, doubly-charged ion and gets rejected almost completely; nitrate is small and singly charged, so more slips through; As(V) is charged at normal pH, while As(III) stays uncharged below about pH 9.2 — exactly what a charge-rejecting membrane handles worst.
| Contaminant | EPA benchmark | Typical RO reduction | NSF/ANSI 58 claim to look for |
|---|---|---|---|
| Lead | 15 ppb action level (MCLG 0) | 95–99% | "Lead reduction" — 150 ppb in, ≤5 ppb out |
| Arsenic, As(V) / arsenate | 10 ppb MCL | 85–95% | "Pentavalent arsenic" — 300 ppb in, ≤10 ppb out |
| Arsenic, As(III) / arsenite | 10 ppb MCL (total As) | 50–70% | No claim exists — oxidize to As(V) first |
| Nitrate / nitrite (as N) | 10 mg/L and 1 mg/L MCL | 60–90% | "Nitrate/nitrite reduction" |
Read the third column against the fourth. A system with no lead claim may still remove lead well, but nobody verified it under a protocol, and "up to 99%" on a carton is measured on whatever the manufacturer chose. The free NSF listings at nsf.org are searchable by brand and model — that listing, not the box, settles it.
Scope is the other half. All three are ingestion concerns and none matter much in the shower, which is why a point-of-use unit at the kitchen sink beats treating every gallon that ends up in the laundry.
| Approach | Where it fits | How it works | Typical cost |
|---|---|---|---|
| Point-of-use RO, NSF/ANSI 58 | All three, at one tap | Membrane rejects dissolved ions | $150–500 + $150–400 install |
| Anion exchange, nitrate-selective | Nitrate, whole house | Resin swaps nitrate for chloride | $1,200–3,000 |
| Adsorptive media (iron oxide, activated alumina) | Arsenic As(V), whole house | Arsenic binds to the media bed | $1,500–4,000 |
| Oxidation stage ahead of RO or media | Wells with As(III) | Converts As(III) to As(V) | $300–1,500 |
| Carbon pitcher or under-sink, NSF/ANSI 53 | Lead only | Adsorption plus ion-exchange media | $8–20 per cartridge |
| Countertop distiller | All three, small volumes | Evaporates and recondenses | $100–400 |
| Water softener | None of the three | Removes hardness ions only | — |
Why the RO under your sink may still miss it
Feed pressure. Membranes are rated at 50–60 psi. Below roughly 40 psi — common on wells set at 30/50 and on upper floors — output slows and rejection falls with it. A permeate or booster pump runs $80–200 and is the fix, not a bigger membrane.
A membrane past its life. TFC membranes last 2–5 years and are destroyed by chlorine, which is what the carbon prefilter exists to stop. The test is arithmetic: TDS in versus TDS out, and if rejection has fallen under about 75%, it's done regardless of the calendar.
Arsenic speciation, again. A household with 20 ppb of mostly As(III) installs a certified system, retests, and lands at 8 ppb — under the MCL, but nowhere near the 90% the spec promised. An oxidation step ahead of the membrane restores expected performance.
Nitrate math. 60–90% is a percentage, not a fixed output. Feed water at 25 mg/L through a 70% system gives roughly 7.5 mg/L — under the 10 mg/L MCL, but not by much, and farm-country nitrate swings seasonally with irrigation and rainfall.
Common mistakes
- Reading "removes 99% of contaminants" as a certified claim. Under NSF/ANSI 58 only TDS reduction is mandatory; lead, nitrate/nitrite and pentavalent arsenic are separate optional claims. Confirm each by name in the NSF listing.
- Testing for total arsenic and stopping there. Without the As(III)/As(V) split you can't tell whether RO gives you 90% or 55% — the $50–150 speciation test keeps you from buying the wrong $2,000 system.
- Using a TDS meter as a safety check. Arsenic at 10 ppb is 0.010 mg/L and lead at 15 ppb is 0.015 mg/L; both vanish inside a normal reading. TDS proves the membrane still works, nothing more.
- Boiling the water while you wait for equipment. Boiling drives off water and leaves all three behind at higher concentration. With an infant in the house and elevated nitrate, EPA and state health departments point to bottled or certified-treated water.
FAQ
Does reverse osmosis remove nitrates from well water?
Yes, typically 60–90% on a system carrying the NSF/ANSI 58 nitrate/nitrite claim. It's a percentage, not a fixed output: 25 mg/L through a 70% system still leaves about 7.5 mg/L against the EPA's 10 mg/L MCL. Well nitrate swings seasonally, so retest rather than assume.
Does reverse osmosis remove arsenic?
It removes arsenate, As(V), at roughly 85–95% — the NSF/ANSI 58 protocol challenges certified systems at 300 ppb and requires product water at or under the 10 ppb MCL. Arsenite, As(III), is uncharged at normal pH and can fall to 50–70%, so a speciation test and an oxidation step ahead of the membrane matter.
Does reverse osmosis remove lead?
Yes — lead is what RO handles best, typically 95–99%, and a system certified for lead reduction must take 150 ppb down to 5 ppb or less against an EPA action level of 15 ppb. It protects one tap only: the bathroom sink and the line to the water heater stay unfiltered.
Do I need whole-house treatment or just the kitchen sink?
For all three, a point-of-use RO at the kitchen sink covers drinking and cooking — a $150–500 decision instead of $1,200–4,000. Whole-house anion exchange or adsorptive media makes sense at persistently high nitrate or arsenic, or when other well problems already need a whole-house system.
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General information about home water quality and treatment equipment, based on NSF/ANSI standards, EPA and CDC guidance and manufacturer specifications — not independent lab testing, and not medical advice. Removal rates vary with water chemistry, feed pressure, membrane age and contaminant form. Nitrate, arsenic, lead and bacteria can only be measured by a state-certified laboratory; questions about health effects belong with your doctor or state health department.