Entry of sewage into an open well - Need advice regarding prevention and treatment

I need your help in solving a problem related to groundwater (Openwell).

Ground water (Well) is being contaminated with Sewage water (mostly) and we are unable to trace from where the Sewage is seeping in.

I have 2 questions;

1. How to identify the source and take some preventive action.
2. Is there an appliance which would treat this water and make it safe for use.

Do let me know about the details of both options including the pricing,

Narendra B.V