Ambient air contains moisture.

Condensation is a variable, dependent on the weather - no tank is airtight so every time you use fuel, ambient air is drawn into replace the volume and that ambient air contains moisture.

All diesels need a water trap, which needs emptying at the recommended interval - failure to do so is likely to lead to major mechanical issues.

Petrol cars also get condensation/water in the fuel but that easily evaporates during the combustion process, indeed it helps it slightly as a make-shift water injection system.

How much water arrives at service stations in the tanker, how much gets in during delivery, and how much while customers fill up (in the rain) ?

I guess it must build up at the bottom of the storage tanks - what is done to remove this, and how often? Is it a frequent problem for drivers?

I ask these questions after reading the thread about changing fuel filters, and whether water is ever found there.

I can only speak for the supermarket petrol station where I work part time to supplement my small business income:

NO water is transferred from the tanker to the underground storeage tanks. There is a water/moisture sensor in each tank and we take a reading at the start of each day, before a tanker is due and immediately after a delivery. We've never had water in any tanks in the 3 years I've worked there, but there is a procedure in place if there is: The contaminated tank is shut down immediately.

In addition, all tanks are thoroughly cleaned by a specialist contractor every 6 months.

I doubt whether much rain water gets into the car's tank during re-fueling. Aside from most petrol stations having canopies, the gap around the nozzle must be tiny.

Having said all that, a customer only this week told me of a Texaco station which somehow had water contamination just before Christmas and was facing huge bills for draining and cleaning customers' fuel tanks, having admitted full liability.