Question - Why is important that the natural material underlying a septic drainage field is permeable, but not too permeable?

Why is important that the natural material underlying a septic drainage field is permeable, but not too permeable?

Answer: 

Liquid wastes from a septic system will be filtered and decontaminated on passing though appropriate natural or imported surficial materials (e.g., sand or fine gravel). This will be most effective if the materials are sufficiently impermeable to allow liquids pass through slowly.  If the materials are too impermeable the liquid will not be able to drain away adequately and may start to pool at surface.