In very turbulent systems like shallow lakes and coastal waters with extensive contact between water and sediments with a high sorption capacity, the mass transfer across the sediment-water interface can be approached by a diffusion-advection term and an equilibrium term:
where C is the element concentration in the porewater, Ce is the equilibrium concentration, K is the sorption/ desorption rate, D is the effective diffusion coefficient, U is the advective transport of pore water, and z is the depth in the sediment. The steady-state solution gives
and if the seepage is neglected (U> 10~7ms_1) the equation is reduced to
Ifwe assume a film model for diffusive transport and ifthe flux at z = o is calculated, we can derive at a mass transfer KL equation:
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