I say, and with God's success: If there is impurity in the well, it is either known that the impurity has occurred in it, or it is not known: If it is known that the impurity has occurred, then the well becomes impure from the time the impurity occurs if it is known with certainty or with a preponderance of suspicion. If it is not known that the impurity has occurred and the water is being used, then it is examined: If the thing that fell into the water has swelled or decomposed, it is ruled that the well has been impure for three days and nights; because swelling is evidence of the passage of time, and it is required to repeat the prayers for that period if one has performed ablution from it, or bathed from impurity, or washed their clothes from impurity with it. However, if they washed their clothes from impurity and did not perform ablution from it, they are only required to wash them correctly; because it is akin to the presence of impurity in the garment, and they do not know when it occurred, nor do they repeat their prayer. But if it has not swelled and the thing that fell into it has not decomposed, it is ruled that the well has been impure for one day and night; as a precaution. See: Rad al-Muhtar 1: 147, Al-Wiqayah p.102, Maraki al-Falah p.42, and God knows best.