Pool care in the rainy season in Costa Rica

Close to four metres of rain falls on this coast in an average year, according to Costa Rica's national weather institute. An uncovered pool receives all of it. Most pool problems on this coast follow from that one fact.

What rain does to pool water

Rainwater is naturally slightly acidic and carries almost no minerals. Every heavy shower dilutes the chlorine, the stabiliser, the alkalinity that holds pH steady and the calcium that protects plaster and grout. At the same time it washes leaves, pollen and soil into the water, and those use up what chlorine is left.

Less chlorine and more for algae to feed on is the whole explanation for a pool that goes green after a week of storms. It is predictable, which means it is preventable.

Heavy rain also raises the water level. Once it is above the skimmer opening, the skimmer stops pulling debris off the surface, so the level has to be brought back down after big storms.

The numbers worth knowing

The United States Centers for Disease Control and Prevention recommends keeping pool water at a pH of 7.0 to 7.8, with free chlorine of at least 1 part per million, or at least 2 where a stabiliser is used. Chlorine kills germs less effectively as pH rises, and water that is too acidic corrodes pipes and equipment.

Stabiliser, sold as cyanuric acid, protects chlorine from sunlight. It is useful, and it is also the cause of a slow problem. It does not evaporate or break down, and the common chlorine tablets add more of it with every tablet. Chlorine works more slowly as the level climbs, and the only way to bring it down is to replace part of the water.

Then there is balance. Water that is low in calcium and alkalinity, which is where months of rain push it, is what pool technicians call aggressive: it dissolves calcium out of plaster and grout to make up the shortfall. A pool surface that has turned rough is often showing the result.

Salt pools are still chlorine pools

A salt system makes its chlorine from salt dissolved in the water. Rain dilutes the salt along with everything else, and a cell running below its working range produces little or no chlorine. Salt level is one more thing to test after heavy rain, not a reason to test less.

Never drain a pool in the wet months

A full pool is held in the ground by the weight of its water. After weeks of rain the ground around it is saturated, and groundwater pushes up on the shell. Empty the pool then and that pressure can crack the shell or lift it out of the ground. When water has to be replaced, it is done in part, in dry weather, by someone who knows the site.

What a green season service should include

Testing and recording pH and chlorine at every visit, and alkalinity, calcium and stabiliser on a schedule. Emptying the baskets, brushing the walls and cleaning the filter. Correcting the water level. And looking at the equipment itself: the pump room for flooding, the fittings for corrosion, the timer and breakers after any power cut.

Records matter more than any single reading. A log shows a pool that is drifting weeks before the water shows it.

The chlorine and pH figures are the CDC's recommendations for home pools. Your pool's own targets depend on its surface, its equipment and whether it runs on salt, and the technician who services it should be able to tell you what they are.