I don't know if you want to believe the oil refining industry, but is on their site as the official version of "How water gets into Diesel".
This one is from Exxon Mobil's site, but is based on studies by Advanced Fuel Solutions (AFS):
www.exxon.com/USA-English/GFM/Products_Services/
Water in a fuel storage tank can cause operational problems
Source: Advanced Fuel Solutions
Tank maintenance is an essential part of any successful premium diesel or heating oil program. As little as .01% (100ppm) water in a fuel storage tank can cause operational problems. Water cannot be completely eliminated from distillate fuels. It can get into the fuel at various stages as it progresses through the distribution network from the refinery to the end-user. Water can get into the fuel during its production, when the hot fuel is in contact with process water. Most of this water is removed in the stripping units at the refinery and more will separate as the fuel cools, but still, some water remains in solution with the diesel. This entrained water may cause a haze in distillate fuel.
Most of the water present in fuels will drop out as it is heavier than the fuel and will sink to the bottom if given time. The temperature of the fuel has an impact on its water-shedding tendencies as well. Warmer fuel can hold more water in suspension than can colder fuel. Water can also be introduced during transportation and storage of the fuel as well. Sea-going vessels can sometimes introduce water into distillate fuel, which can then drop out into storage tanks. Tanks themselves have inherent problems at roof seals and vent pipes which can allow rain water to seep into the system. During fuel withdrawals, tanks can breathe in large volumes of humid air. Moisture in the air will condense when the ambient temperature falls, collecting in tank bottoms. This is particularly prevalent in the spring and fall when the day-night temperature fluctuations can be extreme.
There are numerous ways in which the water can get into the fuel storage and delivery system, now let's look at the problems it can cause.
In cold weather, many water-related problems are incorrectly attributed to the fuel. Water in tanks can freeze 20oF - 30oF above the temperature at which fuel-related problems begin (cloud point). Ice crystals can build up on filters, restricting flow and compromising performance. They can also restrict fuel flow in tank pumping lines.
In warmer weather, the presence of water in tanks may encourage the growth of fungi or bacteria which live in the tank water bottoms and feed on the fuel. Under the power of a microscope, these bugs look like deep-sea creatures. To the naked eye, these bugs show up as slimy mats of substance that can be any color from green to black. Under ideal conditions, these bacteria can double in number in as little as four hours. When left unchecked, they can be drawn out through suction lines and plug filters. In addition, the by-products of their fuel consumption are very acidic and can cause pitting and decay in tank bottoms. Many tanks go unchecked for years, accumulating water from any number of sources. When fuel inventory is low, water bottoms can be stirred up during deliveries, and if not allowed to settle out, can be suctioned out into a transport truck or into a diesel vehicle's tank.
Water also causes corrosion in storage tanks and engine systems. The by-products of this corrosion, including scale and rust, can all lead to filter plugging or injector fouling and can help make a stable emulsion.
Fuel injection pumps are often times lubricated only by the fuel they are pumping and are, therefore, very susceptible to seizing if water gets into them. These high-pressure pumps are not at all tolerant of dirt, debris and organic deposits, all of which can be carried into them with water. The barrel and plunger clearance is often times only 1-2 microns. This tight tolerance is necessary to maintain fuel injection pressures and ensure minimal leakage past the plunger shaft.
