Hydrogen Sulfide Gas Buildup

Hello

I recently purchased a piece of property in VT. This property has an existing well and septic. The house on the property burned down over 3 years ago which means the septic and well have to get re permitted which involve an engineers design. This can get very expensive. I just had it inspected and while it shows no signs of failure the inspection states there are signs of hydrogen sulfide gas build up. Just looking for some info on this and how bad it is.
Also the tank is full to the top. Is that just a sign that it was never pumped out?

Thanks Mike

Shouldn’t be full to top. Gonna need a couple of engineers about the gas build up. Not familiar with that type of build up. But septic level should expose pipe on both ends.