Saturated and unsaturated salt transport in peat from a constructed fen

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.

Effect of anisotropy on solute transport in degraded fen peat soils

“…The ranges of all the calibrated parameters ( D , β and ω ) are within the span reported by Liu et al () for fen peat. The β values we found here generally smaller than those reported by Simhayov, Weber, and Price (), who estimated that the values of β was almost equal to one but with a great variance. One possible reason for the differences in parameter values may be that the fen peat soils in their study was less degraded (bulk density of 0.12 g cm −3 ) compared to soils in this study (bulk density of 0.20 g cm −3 ).…”

Microtopographical and hydrophysical controls on subsurface flow and solute transport: A continuous solute release experiment in a subarctic bog

“…Sodium chloride (NaCl) was used as the contaminant source. Whereas sodium (Na + ) is reactive, specific conductance ( SC ) provides a reasonable estimate of chloride (Cl − ) (McCarter & Price, ; McCarter, 2014, unpublished data; Simhayov, Weber, & Price, ), likely because Na + can cause the release of competitive cations, resulting in a solution with similar ionic strength.…”

Heterogeneity of the peat profile and its role in unsaturated sodium chloride rise at field and laboratory scales

“…However, its passage can be slowed when solutes diffuse from this mobile fluid into closed and dead-end pore spaces (Hoag & Price, 1997;McCarter & Price, 2017b;McCarter, Rezanezhad, Gharedaghloo, Price, & Van Cappellen, 2019;Rezanezhad et al, 2012;Ronkanen & Kløve, 2007) common in Sphagnum peat (Hayward & Clymo, 1982;Lewis, 1988). Reactive solutes such as cations may further accumulate as they adsorb to the peat (Simhayov et al, 2018;McCarter & Price, 2017b;Rezanezhad et al, 2012Rezanezhad et al, , 2016.…”

“…represent the layered heterogeneity of a natural peat profile that is so important to peatland hydrology (Waddington et al, 2015). Moreover, the peat used by Simhayov et al (2018) was sufficiently degraded that it did not exhibit the dual porosity transport behavior commonly reported for Sphagnumdominated peat (Weber, Iden, & Durner, 2017). Therefore, the purpose of this research is to better understand the potential for solutes, and in particular NaCl, in the saturated zone to migrate to the living, growing surface of Sphagnum peat profiles, based on their characteristic hydraulic structure and ambient conditions.…”

“…The movement and accumulation of solute above the water table in natural peat, at both laboratory and field scales, is not well documented. Solute breakthrough experiments have been conducted under saturated conditions (Hoag & Price, 1997; Ours, Siegel, & Glaser, 1997; Price & Woo, 1986; Rezanezhad et al., 2012); however, to our knowledge, only one transport experiment has been executed in an unsaturated peat column (Simhayov et al., 2018). That unsaturated transport experiment used homogenized, moderately decomposed fen peat with the large inclusions removed, so it does not represent the layered heterogeneity of a natural peat profile that is so important to peatland hydrology (Waddington et al., 2015).…”