If there is no other reliable water, and only doubtful donkey or qualifying mule leftover is available, both wudu and tayammum are performed.
Wudu addresses the possibility that the water purifies; tayammum addresses the possibility that it does not. The combination responds to both legal possibilities.
Opening interactive controls...
Su’r: Categories, Purity, and Juristic Differences
If there is no other reliable water, and only doubtful donkey or qualifying mule leftover is available, both wudu and tayammum are performed.
Wudu addresses the possibility that the water purifies; tayammum addresses the possibility that it does not. The combination responds to both legal possibilities.