How is water equal to two qullahs purified from impurity?

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If water is equal to two qullahs and is not changed by the impurity, it is purified only by augmentation as described above (reaching a volume exceeding two qullahs). If the water has changed due to the impurity, purification is achieved by either augmentation that removes the change, or by leaving it undisturbed until the change naturally dissipates over time.