Two current issues: small circular brown stains and white scaling
First a little background. Our pool was installed Dec 2010. As per my signature this is a 20k gal gunite pool with pebblefina plaster and SWG. The pool builder did the startup and did not follow some of the recommendations found on this site for same.
We had white residue buildup that began shortly after pool startup. It gave the plaster (Pebblefina - bella blue) a mottled look and we could see "tracks" from the Polaris. The installer, Modern Method Gunite, came out a few months ago and drained the pool, then did an acid wash and it looked great. No comments made about water chemistry at that time.
Recently, the white residue was starting to show up again so we called them to take another look at that plus a "rust" stain in the spa. The MMG rep that came out said it was scaling. He did some water tests and said TA was 50-60 which according to him is too low and was causing scale buildup. He said TA should be 100-110. I mentioned that previously it was in that range and we had scaling. I tested TA myself and found it was 60. The spa “rust” stain was apparently from a small bit of metal in the plaster. They came out and took care of that, looks OK. I added some "alkalinity up" to raise TA to 80 (current reading), middle of TFP recommended range for salt water pools with plaster.
I need input from the TFP community on confirming the cause of scale buildup, removal and prevention of reocurrence.
2) Brown stains
The small circular brown stains we had previously, and resolved ourselves a few months ago. Did the Vitamin C test at that time and it worked on the test spots so we dropped chlorine to zero, added ascorbic acid, followed by sequestrant. Stain spots cleared up and everything was fine (so we thought). Recently, the brown spots were coming back and looked exactly as before. We dropped chlorine to zero, added 2 lbs ascorbic acid followed by sequestrant. Ran the pump for 24 hours and checked the results - no change! This time we had not performed the Vitamin C test first, just assumed it was the same problem. Oops!
Took pool water sample to pool store for testing and any input on stain identification and removal. Ended up buying the Jack’s Magic Stain ID kit. I performed the tests according to the instructions and had negative results each time with regard to these small brown spots. I used stockings to put the chemicals in despite my test area being the beach entry, which is obviously quite shallow. I had a couple of brown “streaks” on either side of the beach entry and out of curiosity I tried the Jack’s #1 and #2 on those two areas (separately) and both of them seemed to work. This secondary testing was used with the same stockings/chemicals after the timed test on the small brown spots.
Where to from here?
a) Per pool store (Warehouse Pool Supply) test on 8/3/11 (after failed ascorbic acid and sequestrant treatment):
Chlorine (free/combined/total): 0.0 (intentional - had turned off SWG to drop it for AA treatment)
pH: 7.0 (I believe it was approx 7.4 before ascorbic acid treatment)
Alkalinity: 120 (don’t believe this as MMG rep had found 50-60 and I found 60 confirming his numbers)
Total Dissolved Solids: 1300
b) per TF-100 test kit 8/8/11 (i.e. yesterday evening)
Total Chlorine: 2.0
Combined Chlorine: .25
Free Chlorine: 1.75
Alkalinity: 80 (added some “Alkalinity Up” after ascorbic acid treatment to put in middle of recommended range for salt water pools)
CYA: approx. 55
Salt: 2800 ppm per AquaPure PDA (normally keep it around 3100-3200). I just backwashed the DE filter on Sunday and we’ve had to add a fair amount of fresh water due to evaporation and no recent rain. Will add salt this evening.
Lastly, we’ve had difficulty maintaining chlorine level following the AA treatment. We’ve added several gallons of bleach (at intervals) over the last 1 ½ weeks, have run the filter pump for extended time with SWG at 100%. I checked the SWG cell on Sunday, there was a very small amount of scaling which was easily cleaned. Also checked the sensor and didn’t find any buildup there. Keep having to add more bleach. Just added conditioner on Sunday. Prior to AA treatment, we’ve been running the filter pump for 9 hours / day with SWG at 80%. Chlorine level has remained pretty constant 3-4 range all summer up til now. Last week we did see a little bit of algae showing up on the moss rocks at the waterfall but none on the pool walls.