Sentence:  Recent studies by Weston and Stack had indicated that a turbine aerator could be added to an oxidation pond to increase the rate of <WSD>oxygen</WSD> transfer . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust