In water distribution systems design and operation of storage facilities have been based primarily on cost and hydraulic considerations. Little consideration is given to the impact of choosing a standpipe versus an elevated tank on distribution system water quality. Only recently, with enactment of the Surface Water Treatment Rule, have the mixing characteristics of water treatment plant clearwells been studied. In this research, the effects of tank geometry and tank operation on the mixing characteristics of storage facilities are investigated and discussed. The effects of inlet configurations are not considered here but, along with tank geometry and flow regimes, are being investigated in ongoing work. Results of field studies and model simulations reveal that the behavior of tanks with respect to water quality depends on geometric characteristics and daily operation. Shallow tanks with large surface areas, which experience substantial volumetric fluctuations will act as completely mixed reactors. Tall tanks with small diameters, such as standpipes, are not well mixed.