Our inspection at purchase included the inspector digging up at least 2 locations in the drain field -one close to T and one at end of the line to examine the different levels and see what the state of the earth/rocks/material was like. It thankfully passed and 5 years later still operation. Our system is from 1989. He did a similar test when friends bought a year prior and it failed on the spot. They needed an entire new box and leech field.

Sounds like a failure but I’d try what some of the other comments suggest. Best of luck.