Why metal stains?

[mikech]

Had a couple of dime sized stains when I took over the pool. They disappeared when I got the pool balanced, keeping the pH around 7.5/7.6 July through October. I let the pH rise to 7.8/7.9 through the colder months with no stains.

In the last couple of weeks I've dropped the pH back down to 7.5/7.6 as the temp has risen. In the last week about 30 small stains have appeared, mainly on the shallow end steps.

That doesn't make sense! The only factor I can think of is that as about 6 weeks ago I raised CYA to 50 with 4 pucks dissolving over 3 weeks in a Rainbow feeder half open. FC remained consistent at 7, but pH dropped to 7.2 so I added pH up. Added way too much and had to add MA to bring it back down, all within a couple of hours. Stains have only appeared in the last week, and do appear to fade after MA addition.

Why am I getting these stains at the lower pH? Am I heading for an AA treatment or should I drop pH to 7.2 for a few days and see what happens?

Last nights test results:

Temp 78
FC 7.5 (after small Cl addition)
PH 7.5
TA 80
CH 350
CYA 50

Mike

 

[tim5055]

Are you sure try are metal stains?

 

[mikech]

Yes, checked with vitamin C tab.

 

[JasonLion]

They are probably iron stains.

Iron staining happens when PH is higher, when water temperatures are higher, and when FC levels are higher, or some combination of those three.

AA will help remove existing stains, but to prevent the stains from reappearing you either need to remove the iron from the water (not always possible) or use sequestrant. Sequestrant will bind to the iron and prevent it from forming stains. However sequestrant breaks down slowly, so more needs to be added regularly.

 

[mikech]

pH has been lowered from cold water values, FC always kept within correct CYA range. Seems to be iron spots in the surface of the plaster around the steps.

 

[JasonLion]

Staying in the recommended ranges is not sufficient to prevent iron staining when there is iron in the water.

I forgot to mention, the iron level in the water also effects the chances of iron staining.

One more possibility, if the stains always show up in the same few spots they might be caused by iron inside the plaster that is too close to the surface. This would not apply if the stains are extensive or appear in different places each time.

 

[chem geek]

The pH for water above the steps if they are shallow will be higher than the bulk pool water unless you've got continuous water flow over those steps. The reason is that carbon dioxide outgassing will raise the pH and in a shallow area with little water mixing the pH will stay high. Higher pH precipitates more metal stains.

 

[mikech]

Sounds plausible. I just measured pH 1ft out and 2 down from the deep end (6ft) and right in the corner of the shallow end steps. 7.4 at both (I dropped it a little after last nights test). Pump hasn't been on for 3 hrs. I'll add my once every 2 days MA (about 5 oz) in front of the return jet opposite the steps at the shallow end to see if that helps.