What is the ruling on a large quantity of water in which some part has become impure and changed in quality?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
If a large quantity of water, exceeding two qullahs, has some portion rendered impure and changed thereby, and the unchanged portion is less than two qullahs, the entirety of the water becomes impure. This is because the changed portion is impure due to the change, and the remaining portion becomes impure by contact with it. If the unchanged portion exceeds two qullahs, the entire water remains pure. The basis for this ruling rests on the statement of the Prophet, peace be upon him, "If the water reaches two qullahs, nothing renders it impure," and his statement, "Water is pure and nothing renders it impure." Furthermore, the unchanged portion, even if less than two qullahs, is covered by the general ruling of purity if it has not changed, and purity is affirmed because it is a large body of water not altered by the impurity. The effective cause (*'illah*) for impurity in large quantities of water is the change itself; therefore, the impurity is restricted to the location of the change.
Supporting text
Ibn 'Aqil and some Shafi'is maintain that the entire mass is impure even if it is vast and its extremities are far apart. They argue that since some part of the stagnant water is impure, the whole body becomes impure, similar to when the parts are close. They further assert that the changed portion, being an impure liquid, contaminates what it touches, and subsequently contaminates all contiguous parts.