When calculating the Saturation Index (SI), does a single low water parameter such as calcium hardness (CH), or a low carbonate alkalinity (C-ALK), make the water automatically aggressive to pool plaster, even when other water parameters are high enough to balance the SI?
To answer that question, an experiment was conducted. Two quality pool plaster coupons were made and cured in balanced water for 90 days. At that point, plaster coupon #1 was placed into SI balanced water, but with a low CH of 90 ppm. The C-ALK was maintained at 110 ppm and the pH from 7.9 to 8.2, which off-sets the low CH and achieves a balanced SI of -0.1 to +0.2.
Coupon #2 was also placed into SI balanced water that had only 45 ppm of C-ALK. The CH of this water was 360 ppm and the pH was maintained between 7.7 and 8.0, off-setting the low C-ALK and achieving a balanced SI of -0.1 to +0.2.
After six months in the water, the coupons were removed, and the water they were in was tested for the calcium content to determine if any dissolution or etching of plaster surface material occurred. Of course, any increase in calcium from the submersion water’s starting point would indicate that calcium had been dissolved from the coupon, which was the only available source of additional calcium.
The result? There was no increase of calcium in either water container.
Therefore, these results indicate that if the calcium hardness or the carbonate alkalinity is low, but the water is still determined to be SI balanced, the water is not aggressive. This indicates that the SI is applicable for plaster swimming pools.
This experiment was conducted because some in the pool industry claim that pool water with a low CH or a low C-ALK is automatically aggressive despite what the SI actually is. Often, when a new pool plaster surface has undergone severe gray mottling, white spotting, streaking discolorations, flaking, nodules, or other defects actually attributable to plastering errors, the finger is pointed at aggressive water chemistry instead. Specifically, a CH or C-ALK below the APSP “Ideal” minimum is blamed, even in cases where the actual LSI is balanced.
It is interesting to note that the current National Plasterers Council (NPC) 7th Edition Technical Manual states that if any isolated, individual water parameter (pH, C-ALK, or CH) is lower than the “Ideal” range as defined by the APSP, the water is aggressive. Therefore, the NPC is (incorrectly) stating that when the C-ALK is less than 80 ppm (while the total alkalinity may be above 80 ppm), or when the CH is less than 200 ppm, or when the pH is below 7.4, (which are the lower ends of the APSP’s “Ideal Standard”), the water is automatically aggressive and considered to be detrimental to the plaster finish (even though the SI may be balanced).
This is a complete departure from the NPC text on this topic in their 5th Edition of the Tech Manual. It stated that pool water needed to be within the “acceptable tolerance” range as established by the APSP, which set the “minimum” for C-ALK at 60 ppm, CH at 150 ppm, and a pH of 7.2. It is apparent that the water balance requirement by the NPC is more restrictive now. Why did the NPC make that change? Where is the study documenting that a low CH or C-ALK is automatically aggressive when the LSI is 0.0 or higher? Has the National Pool Industry Research Center at Cal Poly (NPIRC) proved that? No.
Now, we are not necessarily promoting the concept that pool water should be maintained on a regular basis with the pH, C-ALK or CH below the minimum APSP targets. We are not even suggesting that the ranges below the ideal are always the optimum. We would recommend that pH and C-ALK levels be maintained above the ideals, not below, whenever possible. However, we understand two things: that there are times when lower pH, CH or C-ALK are unavoidable, and that chemistry is chemistry – the SI “balance” means exactly that… it is not a tool for blaming plaster defects on water chemistry.
As the above plaster study demonstrated, pool water with a single low water parameter (even below the APSP’s “Ideal” or “Minimum” standard) can be non-aggressive if the SI is balanced.
onBalance
Comments
Richard, that is a good point, and I agree that a lower TA level can help to maintain a more consistent pH with saltwater chlorine generators and help service techs simplify their chemical maintenance. Those who work with SWCG should consider that option, and perhaps it may help combat possible scaling problems. I would consider doing that if I was servicing salt pools.
Just wanted to link to the Is the Saturation Index always Reliable? thread at Trouble Free Pool (TFP) since we're carrying on more detailed technical discussions there (that forum allows for long-term editing of comments and allows for subscripting/superscripting).
There are situations where a low Total Alkalinity (TA), even as low as 50 ppm or so, make sense to significantly reduce the rate of pH rise from carbon dioxide outgassing. Saltwater chlorine generators as well as aeration sources such as waterfalls, fountains, spillovers in pools and aeration jets in spas all contribute to a rise in pH when the TA is higher. TA is a SOURCE of rising pH in its own right because it is mostly a measure of the bicarbonate in the water and that is an indication of over-carbonation. Of course, the rise in pH may not be seen if net acidic sources of disinfectant or oxidizer are used (e.g. Trichlor, Dichlor, bromine tabs, MPS), but is seen when a hypochlorite disinfectant is used (e.g. chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite).
With a lower TA, one can confidently use a higher Calcium Hardness (CH) and/or a higher pH target to protect plaster surfaces. Your experiments confirm that. If one wants additional pH buffering without using carbonates, then one can use boric acid such as found in ProTeam® Gentle Spa or ProTeam Supreme PLUS or one can buy boric acid directly or use a combination of 20 Mule Team™ Borax with Muriatic Acid. For spas, sometimes a phosphate buffer is used, but that lowers CH as well so cannot be used with plaster surfaces.