Need a solution to prevent seepage of dirty water into groundwater


We have dug a borewell which is 175 feet deep. The water is available at 60 feet and a pump is fit at 165ft. The borewell has a casing for 30ft.

For some strange reason, the dirty water at the surface is seeping into groundwater making it unusable for household purposes and gives a dirty smell too. We have tried cleaning the borewell but didn't help much. The problem was resolved for a few days but we again faced the same issue.

Can anyone guide us by giving a permanent solution please?