The All terms are in units O2 mass transfer is the O2 gas, and the sink is flowing water. The model development requires three primary assumptions : 1 the process is at steady state 2 the concentration profile of interest is along the in z-direction only and represents the local bulk concentration and fluid properties, 3 there is no reaction of O2 in the water, and 4 the process is dilute with respect to dissolved O2. Under these assumtions, the differential material balance is