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. Peat has a dual-porosity matrix with inter-connected pores that actively transmit water, and dead-end and closed pores formed by the remains of plant cells. The proportion of dead-end and closed pores increased at depth, where the state of decomposition of organic material is more advanced. These dead-end and closed pores act as a sink for solute. Breakthrough at C/Co = 0.5 occurred much later than the total active pore volume in the column, indicating that solute retardation occurred. This retardation was attributed to diffusion of the 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 higher 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 at lower flow velocities, and dispersion increased with depth. Peat effectively attenuated the conservative solute through matrix diffusion, and heterogeneity in peat properties influenced the effectiveness of this retardation.