Ascorbic versus Oxalic Acid / CYA pre-dilution

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The pool is gunite, holds about 9000 gals and is located in Hawaii. Last week I attempted to remove, using ascorbic acid, 10-year-old red dirt stains - oxidized iron/rust -these look yellow in the pool.

Red dirt stains comes from airborne volcanic dust, on the island of Kauai they sell red dirt T-Shirts.

The following method was used. The chlorine was reduced to zero via chlorine reducer. Algaecide 60 was used to stop a bloom. The ph was dropped to 6 and three times the normal amount of metal-out was introduced. Then 1 1/2 lbs of Ascorbic Acid was put in and left for 2 days. The pool was then drained and refilled and the water balanced.

I am now using liquid chlorine with a CYA of 25. The ascorbic acid did a great job of removing most of the yellow stains.

Interestingly after 24 hours it stopped removing stains leaving some areas of the copping tile with the stain and on the neighboring tiles it was completely gone! I removed these with 600 grade wet/dry sand paper, worked great. My theory is that the ascorbic acid reached some maximum absorption point, any theories on this?

There are still some light yellow stains around the pool, below the pool light there is a long narrow yellowish stain which could be iron so the question is should it be retreated with ascorbic acid or should oxalic acid be used? What is the difference between the two? Or is this a waste of time and money because the pool is 10 years old?

The plaster is still good and the pool looks 90% better than before. If I do this again I would like to pre-dilute the cyanuric acid in a bucket of water so I do not have to wait 3 to 5 days before the levels rise. Is this dangerous or even possible and is there any special procedure such as dropping the ph of the tap water?

Finally using Ben's CYA chart what would be the ideal minimum and maximum CYA level for 25 parts? The reason for asking this question is that below that too much chlorine is lost and above that chlorine needs to be maintained at a minimum of 3 which United Chemical states, on their website, can cause staining or is that sales talk?

Any help would be appreciated.

Follow up: It seems that after 5 days some of the iron is settling back into the pool, not bad but still noticeable.

Would it have been better to use the Cal Hypo method as described under "Wayyy too much copper--my fault!" to maintain the CL at 1.5 for a few weeks and then raising it to 5 - 10 rather than Clorox Regular , instead of 3 with clorox? Then not using everything else listed in the following except Calcium Hardness, Algaeside 60 (poly)?

Additional info follows; when I refilled the pool I adjusted it by adding Calcium Hardness to bring the water from 70 to 175, alkalinity increaser to raise it from 60 to 90, 3oz Algaeside 60 (poly), added CYA to bring it up to 25, one treatment of metal-out, then 2 days later one treatment of United Chemicals "Stain Remover," and kept the chlorine with clorox at 3. It seems that after I added the "Stain Remover" the iron started to settle out; this may only be a function of time.

The pool now looks 60% better than when I started.

Oh I would dearly like to solve this problem and I love this site.


Further Update:

After adding the "Stain Remover," which I think has 12.5% oxalic acid in it, my CL dropped to near zero, in the early morning. I then added a great deal of chlorine to bring it back to 3. Then thinking I might get an algae bloom I added Polyquat 60 to the maximum recommended. I did not realize that doing this would zero out my chlorine which is exactly what it did. I then added more chlorine and put pucks in my feeder and turned it up to max. This has brought the chlorine back to 2 and has stabilized there. Hopefully it will go above 4. I also added more CYA so that level might reach 30 from 25 as I read on this forum that below this number it is very hard to keep up with the loss. The water is glistening and it seems that the light stain that deposited itself is going away. Could it be the Polyquat 60? Any ideas as to why? Thank goodness I am working at home and able to keep an eye on it, or I might have a nice algae bloom.

Aloha,


Paul