Hello, its very hard to find an online (distance) PhD at all, but I would imagine it would be even harder in this field. You might need field or lab equipment that your host institution would provide, field assistance etc.
Whilst a lot of people I know in the Environmental Sciences might be away for large periods of time on fieldwork etc, they live and return to the host institution.
I know many circumstances mean a lot of people can't move, but it may be better to look at Universities within a commuting distance? I think everybody on here will tell you how important it is to be around a Department, academics and other students (all your best ideas will come by chatting to them at coffee break) and all the facilities a University provides.