Diffusion plays an important role in the exchange processes between la
ke sediments and the overlying water. Compounds entering the sediments
usually have to penetrate through a certain mud thickness, where a re
action may then occur, while compounds being released have to diffuse
through the interstitial water before escaping into the lake water. In
this article mathematical functions are given which describe certain
cases. They are all based on the differential diffusion equation of Fi
ck, but for different boundary conditions different mathematical solut
ions of partial differential equations are needed. Four of these are p
resented in this paper, covering the cases of a compound diffusing int
o the sediments, with or without a chemical reaction occurring in the
sediments, combined with either a constant or a non-constant concentra
tion in the water. Furthermore a numerical approach is proposed in whi
ch the calculations are made by an iteration process over time and spa
ce. The results are presented as a series of concentrations as a funct
ion of time and depth in the sediment layer. It is shown in the first
place that the time and the space steps must be sufficient small in ag
reement with the dimensions and time scale of the processes studied in
order to obtain a satisfactory precision. The results can be fitted t
o a simplified exponential equation of the form (A . e(-alpha . tb) -
B), which can be used for a quick assessment of special cases. a depen
ds on porosity, ratio between water and mud height; b approximate to 0
.667. Furthermore this equation can be used to extrapolate from labora
tory experiments with the sediments of a specific lake to results vali
d for the lake itself. The numerical model has also been used to descr
ibe the backward diffusion of an eventual product of a chemical reacti
on, which will diffuse further downward, but also upward. This is e.g.
the case for the N2O production during denitrification experiments wh
en acetylene is applied as an inhibitor. Finally, the application of s
o called peepers and benthic chambers is discussed, while many of thei
r disadvantages are explained.