Leaking intake manifold gaskets on these cars are common. Just replace the intake manifold gasket and flush the radiator and block. Do not use Dex-Cool antifreeze anymore. It is a fact that the Dex-Cool reacts with the material the intake gaskets are made with. Check out this website full of compliants.
2002 Chevrolet Impala Leaking Coolant, Intake Manifold Gasket Failure | CarComplaints.com