Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.002B.

This involves water whose form has been altered because of its being in a place for a long period of time, or because of the place in which it is located, or because of its being mixed with a substance that cannot be completely removed from it (i.e., water mixed with algae, tree leaves, and so on). The scholars agree that this type of water falls under the heading of mutlaq water.

The rationale is simple: everything that falls under the general term of water, without any further qualifications, is considered pure, for the Qur’an says, “…and if you find not water, then go to clean, high ground…” (al-Ma’idah 6).