where the y-axis represents the contaminant concentration (ppm or mg/L) and x-axis displays the relatives contact area for mass transfer Mass transfer takes place from the contaminant-rich process stream to the water stream and the vertical concentration gap represents the driving force for this water-using operation Consequently as the two streams contact along the pathway of the operation the contaminant concentration in the process stream representing the operation must be greater than that in the water stream for mass transfer to occur