Go to Australia for graduate study, and study environmental engineering or chemical engineering.

1) If you don't aim at immigration, I suggest you continue to study environmental science. In this respect, the level of science, technology and education abroad is much higher than that at home. If you are lucky enough to find a job in Australia in the future, you can decide which occupation to nominate and evaluate according to the nature of your job (if you find a job related to chemical engineering, you can apply for professional evaluation of chemical engineers, and I have also seen examples where environmental scientists engaged in water supply and drainage work successfully applied for immigration by nominating "water supply and drainage workers"), so as to maintain the consistency of knowledge and be conducive to returning to China in the future.

2) If you really want to emigrate, study chemical engineering. It's easy to find a chemical-related job in Australia after graduation. You can nominate a "chemical engineer" with a bachelor's degree, and then you can immigrate.