If water, food, land, or a tool is contaminated, a harmful substance has already been added to it. Flood water in a well, food left in a dirty kitchen, soil near an old factory — all can be contaminated. It is the -ed form of contaminate: both the describing word and the past of the verb. Though actually, people also use it loosely, the way they use 'poisoned,' when they only mean 'I do not trust this,' with no test. The word feels scientific and serious. Dirty is the everyday cousin, but a dirty shirt is not the same as contaminated drinking water. Once food is contaminated, you usually throw it away; washing may not be enough. Courts talk about contaminated evidence when something has touched it that should not have, so you cannot trust the result.