Smelly borewell water

Advice needed for smell in borewell water in Chennai

In the beginning, the borewell water showed a good ph of 8.5, but was greyish color as there was some grey sand getting mixed in it, so we put some pebbles to block the sand from getting in. But now, I am getting a sewage and chemical kind of smell from the water.

I'm really starting to get annoyed, advice is deeply appreciated. My bore well is currently 80 feet, had not used it for 5 years and just got it fixed. We have cleaned the new pump twice, and I have spent around 18K on it. Now the water is kind of whitish with a slight sewage like smell coming from it.