1. Prove the water is dirty.
2. Prove the dirty water is dangerous.
3. Prove the city is responsible for making the water safe.
4. Contact the regulatory board responsible for overseeing water purification, show them your findings, and ask them to do their job.
5. If no response occurs, contact the media and tell them your story.
6. Use momentum from the media attention to build a campaign to have yourself elected into an oversight position.
7. Hire smart engineers to figure out why the water is dirty and how best to clean it.
8. Create a project plan to implement the engineers' recommendations.
9. Get your project plan approved.
10. Raise the funds required to execute your project plan.
11. Hire capable people to execute your plan and deal with anything that goes wrong.
12. Hire honorable people to oversee the execution of your plan.
13. ????
14. PROFIT

You would get it tested then go to your local government’s meetings and ask them to address it.