Environmental investigations requires several different roles: management, data collection, health and safety, quality control, etc. The division of roles, how much they overlap, and the level of expertise/cost of each person will vary. But you always have a field manager who's in charge in the field and a project manager (PM) who's in charge of the entire project. You also have an in-field safety lead.
The safety person is the person focused on making sure everything is safe, and is the point person for all emergencies. Large investigations (multiple crews, lots of people, high-risk work) may have a dedicated safety person, but this is rare. In my experience, the safety person can be anyone from the organization running the work who is in the field, and if they leave for any reason, the role is rotated to someone else physically on-site. The best practice is to not use the field manager as the safety person, because the field manager is primarily focused on doing the work under the direction of the PM. The safety person may report to the PM, but if there's any question of safety that isn't adequately addressed by the field chain of command, they ultimately work for corporate health and safety, and not the project. Of course, if I'm the only field person from my institution, I wear all the hats. If there's two of us, one is the field manager and one is the safety manager.
Field management is often a stepping stone to overall project management. Lower-level PMs often have small, uncomplicated sites that they also do the fieldwork for. It makes sense - if you've been running the fieldwork, it's easy enough to continue doing so even after you've had other responsibilities added. And it's really easy to coordinate your own fieldwork, data collection, analysis, report writing, etc. The problem is that, as PM, it's a little too easy to adjust things in the field based on your understanding of the client and what they want. Or because you know the budget is a little tight right now...
Far better, from a technical and legal standpoint, to have separate roles for the different types of managers.