All fuels contain some water in suspension. Temperature changes can cause suspended water in the fuel to coalesce and settle out. External seepage or leakage during rains may also cause water to enter the tanks. Particularly in underground tanks, during ice or snow buildup or very heavy rains, leakage has been a problem. The requirements for containment dikes has contributed to the problem when drains become stopped up and do not allow water to get away.
Water is more dense than fuel, so it always settles out to the bottom of the tanks .
Pooled water introduced into the fuel system is of no benefit to operating engines. In fact, it is possible to cause engine shut down or significant loss of power when water is introduced. Fuels delivered into tanks will be acclimatize with water pooling at the bottom of the fuel tank in 72 hours or less. A clean burning, water-free fuel allows the fuel's natural lubricants to work, thus helping prevent wear and eventual failure of engine parts . Slugs of water cause sudden cooling in the engine, and may result in shortened engine life.