What defines the amount of water in a well or pool that is considered undrainable, thus exempt from impurity rules applied to still water?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
There is no precise textual definition from the Imam or his companions for water that cannot be drained, other than comparing it to the Masani' of Mecca. Ahmad indicated that the prohibition concerning the wells of Medina applied because the Masani' did not yet exist. He stated that urine or anything else does not pollute the water if it is abundant, resembling the Masani' reservoirs. If a well has a continuous source but is still (not flowing), it is treated as stagnant water.
Supporting text
If a person urinates in a deep well, it must be drained until the people cannot manage to drain it further. A small pool ('Ghadir') where urine falls is considered easier (less severe ruling). If the well has a source but is not flowing, it is considered subject to impurity from urine if it can be drained.