There are two ways to remove 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
Also be sure to use a Sewage Treatment Plant
More tips to clean 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.