NOTE ON COMMON MIXING CELL MODELS

Different types of mixing cell models are analysed in detail. One type is found to be second-order accurate when reactions are not considere d but reduces to first-order accuracy when a sorption reaction is take n into account. The second type is only first-order accurate even when a reaction is not considered. The most commonly used mixing cell mode l is extended to account for different surface boundary conditions, in cluding that for a landfill. The exit conditions for a finite spatial domain were taken into consideration by making use of boundary layer c orrections. It is found that the results obtained using the mixing cel l model coupled with a non-linear isotherm are very accurate when comp ared with results obtained from a Crank-Nicolson scheme. It was observ ed that the accuracy of the results is dependent on the spatial step s ize. Best results are obtained when the spatial step size is close to twice the dispersivity. Application of the mixing cell model with a bo undary layer correction was demonstrated by comparison with experiment al data from a laboratory Na-Ca exchange experiment in which a solutio n containing Na was passed through a column filled with Ca-saturated l oamy sand. The mixing cell model was also applied to breakthrough curv es of two organic compounds (carbontetrachloride and tetrachloroethyle ne) observed in a field experiment conducted at Borden, Ontario, Canad a. The mixing cell model described satisfactorily the Na transport in loamy sand and also the transport of organics in a Borden aquifer.