The environmentally friendly way to dispose of paint is not to throw it in a landfill. You would take some old cardboard and use the old paint to paint the cardboard until it is gone. Then once it is dried you can throw the cardboard and your paint cans in the garbage. It’s the liquid that will seep into our drinking water and is toxic. By getting rid of the extra paint and throwing it away dry you prevent it getting to our water supply.