My ex and I use to own and operate a septic tank cleaning company so I can confidently say that what you are smelling is septic gases that are escaping the tank. You may want to have a septic tank cleaning company come out and see if the tank needs emptying, if it is full or close to it and it is not cleaned…it will have more gases…and eventually it will back up into the house (which is not a good thing). Most cleaning companies will give a discount if the owner digs up the lid of the tank. You can also check the fullness of you tank yourself, but be prepared for it…it is not pretty. To do this…locate the lid by sticking a rod into the ground until you tap it, then dig to access the lid (or lids-sometimes there are two-sided by side), use a crowbar to remove the lid(s)…if the tank is 3/4 or more full you may want to have it pumped out. If it is not, replace the lid(s) recover the holes. If the tank doesn’t need cleaning, since you have allowed the area to dry, once you replace the dirt the fumes should be eliminated. One more thing that may help lengthen the need for pumping and back up problems, is to use one of the septic enzymes that are sold. Or even regular baking soda works too. What these products do is add enzymes to the tank so that the “waste” is broken down faster into a liquid so that it can be dispensed through the drain lines through out the yard. If you use a lot of bleach products, it will lower the natural amount of enzymes in the tank, and cause it to fill faster and have more fumes.
[Edited by Deecent on 18 August 2001]