Some common contaminants found in today's fuels include water, microorganisms, wax, hard particles, debris and other sediments.
As with most things, time has an adverse affect on diesel fuel. The shelf life for diesel fuel is only six to twelve months but in extreme conditions, that time could be much shorter. Furthermore, the longer it is stored, the more opportunities will present themselves for contaminants to enter the fuel.
Other than water from rain or snow and dirt from the wind entering the storage tank, temperature and humidity have adverse affects. Warm moisture laden air coming in contact with cold diesel fuel or a cold tank can cause the water in the air to condense. The amount of water in the air, or humidity, can greatly affect the amount of water that is added to the fuel from condensation as the tank breaths. Also, warmer ambient temperatures can speed up the natural deterioration of diesel fuel. The composition of diesel fuel can vary dramatically depending on the region and temperatures. Fuel intended for warm weather will not store properly in cold weather.
Diesel fuel in a storage tank is a smorgasbord to many forms of micro-organisms. These bacteria and fungi live in the ever present water and feed off of the hydrocarbons found in diesel fuel. Their living and dead bodies are contaminants. Any opening, whether it is the fill port, ventilation or hole caused by damage, is an easy, direct path for contaminants to enter the fuel. Any damaged caps, seals or gaskets are also perfect opportunities for contamination.