| Popis: |
To determine the underlying processes of solute transport in peat an experimental constructed fen peatland, soil hydraulic properties were measured and saturated and unsaturated solute breakthrough experiments were performed using Na+ and Cl− as reactive and non-reactive solutes, respectively. We tested the performanceof three solute transport models, including the classical equilibrium Convection-Dispersion Equation (CDE), a chemical non equilibrium one-site adsorption model (OSA) and a model to account for physical non-equilibrium, the mobile-immobile phases (MIM). The selection was motivated by the fact that the applicability of the MIM in peat soils finds a wide consensus. However, results from inverse modelling and a robust statistical evaluation of this peat provide evidence that the measured breakthrough of the conservative tracer, Cl− could be simulated well using the CDE. This is demonstrated by a very high Damköhler number (→infinity) suggesting instantaneous equilibration between the mobile and immobile phases; this underscores the redundancy of the MIM approach for this particular peat. Scanning electron microscope images of the peat show the typical multi-pore size distributions structure have been homogenised sufficiently by decomposition, such that physical non-equilibrium solute transport no longer governs the transport process. This is corroborated by the fact the soil hydraulic properties were adequately described using a unimodal van Genuchten-Mualem model between saturation and a pressure head of ~ −1000 cm of water. Hence, MIM is not the most suitable choice, and the long tailing of the Na+ breakthrough curve is caused by chemical non-equilibrium. Successful description was possible using the OSA model. To test our results for the unsaturated case, we conducted an unsaturated steady state evaporation experiment to drive Na+ and Cl− transport. Using the parameterised transport models from the saturated experiments, we could numerically simulate the unsaturated transport using Hydrus-1D. The simulation showed a good prediction of observed values, confirming the suitability of the parameters for use in a slightly unsaturated transport simulation. The findings improve the understanding of solute redistribution in the constructed fen. |
