Septic fields are usually vented, not the tanks directly. However, what happens is there is air flow from the field vents, through the tank, and up through the vent pipes on the top of your house roof. When the breeze is from the "wrong" direction, and you are outside, you can smell the fumes. Not pleasant. What I have done for myself on a temporary basis, but do not recommend as a permanent "fix" is to cover the field vents with plastic bags to stop the air flow. I have only done this when friends were to be over and I didn't want the smell to be around while we were outside cooking, etc.