Strange Test Results with High CH

[chem geek]

I did a full pool water chemistry set of tests today and noticed something very unusual. I have a mostly opaque electric safety cover, but I have a pool cover pump where I dump rain water from the cover into the pool which then overflows to a drain so that I can get water dilution over the winter. I do that to prevent a buildup of salt and to generally refresh the water each season. This year had less rain than others, but even so I got an expected drop in Total Alkalinity (TA) from around 90 to 55, a drop in Cyanuric Acid (CYA) from 25 (at end of season -- was higher earlier) to around 10 (guess based on turbidity, but could still see black dot at 20 mark), and salt level drop from around 1700 to 1300. Based on the rainfall, I'd expect somewhere in the neighborhood of a 25-35% dilution and for the most part that's been true, BUT my Calcium Hardness (CH) only dropped from around 400 to 380 ppm. There were some problems with one of the test reagent bottles, but I had 3 kits and got consistent results with 2 of them (the 3rd was too high).

I noticed this past year that I was using more acid than usual, needing to add it every week or two instead of once a month (remember I have a cover on most of the time) even though the TA was not unusually high. I also noticed that the plaster, which is now 9 years old, seemed to have tiny holes in some areas, though it does not feel rough, and these areas aren't only where chemicals are added over a return. The saturation index was near zero during the summer yet having the pH rise requiring more acid and now seeing that CH must have gone up has it seem as if there was dissolving of calcium carbonate. However, that would have resulted in the TA rising even after acid addition. The TA did rise a little, but not very much. If calcium hydroxide were to dissolve, then the CH would rise but the TA would not (after adjusting pH back down using acid).

I suppose it is possible that the lower saturation index during the winter rain dilution dissolved some calcium carbonate (though that does not explain the increased acid usage during the summer) but the temperature was fairly low around 50ºF (until I turned on the solar recently) and even today when I measured everything the saturation index is only at -0.2 so not very low (and during the winter with the cold water temps the pH was kept at around 7.8). Also, this problem didn't seem to happen in previous years. Does slightly older (9 years isn't very old though, is it?) start to have issues? Anyone have any thoughts about this?

 

[JamesW]

It is my opinion that any time that the CSI is negative, there will probably be some plaster dissolution. You can also get plaster dissolution from any mechanical abrasion, such as from brushing etc. or even from water currents over time.

 

[chem geek]

I posted a plaster photo of the top ramp that only has about 1/4" to 1/2" of water over it since it's easiest to focus on it. It's a high-res photo (that I sharpened) that you can zoom into with a browser. To the far right is the edge of an embedded dark tile (denoting the edge of the ramp) so ignore that (you can see a wider perspective photo on a day with wind). The area in the lower part of the photo shows a mottling (lighter and darker plaster) that shows up elsewhere in the pool in larger patches, but feels smooth so is more cosmetic and seemed to show up fairly early on after a year or two so is likely a different issue. The area in the upper part of the photo shows the pitting holes I was talking about and doesn't feel rough, but it is slightly noticeable to the touch and appears to only be in the areas of lighter plaster. This ramp area is much worse than the bulk pool surfaces and this effect is only in some places in the pool, but this ramp area makes it easier to show the issue in a photo.

I found this thread that would also indicate that even a somewhat negative saturation index over a long enough period of time can result in pitting. As I had written before, a -0.3 saturation index means that there is half as much calcium carbonate in the water as compared to saturation. One thing I can try this summer is to have the saturation index be > 0 and see if the rate of pH rise and acid demand are reduced having the TA low (70) and pH higher (7.7) to minimize carbon dioxide outgassing (TDS is around 1500). So I'll target a higher CH than usual (500) to see if I notice any difference. What is interesting is that I didn't think that this past summer or two was different than previous seasons in terms of water chemistry balance.

 

[JamesW]

I think that pitting can be caused by:

1) Marble aggregate dissolving and leaving pits.
2) Marble aggregate being dislodged by mechanical means, such as equipment going up and down the ramp, brushing, vacuuming, automatic pool cleaners and even normal walking by swimmers.
3) Marble aggregate coming loose when the surrounding cement dissolves enough to allow the grains to become dislodged.
4) Cement dissolving and leaving pits.

Marble aggregate is often described as 200 mesh, which is about 74 microns, which is about the size of a grain of salt. However, there are often larger particles as described here:

Typical Size Distribution
¬ % larger than 12 mesh 0.5%
¬ % larger than 16 mesh 6%
¬ % larger than 50 mesh 55%
¬ % smaller than 200 mesh 10%
http://www.hubermaterials.com/userfiles ... 202011.pdf

If you have ever used a floater, the floater could have caused locally low pH. If it tended to park over the ramp, then that could account for some of the increased pitting on the ramp.

