It is good to have mercury, arsenic, cadmium in drinking water?

As you have probably figured out by now, the answer is no. These elements are sometimes called 'heavy metals'. They are all poisonous to humans, and they have the nasty property of bio-aculation...they build up in your body faster than they can be dealt with (excreted or metabolized). All municipal water supplies test regularly for these toxins, to make sure they don't exceed a 'safe' level.