Tim, I don't get the 'heavy rains' part. I've never owned a diesel so this is all new to me. But should it not matter if it rains or not because the diesel is stored in a underground cell, so how could the water get into it? Or is there something else that I'm not aware of? Please explain. Thanks!
Well, I live in South Florida and we have high humidity and lots of rainfall. So, first don't assume that the tank is a closed system. As you pump fuel out something must come in otherwise you create a vacuum within the storage tank. The replacement air comes from outside, in my case can be very hot humid air. The warmer humid air goes in to a slightly cooler tank where the water condenses on the surfaces and goes to the bottom of the tank (where the fuel pump pickup is located).
A second way water can get in, is from the the fill cap at the fuel station. These caps are typically under a metal lids in a concrete apron that is sloped to contain a spill. If the cap is not secure, or the gasket removed/deteriorated during a heavy rain event, the apron area floods and water finds it's way into the tank.
A third way is from ground water intrusion on older rusted tanks, many of these have already been replaced due to environmental laws.
Please note that all the above is very rare in occurrence, but I don't want to be part of that statistic!