Build-up of biomat over the life span of a septic system.
What is biomat in the septic drainfields (a.k.a. leachfields)?
Biomat is composed of anaerobic micro-organisms (organisms that don’t require high levels of oxygen). These organisms anchor to the soil and gravel surrounding the drainfields.
“The biomat forms first along the trench bottom near the perforations [in the drainfield piping which delivers the effluent along the trench] where effluent is discharged, and then up along the trench walls.” https://inspectapedia.com