Also, it's hard to be sure, but it looks like there might be some light copper staining.

I think that maintaining the CSI in the 0.0 to 0.3 range would give you the best results.

Has your water always been in balance, or was it somewhat unbalanced before you learned about pool chemistry?

 

[chem geek]

During the first year and a half I used Trichlor pucks in a floating feeder, but that was tethered with a cord to stainless steel bars away from the ramp I showed. Nevertheless, the feeder would sometimes park itself near those bars and rusted the two mounts closest to it so I do have some iron staining from that. Perhaps that is what you see, though I've done ascorbic acid treatments in the past and haven't measured any metal and CuLator didn't seem to pick much up either and my water is diluted each winter. Generally, I don't see major metal stains. When I used Trichlor, I used pH Up to keep the pH near 7.5.

Anyway, the water chemistry has been good for the last 7-1/2 years and the only things that more recently changed in the last few years (3-4) were the use of 50 ppm Borates and one (free) experiment with a lanthanum chloride phosphate remover treatment (as well as some enzymes, but those wouldn't remain). The lanthanum isn't very much and would be diluted over time. The borates are replenished each year.

The pitting is worst on the ramps so areas closest to the surface that would get the worst circulation and again these are not very deep pits -- only noticeable with the tip of a pin and a slightly rougher feel with the fingers. Ironically, the very shallow water with poor circulation would normally be of higher pH from localized carbon dioxide outgassing and that same ramp has had a few small nodules that I needed to remove to make smooth, but only once every 3 years or so.

The bulk of the pool has some of the blotchy discoloration you also see in the photo (lower half), though it's not objectionable. It doesn't look nor feel like most of the pool has the pitting though I can't see the depth as clearly, but the sides don't seem to show it and feel very smooth. So this isn't a serious problem, but I'd like to get on top of it before it gets any worse. Also, the increased acid addition the last season or two and the unexpectedly high CH after presumed winter dilution were of concern.

 

[JamesW]

I think that maintaining the CSI at slightly positive level will help and you should be able to get at least 10 more years out of the plaster.

If you ever wanted to clean and smooth the plaster, a drain and polish using diamond abrasive pads might be a good option.

Since you keep the pool heated for significant periods of time, any plaster dissolution caused by negative CSI will be accelerated.

 

[onBalance]

Richard, is the "pitting" occuring in specific areas, and not throughout the entire pool? Is it uniform everywhere? My thought is that it isn't. Therefore, let's not assume that plaster finish is perfect and uniform throughout, because it is not likely that it is.

Plastering is handcrafted and the quality can vary from one area to another. There are 3 to 4 different finishers when the plastering takes place. Also, the steps are usually over-troweled, creating weaker areas just by doing that. Adding water while troweling also weakens the surface. Walls are not generallly over-troweled and do not have excessive water added to finish. In general, walls are more durable than the floor areas.

The pitting can be caused by weak cement paste (caused by excessive water troweling) that surrounds an "aggregate" particle, and in time, the water (even balanced water) can dissolve some of the more soluble compounds, creating porosity, and therefore, weaken the area to a point to cause the aggregate to literally fall out, which JamesW pointed out.

I also see evidence that calcium chloride was added to the mix. Although the plaster finish is now 9 years old, it is still possible to get an idea of how much was added. If you are interested, if you were provide me a small chunk of plaster, I could analyze it for the CC content. Of course, you may not want to do that. Calcium chloride quickly hardens the cement paste, but weakens the long-term durability. Excess CC also causes shrinkage, thereby allowing water to penetrate and dissolve CC and calcium hydroxide from within the cement matrix. Porous areas are light (vertually white) in color, and smooth dense areas are darker shades of white.

It may be that a slight a slight positive CSI would help prevent the pitting from happening, but it is also possible that it wouldn't stop the pitting. I guess it all depends on just how poorly the plastering is in certain areas. A positive CSI could "scale" the surface where the plaster is hard, smooth, dense, and durable, but not prevent the dissolution or pitting elsewhere where the plaster is weak. Tough call.

At the moment, I do not have a good answer on why you need to add more acid to maintain the proper pH for the past two years. But I have to say, there can be other variables that could be at play here. I will continue to think about that, and will respond if I can come up with some good possibilities.

 

[chem geek]

I looked throughout the pool and though the lighter and darker areas occur throughout (and we're talking about subtle differences here), the pitting in the lighter area appears to only be on the ramps and seat areas and some on the floor though not nearly as much. There is none I can see on the walls except right above part of the seat and the amount on the ramp and seat seem much worse than on the floor. It also seems worse on the floor in the area where acid is added over a return so the problem is likely exacerbated by negative CSI even when not continuous (I generally brushed in the area where I added acid, but not always). I tried to take a picture now of the floor, but there is too much water movement between the pump and a light breeze for the picture to be clear and the auto-focus on the iPhone camera gets fooled as well. I'll see about taking one at night with interior pool lights that might show more.

