Simple diffusion models are developed to describe vertical diffusion in Powell Lake, a meromictic former fjord containing trapped seawater. Measurements of the vertical distribution of chlorinity are used to determine the dependence of vertical diffusivity on depth. Near the bottom of the lake the vertical diffusivity is about 4 times that of the molecular diffusivity of salt. On this basis, ions with different molecular diffusion coefficients will be lost from the bottom of the lake at different rates, which partially explains differences observed in the ratios of concentrations of ions in saline bottom water from ratios characteristic of seawater. The observed increase in temperature with depth is shown to result from the geothermal heat flux through the lake's bottom.