In this day and age we use so many chemicals such as bug sprays, perfumes, the strong paints for houses, bleach and other household solvents all these must be disposed off responsibly to avoid contaminating the water supply. 

Nowadays companies sell organic and biodegradable cleaners so you can shop for these less harmful products to ensure no risk of any contamination. 

Things like fat and grease must never be poured into the drain not only will it clog them but it can get into the water supply and oil never dissolves in water. 

Dispose of medical waste properly or the chemicals can and will infiltrate the ground to things like the groundwater sources.

Try to stop your vehicle from leaking oil and other fluids also volunteer to clean up at beaches and river sides to reduce even a little of the surface pollution. 

And finally Support some local  environmental charities which work towards clearing environmental pollution especially water pollution in your local supply.