It definitely depends on what you want to do. Now obviously, chemical engineering and environmental engineering go hand in hand because chemistry can be used to make things more environment friendly, and efficient. But chemistry has several other applications too, but you need to be interested in those applications. So definitely, your thinking is right, but it won't be useful if you have no interest in the field. Make an informed decision, consider the other subjects of Post graduation also, before deciding to do PG in Environmental Engineering. All the best!

Hello aspirant,
Both the courses you talking aboutgo side-by-side,so its upto you,but as a suggestion if you are good at field currently you can apply for Gate for M.tech and then Ph.D if you have an interest in field of research.
Rest it depends on your interest.
Best of luck.