Diffusive boundary layer and its resistance on salt release from deposited sediments in a polder reservoir

The diffusive boundary layer (DBL) is a zone for exchanging matter across sediment-water interface between the mixture of solid sediment and interstitial water from an overlying body of water.The exchange of substances between deposited sediments and overlying water must also pass through the DBL.In this study,two series of flume experiments with and without wind blowing conditions were conducted to study the salt release process from sandy sediments in a polder reservoir.The thickness of DBL was directly measured.The experimental salt fluxes through the DBL under the two wind conditions were inter-compared and compared with the estimations using the Fick's first law in an effort to explore the role of DBL in contact inhibition of salt release.Results indicate that the observed salt fluxes under the two wind conditions show a marginal difference.Under the steady wind condition,a DBL with a constant thickness of 3.5 cm is observed right above the deposited sediment.The molecular diffusion is found to be the dominant mechanism for salt transport within the DBL.The latter can suppress the hydrodynamic enhancement on the exchange of substances between deposited sediments and overlying water.The thicker the DBL is,the stronger the inhibition will be.Thus,the presence of DBL in a polder reservoir can result in a delay of natural clearing process of polluted water.The effect of salt release from deposited sediments on the water quality will be a long lasting process.