In towns and cities, what is the major cause of pollution of drinking water sources?

The major source of pollution in drinking water sources is rainwater that flows into street catch basins (called urban runoff or stormwater runoff). While this rainwater alone is not necessarily harmful, it frequently carries untreated waste products from our streets and yards directly into rivers, streams, and lakes — our drinking water sources.
Other potential sources of drinking water pollution are products that are flushed through the sewer system, not removed by waste treatment, and then passed back into our surface water. They are still not removed after being run through municipal water treatment.