INFLUENCE OF RADIAL PRESSURE-GRADIENTS ON SOLUTE EXCHANGE IN STIRRED BENTHIC CHAMBERS

Bromide was used as a conservative tracer for evaluating the effect of stirring-induced radial pressure gradients in 2 typical benthic chamb er designs. One chamber was square, with side lengths of 30 cm (volume 9.0 1), and the other was cylindrical, with a diameter of 19 cm (volu me 2.5 1). It was demonstrated that radial pressure gradients associat ed with chamber water rotation could induce advective porewater transp ort in both chambers. The intensity of the advective porewater transpo rt was a function of sediment permeability and stirring rate. Stirred at 12 rpm, solute transport changed from diffusive to advective at a s ediment permeability >2x 10(-12) in the square chamber and >5 x 10(-12 ) m(-2) in the cylindrical chamber. The sediment permeability at which solute exchange was still controlled by diffusion could be increased by almost 1 order of magnitude in the square chamber by decreasing the stirring rate to 7 rpm. The sediment permeability values can be used as a guide to when benthic chamber experiments can be performed withou t introducing a serious stirring-induced artefact.