The alkalinity level you ...

The alkalinity level you have is probably all bicarbonate at the initial pH.  Air contact continually will remove the CO2 (pseudo-equilibrium partial pressure due to hydrolysis of bicarbonate).

Over time the balance of the alkalinity will shift away from all three species, to just two, namely bicarbonate and carbonate, which reaches equilibrium at a much higher pH.  If there is any chloramine in this system, it too will upset the pH balance as COD, and BOD reacts with this, leaving chloride and ammonia behind.  This sort of pH shift has been observed in water softener vessels where chloramine containing source water stagnates in the offline vessel(s), and decomposes to ammonia and chloride.  It only requires the 2-3 ppm chloramine to cause a shift from 7.8 pH to 8.5 pH.