Filling a Pool With Well Water: What to Test and Treat
Test the well water for metals before you fill. The Pool & Hot Tub Alliance (PHTA) says to treat source water with total metals above 0.2 ppm before it goes in the pool, and to avoid untreated water above 1 ppm. Fill through a filter, then protect the water with a metal sequestrant.
On this page (9 sections)
Why well water is different
City water is treated and tested before it reaches you. A private well is not. The EPA does not regulate private wells, and the owner is responsible for the water. A PHTA technical note says the main problems with well water in pools are high levels of iron and copper, tannins (plant stains), and iron bacteria. These cause stained surfaces, cloudy or colored water, and a lot of chlorine use. PHTA also notes that untreated well water is especially known for high levels of heavy metals.
The trap is that well water often looks clear in the hose. The metals are dissolved. Chlorine oxidizes them, and they turn brown, green-brown, or tea-colored or settle on the walls. That is why a pool can look fine on day one and brown on day two. The page on pool stains has a color guide for telling iron, copper, and organic stains apart.
Test before you fill
Take a sample from the hose bib you plan to use, after running the water for a few minutes. PHTA recommends testing source water for metals before filling a pool or hot tub, and says to repeat the source water chemistry at least once a year. Here is the pre-fill list.
| Test | Why it matters | What to look for |
|---|---|---|
| Iron, copper, manganese (total metals) | Stains, brown or green water, high chlorine use | PHTA: remediate above 0.2 ppm total; avoid untreated water above 1 ppm |
| Copper | Blue-green stains; with high stabilizer (CYA) can form purple copper cyanurate | PHTA notes CDC says copper in pool water should not exceed 1.3 ppm; staining may occur at lower levels |
| pH and total alkalinity | Sets your starting chemistry and how easily metals stay dissolved | Compare to your pool chemistry chart targets |
| Calcium hardness | Scale if high; plaster etching if low | Orenda: where it exceeds 1,000 ppm, find another source of water |
| Nitrates | Common in rural groundwater (fertilizer, septic); feed algae | PHTA: nitrates above 25 ppm may lead to excessive algae. No practical removal except dilution |
| Phosphates | Algae food; may be high in well water | Orenda says well water may be high. See pool phosphates |
| Smell and color | Rotten egg smell points to hydrogen sulfide; rust-colored sediment points to iron | Do it with a clear glass in daylight |
| Bacteria (coliform) | Wells near septic tanks or runoff can be contaminated | Use a state-certified lab. PHTA notes well water can be contaminated by nearby septic tanks and runoff |
Use a two-step total metals test. PHTA explains that most one-step iron and copper tests measure only the free dissolved metal, and miss metals that are already bound up or oxidized into particles. Iron bound in iron bacteria may not show up at all, per the same PHTA note, so brown water with a low iron reading is possible. For anything surprising, send a sample to a certified lab. Your county health department can usually point you to one.
What to do based on the metal reading
| Total metals in fill water | What PHTA says | Our plan |
|---|---|---|
| Below 0.2 ppm | Its treatment cutoff starts above 0.2 ppm, so no treatment is called for below it | Fill, then use a sequestrant on the label's schedule as a precaution if you have iron-bearing stone or a copper heat exchanger |
| 0.2 to 1 ppm | Remediate before adding the water to the pool | Fill through a sediment filter and a metal-removing filter, or use trucked water, then add a sequestrant |
| Above 1 ppm | Avoid using untreated well water to fill or top off | Treat at the source or use another water supply |
PHTA names three ways to lower dissolved metals: replace part of the water with metal-free water, filter through reverse osmosis (RO), or use a water-insoluble adsorbent that binds the metals. Sediment filters of 10 microns or finer remove most of the larger oxidized metal particles, and filters with metal adsorbents reduce dissolved metals. A hose-end metal filter is a pool-industry product made for this purpose. We have not tested any brand and do not recommend one.
The ppm cutoffs are from a PHTA fact sheet written for pool professionals. They are a consensus view, not law, and your pool's builder may give stricter limits.
Sequestrants: what they do and what they do not
A sequestrant (also called a metal control or stain preventer) is a liquid chelator. It grabs onto dissolved metals and slows the chlorine from oxidizing them, which limits staining. PHTA is plain about the limits:
- Sequestrants do not remove metals from the water. The metal particles are too small for a pool filter to catch.
