Correct way to close borewells

I am working for an NGO called 'Water Harvest' in the bioregion of Auroville, Tamil Nadu. We have an ongoing project about closing a 100 or more borewells drilled up to 150 metres depth. My question is: How do we close the borewells correctly and prevent intrusions of surface water and other sources of pollution?

Any help is appreciated!


Jos Eisberg