I do not know who gave Cherry and Neal "thumbs down." They gave good answers. Whoever it is will probably give me a "thumbs down" too. :) That's fine, just don't give a decent answer a poor rating for spite. That is unreasonable. People like me will just come back and undo it. So, you are wasting your time. :)

...or "to pollute"

" to contaminate "

"Impurify" is hardly a common word, so I'm not surprised English speakers will give far better suggestions. I had a search, and "impurify" turns up only in the phrase, "...a Communist conspiracy to sap and impurify all of our precious bodily fluids" from the comedy movie Dr Strangelove: http://videosift.com/video/Dr-Strangelove-In-defense-of-our-precious-bodily-fluids (hope this site works in China, apparently Youtube doesn't work there?) This has been re-quoted often, so I suppose it's drifting into mainstream English. However it's always used satirically, and only in the phrase "sap and impurify (our precious bodily fluids)". The thing is, it's a nonsense phrase. The character who says this in the movie is insane. I also suspect "im+purify" was specifically constructed to make the phrase even more ridiculous. So I agree it's far better to use "pollute" or contaminate", eg "the rubbish pollutes the lake".

to impurify (third-person singular simple present impurifies, present participle impurifying, simple past and past participle impurified) Impurity or impurities (noun) is a the most common way you will find this word. In my opinion you should rephrase your sentence with impurities used as a noun. "there were impurities in the water"