IPvFletch,
TA is a measure mostly of the bicarbonate in the water and that is in essence a measure of the carbonation of the water. Yes, a pool is very much like a carbonated beverage. Carbonation is intentionally added to the pool (via sodium bicarbonate, or baking soda) in order to provide a pH buffer and for carbonates to protect plaster (along with calcium that is also added to protect plaster).
Just as a carbonated beverage goes flat when you blow bubbles into it, so a pool also loses carbonation via aeration. This process makes the pH go up (because carbonic acid is removed from the pool for technical reasons I won't get into here) and then when you add acid to lower the pH, the TA gets lowered as well. The fact that a higher TA leads to a higher rise in pH and that a lower TA leads to a lower rise in pH is one of the most counterintuitive things in pool water chemistry, but it's true. It's simply that the effect of higher TA, and therefore higher carbonation of the water, causing outgassing of carbon dioxide which makes the pH rise is larger than the effect of the higher TA being a greater pH buffer.
If you've never done this experiment as a kid, take some baking soda (or Alkalinity Up) in a cup or jar and add some vinegar to it (you can add some water to the baking soda first if you want to avoid a mess). Notice all the bubbles -- that's carbon dioxide. Vinegar is acetic acid so is acidic and adding that to baking soda which is sodium bicarbonate causes carbon dioxide to outgas. The same thing happens in a pool -- lower pH and aeration cause carbon dioxide to outgas faster.
Just follow JasonLion's advice and lower your TA level and see what happens. If you add enough acid to have the pH be lower towards 7.0 (or even 7.2), then the outgassing will be even faster and the pH will rise more quickly thus accelerating the process of lowering the TA (assuming you continue to add acid as soon as the pH rises). It is the combination of aeration at low pH combined with acid addition that lowers the TA. Once you lower the TA to 80 or below, you should see a slower rise in pH and also a lot less acid that needs to be added over time.
HOWEVER, new plaster makes the pH rise as part of the curing process and that may overwhelm what you do above (it's not that lowering the TA won't help, but you might not notice that much of a difference). As much as you love your waterfalls, you can just turn them off for a while (a few days) and see if it makes any difference in the rate of pH rise. As for the curing of plaster, that will taper off quickly -- the first month is really strong, then it's less for a few months, and after 6 months it'll be a lot less. After a year, you'll be close to where you'll end up though some report continued pH rise for a second year though not nearly as strong an effect.
Richard
