I know several mechanical engineers working in environmental fields and one very senior person who started a biomedical program had a degree in mechanical engineering. So, I think mechanical engineering offers you an opportunity to go into either environmental or biomedical projects. You may find it better to get into fluid mechanics if your inclination is towards environmental or into mechanics of materials and advanced mechanics of materials to get into biomechanics. I personally am inclined to recommend mechanical engineering or electrical engineering over biomedical for an undergraduate for people wanting to do biomechanics or bioinstrumentation because mechanical and electrical cover the core that you can apply in any field of biomechanical. One question to ask yourself is how much you liked statics and mechanics of materials. That will help you decide fluid mechanics or solid mechanics (should that be the choice)