SIMULATION OF SIMAZINE TRANSPORT THROUGH SOIL COLUMNS USING TIME-DEPENDENT SORPTION DATA MEASURED UNDER FLOW CONDITIONS

In a single-grained and extremely sandy (98% sand) soil asymmetrical b reakthrough curves (BTC's) of simazine, with earlier breakthrough and a delayed approach to equilibrium relative concentration (generally re ferred as tailing) were obtained at varying pore-water velocities usin g laboratory columns. These BTC's were successfully described only whe n the bicontinuum approach of two sites/two regions was used with the convective dispersive equation. The models using this approach need tw o additional parameters (namely fraction of type-1 sites or mobile pha se f, and the rate parameter for type-2 sites or immobile phase alpha) and these are generally obtained by curve-fitting procedures. However , considering the importance of independent measurement of input param eters for the numerical simulation of these BTC's. the rate parameters describing the time-dependent behaviour of simazine sorption were sou ght from a separate experiment performed under realistic (flow instead of batch) conditions. In this experiment sorption values were calcula ted from retardation factors of pulse BTC's as affected by pore-water velocities. The sorption of simazine under flow conditions was found t o be much slower than occurs under the shaking employed in the batch m ethod. Furthermore it followed a linear relation with square root of t ime. The sorption vs. square root time relation provided one of the tw o parameters (the fraction of type-1 sites) needed for simulation. Con sistent with numerous reports in the literature, the rate parameter fo r type-2 sites (or mass-transfer coefficient for immobile region alpha ) was found to be dependent on pore-water velocity. Furthermore, the f raction of type-1 sites (or fraction of mobile phase) was also found t o vary with pore-water velocity, which has not commonly been observed. Since the organic matter in the soil used here was the predominant so urce of sorption sites, intra-organic matter diffusion may have been t he most likely cause of nonequilibrium conditions during simazine tran sport. The independent measurement of the rate parameter alpha could n ot be accomplished. However, the alpha value from this study as well a s those from a number of published studies were found to have essentia lly similar relationship with pore-water velocity.