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

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.