Anything impure is contaminated or corrupted. Water that has sea monkeys swimming in it is impure. Don’t drink it!
Drinking water that is impure requires distillation or another method or purification to get rid of pollutants like bacteria; otherwise people might get sick from consuming it. Another way to use impure is to mean "spoiled by sin" or "lacking morals." In the fifteenth century, impure mainly described "muddy" wine, from the Latin impurus, "not clean, foul."