Leaking from under the handle
If your lav faucet is leaking from under the handle then there are 2 possible causes:
1. Cartridge securing nut is loose. Using a wrench, simply make sure part #5 from the following diagram is tight. F701-1 Parts breakdown
2. Calcium build up on cartridge seating. This can occur over time when hard water is present. To fix this remove parts 1-6, clean out cartridge seating, thoroughly flush out faucet, and reassemble parts 1-6.