- The sanitizer slowly destroys them. PHTA says frequent maintenance doses are needed in addition to the first dose.
- They are a holding action. Fill-water filtering or dilution is the way to lower the metals.
We give no dose here. Strength varies by product, and the dose depends on your pool volume and metal level. Use the label's dose for your pool size. If you do not know the volume, use the pool volume calculator. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling. Also check the label for when to add it in relation to chlorine. We could not confirm one rule that covers every product.
Fill steps
- Test the source water, as above. Write down every reading.
- Attach a sediment filter and, if your metals are above 0.2 ppm, a metal-removing filter to the hose. Follow the filter's flow rating, because a slow fill is normal.
- Start filling through the filter. Do not add chlorine or shock yet.
- Add the sequestrant at the label's dose for the volume of water in the pool.
- Run the pump. Keep the filter clean, since iron particles can load it quickly. Backwash or rinse the filter as the manual directs.
- When the pool is full, test pH, alkalinity, calcium hardness, and free chlorine. Adjust with the pool chemical calculator.
- Retest the water the next day for metals and color before swimming.
The cost of filling a pool by hose from a well, against trucking water in, is on the pool fill cost calculator.
The next point matters most: wait before the first chlorine. If you shock well water full of iron with no sequestrant, the iron oxidizes at once and turns the pool brown. This is our guidance from the PHTA description of how oxidation causes stains. A sequestrant first lets chlorine do its job without turning the metals into particles.
Staining risk by pool surface
We could not find a published ranking of which surface stains most with well water. What PHTA does say:
- Elevated metals can stain pool surfaces. Which stain you see depends on the metal.
- Plaster and pebble finishes can also add metals when the water is under-balanced. Low pH and low calcium or alkalinity can draw calcium and carbonate out of cement surfaces and release the metals in them.
- Iron bacteria stains can disappear when treated with ascorbic acid and return once chlorine is restored.
Vinyl, fiberglass, and plaster can all show metal stains. Light-colored finishes show them most. That is general experience, not a published ranking. If a stain appears, test it first: PHTA says spot-testing the stain with ascorbic acid or citric acid helps tell metal stains from organic ones. PHTA warns that removing metal stains with muriatic acid should be done only by an experienced pool service professional, because it can permanently damage the surface. See pool stains for the safe steps.
Safety. Never mix chlorine and acid. Add chemicals to the water, never water to chemicals. Wear gloves and eye protection, and keep chemicals stored apart. Poison Control: 1-800-222-1222.
Keep the water healthy after the fill
- Keep pH and alkalinity in range. PHTA says pH is the most overlooked factor in keeping metals from being released from equipment and surfaces.
- Hold free chlorine above the minimum for your stabilizer level. See pool chlorine levels. Algae feed on nitrates and phosphates from well water.
- Top off with the same filtered water. A topped-up pool gets a fresh dose of metals each time.
- Check the level regularly. See how high pool water should be.
- Avoid copper-based algaecides in a pool prone to staining. PHTA notes copper-based algaecides add metals.
FAQ
Can I fill my pool with well water?
Yes, if you test it first and treat it. PHTA says to treat water above 0.2 ppm total metals before it goes in and to avoid untreated water above 1 ppm.
Why did my pool turn brown after filling with well water?
Dissolved iron turns brown when chlorine oxidizes it. Test for iron, add a sequestrant as the label says, and hold off on shock until you have. A pool service can help if the brown does not clear.
Will a sequestrant remove metals from my pool?
No. PHTA says sequestrants only chelate metals. They hold them in solution to limit staining, and they get used up, so maintenance doses are needed.
Does well water make a pool turn green?
Copper can give water and stains a blue-green tint, and nitrates and phosphates in well water feed algae. Test for metals first, because the treatments are different.
How often should I test well water for the pool?
Test it before each fill. PHTA says to monitor source water chemistry at least once a year, and annual water reports are available from city suppliers but not private wells.
Sources
- PHTA Recreational Water Quality Committee, Fact sheet: Metals and metal staining (June 2020; 0.2 and 1 ppm limits, sequestrants, filtering): phta.org
- PHTA Recreational Water Quality Committee, Source water and its effects on pool and spa maintenance, Aqua Magazine Tech Notes, January 2020 (well water, nitrates, iron bacteria): PDF
- Orenda Technologies, Test the tap water (what to test in fill water): blog.orendatech.com