The below - grade ground between the tank and the foundation (sometimes about a seven to ten foot distance depending upon the age of the system) becomes saturated with smelly moisture over time and the excess then leaches downward into the foundation weepers (which in some areas with high water tables are always somewhat full) which then empties into an in - basement sump pit in homes not on municipal sewers. (remonline.com)
It is the first home we have ever owned that had a sump pit & pump. (remonline.com)