Based on the Shulchan Aruch, Yoreh De'ah 201:25, if the wine changes the color of the water (e.g. if the wine is red), then the Mikvah will be Pasul (even if the wine comprises less than half the Mikvah). But if it does not (e.g. if the wine is white), then, as long as the Mikvah contains forty Sa'ah of water, it will remain Kasher (even if there is more wine than water).

be'Virchas Kol Tuv

Eliezer Chrysler.

Shalom and thanks for your inquiry.

If wine falls into the Mikveh it will disqualify it, if and only if it changes the color of the Mikveh. In the case of red wine this will occur long before the wine is majority of the Mikveh. In the case of white wine, it will never disqualify the Mikveh since it will not cause a color change.

However, the wine cannot be considered towards the required forty Se'ah. Thus if the Mikveh contained thirty nine Se'ah and a Se'ah of white wine was added, the Mikveh still requires another Se'ah of water, while the wine does not disqualify it.

For more details please see Rambam Mikva'os, chapter Seven.

Yehuda Landy