Similarity of whole‐sediment molecular diffusion coefficients in freshwater sediments of low and high porosity

Abstract

Whole‐sediment molecular difusion coefficients ( D s ) for tritiated water in pore waters of various lakes were determined experimentally by adding 3 H 2 O 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 transport and on the predictability of D s . We found that D o /D s did not change as much as expected with increasing porosity, i.e. in low‐porosity sediments the average D o /D s was 1.8±0.1 and in high‐porosity sediments it was 1.5±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 result means that in most freshwater sediments, the sediment diffusion coefficient can be predicted reliably from the molecular diffusion coefficient at in situ temperature.