it's always about how much. to take the example of radioactivity see https://xkcd.com/radiation/. please provide quantities necessary for these conditions and the quantities present in water to give a honest image about this. mind that too low quantities of certain elements can be just as dangerous, so just saying something can cause something is not enough. just being careful seems common sense, but can actually cause more damage. everything becomes a poison at a certain quantity, so you can have this argument about anything.