A 1m clay wall separates two contaminants, A and B, dissolved in water. The diffusion coefficient of contaminant A is D*= 2.3 x 10^-9 m² /s. After 3 years the two contaminants meet in the clay wall (i.e. the concentration of both contaminants is 50% of their source concentration at the same point.) Neglecting seepage velocity across the wall, determine where in the wall the contaminants meet and the diffusion coefficient of contaminant B.