Pore-water convection induced by peeper emplacement in saline sediment

"Peepers" (dialysis samplers) rely on exchange across a dialysis membrane b etween fluid-filled cells and the surrounding sediment in order to sample t he pore water at various depths. In the current study, we show that under s ome realistic circumstances, where there is a large density difference betw een the pore water and the cell fluid in permeable sediments, the resulting convective motion in the pore water distorts concentration distributions n ear the peeper. Peepers withdrawn prematurely in these conditions will cont ain seriously misrepresentative samples. We present numerical model and lab oratory experiment results that show that the equilibration dynamics are co mplex and that the time required for equilibration may be far in excess of the duration predicted by a simple molecular diffusion model.