Abstract
Abstract. The underlying processes governing solute transport in peat from an experimentally constructed fen peatland were analyzed by performing saturated and unsaturated solute breakthrough experiments using Na+ and Cl− as reactive and non-reactive solutes, respectively. We tested the performance of 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 (MIM) phases. The selection was motivated by the fact that the applicability of the MIM in peat soils finds a wide consensus. However, results from inverse modeling 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. Furthermore, the very high Damköhler number (which approaches infinity) suggests instantaneous equilibration between the mobile and immobile phases underscoring the redundancy of the MIM approach for this particular peat. Scanning electron microscope images of the peat show the typical multi-pore size distribution structures have been homogenized sufficiently by decomposition, such that physical non-equilibrium solute transport no longer governs the transport process. This result 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 was not the most suitable choice, and the long tailing of the Na+ breakthrough curve was 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 parameterized transport models from the saturated experiments, we could numerically simulate the unsaturated transport using Hydrus-1-D. 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 and imply that MIM should not be automatically assumed for solute transport in peat but rather should be evidence based.
Highlights
A large pool of leachable Na, Ca and S was introduced in the tailings sand used to construct a fen watershed as part of a novel attempt at landscape reclamation in the oil sands region (Simhayov et al, 2017)
We tested the performance of 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 (MIM) phases
The dissimilarity of the electrical conductivity (EC) breakthrough curve to that of Na+ (Fig. 4) demonstrates the limitation of using EC electrodes as an indicator for solutions containing reactive solutes, flowing through reactive mediums. This limitation is due to enrichment of ions in the solution from the soil and cation exchange with the medium, which changes the solution concentration of the cation of interest; EC can be a good estimator for non-reactive solutes but is limited as an indicator for cation transport (Olsen et al, 2000; Vogeler et al, 2000)
Summary
A large pool of leachable Na, Ca and S was introduced in the tailings sand used to construct a fen watershed as part of a novel attempt at landscape reclamation in the oil sands region (Simhayov et al, 2017). In Sphagnum-derived peat, hyaline cells and their skeletal remnants are thought to account for a large fraction of dead-end pores with distinct pore size density distributions (Weber et al, 2017a, b) and a volumetric moisture content (VMC) between 10 and 20 % (Hayward and Clymo, 1982; Weber et al, 2017a, b) This physical structure, along with surface adsorption of reactive solutes (Rezanezhad et al, 2012, 2016) will lead to delayed arrival of solutes which are likely to affect vegetation community development in oil sands reclamation landscapes. In constructed peatlands, the decomposition gradient is destroyed because of the disruption caused by draining, stripping, transport and placement (Nwaishi et al, 2015)
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