Effects of residence time and degree of water saturation on sorption nonequilibrium parameters

This study reports the impact of the degree of water saturation on sorption nonequilibrium parameters. Two nonionic organic compounds (benzene anti dimethylphthalate) and three nonaggregated sandy soils were utilized. Local equilibrium assumptions were found to be invalid for describing the transport of these compounds even at pore-water velocities as low as 0.7 cm/h. Sorption nonequilibrium appeared to be of a diffusive nature rather than due to a slow chemical reaction. Sorption mass-transfer coefficients varied proportionally with pore-water velocity. A strong correlation between the mass-transfer coefficient and residence time was found utilizing present and previously reported laboratory data. A similar relationship was also found for the mass-transfer coefficient between mobile and immobile water regions. Field data indicate that the sorption mass-transfer coefficient may continue to decrease in a consistent way even at residence times as large as 5 x 10(3) h. Variations in the degree of water saturation had no impact on the value of the sorption mass-transfer coefficient other than what would be expected due to changes in the residence time. This suggested that movement into the solid grains of the large emptied pores through diffusion from the water-filled pores into stagnant water covering these grains was relatively fast compared to the sorption rate. (C) 1999 Elsevier Science B.V. All rights reserved.