zagaditi

1. to pollute; to contaminate
   Fabrika je zagadila reku otpadnim vodama. — The factory polluted the river with wastewater.
   Požar na deponiji je zagadio vazduh u celom gradu. — The fire at the landfill polluted the air across the whole city.
2. (figuratively) to poison; to spoil
   Ta svađa je zagadila atmosferu u kancelariji. — That argument poisoned the atmosphere in the office.

The imperfective is zagađivati; the noun is zagađenje.