A monitor device is attached to the water supply line of a consumer where it repeatedly measures a characteristic that correlates to quality of water in the water line. Each of a succession of a water quality values is derived from one or more of the measurements. The process continues at selected intervals to continuously monitor the condition of water in the line. Each new value can be compared to a reference value representing a maximum acceptable level of contaminants. If the water quality value exceeds the reference value, an overvalue signal is produced, indicating an unacceptable level of contaminants in the water. Additionally, the values can be transmitted to a central collection facility where they are correlated with values transmitted by similar devices on the supply lines of other consumers to track the quality of water of a supply system over time.