Need remedy for silt problem in borewell water


A year ago I drilled a borewell using the bit drilling technique.

The contractor used a 10 inch outer pvc casing and we reached the rock around 20 to22 feet. The boring went on till 470 feet during this process strangely we found huge amounts of rock dust which spread all over the neighbourhood. I am not sure but it was strange as I had not seen this anywhere earlier.

After we found water it has silt and the contractor suggested we put casing which we did. My neighbours on either sides have good quality water with no silt complaint.

Till date after 1 year we are not able to use the water as it is dirty (muddy water is pumped at regular intervals of clean water) there is an oily layer on the surface of the water. The water smells weirdly  unpleasant. 

Kindly suggest a suitable remedy so that I can use the water