Study of U.S. oil refineries ranks Chevron El Segundo as worst emitter of two water pollutants
The Los Angeles County refinery was ranked as the top emitter of nitrogen and selenium among 81 oil facilities in 2021, a recent study found. 
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### Instruction: Study of U.S. oil refineries ranks Chevron El Segundo as worst emitter of two water pollutants

### Response: The Los Angeles County refinery was ranked as the top emitter of nitrogen and selenium among 81 oil facilities in 2021, a recent study found. 