You have a couple of options.
1. You could dig down at the outlet end of the septic tank. Many tanks have a lid on the tank over the outlet baffle. If this tank has one, you can access the outlet pipe through the outlet baffle and introduce water here.
2. You can dig to the outlet pipe after the septic tank, cut the line, and install a tee/cleanout in the line and introduce the water at that point.
Hope this is helpful.
PSMA education committee chairman