There are many marshes surrounding the area where I live, and they are full of murky groundwater. Even droughts can’t seem to thoroughly dry up these swampy areas.
When the groundwater does get low, the marshes develop a green slime across the surface of the water. It parts a little after a good rainfall.
What’s cool about a marsh is that it gets replenished from both above and below. As it rains, it receives water, and since the groundwater level also rises, it discharges into the marsh from beneath the surface.