In the attached save, pincha pepperplants complain about irrigation, even though it is there in the pipe leading to it. Removing the hydroponic farm and creating a new one fixes the problem (the one to the left from the cursor in the picture complained before this too). And I am reasonably sure only polluted water has entered that pipe (but even if, I do not see anything else than polluted water reported anywhere).
See the attached save. I don't know how the situation occurred.