Similarity of Whole-Sediment Molecular Diffusion Coefficients in Freshwater Sediments of Low and High Porosity

Whole-sediment molecular diffusion coefficients $(D_s)$ for tritiated water in pore waters of various lakes were determined experimentally by adding $^3H_2O$ to the overlying water of asphyxiated (without bioirrigation) and unasphyxiated cores and measuring the resulting pore-water profiles after a period of time. Our objectives were to determine the relationship between $D_s$ and $D_o$ (the diffusion coefficient in pure water) in sediments with a wide range of porosities and organic contents and to examine the influence of bioirrigation on solute trasnport and on the predictability of $D_s$. We found that $D_o/D_s$ did not change as much as expected with increasing orosity, i.e. in low-proposity sediments average $D_o/D_s$ was $1.8 !pm 0.1$ and in high-porosity sediments it was $1.5 !pm 0.2$. We also found that the effect of faunal activity on the predictability of $D_s$ was only significant in sediments with high (14,000 ind. $m^-2$) invertebrate populations. This results means that in most freshwater sediments, the sediment diffusion coefficient can be predicted reliably from the molecular diffusion coefficient at in situ temperature.