National Water-Quality Assessment Program (NAWQA)
The National Water-Quality Assessment Program (NAWQA) provides an understanding of water-quality conditions; whether conditions are getting better or worse over time and how natural features and human activities affect those conditions.
Monitoring data are integrated with geographic information on hydrological characteristics, land use, and other landscape features in models to extend water-quality understanding to unmonitored areas. Local, state, tribal, and national stakeholders use NAWQA information to design and implement strategies for managing, protecting, and monitoring water resources in many different hydrologic and land-use settings across the U.S.
If you have any trouble accessing the website link above, please find here an archived page. You may find this has limited use. https://web.archive.org/web/20161219072645/http://water.usgs.gov/nawqa/