I think you are right that this is mostly about aggregate (sand-sized) that is falling out from areas of plaster that are weaker and that this might be exacerbated by a slightly negative CSI though this is not certain. The acid demand and increasing CH (relative to what should have happened with rain dilution) are still strange, but I'll see if that goes away maintaining a zero to positive CSI.

 

[HouTex]

FWIW, my pool is about 16 months old and I've had rising CH pretty much from the beginning. I've also had similar mottling that showed up around month 4. I first thought it was calcium scale, but my CSI was and has been mostly negative so I don't think it's calcium scale. Also, as far as how it feels. my plaster is very uniform and relatively smooth (for luna quartz) to the touch.

I've wondered in posts on this board where the CH is coming from and it must be coming from my plaster. We've had a ton of rain this winter/spring and my CH has remained around 475 while my salt, borates, and CYA all have gone down significantly because of the rains. I hope my plaster is not being eaten away by something, but I never let the CSI get below -.3 so I'm hoping it's fine.

[edit] And I continue to have to add MA to keep my pH from rising too high. I'm adding about 28-32 oz. or so of MA every 5 days to keep my pH in the 7.5 range. The SWG, oversized pump, and spillover spa probably all contribute to the rising pH, but I was hoping the acid demand would get better after the first year.

 

[onBalance]

Richard, one thought: Could your liquid bleach that you buy and use contain a higher content of caustic soda to explain the higher acid doses? But that doesn't explain why the higher calcium level or that it hasn't decreased like the other parameters. I am still thinking about that.
Do you use a nylon plastic pool brush or a stainless steel one? It doesn't look like you have an attached spa. BTW, a small chunk of plaster could come from underneath the main drain cover.

 

[JamesW]

I have had good results by allowing the pH to go as close as possible to 8.0 without going over*. I rarely use acid on a concrete pool. I will sometimes use trichlor in an offline feeder, which helps manage the pH, but only enough to maintain the cyanuric acid level in the desired range. When adding acid, I only add enough to lower the pH to about 7.8, and not lower.

If you do have pH rise, you should be careful to note whether or not it is accompanied by TA rise. By measuring the amount of TA rise in relation to pH rise, you should be able to determine how much of the pH rise is from carbon dioxide outgassing and how much is from introduced bases.

*Note: High pH is not a good choice for most people. However, I think that, in some limited circumstances, it can be a good choice.

Of course, the CSI still needs to be balanced to prevent scaling. When using high pH, acid demand can be helpful. Also, metal staining can be an issue for those with metals. However, I think that it can reduce copper corrosion in heat exchangers, which reduces the amount of copper in the water, and therefore it can also help prevent metal stains.

 

[chem geek]

Richard, one thought: Could your liquid bleach that you buy and use contain a higher content of caustic soda to explain the higher acid doses? But that doesn't explain why the higher calcium level or that it hasn't decreased like the other parameters. I am still thinking about that.
Do you use a nylon plastic pool brush or a stainless steel one? It doesn't look like you have an attached spa. BTW, a small chunk of plaster could come from underneath the main drain cover.

I thought about the bleach having more excess lye in it and had contacted Hasa about that possibility, but they monitor their pH and manufacturing process carefully and noted no changes. And as you say, that doesn't explain the CH not getting diluted down over the winter. As for brushes, I use a nylon brush, never a stainless steel one. I don't have an attached spa. As for under the main drain cover, that won't be happening -- too bad that in the skimmer it's gunite and not plastered otherwise that would be easy to get to.

I took some photos tonight with the pool light. One is of the shallow end where the plaster is smooth but has some minor light/dark mottling and some rubbed surfaces from things like the pool sweep. I also notice a few volcano-like eruptions (sort of like blisters) that I'll have to smooth down with a pumice stone. Another photo is of the bench in the deep end (which is across the width of the pool from the light so looks green in the photo, but it's really white plaster). This shows the extensive pitting that is worse than the ramp, but this is near the return where acid is added (it's to the left of this bench) so I suspect that is part of the problem (even a return flow and brushing may not be enough during the time acid is in the area, if poured too quickly).

I found a presentation that shows more extreme examples of plaster problems. On page 22, the mottled discoloration they say is etching. Page 23 shows more extreme etching/pitting. All-in-all I'd say that in my pool some areas show early signs of etching and that I should try setting the saturation index higher and see if I can also reduce pH rise and acid addition and be more careful when adding acid (i.e. pour more slowly and quickly sweep the bench area to ensure thorough mixing).

