Works : Islamic Laws
44. The water of a well which springs forth from the earth, (although its quantity may be less than a Kurr) does not become najis owing to something najis falling in it, unless its colour, smell, or taste changes. However, it is recommended that, in the event of certain najasat falling in it, a quantity of water should be drawn from the well. Details about this quantity are given in the relevant books.
45. If a najasat falls into well water and changes its smell, colour, or taste, it will become Pak as soon as the change in its smell etc. vanishes. But it is better to wait till it is mixed with the fresh water springing from the earth.
46. If rain water is collected in a hole, and its quantity is less than a Kurr, it will become najis if najasat reaches it after the rain has stopped.