Heavy metal stains in the pool: copper, iron and manganese

When a stain resists brushing and shock chlorination, it's easy to assume it's a particularly stubborn strain of algae. But there's another, much less known and surprisingly common cause: metals dissolved in the water that oxidize and deposit onto the surface, forming stains that chlorine alone can't remove.

A sequestrant “traps” the metal

Where the metals in the water come from

Pool water doesn't usually carry dissolved metals in any meaningful amount unless they enter through one of these routes:

• Well or borehole fill water. In many areas, groundwater naturally contains iron, manganese or, less often, copper, in concentrations that go unnoticed when drinking it but that do end up staining a pool over time.
• Corrosion of metal fittings in the system. Heaters, heat exchangers, some pump parts or old pipework can release small amounts of copper or iron if the water is out of balance (very low pH, for example, is especially corrosive).
• Copper-based algaecides used in earlier treatments, either in the pool itself or in the fill water if it comes from another reservoir treated with that kind of product.

How to tell a metal stain apart from an algae stain

The stain's color and behavior give fairly reliable clues:

Each metal leaves a fairly characteristic color: copper stains blue-green (and sometimes gives blonde hair a greenish tint after swimming), iron leaves reddish-brown tones reminiscent of rust, and manganese produces purple or nearly black stains. None of the three lift off with normal brushing, because the metal isn't sitting "on top" of the surface like ordinary dirt, it's oxidized and bonded to it.

How to prevent metal stains

Prevention rests on two complementary fronts:

• Metal sequestrants (stain preventers). These products "grab" dissolved metal ions in the water and keep them from precipitating and depositing onto the surface, even when fill water keeps introducing them. They work preventively, they don't remove stains that are already set.
• Controlling the source of the metal. If the fill water comes from a well, it's worth knowing its composition before using it regularly. If the source is the system itself, checking the condition of heaters, heat exchangers and metal pipework helps stop the problem at the root, along with keeping pH in its correct range to reduce corrosion.

When it's worth calling a professional

A recent, isolated metal stain can sometimes be resolved with a well-applied specific treatment. But it's worth calling in professional help when the stain is extensive, covers much of the pool, keeps coming back, or when the source of the fill water isn't known for certain, in those cases, correctly diagnosing the metal involved and its source avoids blind treatments that can make things worse.

Frequently asked questions

How do I tell a metal stain apart from an algae stain?
By color and by how it reacts: algae is usually green and lifts off when brushed or clears up with shock chlorination; metal stains (blue-green from copper, reddish-brown from iron, purple or blackish from manganese) barely change with brushing and can sometimes even darken if more chlorine is added before the metal itself is treated.

Where does copper or iron come from if I've never added any metal to the water?
The most common sources are well or borehole fill water (which in many areas naturally carries dissolved metals), internal corrosion of heaters, heat exchangers or metal pipework, and some copper-based algaecides used in earlier treatments.

Does a metal sequestrant help if stains are already there?
It helps keep dissolved metal in the water from precipitating and forming new stains, but it won't on its own remove stains already fixed onto the surface. Those usually need a specific treatment, and heavy or unexplained staining is worth having a professional assess.

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