Transport of chloride and deuterated water in peat: The role of anion exclusion, diffusion, and anion adsorption in a dual porosity organic media

C P R McCarter; F Rezanezhad; B Gharedaghloo; J S Price; P Van Cappellen

doi:10.1016/j.jconhyd.2019.103497

Transport of chloride and deuterated water in peat: The role of anion exclusion, diffusion, and anion adsorption in a dual porosity organic media

J Contam Hydrol. 2019 Aug:225:103497. doi: 10.1016/j.jconhyd.2019.103497. Epub 2019 May 10.

[Article in French]

Authors

C P R McCarter¹, F Rezanezhad², B Gharedaghloo³, J S Price³, P Van Cappellen²

Affiliations

¹ Ecohydrology Research Group, Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave West, Waterloo, ON N2L 3G1, Canada. Electronic address: colin.mccarter@utoronto.ca.
² Ecohydrology Research Group, Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave West, Waterloo, ON N2L 3G1, Canada.
³ Department of Geography and Environmental Management, University of Waterloo, 200 University Ave West, Waterloo, ON N2L 3G1, Canada.

PMID: 31102982
DOI: 10.1016/j.jconhyd.2019.103497

Abstract

The dual-porosity structure of peat and the extremely high organic matter content give rise to a complex medium that typically generates prolonged tailing and early 50% concentration breakthrough in the breakthrough curves (BTCs) of chloride (Cl^-) and other anions. Untangling whether these observations are due to rate-limited (physical) diffusion into inactive pores, (chemical) adsorption or anion exclusion remains a critical question in peat hydrogeochemistry. This study aimed to elucidate whether Cl^- is truly conservative in peat, as usually assumed, and whether the prolonged tailing and early 50% concentration breakthrough of Cl^- observed is due to diffusion, adsorption, anion exclusion or a combination of all three. The mobile-immobile (MiM) dual-porosity model was fit to BTCs of Cl^- and deuterated water measured on undisturbed cores of the same peat soils, and equilibrium Cl^- adsorption batch experiments were conducted. Adsorption of Cl^- to undecomposed and decomposed peat samples in batch experiments followed Freundlich isotherms but did not exhibit any trends with the degree of peat decomposition and sorption became negligible below aqueous Cl^- concentrations of ~310 mg L^-1. The dispersivity determined by fitting the Cl^- BTCs whether assuming adsorption or no adsorption were significantly different than determined by the deuterated water (p < .0001). However, no statistical differences in dispersivity (p = .27) or immobile water content (p = .97) was observed between deuterated water and Cl^- when accounting for anion exclusion. A higher degree of decomposition significantly increased anion exclusion (p < .0001) but did not influence the diffusion of either tracer into the immobile porosity. Contrary to previous assumptions, Cl^- is not truly conservative in peat due to anion exclusion, and adsorption at higher aqueous concentrations, but the overall effect of anion exclusion on transport is likely minimal.

Keywords: Breakthrough curve; Conservative tracer; Equilibrium adsorption; Pore structure; Solute transport.

MeSH terms

Adsorption
Diffusion
Porosity
Soil*
Water Movements*

Substances

Soil