Hydrogeology is the study of water in the ground. When it rains, some of the water soaks into the ground, and that water is called groundwater. Hydrogeologists study where the groundwater goes and how it moves through the rocks and dirt underground.
Think of a sponge as the ground. When you pour water onto the sponge, it soaks in and gets trapped between the sponge's holes. The ground is like a sponge, and the water is soaking into the ground, and there are different layers in the ground called aquifers, which are like the different layers of a cake.
Hydrogeologists help us find water that is safe to drink and use for things like watering our plants, swimming in a pond or lake, or washing things. They do this by testing the water to see if it is clean and safe to use, and by looking for places in the ground where the water can be found.
In summary, hydrogeology is like being a detective who looks for water underground, and ensures that it is clean and safe to drink or use.