The survey is at this thread (it was actually leejp that asked the question). Unfortunately, what was not asked for the table was the FC level when the additional chlorine was added. This is important because the absolute amount of chlorine that is lost through sunlight is proportional to the amount of free chlorine (FC) in the pool. If you have twice the FC level, you will lose twice as much chlorine.

Sorry I misinterpreted your numbers. Going from 15 ppm to 10 ppm (a 33% loss) is quite normal as is 18 ppm to 14.5 ppm (a 20% loss).

I hope you are right that it is just dirt or pollen. That certainly a lot easier to deal with!

As for your dog (I assume that's the 4-legged creature), the EPA reference limit for chlorine of 0.1 mg/kg/day so for a 10 kg (22 pound) dog that comes to a limit of 1.0 mg/day so at 15 ppm (mg/l) FC that is 67 ml or about 2.3 fluid ounces per day. This is the limit where there were no adverse side effects whatsoever -- it's not a limit for chronic toxicity which is much, much higher (14.4 mg/kg/day) -- and is worst when inhaled rather than drank. The fact that there is CYA in the pool probably doesn't matter at all since the chlorine will get released from the CYA as it is used in the body.

For boron (borates), in dogs at 0.44 mg/kg/day there was noticeable decrease in spleen to body weight in males, but that was the only effect and is the lowest dose with any known effect. Boron in humans actually has potential nutritional importance, at a requirement rate of around 0.75 kg/day with normal safe amounts of around 1-13 mg/day (this translates to around 0.015-0.2 mg/kg/day). So using the 0.44 mg/kg/day as a limit, then for a 10 kg dog this comes to a limit of 4.4 mg/day so at 50 ppm Borates (which is actually measured as ppm Boron by convention) that is 88 ml or 3.0 fluid ounces per day.

So the bottom line is that both the high chlorine level of 15 ppm and a 50 ppm Borates level are both a problem for dogs drinking regularly from a pool if they do that every day drinking, say, a cup or more every day and the technical limit below which there are no symptoms would be less than half a cup every day. I again want to stress that the amounts are for the lowest limits below which there are no symptoms or measured body changes (even after years of exposure at these levels).

City water, by the way, probably doesn't have any chlorine in it at all since most municipalities have switched to using monochloramine as the residual instead, mostly due to newer strict requirements at reducing the amount of disinfection by-products in drinking water. Also, if there was residual chlorine in the water, it is likely to be at very low levels less than 1 ppm FC. In fact, you can easily measure the FC level of your drinking water and if monochloramine is used instead, then your FC will measure near zero, but your Total Chlorine (TC) will measure high (since monochloramine measures as combined chlorine). There are usually no Borates (or Boron in any form) in drinking water.

Richard