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For unfractured clayey and silty soils the primary transport mechanisms will be generally be advection and diffusion (i.e., advective-diffusive transport). The flux of mass, f, is obtained by adding the advective flux and the diffusive flux viz.:

 

f = n v c - n De dc/dz

 

and the total mass, m, transported from the landfill is given by:

 

md = A ʃ ( n v c - n De dc/dz ) dτ

 

where the parameters are the same as those defined previously. By convention if the velocity is positive the flow is out of the landfill, and if the velocity is negative the flow is into the landfill. The direction of transport for diffusion and advection can be in the same direction or in opposite directions. If the direction of diffusive transport is in the same direction as that of advective transport, then diffusion will increase the amount of contaminant transported and decrease the time taken for the contaminant to move to a given

point. Diffusion can also occur in the opposite direction to advection. For example, even if groundwater is flowing into a landfill, the high concentration of contaminant in the leachate can cause diffusive transport out from the landfill. Thus, even though the groundwater flow is into a landfill contaminants can still escape from the landfill by diffusion.