Thanks for writing. Without being able to see your geraniums in person, I can't say why their leaves are dying.
I can tell you that geraniums do best in a hot, sunny spot with well-drained soil. If you grow them in the ground, let the top inch or so of the soil start to dry out before watering them again.
Also: If you can, use a soaker hose or drip irrigation system to water. Getting the leaves wet encourages some types of fungal diseases. These diseases can cause leaves to die early.
---Justin, Senior Garden Editor, BHG.com