Chlorine Decay Rate

Dear Everyone :

I am trying to model the available free chlorine concentration in our municipal water network. But I have no info on the decay rate ! Is chlorine decay rate a constant in a fixed temp. pH environment, just like the half life of a radioactive substance ? Or it depends on pipe age, configuration, and many else. What are the typical values of decay rate?

What kind of chemical process actually take place so the concentration of free chlorine decays?? Any wonderful reference from EPA that explains in details.

Your help will be very much appreciated
Thanks
Yao Lan