Keyword frequency restrictions are put forth by publishing sites quoting that, excessive use of keywords means articles are written for search engines and not for readers. But, how can a writer reduce the frequency of keyword "water" when he is writing an article on "water pollution"?
Actually we should try not to overuse the keywords. In water pollution, the author will be describing the reasons which are leading to water pollution for eg dumping of factory waste in rivers. So I think it is within our control how many times we use keywords