I believe I have in my backyard what appears to be a severed septic drain field line (no longer used).
What I want to know is why would it still be draining water if I am now on the county sewer? Once my house was put on the county water and sewer they should be all dried out and no longer connected to my existing sewer system, correct?
A buried perforated plastic line in the ground may not necessarily be part of a septic system drainfield.
It could be a drain line for roof gutters, or part of a French drain system, or part of some other underground drainage system designed to carry away excess storm water to keep it from accumulating on the surface and backing up under your home away from the basement or crawlspace.
Normally, an unused,shallow septic drainfield line would dry out unless it is still getting storm or ground water intrusion.
If it aint broke, don't fix it. However, if it were me, I would have the septic tank pumped out and then backfilled with compacted fill dirt, if you don't need to use it for a "dry sump" for excess storm water.