Introduction
Montijo and Alcochete share a particularity almost no owner connects to their pool: the water table.
We are on the edge of the Tagus estuary, on low ground, with salt marsh nearby and groundwater at shallow depth. That has a consequence which can cost you an entire pool: draining a pool in this area can destroy it.
This article explains the mechanism, what to do instead, and the two other local factors — marsh insects and well water — that define maintenance on a pool in Montijo, Alcochete, Samouco, São Francisco, Afonsoeiro or Sarilhos Grandes.
The Risk of Draining: Why an Empty Pool Can Float
A pool is a hollow structure buried in the ground. When it is full, the weight of the water balances the pressure of the groundwater outside. When it is empty, that balance disappears.
On ground with a high water table, groundwater exerts hydrostatic pressure from below and from the sides. With no weight of water inside to oppose it, that pressure can:
- Float the structure, in the case of fibreglass or lightweight concrete pools. It is called uplift, and it is the same principle that floats a boat.
- Crack the walls or the floor of a concrete pool.
- Lift and wrinkle a liner irreversibly.
- Displace the shell on fibreglass pools.
The risk window is widest in winter and spring, when the water table is highest. In Montijo and Alcochete that means the months when you feel like draining to do work are precisely the worst ones.
💡 Dica Profissional
Before draining a pool in this area, check whether the installation has a hydrostatic valve (a floor valve that lets groundwater in to balance the pressure). Many older pools have one; many recent pools do not. If it does not, or you do not know, do not drain without talking to a professional.
What to Do Instead of Draining
In almost every case, draining is not necessary.
| Situation | Alternative to draining |
|---|---|
| Very high hardness or cyanuric acid | **Partial dilution**: drop the level 30 to 40% and refill |
| Green or badly degraded water | Recovery treatment with continuous filtration. It almost always works |
| High iron or copper | Sequestrant and partial dilution |
| Staining on the lining | Localised treatment with the pool full or partly lowered |
| Repairing a crack above the waterline | Lower the level only as far as needed |
| Liner replacement or structural work | Here it is unavoidable — and it is done with supervision, at the end of summer |
The practical rule: lower the level only as far as the work requires, and only for as long as it requires. A pool lowered 40 cm for two hours is a different risk from a pool empty for three days.
And if the work does require the pool empty, do it at the end of summer or in early autumn, when the water table is at its yearly low — not in February.
Insects: The Marsh Next Door
Montijo and Alcochete sit beside wetlands — salt marsh, salt pans, flooded fields. That means a mosquito and insect population higher than in areas further inland, particularly between May and September.
The pool plays a double role here: it can be a victim, or it can be part of the problem.
A treated pool does not breed mosquitoes. Chlorine and circulation prevent larval development. An abandoned pool, or one with the circulation stopped, does — and becomes a breeding site within days.
Standing water around it is the real problem. Buckets, pot saucers, tarpaulins with pockets of water, the cover reel itself, an open well. These generate mosquitoes, not the pool.
What works:
- Daily circulation, even on a reduced regime. Moving water does not allow the larval cycle.
- Eliminating standing water within several metres. The most effective measure of all.
- A cover pulled taut, with no pockets. A tarpaulin with water pooled on top is a perfect breeding site.
- A surface net at the end of the day. Dead insects in the water are organic load.
We covered the subject in Eliminating Insects and Mosquitoes from the Pool.
Well Water: Very Common Along This Strip
In Montijo, Alcochete, Samouco and Canha, wells are frequent — and this area's groundwater typically brings iron, hardness and, in some spots, salinity.
Three specific points:
Iron is the main problem. Dissolved, it is colourless; it oxidises on contact with air and chlorine, and stains the lining brown. Which is why, on a well-water fill, the metal sequestrant comes before the disinfectant.
Salinity deserves a measurement. In areas near the estuary, some wells have higher conductivity. It does not prevent use, but if you have a saltwater pool with a chlorinator, measure the salinity before adding salt — it may already be higher than you think.
The composition changes through the year. At the end of summer, with the water table low, well water is more concentrated. A top-up in August brings in more iron and more hardness than the same top-up in May.
The full protocol is in Well Water in Moita: How to Treat It — our base is a few minutes from here and it applies in full.
Humidity and the Equipment
The estuary's humid air has a discreet but real effect in the plant room: condensation, and with it corrosion on electrical panel terminals and metal fixings.
It is not salt air — the chloride load is far lower than on the ocean strip — but it justifies two routines:
- Open the electrical panel once a month and check the terminals.
- Make sure the plant room is ventilated. A closed, humid space concentrates the problem; a ventilated one disperses it.
The Plan for a Riverside Pool
| Period | Local priority |
|---|---|
| March-April | Opening. Never drain in this season: high water table |
| May-June | Insects increasing: eliminate standing water around |
| July-August | Frequent top-ups: measure hardness and iron. Daily circulation |
| September | The year's best window for work needing the level lowered |
| October | Closing. Cover pulled taut, no pockets of water |
| Winter | Periodic circulation. Check the electrical panel |
Common Mistakes
Draining the pool to clean it. In this area, the error that can cost you the structure.
Draining in winter. That is when the water table is highest.
Leaving the pump off for weeks in winter. Standing water in marsh country is a breeding site.
A cover with pockets of water on top. It creates exactly the problem the cover was meant to prevent.
Adding salt without measuring salinity, with a well near the estuary. It may already be high.
Frequently Asked Questions
Can I drain my pool in Montijo?
With great care, and preferably never completely. This area's high water table exerts hydrostatic pressure that, without the weight of water inside, can crack, displace or float the structure. If it is unavoidable, do it at the end of summer, with professional supervision, and check whether a hydrostatic valve exists.
How do I reduce hardness or cyanuric acid then?
By partial dilution: drop the level 30 to 40%, refill with fresh water, and repeat if necessary. You get the same effect without the risk of draining.
Does my pool attract mosquitoes?
A treated pool with circulation does not — chlorine and movement prevent the larval cycle. The real breeding sites are the standing water around it: buckets, pot saucers, pockets of water on the cover, open wells.
What is a hydrostatic valve?
A valve in the pool floor that, when the pool is empty, lets groundwater in to balance the external pressure. It exists in many pools built on ground with a high water table — and its absence is a reason not to drain.
Does my well water contain salt?
In areas close to the estuary, some wells show higher conductivity. It does not prevent using the water, but if you have a salt chlorinator, measure the pool's salinity before adding salt.
When should I do work on the pool?
At the end of summer or in early autumn, when the water table is at its yearly low. It is the reverse of intuition — most people want to do work in winter — and in this area intuition is wrong.
Conclusion
In Montijo and Alcochete, the most important decision an owner makes about the pool is the decision not to drain it.
Key points to remember:
- High water table: an empty pool can crack, shift or float.
- Never drain in winter or spring. If it is unavoidable, end of summer.
- For high hardness or cyanuric acid, use partial dilution rather than draining.
- Mosquitoes come from standing water around the pool, not from a treated pool.
- Well water: sequestrant before disinfectant, and measure salinity if you have a chlorinator.
If you have a pool in Montijo, Alcochete, Samouco or São Francisco and are thinking of draining it or doing work, the on-site technical diagnosis assesses your installation's hydrostatic risk and tells you in writing what can be done without draining. It costs €45, deductible from your first month under contract.
