Black Staining Over Main Drain

[clevereuphemism]

Over the wonderful winter we had in the Northeast, our pool cover failed somewhat (we use a tarp, do not have safety cover yet) and we had some leaves and debris (including a dead snake, yikes) fall into the pool and settle over the drain in the deep end. When we opened the pool the water was still pretty clear and some skimming by the pool company and a few rounds with our Polaris cleaned up most of it. However we have black stains left around the main drain. No problem, just organic stains, I thought, we can handle that. We added pool shock, kept the chlorine levels up, brushed and brushed, and no change. Then I thought it might algae, but there is no plume or cloud or dust at all when the stain is brushed like when we had algae. It may as well be paint down there for all it moves. After two weeks of this, we let the chlorine drop to zero and tried the vitamin c stain remover. There was no change.

After some reading of past threads on here and googling I thought it might be calcium scaling over organic stains but we have public water and have never had an issue with calcium scaling but I am at my wit's end so who knows. I went the pool store for help and they sent me home with descaler/stain remover. Before I dump this in my pool I thought I might see if anyone else has any ideas. I thought between using the shock and the vitamin treatment I would have seen some improvement to guide me towards using one or the other. Is it possible I would have to do multiple treatments with the vitamin c powder on a tough stain like this? This is the only stain we have in the pool from the winter. I can see why people move to the South.

We have a freeform plaster pool, it is approximately 30 years old and I do not think it has ever been replastered (we have only lived here four years). I don't know if the age of the plaster makes it more prone to staining, but as I said it is not stained anywhere else. As far as I know the prior owners took good care of the pool. DE filter.

FC: 0
TC: 0
(These are zero because I just did the stain remover treatment). I have turned our chlorinator back on so they will be going back up.)
Calcium Hardness: 230
CYA: 70
TA: 110
pH: 7.6
Copper/Iron: 0 (Just had tested at the pool store)

Sorry for the wavy water, I should have turned off the pump but sometimes there are snakes back by the equipment and my husband isn't home right now. Any suggestions/ideas are welcome. Thank you!

 

[basod]

Try a 3" trichlor puck on the lighter outer area of the stain and move it every few minutes with your net/pole.
it may take longer, but don't leave it there for extended periods of time(like over night)

 

[windowman]

Had the same staining (worms and leaves). Tried acid, no change. Have put in a 3"triclor puck in sock and have left for a few days, moving it around. Stain is almost completely gone. What is potential issue with leaving puck in one position too long?

 

[duraleigh]

What is potential issue with leaving puck in one position too long?

Trichlor is VERY acidic. It can easily do damage to a vinyl liner and there is a good chance it can damage plaster as well.

Now, back to OP's question. basod has given you good advice. Probably around 5 minutes with the puck should be long enough. I'll bet it lightens it. Post the results and we'll have some ideas how to get rid of it.

 

[basod]

Expanding on Dave's answer.
Your gunite is dry nozzle mixed concrete. Concrete is a caustic material(high pH) when you want to paint/roughen concrete acid is typically used - it will literally smoke when applied even under dilution
We all know anytime baking soda(caustic) and vinegar(acid) come in contact a reaction occurs - ions are changing hands

Leaving the acidic Trichlor puck(contains CYA) on the surface will weaken the concrete bonds, causing etching - roughness that is a future place for algae to grab hold and grow into the concrete.
This cycle will repeat until your plaster needs resurfacing again.

Within the trichlor puck is also bonded chlorine - that's what is oxidizing the organic stains in direct contact with the dark bacterial/algae growth that have worked into the concrete matrix.

There is also an option of using cal-hypo in a dry sock on organic stains, most people have or can buy a couple pucks for stain removal which is the easiest first option

 

[clevereuphemism]

To report back, we tried the puck and it didn't make much of a difference, but it's been a month and the stain has lightened considerably with some extra brushing every time we go for a swim. There are still some dark areas around the drain (the very dark areas in the pic) but all the lighter gray areas are gone. We are hoping with time the rest of the stain will disappear. So I guess it was just bad organic staining from the craptacular PA winter we had? Thank you all for your advice.

 

[basod]

It seems like you might have organic staining that has worked into the plaster. Is the main drain in working condition?
You could try some cal hypo in a sock over the area again moving it around every few minutes