One of the most difficult chemistry parameters to control in a cooling water system is Free Halogen levels.  There are many reasons for this but primarily Halogen is different from every other type of water treatment because unlike other water treatment chemicals halogens are continuously being consumed or depleted.  Chemicals that make up free halogens like Chlorine, Bromine, and Chlorine Dioxide are very strong oxidants and as such are actively depleted by things like UV from sunlight, microbiological activity in the water as well as other treatment chemicals.

14-08: Comparison of Chlorination Monitoring Methods in Cooling Water Systems