Borehole water and pool stains in Phuket: why pools turn brown or green

Why the water changes colour

Water from a borehole, lake or river can look clear as it comes out of the pipe. Even so, it can carry dissolved iron and manganese. While these metals stay dissolved, you see nothing.

Once the water is in the pool and meets the pool's water treatment, the metals can come out of solution. Depending on the source, the water can turn brown, yellow, green or darker. The metals can then settle as stains on the finish, the grout and the fittings. The same thing can happen more slowly when untreated water is used to top up the pool through the season.

Iron tends to show as brown, rust-coloured or yellow marks. Manganese often leaves darker, greyish staining. A green or yellow tint straight after filling can come from metals rather than algae. That is why adding more sanitiser does not clear it and can make it worse: the oxidiser pulls more metal out of solution and onto the surfaces. Colour is a clue, not a diagnosis. A test of the source water confirms the cause.

Acidity matters too. Lake, river and borehole water is often acidic when it arrives. Mains water and truck-delivered water usually are not.

What to test before filling

Find out where the fill water will come from before the pool is filled, not after. Have the source water tested, not just the pool water once it is in.

We can order a test of the incoming water from an approved laboratory we work with, the same kind condominiums use for their regular water reports, and design the filtration system from the results.

Keep the results with the pool's operating records. If you plan to use lake, river or borehole water, tell us in advance. It changes what we ask for at handover.

• Dissolved iron and manganese, the metals behind this kind of staining
• Acidity, total alkalinity and calcium hardness, which together decide whether the water attacks grout and cement-based finishes
• Sediment and cloudiness, which clog the pool filter
• Where you can, a second test at another time of year, as some sources change with the seasons

Filtering the incoming water for the pool and the house

The practical answer to metal staining is to take iron and manganese out of the water before it reaches the pool. We install incoming water filtration on the pool fill line and the automatic top-up line, so every fill and every top-up passes through it.

You can also filter the main supply into the villa. Bathrooms, kitchens and laundry equipment then receive treated water from the same plant, and fittings and sanitaryware are less likely to stain. We inspect your raw water, tanks and plant space. Then we agree with you whether to filter the pool fill line alone or the whole property.

The filters are fitted in the pump room or beside the storage tanks, and in most cases the pool stays in use. An inspection confirms whether the water supply will be interrupted.

• Filtration vessels with automatic backwash controls, chosen for the sediment and metal content of your water
• Lining of concrete water storage tanks and balance tanks where needed
• Regular servicing, including valve checks, backwash settings and replacing used-up filter media when due

What acidic fill water does to a new cement-based finish

This matters most on the first fill of a new or renewed PebbleCrete finish. A cement-based finish goes on curing under water, and while it is still curing it is least able to resist aggressive water. The pool is filled as soon as the finish is ready, so the first fill water is the water the new cement meets at its weakest. Balanced fill water lets the cement cure as intended.

Acidic or very soft fill water does the opposite: the cement meets aggressive water at the worst possible time. Adding sanitiser to water that has not been corrected first makes the problem worse, and the pool can look normal while it happens. Checking the sanitiser level and pH alone will not show it; alkalinity and calcium hardness matter as much. Damage that appears long after filling can have started here.

The target figures for water care belong to the material manufacturer and are kept up to date on its site. We do not repeat them here, and our water treatment page explains why.

• Have the source water tested before the first fill, not just the pool water afterwards.
• Have the water corrected before the sanitiser goes in, not at the same time. Our water treatment page explains why.
• Tell us in advance if the pool will be filled from a lake, river or borehole.
• Agree with your pool operator who is responsible for water testing and treatment.

Restarting a pool after a bad fill

If the pool has already turned brown or green, or stains have appeared, stop topping up with the same water. Avoid adding more and more treatment in the hope that it clears. Take photos of the water and the surfaces, note where the water came from and keep any test results.

Our pool restart service begins with a check of the whole pool and its plant. We look at the interior surface for staining, roughness or hollow areas, and we check the grout and tiles, the equipment and the water source you plan to use. Then we explain the options.

At handover, we give you or your pool operator written care instructions. We also make clear who is responsible for water testing and treatment from then on. Repairs found during a restart are quoted separately, and you see photos first.

Not every hidden problem can be found at once, especially when the pool's history is unknown. A restart can go in stages: we start the pool, find another issue, fix it and start again.

• Where the finish is sound, the pool can be cleaned in a way that suits the finish, refilled with water that has been filtered or tested and found suitable, and brought back into service.
• Where the finish is heavily stained or rough, renovating the shell means draining the pool. An empty shell can be affected by groundwater pressure, and in the rainy season the groundwater is at its highest, so the timing of draining is planned with you first, and with your engineer — or, if you don't have one, with our partner engineer.
• Either way, incoming filtration that is serviced regularly helps stop the problem returning with the next fill or top-up.

What to send us for a first discussion

Send us a few details so we can review your water setup before we visit.

• Photos of the pool water, any staining on the finish and fittings, and your water tanks and pump room
• The property location and the approximate pool size
• Where the water comes from: a borehole, lake, river, the mains or truck delivery
• Any water test results you have, and when the colour change started
• Whether you want filtration for the pool fill, for the whole house, or a restart after a bad fill

Further reading

• Water care that protects the finish
• Water care for a cement-based pool finish