I worked in a gas station from 1974-1977 and we would use a long stick every morning to see how much gas was in our four 12000 and 6000 gallon underground tanks. We also put a white paste on the stick to see how much water was in each tank. If it got over 1/2 inch we would tell the delivery guys and they would at the refinery put a “dry gas” additive to absorb water. Now each tank had an outlet 6-12 inches from the bottom of the tank that would drawl gas out going to the pumps. The problem arises went gas is delivered and dropped into the tanks mixing all the dirt, water, and crud from the bottom of the tank. Are there filters at the pump? Yes there should be but I have always kind of bypassed gas stations went I see a fuel delivery truck dropping fuel.