Who is responsible for accommodations carried out in lab or field situations by a graduate teaching assistant?

This depends upon who has the designated responsibility for overseeing the lab and/or fieldwork. In some cases, the professor of the course has the responsibility to make sure teaching assistants are providing disability-related accommodations. In other cases, another professor or lab instructor is designated to oversee the lab and/or fieldwork. The academic unit and the university are accountable to ensure that reasonable accommodations are implemented.