What these 13 towns have in common
They are all supplied by Canal de Isabel II and, according to the official analyses, their water shares three things that matter a lot in a pool:
- It is soft: calcium hardness at the tap ranges from 30 to 62 mg/l, well below the 150–300 a pool should have.
- It is aggressive: the Langelier index published in the analysis itself is negative in all of them. Water like this tends to “eat” the calcium from grout and surfaces. Work out your pool's with our Langelier index calculator.
- It arrives with combined chlorine: between 0.6 and 1.6 mg/l, with 0 free chlorine. That is how mains water is disinfected, perfect for drinking; but in a pool Spain's RD 742/2013 sets a 0.6 mg/l maximum for combined chlorine.
| Town | Total hardness (mg/l) | Calcium hardness (mg/l) | pH | Combined chlorine (mg/l) |
|---|---|---|---|---|
| Alcorcón | 45 | 35 | 8.3 | 1.3 |
| Arroyomolinos | 68 | 55 | 7.9 | 0.6 |
| Fuenlabrada | 44 | 35 | 7.7 | 1.4 |
| Getafe | 46 | 35 | 7.8 | 1.1 |
| Humanes de Madrid | 51 | 40 | 7.8 | 0.6 |
| Leganés | 30 | 30 | 7.8 | 0.9 |
| Moraleja de Enmedio | 40 | 32 | 8.0 | 0.9 |
| Móstoles | 76 | 62 | 8.0 | 0.6 |
| Navalcarnero | 73 | 60 | 7.9 | 1.0 |
| Parla | 41 | 32 | 7.6 | 0.6 |
| Pinto | 40 | 32 | 7.7 | 1.1 |
| Torrejón de Velasco | 45 | 35 | 7.9 | 1.6 |
| Valdemoro | 45 | 35 | 7.9 | 1.3 |
Calcium hardness worked out from the calcium in the analysis (calcium × 2.5). Each figure is the latest analysis published in SINAC for each town's network, consulted on 30/09/2026.
We have not included El Álamo, Galapagar, Griñón, Majadahonda, Pozuelo de Alarcón, San Martín de la Vega, Serranillos del Valle, Sevilla la Nueva or Torrejón de la Calzada: we could not find a reliable hardness analysis for their network in SINAC (some show 0, which cannot be right). You can look them up in the official SINAC search.
What to do when you fill (or heavily top up) the pool
- Raise calcium hardness with calcium chloride to at least 150 mg/l, in two or three goes. See what calcium hardness is.
- Measure combined chlorine before the first swim and, if it is above 0.6 mg/l, do a shock treatment.
- Adjust pH to 7.2–8.0, then carry on with the normal routine.
Frequently asked questions
Is tap water in southern Madrid hard?
No. According to official SINAC analyses, in Alcorcón, Fuenlabrada, Getafe, Humanes de Madrid, Leganés, Moraleja de Enmedio, Parla, Pinto, Torrejón de Velasco and Valdemoro total hardness is between 30 and 51 mg/l (soft water), and in Arroyomolinos, Móstoles and Navalcarnero between 68 and 76 mg/l (moderately hard). None of them has hard water.
Do I need to add calcium when filling the pool?
Usually yes: tap water arrives with 30 to 62 mg/l of calcium hardness and a pool should have 150 to 300. It is raised with calcium chloride, a little at a time.
Why does tap water contain combined chlorine?
Because that is how mains water is disinfected: published analyses show 0 free chlorine and 0.6 to 1.6 mg/l of combined chlorine (chloramines). In a pool the maximum is 0.6 mg/l, so after filling measure it and, if it is above, do a shock.