All Praise is due to Allah Alone and peace and blessings be upon His Messenger, his family and Companions.
The basic ruling on water is that it is Tahir (ritually pure). If its color, taste, or odor changes due to being mixed with a Najasah (ritual impurity), it will be considered to be Najis (ritually impure), whether the amount is small or large. However, if the Najasah does not alter it, it will be considered as Tahir, but if there is only very little water, it is preferable not to use it for purification, as a precaution and to escape the difference of opinion of the scholars over this. This is based on the Hadith Marfu` (a Hadith narrated from the Prophet with a connected or disconnected chain of narration) narrated by Abu Hurayrah who said that the Prophet said:
"If a dog licks a utensil belonging to any one of you, it (the contents) should be thrown away..."
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.