Is the clay from wetlands adjacent to septic tanks impure (najis), and can it be purified by rainfall?

If it is highly probable that the wetness on damp ground is from altered sewage water, then the ground is ritually impure (najis). This ground becomes pure if rain falls upon it, covering the impurity, or if a flood washes it away, as stated by Imam Al-Shafi'i. However, if it is not known or highly probable that the ground has been altered by sewage water, then the default is that it remains pure, because certainty is not removed by doubt.