Forum Flora Germanica

Might be introduced, at forest road. Leaves like 15 cm. Flowers like 4 mm, to be white. Loc.:Slovenia.

That's highly unlikely, and it's not possible to identify the species without flower and fruit details.
https://floranorthamerica.org/Alisma

Alisma triviale and A. subcordatum are both common in North America (pictures in https://www.flora-germanica.de/en/flora ... ecies-list), so an introduction is always possible. But both are extremely similar to A. plantago-aquatica and, as Dominik said, can be determined only with details. I also consider it unlikely that this is one of the North Americans, the leaf form does not fit.

Michael