How to Tell If a Water Cistern Is Contaminated: Signs, Risks and Solutions

Many homes and country properties in Mallorca still rely on a water cistern, or aljibe, either as their main water store or as backup to the mains supply. It's a practical solution that's widespread across the island, but like any enclosed tank, it needs regular upkeep. When that upkeep is neglected, the water can become contaminated without it being obvious at first glance, and recognising the warning signs early makes the difference between a minor issue and a genuine health risk for the people living in the house.

Smell is often the first clue. A cistern in good condition shouldn't produce noticeable odours when you turn on the tap or open the tank. If you notice a rotten-egg smell, an overly earthy or musty smell, there is likely organic matter decomposing at the bottom or biological growth on the walls. Taste changes too: a metallic, brackish, or simply "off" aftertaste shouldn't be ignored, even if the water is only used for non-drinking purposes.

Turbidity is another clear sign. Water from a clean cistern is usually clear, or at most slightly yellowish depending on its source. If it comes out cloudy, with particles in suspension, or takes a while to settle after standing in a glass, there's likely sediment stirred up from the bottom, dirt seeping in through a poorly sealed opening, or decomposing organic matter. Sediment that builds up at the bottom of the tank over years without being cleaned becomes a breeding ground for bacteria and gives insects and other organisms a place to reproduce.

Colour also offers clues. Greenish tones usually point to algae, which need light to grow — common in cisterns with ill-fitting lids or cracks letting light in. Brownish or orange tones can indicate rust from pipes or metal fittings, while a milky appearance or persistent foam can signal contamination from external products. If you also notice a film or "skin" on the surface, or slippery walls when inspecting the interior, there is active biological growth.

The risks of a contaminated cistern aren't just cosmetic. Stagnant water with organic matter is a favourable environment for bacteria, algae and other microorganisms that can cause gastrointestinal problems or, in poorly ventilated tanks with warm water, favour conditions associated with bacteria such as legionella. Spanish regulations on quality criteria for water intended for human consumption set out parameters aimed precisely at avoiding these risks, and although a domestic cistern isn't always subject to controls as strict as the municipal supply, common sense suggests treating it with the same care whenever the water is used for personal hygiene or consumption.

If you spot any of these signs, the first step is to stop using the water for drinking or cooking until you've confirmed its condition. The next step is to call in a professional who can empty the tank, remove accumulated sediment, clean and disinfect the walls, check the lid and seals to prevent further dirt or light from getting in, and inspect the overall condition of the connected pipework. At Tuberías y Aljibes we've spent more than fourteen years cleaning and inspecting cisterns across Mallorca, and we know that regular preventive cleaning is far simpler and cheaper than dealing with contamination that's already taken hold. If you can't remember the last time your cistern was cleaned, it's probably a good time to have it checked.