How do wetlands filter water?

Question

I'm doing an experiment with my class given to me by the state curriculum--we put celery stalks in water that has been colored with food coloring and you can see how the stalks absorb the blue. But I'm wondering (and the curriculum does not tell me) how the pollutants remain in the stalk but the water comes out clean. The fact that the stalk absorbs the blue implies it also absorbs the water...am I making any sense?

Answer 1

http://www.thirteen.org/edonline/wue/wat...

here is a wetlands project here that may help you. It tells you how it filters the water.