وَالْغَدِيرُ الْعَظِيمُ الَّذِي لَا يَتَحَرَّكُ أَحَدُ طَرَفَيْهِ بِتَحْرِيكِ الطَّرَفِ الْآخَرِ إِذَا وَقَعَتْ فِي أَحَدِ جَانِبَيْهِ نَجَاسَةٌ جَازَ الْوُضُوءُ مِنَ الْجَانِبِ الْآخَرِ؛ لِأَنَّ الظَّاهِرَ أَنَّ النَّجَاسَةَ لَا تَصِلُ إِلَيْهِ.
Meaning: In a very large pond whose one end does not move when the other end is stirred, if impurity falls on one side, wudu is permissible from the other, because apparently the impurity does not reach it.
A very large pond is stagnant water whose two ends are far enough apart for the rule to distinguish them.
The basis is the apparent non-arrival of impurity at the distant side, rather than a claim that contamination at one edge has no significance anywhere in the pond.