The authors describe a field study that examined the effects of hydraulic mixing on water quality variations in a distribution system. Conducted at the North Penn Water Authority (average production of 5 mgd and 225 mi of distribution pipe), the study incorporated a field sampling program that utilized customized automated samplers designed to prevent loss of volatile constituents. As a complement to the field sampling program, computer models that would predict water quality variations in distribution systems were examined and developed. A clear need was shown for obtaining more representative monitoring results than are normally acquired from distribution system sampling. Includes 36 references, table, figures.