On this map, the median of monthly maximum turbidity values (in nephelometric turbidity units) are shown for drinking water facilities of each drainage region. Turbidity refers to the relative cloudiness of water, caused by suspended particles in water. The Survey of Drinking Water Plants collected this monthly maximum turbidity data for surface water sources from facilities reporting turbidity data for at least 10 months in 2013. These facilities served 24 million people and produced 4,091 million cubic metres of potable water from surface water sources in 2013. Source water turbidity was monitored continuously at 42% of these drinking water plants in 2013, daily at 34% of plants and less frequently at the remaining.