Lets see if I can explain and simplify this. Alkalinity and hardness are related but measured differently. Alkalinity measures dissolved carbonates and bicarbonates. Carbonates and bicarbonates are electrically charged and change back and forth due to different living activities (mostly from plants) going on in the water. Carbonates buffer or 'mediate' the pH and tend to keep it more stable. The amount of buffering capacity depends the amount of carbonates dissolved in the water. Limestone (lime) is mostly calcium carbonate.
When you add lime to the water the calcium carbonate separates into carbonate and calcium. Hardness tests measure two of the main minerals dissolved in water - calcium and magnesium. NOTE: Dolomite has calcium and some magnesium carbonates in the reference above by ewest. Limestone is mostly Calcium carbonate.
This is what I think happened in your situation. When you added lime, it dissolved and separated into calcium and carbonate. Then something in the pond absorbed or precipitated the carbonates removing them from the dissolved form or condition. This left the calcium still dissolved and it was measured by the hardness test. New added carbonates were no longer dissolved and were not increased.
Last edited by Bill Cody; 05/09/17 07:58 PM.
