Genocide is unlikely as people can stop drinking after finding out it is poisonous, especially if the poison is fast acting. There has been many cases of industrial mining companies poisoning rivers.
River ecocide will probably alert the authorities first. https://www.waternz.org.nz/Story?Action=View&Story_id=2427
It has to be a slow acting poison and not easily detectable or something so new that no one knows about… yet. Example…PFAS - https://www.youtube.com/watch?v=SC2eSujzrUY
Seems like there has been break-ins though to water supply - https://www.linkedin.com/posts/elisabethbraw_is-russia-trying-to-poison-finlands-water-activity-7251250484294873088-d2YP
It’s time to go hard to force russian to stop trespassing into Ukraine without a visa.