Introduction
Our base is in Baixa da Banheira, in the municipality of Moita. Which is why we have a very concrete opinion about well water: we have been filling and treating pools with it for years, in Moita, Alhos Vedros, Sarilhos Grandes, Gaio-Rosário and all the way across to Alcochete and Canha.
Well water is abundant and practically free, which is why so many people use it. But it is not mains water. It arrives with no chlorine, with dissolved minerals the mains does not carry, and with a composition that changes through the year — harder and more ferrous at the end of summer, when the water table drops.
Fill and treat it as though it were mains water and the result is predictable: brown staining on the floor two days after filling, and an owner convinced the pool has a structural problem.
This article explains what is different about the area's well water, the filling protocol that prevents the staining, and what changes in maintenance through the year. For the general treatment method, see also Treating Well Water for the Pool.
What Well Water Brings That Mains Water Does Not
| Element | Mains water | Well water around Moita |
|---|---|---|
| Residual chlorine | Present | None |
| Dissolved iron | Practically absent | Frequently present, sometimes in quantity |
| Manganese | Practically absent | Possible, and stains blackish-brown |
| Calcium hardness | Known and stable | Variable, typically high |
| Organic matter | Controlled | Possible, depending on the well and its depth |
| Composition through the year | Stable | Changes with the water table |
The first two are what ruin fills. With no residual chlorine, the water arrives biologically active. With dissolved iron, it arrives transparent — and that is what fools people.
Why the Water Runs Clear and the Pool Turns Brown
This is the mechanism behind almost every call we get after a fill with well water.
Iron dissolves in groundwater in its ferrous form, which is colourless. The water comes out of the hose transparent and the owner reasonably concludes it is fine.
Then two things happen. The water meets oxygen from the air and from circulation, and it receives its first dose of chlorine. Both oxidise the iron, which turns to its ferric form — brown and insoluble. It precipitates, settles on the floor and walls, and stains.
The practical conclusion is counter-intuitive and it is the key to everything: in well water, chlorine goes in after the iron is dealt with, not before. Shocking a freshly filled pool of ferrous water is the fastest way to stain it completely.
If you already have staining, treatment depends on whether it is iron or manganese, and the procedure is in Brown Stains on the Pool Floor.
Filling Protocol for Well Water
In order. The order is the whole article.
1. Analyse the well water before filling. Iron, manganese, calcium hardness, pH. A laboratory analysis of the well is the best money you will spend on the pool, and it stays valid for years.
2. Fill through an in-line filter on the hose, if iron is high. A sediment cartridge at the inlet catches part of the already-oxidised iron. It does not replace treatment, it reduces the load.
3. Correct alkalinity and pH before any disinfectant. Alkalinity 80 to 100 mg/L, pH 7.2. With the pH set, the next step works.
4. Apply a metal sequestrant. This is the step almost everyone skips. The sequestrant keeps iron and manganese in solution, stopping them precipitating and staining. It goes in before the chlorine, with filtration running.
5. Only now, the disinfectant. A normal dose, not a shock. With the iron sequestered, chlorine does its work without oxidising metals onto the lining.
6. Continuous filtration for 24 to 48 hours. This is the phase in which the water clears and the filter catches what precipitated.
7. Backwash, and measure again. The filter will be loaded. A full backwash and a fresh reading of every parameter.
💡 Dica Profissional
If you are filling a pool with well water for the first time, fill it in April rather than June. If something goes wrong — and with new well water something goes wrong in roughly half of cases — you have two months to correct it instead of two days.
What Changes Through the Year
Well water composition is not constant, and this is the local detail that makes the difference.
| Period | What happens in the well | Consequence in the pool |
|---|---|---|
| Winter and spring | High water table, more dilute water | Top-ups with less impact |
| June to July | Table falling | Hardness and iron start climbing |
| August to September | Water table at its lowest | More concentrated water: iron and hardness at the year's peak |
| October | First rains | May wash organic matter in and cloud the well |
That has a direct consequence: an evaporation top-up done in August with well water brings in more iron and more hardness than the same top-up in May. If your pool "goes wrong" every August, this is the first hypothesis to consider before blaming the treatment.
Hardness and Scale: The Second Problem
Beyond iron, the area's well water is usually hard. That means deposits at the waterline, in the filter and — most expensively — in a heat pump's exchanger and on a salt chlorinator's plates.
In pools filled with well water we measure calcium hardness at least monthly, and we work with pH and alkalinity at the low end of their ranges to reduce the tendency to precipitate. We covered the subject in detail in Hard Water in Setúbal.
Does Well Water Make Sense?
The honest answer is: yes, in most cases, if the treatment is done knowingly.
In favour: the cost of filling and topping up is practically nil, and in a summer with heavy evaporation the difference against mains water is significant. In an area where many people have a well, it is a resource that exists and there is little sense in not using it.
Against: it requires a permanent sequestrant in the maintenance plan, an initial analysis, closer attention to hardness, and a fill done in the right order. It is not more work every day — it is more knowledge once.
Where we do not recommend it: a well with very high iron on a pool with a pale, porous lining, particularly if the pool is in tourist use. The staining risk is permanent, and a stain at a holiday rental is a photograph in a review.
Common Mistakes
Shocking the pool right after filling. The classic error. It stains the whole pool within hours.
Not using a metal sequestrant. Without it, every top-up brings fresh iron that will precipitate.
Brushing iron stains. They do not come off mechanically and brushing scratches the lining. The treatment is chemical and specific.
Assuming the well is always the same. Analyse it once, and be suspicious in August.
Filling in June. No margin to correct.
Frequently Asked Questions
Can I fill the pool with well water?
Yes, and around Moita, Alcochete and Canha it is what many people do. What changes is the protocol: analysis, pH correction, metal sequestrant and only then disinfectant, with continuous filtration for the first 24 to 48 hours.
Why did the water run clear and the pool turn brown?
Because dissolved iron is colourless until it is oxidised. Contact with air and the first dose of chlorine oxidise it, it becomes insoluble and precipitates brown. It is the most common staining mechanism in well water.
Is a metal sequestrant always needed?
In a pool fed from a well with iron present, yes, continuously — every top-up brings fresh metal. The dose and frequency are set from the analysis result.
Is well water actually cheaper?
Filling and topping up are practically free, but there is the cost of the sequestrant and of a periodic analysis. In most cases it still works out cheaper, especially on large pools or where evaporation is high.
Do I need a laboratory analysis or is a kit enough?
A home kit measures pH, chlorine, alkalinity and hardness. It does not measure iron or manganese reliably, and those are precisely what decide the protocol. One laboratory analysis of the well settles it.
The pool goes cloudy after every heavy rain. Is that the well?
It can be: the first autumn rains wash organic matter into the well, and a top-up in that period brings that load into the pool. We also covered the general case in Green Pool Water After Rain.
Conclusion
Well water is a good resource treated with the right method, and a source of staining treated like mains water.
Key points to remember:
- Dissolved iron is colourless until oxidised. That is what fools you at filling.
- In well water, the metal sequestrant comes before the disinfectant.
- Never shock a pool right after filling it with well water.
- The well's composition changes through the year: in August iron and hardness peak.
- Fill in April. Two months of margin is worth more than any product.
If you are filling with well water this year, or if stains appear every year that you cannot explain, the on-site technical diagnosis includes a water analysis and a written protocol for your well. It costs €45, deductible from your first month under contract, and our base is ten minutes away from you.
