Post return line repairs: Small spots on liner

Long time lurker / New Member / First post --

I have an inground 18x36 pool with a vinyl liner, that came with the house I bought a few years ago. I've been learning through this site and trial and error how to effectively manage the pool.

We recently had our return lines repaired under the concrete deck - they cut into the deck and replumbed with new fittings and pvc. (The lines were cracked)

Immediately after we've had two weeks of solid rain, and I've been unable to vacuum the pool, but I've been testing and brushing down regularly.

I got out to work on the pool today because I had the day off and it was not pouring rain.

After I vacuumed and brushed, I noticed small black spots on the pool liner (I say black, but they are really just dark spots) - They only appear near the two areas that they had to cut our portions of the concrete ( near the returns, one in the deep end, one in the shallow)

Now some concrete sediment did get into the pool and I vacuumed it today best I could.

The black spots were not coming off from brushing the liner.

I got into the water, and the spots had the consistency of grit/sand paper. I scraped some off with my finger nails, but it was difficult to get it out of the water - the one I did appeared to be hard. It would not squish between my fingers or stain green.

Even after I scraped off the bits giving a coarse texture (in the small area) the liner still had spots, just no texture.

Today's test (before adding more chlorine -

FC 1.2
CC 0.0
Ph 7.6
TA 80
CYA - ballparking between 20-30. It's low because we had to lower our water level about 18 inches to get the returns fixed. The dot wasn't completely obscured at 30.

Any advice would be helpful. There seems to be causality with the repairs. I would like any info if possible, particularly if I need to confront the company that cut into the concrete.

As a note, the concrete decking did have support metal under the structure.

I had the thought that it might be dust from the blade/support metal that rusted dark flecks on those areas, but I haven't had the chance to do an ascorbic acid test.

Also the water has maintained FC and hasn't gottoen cloudy, green, etc. this season.

Thanks for any advice,
Trae

Hi Trae;

I can think of two other possible causes:

1. Copper oxide. Could be from a pool heater, or from a copper algaecide. Test by placing or holding a Vitamin C tablet against the spot for 5 minutes. If it lightens or disappears, it's copper.

2. Fungus from under the liner. Sometimes fungal colonies will develop under the liner. This usually happens when the ground is very wet and warm. There's no real test, except the color is IN the liner, rather than ON the liner. Unfortunately, there's no fix, either, since the source of the stain is fungal pigments bleeding through the liner from the dirt side.

If this is the problem, there are antifungal treatments you can do to the dirt / sand / vermiculite the next time you replace the liner.

By the way, if it's fungus, it usually will NOT be against the pool wall, but against the dirt / sand / etc.

I will try the vit c. test tomorrow.

I have no pool heater, and haven't used any copper algaecide. It did have a sandpaper grit consistency and seemed to appear after the deck was cut into and concrete dust was scattered.

(The dots seem clustered around the two areas that had cut concrete.)

Is there any kind of reaction concrete dust can have that would bond it to vinyl?

Is it possible that the mini-rebar supports / saw blade cast off could have oxidized and made the pattern? If so the vitamin C test should help identify.

Lastly there is still some very tough grit that is stuck on the liner. Would getting a 100% nylon scrubbing pad and using that on the liner be safe for the vinyl? The pool brush just isn't helping at all. I was only able to scrape it off with fingernails.

Bits of steel will absolutely stain a liner. Usually the stains will be orange, not black, but black is possible. If you suspect iron from that source, Iron Out offers a better test.

Buy a bottle of Iron Out (detergent section of Walmart, etc) and put 1/2 cup in a snack bag. Open the bag against a stained area, so the Iron Out gets wet, but is not washed away. Again, 5 minutes of contact should be sufficient to produce visible change.

Test grit with the snack baggie method, but use sodium bisulfate (dry acid or pH Minus). Wear plastic gloves.

I'm still working on this.

I verified the vitamin C test didn't work.

I have purchased the Iron out, but haven't had time to apply it as instructed.

Will update after that.

Thanks!

OK.

I just reread the thread -- it's possible for concrete to fall through the water and stick tightly to a liner. But, it won't usually be black. Just thought I should mention that.