Need help with Stain ID

mucoy

Member
Sep 27, 2010
18
Urbana, MD
Looking for advise from the forum chem experts on identifying metal stain I’ve been dealing with for a couple of years. I have large brownish stains mostly in deep end around the bottom drain. At first I thought they were organic stains resulting from the accumulation of earthworms that always seem to find their way into the pool during the winterization period. However they did not fade with chlorine and I found I can remove them by placing a sock with vitamin C powder on the stains. The vitamin C seems to oxidize the stain and it first turns very dark purplish before eventually fading completely away.

Gradually the stains return to the exact same location. I have not been keeping proper level of sequestering agent to keep metals in suspension and instead been content repeatedly spot treating with low cost AA powder. Because the AA removed the stains I have assumed them to be Iron. I did have a rusted pressure gauge on top of my CCP filter that I replaced last year which may have been the source.

Planning to do AA treatment this spring and follow with proper Sequestering agent protocol, but have a couple questions.
1. Is it normal for iron stains to turn from brown to dark purple before fading? I haven’t seen this described in anything I have read on the topic.
2. I understand the concept that the AA lifts the metal from the plaster and if the ions are not sequestered or removed, the metal will precipitate back into the pool plaster. However, why would this happen in the exact same location vs randomly recurring in different locations?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,224
Tucson, AZ
No. That doesn’t sound normal at all. Purple coloration is usually associated with manganese (often comes into water with iron).

Are you on well water?
 

mucoy

Member
Sep 27, 2010
18
Urbana, MD
No, municipal water. Stain is brown, gets dark-pretty much black to purplish black when I apply the vitamin C, then gradually fades completely away on its own over a 30 min period.
 

mucoy

Member
Sep 27, 2010
18
Urbana, MD
Read up on manganese after the suggestion and what I read leads me to believe it unlikely. I learned that manganese tends to change pool water color and that associated stains can be addressed via chlorine slam. Neither of these are symptoms that I am experiencing with this stain. In fact, chlorine slam will bring the stain back faster instead of helping to remove. I'm still planning AA treatment in next couple weeks after I open and have water balanced.

I've been following the tips given on this board for 10 years and has kept my pool sparkling with no issues other than these stains that started several years ago and have been getting worse. As mentioned in my original post, I have not been adequately sequestering and was content AA spot treating when I got tired of seeing them (remove,return... remove, return... etc...). That has grown old and now ready to go to war on them :). Prepared to maintain sequestrant this year and add Culator to the skimmers following the AA treatment. Hopefully this helps. Ultimately would like to confidently understand the source so that perhaps I can resolve and don't need to keep spending $$'s on sequestrant every pool season. If that is a drain/refill so be it, but would want to know what I need to do to prevent reentry before I go that route. My current theories are:

  1. Rust - a couple screws the pool builder used on a the Parmount MVFuse deck lid rusted over the years before I removed them. Pressure gauge on top of the Pentair CCP420 filter rusted and I naively didn't replace for a couple years assuming it was not at risk of contaminating the the water inside the filter (doh!). These two events have me thinking iron is the culprit along with the fact that vitamin c vanishes the stains.
  2. Iron Bacteria - maybe in municipal water, maybe from abundance of fall tree leaves? Many years I have had to fish out a ton of fall leaves after storms... I like to close late and it's always a game Russian roulette on getting the cover on just in time. I'm sure my phosphorus levels are way high as a result, but as often mentioned on this board I don't worry about that and do not have any issues with algae blooms.
  3. Copper - seems unlikely from my novice view... stains are not green colored nor do I have any water coloration issues. Pulled the yearly municipal water tests from last 10 years and shows 0.2ppm of copper or lower every year. Think the pool builder might have added a copper based algeacide in year one as well... I have no need for algaecide and haven't used any since. The first couple years my young daughter threw coins (pennies and nickles) in the pool to retrieve and I found that some were left on the bottom for a few weeks before I eventually shut down that practice. The coins never left any stains but perhaps there was some oxidization.
Testing for metals is the most likely next step for me but still wondering if anyone on the board has any experience with the similar stain behavior that might help. Also, not sure if it is typical behavior for the lifted stain to precipitate back to the exact same spot and shape. Seems to me there would be more random pattern.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,224
Tucson, AZ
You can rule out iron bacteria. If it was in muni water the EPA would shutdown the facility. It also doesn’t come from leaves.
Get a stain ID kit from Jacks Magic.