honestly, this is something i remember hearing about back around when obama was still office; not by the border patrol but rather by some militia/vigilantes. i tried to find a source when asked, but google really insists on directing me to some recent report about the actual border patrol slashing water jugs and pouring water out, and border patrol providing detained prisoners with water that smells like bleach, which is heinous and criminal, but neither of those are what i was referring to at all
being unable to find a source now, it may have been false all along, or possibly just a few incidents at the time. the claim was specifically that bleach-tainted water bottles would be placed at key points in the desert to either poison people or sow distrust in all water supplies. i can't currently find anything about specifically bleach being used, which is the detail i remember most vividly
there are humanitarian reports of border patrol or militia writing "venino"/"poison" on water bottles left by humanitarian groups, but i'm unable to find confirmation on actual poisoning attempts - the claims may have stemmed from those incidents of bottles being marked as poison.
this is a youtube channel for a humanitarian org called "No More Deaths", who have been making extensive reports on the crimes against humanity that have been occurring. here's a shorter video that mostly discusses the water/food vandalism and shows a recording of border patrol doing it: