Mustard stains but not algae, what is it?!

Stains on the pool surfaces, pool equipment
or on the swimmers, or off-color swimming pool
water. Discolored but clear pool water.
corsara
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My Pool: 36 x 18, vinyl liner
Location: Ontario, Canada

Mustard stains but not algae, what is it?!

Postby corsara » Thu 27 May, 2010 06:46

Well, my pool is a 36 x 18 with vinyl liner.

A little background: About a week ago, I discovered that chlorine level had dropped to almost 0, so I immediately restored the chlorine. What I did in addition to other times was to add this chlorine stabilizer (as instructed by the pool shop)...

Anyways, a couple of days ago I noticed mustard color stains on the bottom on the pool. The funny thing is that those are everywhere, BUT above the direct line of the water inlets, as well as not in the bottom flat part of the deep end. I thought I got myself this yellow/mustard algae... However, the thing does not respond to brushing at all, and everywhere on the internet people say that the mustard algae is easy to brush off (at least temporarily). With mine, I can be brushing the spots all day, and they won't even change a notch..

The water is clear, the pH, Alkalinity, chlorine---all good! Last night I poured some algaecide i bought in Walmart (probably a very low quality, but I put more, it is the kind that makes a bit of foam), I let it rest during the night (no circulation) in hopes I can see improvement in the morning. This morning I vacuumed, stained bottom still there, not responding to brushing or anything.. Looks like sand if that would help, but won't come off at all :(

It is really disturbing, seeing the liner perfectly light blue in some parts, and then the rest (bottom of shallow end and sides of bottom end) being this sandy/mustardy stained....

Please help me diagnose, this is my first season with my pool, bought the house in the winter.

PS.I plan to pour a ton of liquid chlorine tonight when I come back from work. I just put some pH- to lower the pH, it is 7.6 right now, hope to get it down to 7.2 to allow the chlorine to work better. Could it be that the chlorine stabilizer is making the chlorine useless and I'm developing some crazy kind of algae, that is clinging to the bottom, or what?! Please help!


chem geek
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Mustard stains but not algae, what is it?!

Postby chem geek » Thu 27 May, 2010 12:16

If it doesn't brush away, it's probably not algae (even black algae in plaster can at least have the heads scraped off with a brush). It's probably either an organic stain or a metal stain. You can try taking an ascorbic acid (Vitamin C) tablet and rubbing it on one area to see if it lightens up at all. If it does, then this could be an iron metal stain and you can use an ascorbic acid treatment and metal sequestrant (see this post ). If the stain is instead organic, then high chlorine levels should eventually fade it, but organic stains are usually in specific places where leaves or other organic material settle and decay.

When you say this is like sand, do you mean that it feels rough? You said your water parameters were OK -- otherwise if the pH, TA and/or CH were high then it could be scale though that's usually white or gray and not mustard in color. What does the stain feel like? If it's slimy, then it could be algae; if it's squishy then it could be pollen; if it's gritty then it could be dirt. However, all of these should come off with brushing which is why I think it more likely to be either a metal stain or an organic stain (like tannin from decaying leaves).
corsara
Pool Enthusiast
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Posts: 20
Joined: Tue 04 May, 2010 10:42
My Pool: 36 x 18, vinyl liner
Location: Ontario, Canada

Mustard stains but not algae, what is it?!

Postby corsara » Thu 27 May, 2010 15:26

Thank you very much for your reply, <Chem Geek>!

I like your line of thought that "if it doesn't brush away, it's probably not algae". I have a few questions I would like to ask you:

1) If it is a metal stain or organic stain, is it safe to swim?

2) Unfortunately I currently don't know if it is mushy or rough, it is underwater on the bottom of the pool. Water is a bit cold after work, so I guess I'll have to wait for the weekend and check this. I guess, same applies for trying with Vitamin C tablet---need to get underwater in order to try this..

3) you mention "scale" is possible if water parameters are not right. To be honest, I am not sure what "mustard" color exactly should look like, in fact now that I think about the pool bottom color, it does kind of look greyish.. and also, there were a couple of days with very low chlorine level...hmm. So I guess "scale" is also an option?

4) Since I've put a lot of Chlorine stabilizer about 10 days ago, now I wonder if I should shock the water tonight. I really want to be able to swim in the pool in the weekend, but if chlorine level doesn't dissipate quickly, then I won't be able to swim, is that right?!

So, I guess the conclusion is that I have one of these things:
1) metal stains (test with Vitamin C)
2) organic stains (confirm how?)
3) scaling (confirm how?)

Thanks a lot! I really appreciate your help and advice!
chem geek
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Mustard stains but not algae, what is it?!

