The white residue that you are seeing is the results of dissolved minerals found in water. Some common dissolved minerals are calcium and magnesium and they are directly related to the hardness of the water. "Soft water" is said to be soft if it has a low concentration of calcium and magnesium ions in it, as opposed to “hard water” which has a high concentration of calcium and magnesium. These dissolved minerals do not pose a risk to human health but can build up on surfaces over time and become a nuisance. Commercial products are available to remove white residue caused by minerals in water. The city uses hard water over soft water as part of its corrosion control program as soft water tends to be more corrosive and can break down the piping throughout the system. The average level of hardness for the city is roughly 115 ppm or 6.7 grains per gallon (gpg), which is classified as moderately hard to hard water. Total dissolved solids, which include hardness, does have a non-mandatory secondary maximum contaminant level (SMCL) of 500 mg/L (ppm) which is equivalent to 29.2 gpg. These secondary contaminant limits are set as voluntary limits for contaminants that are deemed to not be a health concern but are a concern for the aesthetics of the water itself (taste, color, odor etc). For more information on secondary contaminants and their SMCL please visit the EPA's Secondary Standards.