 

[onBalance]

Richard, your link to the "presentation" is a power point presentation. I have the research report with the complete text along with the pictures in that power point. The text mentions that the pool plaster in question also contained more than 2.5 percent calcium chloride. It mentions that certain areas of the plaster surface was very porous, with a high water/cement ratio, and the researcher mentions the possibility of poor workmanship practices leading to that porosity. You will note in the picture that the surface had darker gray areas along with the lighter areas. That researcher acknowledges (in other parts of that report), the darkening effect and shrinkage of the plaster by adding high doses of calcium chloride.

Also, I am in agreement JamesW on the benefits of maintaining a higher pH.

 

[HouTex]

Richard, thanks for posting the link to the presentation. The etching on page 22 has the same basic look as mine, but my marks are rounder.

The range of suggested pH and TA in the chart on page 21 has me wondering whether my TA has been too low.

I've been trying to keep my TA in the 60-70 range to help with the ever-rising pH and to reduce my acid additions. Because my CH remains in the 475-500 range with the low TA and pH in the 7.8-8.0 range I'm still keeping my CSI around the zero to slightly negative range. And it never goes anywhere near -.3 or over +.3. So I've always thought the issue with my plaster was not my water chemistry. Now, I'm not sure. Should I target a higher TA and just keep adding MA to keep pH in the 7.4-7.6 range? I'm thinking that would entail pretty much constant additions of baking soda and MA.

If this should be a separate thread please let me know.

 

[bg22]

Interesting read in the presentation from Richard's post. I think I may have some etching on my new plaster as well now. Of course early on I was not on top of keeping my water as balanced as I am now. Plaster is Sunstone and is about 9 months old now (new pool build). For the last 4-5 months I've been trying to maintain a CSI between 0.0 and -0.3. Maybe I should let it drift up to closer to 0 and try to keep it around there to see how things work. I also started developing scale on my spill way from the spa into the pool. Probably from before having not kept on top of keeping water balanced. I also just did a partial drain and refill because CH was constantly over 430 and I now have it down to ~320. Trying to keep TA around 80 as well now and PH around 7.6.

 

[bg22]

After reading onBalance's post on Bi-carb start-up, maybe my etching was due to the acid startup performed on my plaster.

 

[onBalance]

HouTex, I wish I could convince you that the round white spotting is not an etching condition. Those type of spots are very porous and have loss some material because of the weakness caused by improper plastering. One cause is heavy water troweling.
A -0.3 CSI will not cause visible etching within a few short months, such as your new pool has experienced. Also, aggressive water will not cause small spots or areas to be etched and leave the plaster surface immediately surrounding those spots completely unaffected if the plastering was done properly.

The (gray) mottling is caused by late hard troweling and usually adding too much calcium chloride to your plaster mix when the plastering was perform. All of this has been documented.

Without going into great detail on some of interpretations contained in the above "presentation link" report, the bottom line is that the report does acknowledge the above plastering issues as likely leading to the visual conditions (spotting and severe gray mottling) seen in your own pool.

I feel that Acid start-ups are the wrong thing to do. Acid start-ups usually result in a CSI of -4.0 or even lower sometimes, and yes, it very slightly etches (uniformly) the plaster surface as seen under magnification. But here is the point, acid start-ups, or maintaining slightly aggressive water over time, doesn't result in round white spotting and also doesn't cause gray mottling.

It is some of the pool plastering people that put out such nonsense. And the solution they often recommend to correct the above plaster discolorations, is to perform an acid wash! Is that not contradictory?

 

[chem geek]

HouTex,

I agree with onBalance. I wouldn't worry about a lower TA if you have a higher CH to compensate. It's the CSI that matters, at least for water chemistry. The faster problems you are seeing and the nature of those problems with fairly balanced chemistry aren't a water balance issue. Even for my pool it's not clear whether or not it's water balance or whether the plaster was troweled too thick on the ramp and seat areas 9 years ago. I'm just trying to catch a potential issue early, especially in light of the unusual chemical readings (high CH and higher acid demand) I had. Even these pitted areas still feel relatively smooth, though the difference is slightly noticeable to the touch. I will certainly report back over this season and next on how things go with the somewhat higher saturation index.

 

[HouTex]

Richard and onbalance thanks for the comments. My plaster is not looking any worse since the etching appeared about a year ago so I'll keep doing what I've been doing.

 

[HouTex]

But a follow up. Maybe I'm not getting it from your responses, but what is causing my CH to increase? During last year's drought I thought (and many here agreed) that it was because my fill water is CH 150. But I've barely had to add water to my pool since November 2011 and the CH remains at 475-500 even though heavy rains have caused my salt, borates, and CYA to fall dramatically.