The hypothesis that a sample of river water taken upstream is less polluted than a sample of river water taken downstream is proven to be true.
Try testing the quality of the water in lakes and ponds located at different distances from urbanized/densely populated areas.
Water pollution - http://en.wikipedia.org/wiki/Water_pollution
Common water measurements - http://ga.water.usgs.gov/edu/characteristics.html
Earth’s water- rivers and sediment - http://ga.water.usgs.gov/edu/earthriverssed.html
Basic safety requirements