Is the measurement of the expanse (proximity or distance between extremities) of the water relevant in determining its purity when only part is changed?

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The measurement of the expanse, whether the extremities are far apart or close, is irrelevant. The decisive factor is whether the unchanged portion is considered little or great (less or more than two qullahs). The purity of the water adjacent to the impurity is not precluded, as evidenced by cases where a dog or carcass is in the water; the immediate contact point is pure. If that contact point is deemed impure, then the part adjacent to it remains pure. Ruling against this opinion would imply that the entire sea becomes impure if one side changes, or all running water, which no one asserts. Imam Ahmad ruled that the waters in the cisterns along the road to Mecca are not rendered impure by anything.