If impurity falls at the eastern side of a sufficiently extensive pond while ordinary movement does not reach the western side, the stated large-pond exception may apply at the opposite side.
The basis is the apparent failure of impurity to reach there; an observed effect at that far side would call for a separate assessment.
Opening interactive controls...
Impurity in Stagnant and Running Water, Large Ponds, and Aquatic Creatures
If impurity falls at the eastern side of a sufficiently extensive pond while ordinary movement does not reach the western side, the stated large-pond exception may apply at the opposite side.
The basis is the apparent failure of impurity to reach there; an observed effect at that far side would call for a separate assessment.