Black patches after adding shock

[Aquamore]

I just had a nasty surprise after adding pool shock ("Super Zappit" by PPG Industries, 73% Calcium Hypochlorite, 27% inerts) .... a lot of black patches appeared in a pool that I had thought was under control (except for some light grey patches which appeared after some inattention last October. The black (sometimes very dark red) patches are very resistant to scrubbing - essential immovable. Photo attached. The pool is salt water chlorinated and I have a trickle from chloring tablet chlorinator as back up. Ph is 7.2 and a high chlorine content was indicated by the bright yellow in my test kit even prior to introducing the kit. What do you think is the cause and how can I restore the pool to its former glory?

 

[duraleigh]

Aquamore,

Welcome to the forum.

If your thinking the Cal-Hypo contributed to the black....I'm sure it didn't.

Without seeing it first hand, it sure sounds like black algae...difficult to deal with. You can't get it out with a pool additive.

You'll need to run your chlorine level up pretty high (can't tell you how much 'til you post some test results for CYA) and KEEP IT THERE ALL THE TIME. Then, brushing those black spots with a deck brush on a daily basis will expose it to the chlorine and, in time, get rid of it. Then, and only then, can you let your chlorine levels recede to normal.

All the above ASSUMES black algae. Is it slimy to the touch?

 

[Aquamore]

Thank you for the quick response.

No, the black patches are not at all slimy - quite dry, more powdery but very well adhered to pool surface. Very difficult to brush ......... more impression made by a firm finger wiped across it.

The appearance after adding shock is repeatable........ the black (now appearing more dark red) appears within one minute of adding the shock. I agree that the shock may not be the cause but is certainly making the problem, whatever it is, more visible. This would be an interesting project for someone living near Katy!

 

[Guest]

Staining appearedd after shocking? Sounds like metals to me! Highly oxidized copper can cause black stains and sometimes copper can create a dark red-brown stain, manganese can cause greyish stains. Have you ever treated the pool with a copper based algaecide? Do you have a heater with a copper heat exchanger? Have you tested your water for metals?
Do your stains look like any of these (From the Jack's Magic website)?:
copper stain1

copper stain 2

copper stain 3

copper stain 4

copper on fiberglass

It's really hard to tell without seeing your pool what the stains are but stains that appear right after shocking are usually metal. Oxidized copper stains are not easy to get rid of but it can be done. First step is to ID the stain and see what will remove it. If your staining looks anything like these picures I can tell you some things that might work but please post a full set of test results for FC, TC, CC, pH, TA, CH, and CYA and if possible test results for iron and copper (however, if the metals have already plated out as stain they might very well test at 0 in your water). Have the testing done with liquid or dry reagents and NOT strips.

 

[Aquamore]

Thanks. Yes, the photos that you referenced do look similar to my situation (sorry, I'm having difficulty attaching photos of my pool). Yes I do have a pool heater (Jandy/Lars LT400N-LO) which I would expect to have a copper coil. I haven't used copper sulphate in the pool. I'll try to get the analysis that you suggest and post. it. On the bright side, if it is a copper compound, I guess I won't be getting an algae problem anytime soon!!

 

[Guest]

If the copper is coming from your heat exchanger that is NOT a good thing. It's mostl likely because the pH of your pool has been too low for period of time, you have been pouring acid into your skimmer, or your heater is not designed to be used with a SWG (many older heaters aren't). I would get your heater checked out also nfor damage to the heat exhanger. Oxidized copper staining is one of the most difficult to remove but it can be done. When you treat the pool for stains you will have to bypass the heater.

 

[lovingHDTV]

The stains you show in the pictures do look like the ones I got last night, however mine just went away. As of this AM they are not there.

 

[Aquamore]

Here is the water analysis by Warehouse Pool Supply yesterday:

Free Chlorine 5.0 ppm
Total Chlorine 5.0 ppm Typically high for my Aquarite set at 75% and some trickle flow from the chlorine tablets. Shock was added three days previously.
Combined Chlorine 0.0 ppm
pH 7.4 A little lower than I typically see. Every 1 to 2 weeks I normally have to add 1 gal Muriatic acid to the pool which is now 2 1/2 years old.
Hardness 270 ppm
Alkalinity 40 ppm
Cyanuric Acid 40 ppm
Copper 0.1 ppm Some debate by the store staff about whether this was 0.0 or 0.1ppm
Iron 0.0 ppm
TDS not run (they said note sufficient sample - I could get this run on another sample if important)

So, the problem could be a copper compound, for which I undersand the treatment will be a strong acidization. I shall need to get my pipework modified to take the heater out of the loop before doing this.

Thanks for your posts and when I have taken the next steps I'll let you know how I got on. This might take several weeks as I need to get others to do things while I am travelling away from home.

 

[JasonLion]

A copper level of 0.1 will not normally cause any problems. Copper generally needs to be at 0.3 or higher before you should need to worry about it. Particuarly with your low PH reading it seems unlikely that it is copper related. A good quick double check would be to rub the stain with some ascorbic acid (crushed vitamin C tablets) wrapped up in a sock or panty hose. If the stain goes away where you rub then it most likely is copper. If the ascorbic acid has no effect then it isn't copper.

 

[Guest]

A copper level of 0.1 will not normally cause any problems.

If the copper has already deposited out as stain then it will not show up on water testing. The low pH combined with the fact there is a heater that probably has a copper heat exchanger makes copper the #1 suspect in my book. If the copper is heavily oxidized (black cupric oxide) then ascorbic acid will have minimal effect on it so the ascorbic acid test will only tell if the metal is easily reducable. Iron and some forms of copper staining are but some heavily oxidized copper stains are not.
Copper generally needs to be at 0.3 or higher before you should need to worry about it. Particuarly with your low PH reading it seems unlikely that it is copper related. A good quick double check would be to rub the stain with some ascorbic acid (crushed vitamin C tablets) wrapped up in a sock or panty hose. If the stain goes away where you rub then it most likely is copper. If the ascorbic acid has no effect then it isn't copper.