Iron in pool water from a well: how to fix it and keep it out

Iron in well water is usually dissolved — ferrous iron — which is invisible. The pool fills crystal clear, and nothing looks wrong until the first dose of chlorine or shock goes in.

Chlorine is an oxidiser, and oxidising dissolved iron is exactly what it does. The iron turns into ferric particles: rust. Depending on how much there is and what else is in the water, the pool goes tea-brown, orange, or a murky green that looks like algae but is not. Some of those particles stay suspended; the rest settle on the floor, the steps and the walls as stains.

That is also why the obvious fix backfires. Adding more shock to a brown pool oxidises more iron and makes it browner. The chlorine is not failing; it is doing its job on something you did not want oxidised in the pool.

Filling from a well points at iron, but it is worth two minutes to confirm before buying anything, because algae is treated with more chlorine and metals are made worse by it.

Spot tests are indicators, not a lab result. A test strip or kit that reads iron and copper in the pool water, plus a test of the well itself, settles it.

Every guide says well water "can contain iron" and stops. The useful number is how much iron a fill carries, because that decides whether a sequestrant is enough, whether a hose-end filter can cope, and whether you should be using well water for this at all.

The arithmetic is simple: gallons times parts per million times 3.785, divided by 1,000, gives grams. Worked through for a typical 20,000 gallon pool:

Grams = gallons x mg/L x 3.785 / 1,000. Top-ups carry the same concentration: replacing 2,000 gallons lost to evaporation and backwashing adds a tenth of the fill figure each time.

Find out your well's iron level→

The other number nobody runs is time. A garden hose off a well delivers whatever the pump and well sustain — often 5 to 10 gallons a minute. At 10 GPM a 20,000 gallon pool takes about 33 hours of continuous running; at 5 GPM, about 67 hours.

On a low-yield well that is a real risk. Running the pump for a day or more can draw the water level down to the intake, and a submersible that runs dry overheats quickly. Fill in shifts, with rest periods long enough for the well to recover, and stop if pressure or flow starts falling off. Sediment stirred up by long pumping also tends to carry more iron.

If pressure is already dropping→

Prevention is cheaper than removal at every iron level, and the right method depends on the number from your well test.

• Check whether your outdoor spigot is treated. On most houses with a softener or iron filter, the outside taps are plumbed before the equipment so the lawn is not watered with softened water. Filling from an inside, treated line — or temporarily from a hose connection after the iron filter — can take most of the iron out before it arrives.
• Use a hose-end pre-filter for low to moderate iron. They are made for exactly this. Compare the filter's rated gallons at your iron level against the pool volume, and have a spare cartridge ready.
• Add a metal sequestrant as you fill. Products based on HEDP or phosphonic acid bind dissolved iron and keep it from dropping out as stains when chlorine goes in. Dose for the full volume per the label before adding any chlorine.
• Keep sequestrant topped up. It breaks down slowly in sunlit, chlorinated water, so it needs regular maintenance doses, and more after every top-up with well water.
• Above about 3 mg/L, stop filtering on the way in. Buy a delivery of water for the initial fill, and use well water only for small top-ups with sequestrant.

How whole-house iron filters work→

Once iron has been oxidised in the pool, the goal is to get it out of the water, or at least keep it from staining, without re-oxidising it every day.

1. Stop adding chlorine or shock. Let the free chlorine drop rather than chasing the colour with more.
2. Bring pH to about 7.2 to 7.4. Metals drop out and stain more readily at high pH.
3. Add a metal sequestrant at the initial-treatment dose on the label, and run the filter continuously.
4. Clear the suspended particles: a clarifier and 24 to 48 hours of filtration, or a floc, letting it settle, and vacuuming to waste. Clean or backwash the filter afterwards, since it now holds the iron.
5. Where the water is still discoloured after that, partial drain and refill — but only with low-iron or treated water, or you are refilling the problem.
6. Raise chlorine back up gradually over several days rather than shocking, keeping sequestrant in the water throughout.

Stains on plaster, vinyl or fibreglass can usually be lifted with ascorbic acid — vitamin C — which converts the rust back to dissolved iron. Pool owners' forums have refined the method over many years, and it is gentler and cheaper than acid washing.

Confirm first: rub a vitamin C tablet on a stain. If it lightens, the treatment will work on that stain.

1. Lower free chlorine to zero and pH to about 7.2. Chlorine consumes ascorbic acid, so any left in the water wastes it.
2. Add ascorbic acid at roughly half a pound per 10,000 gallons, spread around the pool with the pump running. Follow the product's directions where they differ, especially on vinyl liners.
3. Circulate for about 30 minutes and look. Add more in small increments if stains remain.
4. Once the stains are gone, the iron is back in the water. Add sequestrant before doing anything else, or it will re-stain as soon as chlorine returns.
5. Bring chlorine back slowly over several days, keeping pH in range and sequestrant topped up.

A few common moves make an iron pool worse or change nothing, and they are worth avoiding before you spend money.

• More shock. It oxidises more iron, darkens the water and sets stains.
• Baking soda. It raises total alkalinity and pH. Higher pH makes iron drop out faster.
• Draining and refilling from the same well. The new water carries the same iron.
• Relying on the pool filter alone. Sand and cartridge filters catch some oxidised particles, but they do nothing about dissolved iron, which is what comes back.

Read the well test you already have→