9Mass transfer coefficients of the water boundary layer (kw) were measured using alabaster dissolution 10 kinetics in a diffusion cell that was operated at stirring rates between 90 and 600 min -1 , aiming to 11 provide a more robust characterisation of the effect of hydrodynamics on the uptake of polar 12 compounds by passive samplers, as compared with characterisations in terms of stirring rates and water 13 flow velocities. The measured kw helped to quantitatively understand calcium sulphate transport through 14 a poly(ethersulfone) membrane and two water boundary layers (at both sides of the membrane). 15Alabaster based kw were used to understand atrazine transport in the diffusion cell, allowing the 16 conclusion that atrazine transport in the membrane is via the pore space, rather than via the polymer 17 matrix. The merits of measuring alabaster dissolution rates for passive sampler calibration and 18 application in the field is discussed. We propose that passive sampler calibrations be carried out under