The amount of impurity and the quantity of water are separate questions. In ordinary stagnant water, a very small amount of impurity does not automatically become irrelevant.
For an extensive pond, the location of impurity and its reach toward the distant side matter. The judgment therefore depends on whether water is stagnant, how extensive it is, and where the impurity fell.
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Impurity in Stagnant and Running Water, Large Ponds, and Aquatic Creatures
The amount of impurity and the quantity of water are separate questions. In ordinary stagnant water, a very small amount of impurity does not automatically become irrelevant.
For an extensive pond, the location of impurity and its reach toward the distant side matter. The judgment therefore depends on whether water is stagnant, how extensive it is, and where the impurity fell.