A mathematical model of the benthic boundary layer and the porous bottom is presented and explored. Emphasis is placed on the transition between the Ekman layer flow and the Darcian flow deep in the sediments. The predicted flow field in the porous region is verified against analytical solutions. The effective exchange coefficient for a substance with a large Schmidt number, for example, oxygen, is calculated taking different physical processes like turbulence, diffusion and dispersion into account. It is found that vertical transport may be considerably enhanced in a thin layer near the sediment/water system has been investigated for different porosities and consumption rates. ¿ American Geophysical Union 1991