My rocks have turned black... but indestructible.

[Jay Hanig]

I have two small shallow ponds, about 6 X 10 feet in area and about 1 foot deep. The bottom is lined with river rock. Ever since the ponds got flooded three years ago during a hurricane, I've had problems with algae. I used to use bleach to kill the algae and over time the dead algae just turned to black dust, easily swept away with a boat brush or even just a garden hose.

I've resolved the algae finally; no more green water. In fact, the water is crystal clear. The problem is that the rocks appear to have become permanently stained black. I tried draining the ponds yesterday and sprayed the rocks with bleach, on the theory that I was dealing with mold this time. I gave it plenty of time but no joy. Then I got out the pressure washer. Didn't touch it. I'm at a loss. Since there's no fish to protect, I'm not above using chemicals to fix this but have run out of ideas.

It seems like some of the color is coming from dead pine needles that have turned to sludge on the bottom, if that makes a difference. I have vacuumed up a lot of that; the pond is much cleaner now... but the rocks remain stained.

Anybody?

 

[Meyer Jordan]

Basically what has happened is that the rocks have become acid-etched from the acid produced in the decomp of the pine needles.

 

[Smaug]

Yup to the above,its not gonna come off with anything reasonable. Muratic acid would do it though.

 

[Lisak1]

Time for new rocks!

 

[morewater]

Don't use muriatic with bleach. Bad Juju.

 

[Jay Hanig]

Don't use muriatic with bleach. Bad Juju.

If I wanted to clean the rocks, I can get rid of the bleach by flushing it a few times. What will muriatic acid do to my liner?

I have also considered just picking up a few rocks and treating them with the acid separately away from the pond. Would that be a problem if I did it in a 5 gallon bucket? A couple of selected rocks a day would make a difference in the span of a week or two. They're not all black.

 

[Smaug]

Don't use anything caustic on a liner. Take the rocks out and soak them in a tub with the acid. I wouldn't know how much to advise yiu to use per gallon of water though. When I was a mason tender in a past unfortunate life we used it to clean brickwork at a mix of about a cup of acid to 5 gallons of water.

 

[sburtchin]

The acid bottle will give a minimum concentration to be effective. After that it is a function of exposure time vs concentration. The higher the concentration, the less time is needed.

If there is an organic film on those rocks, a simple rinse may not remove the bleach. Bleach + ammonia will kill you. I don't know what bleach + acid would do. Bleach does nasty things to local wildlife.

 

[morewater]

Adding bleach to muriatic acid (HCl) results in the formation of chlorine gas.

 

[sburtchin]

Adding bleach to muriatic acid (HCl) results in the formation of chlorine gas.

Danger Will Robinson!

 

[Jay Hanig]

Well, I finally got around to getting some chemical gloves, a couple of brand new 5 gallon buckets, and some muriatic acid. I dropped a big rock in one of the buckets and covered it with water, then added a cup of HCl slowly. I figure there's probably about 3 gallons of water in the bucket.

It's been sitting in the bucket for an hour now and I can't be sure whether it's lightening or it just my imagination. How long should I expect this process to take? 2 hours? 2 days? 2 weeks? I just want to have some sort of realistic expectation.

The end plan isn't to end up with a bleached white rock; it's just for a natural looking tan stone.

 

[Meyer Jordan]

Length of treatment will depend on type (porosity) of the rock and the depth of the original acid etching.

 

[MitchM]

Will the rocks not return to their natural colour over time?
I would think that UV exposure and bacterial activity would return the rocks to their natural appearance.

 

[Meyer Jordan]

Will the rocks not return to their natural colour over time?
I would think that UV exposure and bacterial activity would return the rocks to their natural appearance.

The acid produced by the bacteria in the biofilm is what originally stained them.

 

[sissy]

I can't believe this ,for black rocks that turn that way naturally .No way would I treat anything like a rock with those chemicals and then put it back in my pond .A rock is a rock and even people will pay extra for black rocks .Plus no one ever saw a pic of these black rocks .They could have been just black algae and that can be treated safely with peroxide and water