From The Kosher Information Bureau: Pesach News – Bottled Water

In Newsletter Items, Pesach Alerts 2020 by kosherqu

Unflavored, Purified Water generally does not require certification for Pesach.  However, when containing a Citric Acid, such as Kirkland brand, which lists “calcium citrate” in its ingredients, the water should NOT be used without reliable Pesach certification.  Unflavored water, with other minerals added, do NOT require certification for Pesach.