Discolouration from scale
Often times, abnormal water discolouration can occur as a result of the formation of scale on pool or hot tub surfaces. Scaling is regularly misdiagnosed as a metal or non-living organic stain. The term scale refers to calcium carbonate. Calcium is a soft, grey alkaline earth metal, and is the fifth most abundant element in the Earth’s crust. When a pool or hot tub’s calcium levels are in excess and/or the water is out of balance for a period of time, calcium fuses with carbonates in the pool and hot tub water and a crystal-like structure forms.
This type of scale can alter the appearance of the pool and/or hot tub surface, and the resulting stain closely resembles a metal stain. The sense of touch can be used to determine if the stain is caused by a form of scale. If the discoloured area feels rough like sandpaper, it is scale and the stain can be handled accordingly for removal. The vitamin C test earlier is also a good way to determine what may be altering the pool/hot tub’s appearance.
Other issues that can cause discolouration
- Blotchy or streaky grey discolouration that can develop during the first few months after completing a plaster swimming pool is called mottling. This type of discolouration is difficult to impact when removal is attempted and is often considered a ‘hydration problem’ or ‘entrapped moisture.’ This type of normal, dark discolouration should not be confused with metal or mineral staining;
- Improperly mixed colour pigment;
- Variations in water content in batches of the finish coat;
- Excessive dry or moist spots (weepers);
- Trowel burn from improper trowelling technique;
- Various surface coating thicknesses;
- Corrosion of structural rebar; and
- Mould under the liner. When a backyard’s water table becomes high, the excessive moisture trapped under the subsurface below a pool’s liner can become an ideal environment for black mould. This mould can grow so concentrated it actually shows through the liner and appears as a dark amoeba-like discolouration. Unfortunately, there is no effective method to removing this type of stain short of replacing the pool liner.
