We're in the only neighborhood in town that isn't on sewer, and folks know it. My realtor calls it 'a cloud' over these properties...getting it fixed is going to be a major selling point. My excavation contractor is going to make it so that the leach fields can be swithced back and forth, making them good for 20 or more years. Definite selling point!
If folks know it, and you've made it good for 20+ years...it's a fantastic selling point. And you discovered its problem by accident, yes? Better you than the new buyer.