Under what condition do animal droppings (like sheep or cow dung) contaminate a small well?

Well Purification: Moderate Drainage & Unknown Decay Timings

• If even a single piece falls in.
• Only if the amount is excessive in the eyes of a viewer, otherwise small amounts are excused due to public necessity ('Umūm al-Balwā).
• Animal droppings are always inherently pure.
• Only if the water changes color.