I say, and with God's success: Well water is considered stagnant water if its area is less than ten arms by ten arms — which equals (25) square meters of water surface area, and the depth does not appear to be from the ground when scooped out — and the depth of the well and its abundance are irrelevant; it is treated like stagnant water if it is little. It becomes impure if impurity falls into it and it is known with certainty or with a preponderance of evidence that it has occurred, even if the effect of the impurity does not appear in it. See: Maraqi al-Falah, p. 39, and God knows best.