The white bucket is a simple but reliable test for detection of significant amounts of water and particulates present in fuel. Water occurs in different forms in the fuel;
Dissolved in the fuel, normally this water can not be removed from the fuel.
Suspended or entrained in the fuel. Entrained water can be detected with the naked eyes. The fine droplets of water in fuel reflect light and in high concentration give the fuel a cloudy or hazy appearance.
Water in high quantity into fuel may be caused by leakage into storage tanks, delivery of water laden fuel, condensation or the coalescence and subsequent settling of entrained water.