Surface waters require a coagulant such as alum for effective treatment. After treatment and disinfection with chlorine, the water can become aggressive. At present there is no requirement to produce a water that is stable (neither scale forming or corrosive) other than having a pH in the range 6.5 to 8.5.Experimental data regarding the measure of the stability of a water with respect to its degree of CaCO3 saturation are presented. Higher concentrations of calcium,total dissolved solids, and alkalinity all promote a greater tendency for scale.