The Irvine Ranch Water District, a large water utility supplying approximately200,000 residents of southern California, has historically experienced repeatednitrification episodes within two portions of its domestic water distributionsystem. As part of a Nitrification Action Plan, this research was conducted tocharacterize both fixed and suspended biomass populations within an activepotable water distribution system and to discover an alternative way to identifythe early beginnings of a nitrification event well prior to an occurrence. Theproject team wanted to design a monitoring program that would reflect actualin-situ conditions. Includes 5 references, figures.