In Situ Measurements of Pore Water Diffusion Coefficients Using Tritiated Water

Abstract

In situ diffusion coefficients in the pore waters of Lake 227 were measured by monitoring the movement of tritiated water into the pore waters. The diffusion coefficients were determined by analytical and numerical modeling of the tritium profiles. During the summer, tritiated water diffuses into the sediment by molecular diffusion and this process is well approximated by a constant diffusivity model. The summer coefficients for diffusion were 1–2 × 10⁻⁵ cm²∙s⁻¹ at a water depth of 1.5 m (20 °C) and 0.3–0.8 × 10⁻⁶ cm²∙s⁻¹ at 8.75 m (4 °C). During fall overturn, clear evidence was found for enhanced mixing of pore waters to a sediment depth of 10 cm at water depths of 0.75 and 3.85 m. This enhanced mixing was not accompanied by mixing of the solid phase of the sediments.Key words: sediments, pore water, diffusion coefficients, tritium, lake sediments