Diffusion and accumulation of chloride and sodium in Lake Ontario sediment

Distribution of Na⁺ and Cl^- in the upper 30 cm of Lake Ontario sediment cores indicates that the two species are being transferred from the lake water into the sediment. The likely cause of the transfer is the strong increase in their concentrations in the lake during the 20th century. The concentration data are compatible with diffusional transport models which lead to the following estimated values of the diffusion coefficients in the sediment-pore water column: D_Cl = 2 × 10^-5 - 3 × 10^-5 cm² · sec^-1 and D_Na = 3 × 10^-6 - 4 × 10^-6 cm² · sec^-1. The diffusion coefficient of Na⁺ in pore water, corrected for the uptake of Na⁺ by the sediment, is D′_Na = 9 × 10^-6 - 1.2 × 10^-5 cm² · sec^-1. The total amount of each species which has entered and accumulated in the sediment-pore water column since the concentrations in the lake began to rise is, for Cl^-, 2.96 mg·cm^-2 and, for Na⁺, 1.31 mg·cm^-2. The amounts accumulated in the sediment are on the order of only 1-2% of the amounts of the two species now present in the lake water.