verb

make (something) impure by exposure to or addition of a poisonous or polluting substance.

the site was found to be contaminated by radioactivity

pollute,adulterate,make impure,defile,debase,corrupt,taint,infect,blight,foul,spoil,soil,mar,impair,stain,befoul,sully,tarnish,poison,radioactivate,vitiate