[The Ruling of Large Pools When Their Odor Changes]

Answer (and Allah grants success): I found in Sharḥ al-Majmūʿ known as al-Minhāj al-Jalī by Imām Muḥammad ibn al-Muṭahhar (may Allah have mercy on him) the following meaning: “It is permissible to perform wuḍūʾ with odorous water by consensus of the Ahl al-Bayt (ʿa).”
Accordingly, purification with such odorous water is valid, as long as it still retains the name “water.” Its change of odor does not harm so long as it remains māʾ.
This may be supported by Allah’s words:
“And We sent down water from the sky in measure, and settled it in the earth. And indeed, We are able to take it away.” — [al-Muʾminūn:18], and His words: “And We sent down from the sky purifying water.” — [al-Furqān 25:48]
Allah Almighty informs us that He sent down purifying water from the sky, and settled it in the earth. Thus, the water that Allah settled in ponds, pools, and valleys remains purifying (ṭahūr). Otherwise, the divine favor of sending and settling it would not be complete.
It is known that water, when long settled in the earth, changes in odor due to the growth of creatures within it, fallen leaves, roots, animals, humans, exposure to the sun, and the winds.
Yes, if water changes to the extent that its very nature is altered—becoming milk, coffee, or the like—it can no longer be called water, and purification with it is invalid, for ṭahūriyyah (purifying capacity) is a quality specific to water and earth.
Hence, changes short of this do not prevent purification.
The criterion for “little water” according to the scholars of the madhhab: that which one presumes impurity upon contact with filth. “Much water”: that which one does not presume impurity upon contact with filth.