Postby chem geek » Thu 27 May, 2010 20:37

If you have measurable Free Chlorine (FC), it is safe to swim, even if you had algae which is an annoying nuisance but is not unsanitary (it can be dangerous, however, if it makes pool surfaces slippery).

If you had a plaster pool, you could put a Trichlor puck over the stain and if it faded it was likely organic, but you can't do that in a vinyl pool since Trichlor is too acidic.

As for scale, you need to enter your water parameters into The Pool Calculator . Unless your fill water is high in Calcium Hardness (CH) or you've been using Cal-Hypo for chlorine, I doubt your CH is high enough to cause scale. You really should get your own good test kit, either the Taylor K-2006 or the TF-100 with the latter kit having more volume of reagents so is comparably priced per test.

I'm not sure how you will easily test items at the bottom of the pool. For ascorbic acid, you could just have a tablet settle at the bottom in still water and then later move it and see if the stain faded at all. Ascorbic acid isn't strong enough an acid to cause damage.
corsara
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Posts: 20
Joined: Tue 04 May, 2010 10:42
My Pool: 36 x 18, vinyl liner
Location: Ontario, Canada

Mustard stains but not algae, what is it?!

Postby corsara » Thu 27 May, 2010 20:48

Thank you so much for the help! You led me towards finding the problem, though I made a mistake and did something wrong (should have re-read your post before that!)

Exploring the idea in your first post, I threw one of the Vitamin-C pills in the pool after work, and in about a minute or so, the area around it was blue and clear as new! Being a complete beginner at this pool stuff and happily rushing towards a solution, I was so sure it's organic matter, and what I did was to pour 10 litres of liquid chlorine in the pool (about 100k liters total)..........only to discover right after going inside the house and reading your post again that what I'm dealing with is not organic matter, but metal stains! :oops: To make things worse, turns out chlorine will make it even worse and I just prolonged the whole procedure! :sick: Apparently now I need to wait who knows how much time for the chlorine to go down, then buy expensive Vitamin-C or some solution of that sort....

Anyway, I am really glad I posted here and got a reply from you! If I hadn't done so, I would have been probably spending a lot of money on algaecide or what not....now at least I know the problem and I can attack it when the chlorine goes down. I just wonder if they have what I need in the nearby pool shop, which is accessible to me only during weekends (due to store hours in conflict with my work)...
chem geek
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Mustard stains but not algae, what is it?!

Postby chem geek » Fri 28 May, 2010 02:01

OK, so this seems like a metal stain, most likely iron. And yes, chlorine can cause more of the copper to leave solution as it oxidizes iron and the high pH forms more solid iron oxide-hydroxide (i.e. rust). Anyway, the chlorine will come down if exposed to sunlight and you should read Metals in the Water and Metal Stains and follow the procedures there.

Unfortunately, it's not easy to remove metal from the water -- you mostly just sequester it to keep it from staining. You should have your fill water tested to see if that's where the iron is coming from.
corsara
Pool Enthusiast
Pool Enthusiast
Posts: 20
Joined: Tue 04 May, 2010 10:42
My Pool: 36 x 18, vinyl liner
Location: Ontario, Canada

Mustard stains but not algae, what is it?!

Postby corsara » Fri 28 May, 2010 06:26

Thanks, Chem Geek, your help is much appreciated! Looks like I have a number of things I'll be doing about that in the upcoming days/weeks.

One last question---this metal in the water, is it safe for me and my family to use the pool despite the iron being in there?
chem geek
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Mustard stains but not algae, what is it?!

Postby chem geek » Fri 28 May, 2010 19:19

Yes, metal in the pool is safe. It just causes staining, especially when the pH gets higher, but it isn't unsafe.
corsara
Pool Enthusiast
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Posts: 20
Joined: Tue 04 May, 2010 10:42
My Pool: 36 x 18, vinyl liner
Location: Ontario, Canada

Mustard stains but not algae, what is it?!

Postby corsara » Fri 28 May, 2010 20:43

Thanks, I learned quite a lot from you in this thread, I much appreciate your replies!

For better or for worse, the mystery continues though: I just had the water tested at the pool shop, and the Iron is 0, Copper is 0, not a trace of them :? I bought a bottle of something for scaling and stuff, but it doesn't make sense... my logic tells me it is either some other kind of metal, or that the metal is in the form of the stains and not free floating... ? Any suggestions?
chem geek
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Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Mustard stains but not algae, what is it?!

Postby chem geek » Fri 28 May, 2010 21:46

You may not show any metals in the water if they have all precipitated (stained) onto the pool surface, presumably when the pH rose at some point. The bottom line is that if the ascorbic acid removed the stain, then the full treatment will work -- ascorbic acid to remove the stain and then metal sequestrant to keep the metal in solution.

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