Pool water is tested four times a day with a photometer and the free chlorine has to sit inside a range or the pool shuts. Everywhere in our pool it holds. In the deep end corner it is gone within the hour, and chlorine only disappears when it has something to react with.
I am the pool plant operator at a school with a twenty five metre pool. Sixteen years, and I hold the qualification you have to hold to do it.
The testing, because this is all numbers.
Free chlorine is the chlorine available to kill things. Combined chlorine is what is left after it has reacted with the ammonia that comes off people, and a high combined figure is what makes a pool smell like a pool. You measure both with a photometer, which is a little machine that shines light through a sample with a reagent tablet crushed in it and reads the colour as a number.
We run free chlorine between one and a half and two parts per million, pH between seven and seven point four, and we test at four points round the pool, four times a day, and it goes in the book with the time and my initials.
The water turns over every three hours through sand filters and a dosing system that injects to hold the figure. If the reading goes out of range and will not come back, the pool closes. I have closed it four times in sixteen years.
The deep end corner, the one furthest from the inlet, on the side by the changing rooms.
Tested half an hour after dosing it reads one point eight like everywhere else.
Tested an hour later it reads nought point two.
Nought point two is not a low reading. It is a pool you would not get in.
Everywhere else at the same moment is one point six or one point seven, which is what three hours of turnover and a dozen swimmers takes off it.
What that means chemically, and this is why it has bothered me for years rather than annoyed me.
Chlorine does not evaporate out of a pool in an hour. It gets used. It binds to something organic, and what it binds to shows up as combined chlorine, and combined chlorine is the other number on the machine.
In that corner the combined reading is nought point one.
So the free chlorine is going and it is not turning into the thing it turns into when it reacts with anything I know about.
What has been checked, and the authority and two contractors have all been in.
Circulation. This was everybody's first answer and it is a good one, because a dead spot with no flow will always read oddly. Dye testing in 2017 showed the corner is slow but not dead, about forty percent of the velocity at the shallow end, which is normal for a pool with inlets at one end.
We added an inlet. Drilled and plumbed, 2018, four thousand pounds, because I asked for it. The velocity doubled. The reading still falls.
The photometer. Two machines, three sets of reagents, and a sample taken in a bottle from that corner and carried to the other end of the pool and tested there, which rules out everything to do with the place I am standing.
It reads nought point two in a bottle in my hand at the shallow end.
A laboratory. The school paid for a sample to go to a lab in 2019, taken by me, sealed, couriered. The report says the sample is within the normal range for pool water in every respect except the free chlorine, which is consistent with a high chlorine demand, and that a persistent localised demand indicates organic contamination and recommends investigating the source.
That is exactly right and it is the sentence I have been stuck on for seven years.
What has been looked at physically.
The tiles in that corner are the tiles in the rest of the pool. The grout was replaced in 2020 as part of a refurbishment and the corner was stripped back and re-laid, and the contractors found nothing behind it but screed.
There is no pipe in that corner, no light, no outlet, no duct. The drawings show plain construction and the refurbishment confirmed the drawings.
The pool was built in 1971 and the ground it sits on was the school field.
What I do about it.
I dose to hold the whole pool, which means the rest of the water runs at the top of the range to keep that corner anywhere at all, and my chlorine consumption is about a fifth higher than a pool this size should use. That has been queried once, by a bursar, and I explained it, and he accepted it.
And the lane ropes go in at an angle now.
That is mine and I did it quietly. The pool is roped for four lanes and it used to be roped symmetrically, and I have set the anchor points so the outside lane is narrower at the deep end by about a metre, which puts that corner outside the swimming area.
It means nobody stands in it, and nobody treads water in it waiting for a whistle, and if a class is told to hold the wall they hold a different wall.
I have never written down why, and no teacher has ever asked. The one who came closest was a swimming coach in 2022 who said the lanes looked off and I said the anchors were worn, which was a lie, and then she said she had always hated that corner and went to get changed.