I bought a new refrigerator that makes ice cubes. The instructions said to discard the first 3 full bins of ice in order to ensure the new water line is flushed clean. I dumped the full bins in my stainless steel sink each time. After the ice melted each time a chalky residue was left in the sink. I emptied a couple of extra full bins and still was left with the chalky residue. I cleaned the sink after each dumping to be certain there wasn't any left over residue from the previous dump. What is the chalky substance? Is it something to be concerned about or not?