The Surface Water Treatment Rule requires maintenance of CT (disinfection concentration multiplied by time) through the disinfection process. The CT calculation requires use of T10, which is usually lower than theoretical detention time and is defined as the time in which 10 percent of the water passes through a basin, system, or process in which a disinfectant residual is maintained. If the CT value needs to be increased, there are a number of options: increase "C" by increasing the disinfectant residual; decrease the CT required by using a different disinfectant or lowering the pH; or increase T10 by increasing the minimum depth of the clearwell, lowering high service flow peaks, modifying inlets or outlets, or installing baffles. Of course, each measure must be evaluated for its side-effects on water quality. The results of a study of 20 tracer tests and clearwell modifications are summarized. The study summarized data from four basic clearwell configurations, including single circular and square, and double circular and square clearwells operating in series.