I say, and with Allah's guidance: If impurity falls into a well, it becomes impure, even if it is little, like a drop of blood; because a small amount of impurity contaminates a small amount of water even if its effect does not appear in it. What cannot be avoided is excused: such as the dropping of camel and sheep droppings, horse, mule, and donkey dung, cow manure, and bird droppings, as they do not contaminate the well due to necessity. The waters of wells are considered stagnant and little as long as their area is less than ten cubits by ten cubits - which equals (25) square meters of water surface area, and the depth from which the ground does not appear when scooping from it - and the depth and abundance of the well have no relation to this ruling. It is ruled like stagnant water if it is little, thus it becomes impure if impurity falls into it and it is known with certainty or with a preponderance of suspicion that it has fallen into it, even if the effect of the impurity does not appear in it. See: Maraqi al-Falah, p. 39, and Allah knows best.