I had a similar situation at my location when I bought this place Boab, septic tank and all... I had to learn how to locate and repair field drains. Divining rods can help, along with a probe which has a drilling in it`s nose, on pushing it down into the soil, anything hard is hit by the probe, and if it is a drain you have hit, you may detect a hollow sound or not..the drilling will contain some trace of the red fireclay surface of the drain tile.
