(a) If the concentration of a species is measured in mass per volume, what units are associated with the following parameters? (i) Concentration gradient (ii) Flux (b) What is the difference between the intrinsic permeability of a sand column and its hydraulic conductivity? (c) Typical molecular diffusivities are 0.1 cm2 s-1 in air and 10-5 cm2 s-1 in water. Explain why the diffusion coefficient is so much smaller in water than in air. (d) Molecular diffusion results from the random motion of molecules in