Planting a Forest in SimCity will reduce ground pollution. To learn how to build a forest, see: Planting a Forest
It is important to reduce Ground Pollution in SimCity 5

There are two ways to clean Ground Pollution

1) Pollution will gradually go away on its own.

2) Plant lots of trees and plants near the area of the ground pollution. This will make the pollution go away faster. This is a good method of removing ground pollution

Also be sure to use a Sewage Treatment Plant to help with reducing the ground pollution

More tips to cleaning ground pollution

Trees and placing a water pump with a filtration system right on top of the pollution. Just make sure you remove the normal water pump and replace it with a filtration pump, or you will get pollution in your water.

About trees: Trees will disappear after the pollution is cleaned. Just replant until ground pollution gets better.