Assalaamu alaykum. I would like to ask a question regarding the eligiblity of water to be used for ablution and ghusl (ritual bath). Here in the Maldives, we get lots of fresh water from wells. Likewise, our well also provides us with fresh well water. The problem, however, is that there is a sanitary tank near our well which I suspect is leaking and affects the color and smell of our well. This is not for sure though as the well is dug deep and the waste leaking might also be filtered before reaching it. Our well frequently changes to a milky color and smells bad. Please help me and note whether this water can be used for ablution and ghusl or not. I await your reply.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

In principle, the water of the well is pure and the impurity that reaches it does not affect it. Abu Abu Sa'eed al-Khudri narrated that the Prophet was asked, “O Messenger of Allaah, can we make ablution from the well of Budhaa‘ah when it is a well into which the bodies of dogs, menstrual rags, and garbage are thrown?” He replied, “Water is pure and it is not rendered impure by anything.” [Ahmad and Abu Daawood]

However, if the water of the well changes due to an impurity, then it is ruled to be impure by the consensus of the scholars. Ibn Al-Munthir said, “The scholars unanimously agreed that little water and much water becomes impure if impurity falls into it and changes its taste or color or smell.”

What we understand from your question is that you suspect whether the water of the well has changed by the impurity of sewage water or by something else. Since the water changed and you doubt that the cause of this change is the nearby sewage water, then this water is not to be used for purification (i.e. for ghusl or ablution). The scholars stated that the wells in villages and cities which are near the toilets, if their water changes and it is not known what caused its change, then it must be avoided. So it is more appropriate that you avoid this water if you predominantly think that it has changed due to sewage water; it is ruled that it is impure water, and it is not suitable to perform ghusl or ablution with it, contrary to the wells found in the desert, as such wells are ruled to be pure if their water changes even if it is doubted that it has changed due to impurity as long as the doubt is equal regarding the cause of the change being pure or impure.

Al-Hattaab, from the Maaliki School of jurisprudence, said in Mawaahib Al-Jaleel about the changing of the water if there is doubt about what had changed it:

“Or that one doubts about what had changed it (the water) and whether it affects it or not: meaning that if water changes and there is a doubt about what had changed it, is it something that negates its purity or not? In principle, it still remains pure…” (Imaam) Maalik said, “It is acceptable if the smell of the water of the well changes because of mud or other things.” 

It is understood from the word 'doubt' that the author used that this ruling applies when the two probabilities are equal, let alone if the probability of purity is more preponderant, but if the impurity or the lack of being ritually purifying is more preponderant, however, then one acts accordingly (based on this basis).” Al-Baaji said, “If one does not know the reason why the water had changed, then he should look at how this water looks and act accordingly; if there is nothing apparent and he does not know the reason, then it is ruled to be pure. 

Ibn Naaji said in Sharh al-Mudawwanah: "With regard to the city wells if its smell changes, then Al-Maaziri said, 'If there is a case of doubt such as about wells that are near toilets, then Imaam Maalik said, 'It should be left for two days, if it becomes good, then it is acceptable, otherwise, one should not perform ablution from it…''

Ibn Rushd said, 'In urban areas, the water is considered to be changed because of the impurity of the toilet drainage that goes through the houses; contrary to the wells in the desert, if its smell changes and one does not know the reason; then in this case, it is considered pure … Nevertheless, if it is known that the smell of the well is not caused by a toilet drainage that is nearby, then there is no harm in using it.' In another context, he added, 'It is considered pure.'” [Abridged]

For more  benefit, please refer to fataawa 91531and 91628.

Allaah knows best.