I need help diagnosing a staining problem in a plaster pool. First, more information about the pool. I hope this isn't too long winded, but I'd rather not have a long series of follow up questions because I left something out. Bear with me...
It is an approximately 10 year old 20' x 40' below-ground white-plaster gunite pool with an average depth of 5.5' located in Santa Cruz, California at an elevation of 500' (moderate climate). Water is circulated by a 1.5HP pump through a Triton TR-100 filter loaded with 600# of sand. The bottom is swept with a Polaris 280 robot that runs nightly. The pool is heated with a Pentair MiniMax Plus 400 kBTU heater and passive solar panels. It is covered with a mechanical cover that rides on two rails under the coping and retracts into a below-grade vault when open. The pool was originally filled with local well water, and although I do not have historical test data, the water is generally known to have high alkalinity and suspected moderate iron and manganese content (although we do not experience iron staining on our dishes or toilets like some other regions of Santa Cruz do). There are no natural features that would induce water aeration.
For the past 10 years, the only "water" maintenance that it has received is that which I was taught by the PB. Chlorination has been with trichlor tablets in a floater as necessary to keep the CL between 1 and 3 ppm, which was all that was required to keep the water sparkling clear. If for some reason the water ceased being sparkling clear, 1 gallon of 12% liquid chlorine was added and this would always fix the problem. The pool sees VERY light use, just my wife and I, with one annual summertime party where it sees a single day of heavy use, after which 1 gallon of 12% chlorine was added. Typically it is heated from late March to early September using mostly the solar and occasionally the propane to a temperature of 88F, and it is not "closed" in any freezing sense of the word, but winterized by draining the solar and turning down the propane heat (the heater is programmed to run 10 minutes a day; we find this helps keep spiders and mice out of the burner tubes). It typically spends most of the winter in the mid 50s F. When not in use, the cover is kept closed, so the pool sees very little light. Acid was added as necessary to keep the pH around 7.4, but as the pool aged the pH would hover around 7.2 without the addition of any acid. Backwashing occurred 1-2 times/year even though the pressure never increased enough to notice, nevermind warrant it.
A few months ago, my wife noted that the pool looked more sparkling green than blue (not an algae type of look to me, crystal clear, but green tinged). I added the obligatory gallon of chlorine, then another, and nothing touched the green. I took a water sample to my local pool store for advise and was told that the green color was probably suspended particles in the pool and that I should change the sand in my filter (given the age) and add some cellulose to improve the filtration. This advise DID fix the color. I was also advised that my alkalinity was "way too high" (I forget the exact number, but probably in excess of 200) and that I should add acid to fix that problem. When I returned with my pH too low, I was sold something to dump in the pool to raise the pH (which also put the alkalinity right back where it was). This caused me to embark on a personal journey of understanding pool chemistry which led me here. I now have the water chemistry under control, but am questioning what I can do about the stains which have slowly accumulated over the past 10 years.
Test results, test results...
My first set of tests with the TFT-100 kit showed:
pH 6.8 (this was after adding the pool supply suggested acid, I doubt it was this way for long)
TA 110 (this was after attempting to lower the TA).
Resigned that I needed to do something about the high CYA first, I did a 50% drain-and-refill on the pool to lower it to a more manageable value. My intention was to divert future rain water INTO the pool, instead of just pumping it off the cover, to continue to lower the CYA. Given an annual rainfall of 20" and an average pool depth of 66", it would have taken many years to get the CYA under control with just rainwater (also, we're on a private well, so my marginal water cost is very low). Note that this refill was accomplished by water supplied with a different well than the original fill, but they are within a half mile of each other, and I would expect the fill chemistry to be similar.
After the refill, the water tested:
I next embarked on a long affair of lowering the TA by acid/aeration. I aerated initially at a pH of 7.2 using the Polaris hose with the sweeper removed to introduce a splash on the surface of the pool. I eventually lost patience with this and sunk 50' of 1/4" "drilled" drip irrigation line connected to a 5HP oilless shop air compressor, and I used a calibrated electronic pH meter to keep the pH around 7.0 (but never below) through this process.
My latest tests:
CCL untested, presumed <.5
...and the water is sparkling clear blue.
Using the Taylor test kits, the Iron and Copper content are currently below the detection threshold. Since this test was done after the 50% water change, this tells me that there was no copper or iron in solution prior to the change and none in the incoming water. I have not added any sequesterents, but do not know if the PB added some during startup. As far as I know no algaecides have ever been used in the pool.
A few observations -- Obviously I poisoned my pool by using Trichlor until the CYA accumulated beyond reason. I think this is under control now. Also, I would say that the pool HAS NEVER been effectively Chlorine-shocked, since a gallon could only be expected to raise the FCL by 4ppm and only once, recently, did I use two gallons, but by then the CYA was most certainly 110ppm!
On to the stains, remember the stains...
The pool plaster in general has a uniform light green to blue green color to it. This discoloration is not objectionable except that it makes the sparkling white of a recent plaster repair stand out. There are also objectionable mottled darker stains, and I'm not quite sure what they are. I've tried parking a Trichlor tablet on them for an hour and it doesn't touch them. I've also tried Vitamin C and it doesn't touch them. My guess, from the color, would be that this was copper staining, but the copper content of the water is below detect. Could this be copper from the heater that has fully precipitated out so that none is left for the water test to see? Any other thoughts?
I don't find the condition of the pool so objectionable that I'd be willing to do something as drastic as an acid wash to fix it, but if there were a simpler, chemical, or balance, solution, I'm all ears. At this point, I just want a diagnosis. I do have some plaster fragments from an area of chipping plaster that I could perform destructive diagnostic testing on, but am reluctant to send them to someone with a mass spectrometer.
The following pictures were taken under direct sunlight, but using an underwater camera. Because sometimes colors don't come across the web quite right, I've included a photograph of a Trichlor tablet as a white color reference. Also note the brown staining running down the stairs is likely metal staining caused by the Trichlor floater "parking" over the stairs; THIS staining readily responds to acid removal and isn't what I'm concerned about.