(2) residual chlorine Explanation: The word "residual" means "remainder" or "that which is left", and as the name suggests the chlorine residual is measure of the amount of chlorine remaining in the water after disinfection. The chlorine residual is usually tested in finished water which is ready to be released into the distribution system, although operators must also ensure that there is adequate residual at the extreme ends of the distribution system. Tests for chlorine residual are probably the most frequently performed tests at water treatment plants.