According to your permit you have plastic pipe about 4 ft deep. Not a root problem and plastic pipe doesn't corrode. Dig up just the wet spot and probably find a broken joint and stopped up with mud. Downstream might still be good.
Do you have any trees in the area of your leach field? Might just be one area that needs to be dug up. I have replaced lines that were completely full of tree roots. Good luck with it.itching