Hope this answers your query:
bcm = bank cubic metre
tonne = bcm x material density. material density in this case is refered to as 'bulk density' since BCM is in-situ measurement
material density = tonnes per bcm
Therefore;
To convert, g/BCM to g/tonne; multiple BCM by density
ie g / BCM can be expressed as g/(bcm x density) = g/tonne
