What is the ruling concerning the purity of water in a well adjacent to another well containing urine or other impurities, when the seepage is suspected but unconfirmed?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
If a well is adjacent to a polluted well and there is doubt regarding the contamination reaching the water, the water remains on its original state of purity. One may perform ablution from it as long as the color or smell is not changed. If the water changes in a manner consistent with the impurity and no other cause is known, it is impure, as adjacency serves as a presumed cause. If water is found changed without a known cause, it remains pure because the default state is purity, which is not overturned by mere doubt.
Supporting text
If the change is inconsistent with the likely effect of the impurity due to its quantity or qualitative difference in color or taste, the water is considered pure, as the perceived impurity has no verifiable cause, resembling a situation where no impurity fell into it.