When liquid paint is put in a landfill it could possibly leak into the ground water. You can prevent this by drying it out. Drying it outside the can is quicker.
A good method is to line a cardboard box with a heavy plastic garbage bag. Then fill the bottom of the bag with a layer of cat litter, oil dry or shredded newspaper. Then drizzle some of the paint on the absorbent layer and give it some time to dry. Repeat the next day as needed. After it is all dry, close and tie the bag and include it with your garbage.