THE EFFECTS OF MATRIX DIFFUSION ON SOLUTE TRANSPORT AND RETARDATION IN UNDISTURBED PEAT IN LABORATORY COLUMNS

Experiments were performed to assess the nature of solute transport in peat by using step-inputs of a NaCl solution in laboratory columns of undisturbed peat. Feat has a dual-porosity matrix with inter-connecte d pores that actively transmit water, and dead-end and closed pores fo rmed by the remains of plant cells. The proportion of dead-end and clo sed pores increased at depth, where the state of decomposition of orga nic material is more advanced. These dead-end and closed pores act as a sink for solute. Breakthrough at C/C-0 = 0.5 occurred much later tha n the total active pore volume in the column, indicating that solute r etardation occurred. This retardation was attributed to diffusion of t he flowing solute into the closed and dead-end pores (matrix diffusion ). Greater retardation occurred at depth, increasing from 2.7 at 0.20 m to 7.3 at 0.62 m, corresponding to the greater volume of closed and dead-end pores there. Retardation was also velocity dependent, with hi gher velocity resulting in less retardation of solute since there was less time available for solute to be abstracted from the flowing water into closed pores. Matrix diffusion was shown to enhance dispersion a t lower flow velocities, and dispersion increased with depth. Feat eff ectively attenuated the conservative solute through matrix diffusion, and heterogeneity in peat properties influenced the effectiveness of t his retardation. (C) 1997 Elsevier Science B.V.