Nitrates

[Oceansol]

I live in the San Diego area, and have a recurring nitrate problem. The obvious answer is urine, but I believe my kids don't per in the pool, and we only have pool parties a couple times each summer. My gardener only uses liquid fertilizer, and I've instructed him to be careful not to get any in the pool. There is a greenwaste recycling plant upwind of me about a mile, and the turn plant material into mulch. Often I can smell the "cooking" mulch, though it is not a very noticeable smell often.

The only solution to nitrates that I am aware of is to drain and refill. My question is about the chemistry of nitrates: do the nitrates stay in the water during evaporation, and then build up? I should also mention that my city water contains approx 1.5ppm nitrates from a desal plant, so I'm continually adding nitrates to what I'm guessing is an increasing concentration of them.

Please help me understand.

[Oceansol]

I mistakenly posted this in spa forum, but have a pool. Sorry! (But question still valid, and I have a pool AND spa anyway!). Sorry!

[chem geek]

Why are you worried about nitrates? Nitrates and phosphates are usually the limiting nutrients for algae growth, but algae growth can be prevented by maintaining a Free Chlorine (FC) that is at least 7.5% of the Cyanuric Acid (CYA) level regardless of phosphate or nitrate level. Stop worrying about the nitrates. Roughly 20% of the ammonia from your sweat and urine will become nitrate so it builds up over time from bather waste, but it's not a problem.

[Oceansol]

Well, my pool guy (the pool supply store) says that excessive phosphates and nitrates are food for algae. Are you saying that I just need to up my chlorine?

I have a salt system, and generally target chlorine at about 2.0-3.0.

[Oceansol]

will I be able to keep up with the algae using a salt system?

[chem geek]

Your Free Chlorine (FC) level is probably too low, but it really depends on your Cyanuric Acid (CYA) level. For an outdoor pool exposed to sunlight a saltwater chlorine generator (SWCG) pool should usually have the CYA at 80 ppm. For SWCG pools, the rule is that the FC needs to be at least 5% of the CYA level so that means 4 ppm FC.

You may need to shock the pool at a higher chlorine level to get ahead of any algae growth that may have started, but once that algae is dead, then the proper FC/CYA ratio will kill algae faster than it can grow. The 80 ppm CYA is to protect the chlorine from sunlight since many SWCG systems for residential pools are somewhat undersized and you can extend the life of the SWCG by lowering the amount of chlorine lost to sunlight.

[Oceansol]

So if my cya and fc are in those ranges, then I shouldn't even worry about phosphates or nitrates?

[chem geek]

Yes, that is correct, but since you've had the FC too low for your CYA (probably -- we don't know your CYA level), you may already have nascent algae growth even if not yet visible. So you may need to shock the pool (raise the FC/CYA ratio higher than normal) to kill off any nascent algae growth. You could just try raising the FC to the appropriate level and see if your SWCG can keep up, but if it can't then you will probably need to shock the pool.

[Oceansol]

Chemgeek, my cya WAS 50 a couple weeks ago, but I added more. Will check again today.

Thanks for your help, and I'll update as I learn more.