Colloid Mobilization and Transport in Undisturbed Soil Columns. I. Pore Structure Characterization and Tritium Transport

Abstract

While it is recognized that preferential flow may increase the transport of colloids, less is known about the actual influence of preferential flow on colloid mobilization in situ. Changes in pore structure upon soil exposure to drying and rewetting may additionally affect colloid mobilization. Information about the pore structure and the active flow volume, as well as the changes in these properties, are therefore important when investigating colloid mobilization. We investigate the pore structure characteristics and the transport of tritium (3H2O) during steady unsaturated flow conditions. A total of 54 soil columns sampled along a natural clay gradient representing six clay contents (12, 18, 24, 28, 37, and 43% clay) were equilibrated to three different initial matric potentials (IMP), ψ = −2.5, −100, and −15500 hPa. Pore structure characteristics were deduced from water retention characteristics and measurements of air‐filled porosity and air permeability. Tracer experiments were conducted at 1 mm h−1 and with a suction of 5 hPa. A mobile–immobile region model (MIM) and a three‐region model (2MIM) with two mobile and one immobile region were used for describing the breakthrough curves (BTCs). The 2MIM model was able to fit the data well and predicted the existence of two mobile flow regions, most pronounced at higher clay content. The 12% clay soil exhibited matrix‐dominated flow behavior, which is probably attributable to a large fraction of drained pores disconnecting the rapidly conducting flow system. Soils with ≥18% clay exhibited asymmetrical BTCs with early breakthrough and tailing and an increasing amount of immobile water, indicating preferential flow. Drying and rewetting, because of associated changes in the pore structure, significantly reduced the degree of preferential flow.

Effect of water content on aggregation and contaminant leaching: the study of an urban Technosol

“…Contrary to heavy metals, leached DOC composed a major fraction of the TOC, as observed previously by several authors (Laegdsmand et al 1999;Kjaergaard et al 2004b). The huge amount of organic carbon leached from the May sample must be related first to the huge release of particles and secondly to DOC.…”

“…Fewer particles were mobilized as the Technosol material dried. Regarding colloids, Kjaergaard et al (2004b) suggested that drying causes a decrease of colloids as a result of inter-particle bonding and cementation of colloids. The explanation given by Klitzke and Lang (2007) is that the decreasing amounts of mobilized particles during drying are caused by three processes: enhanced aggregation, disintegration of organomineral complexes (break-up of weak bonds), and the hydrophobization of colloids.…”

Lessivage as a major process of soil formation: A revisitation of existing data

“…These experiments simulated eluviation. Only 7 of these studies, which summarise 119 different experiments, could be found in the literature (Jacobsen et al, 1997;Kjaergaard et al, 2004aKjaergaard et al, , 2004bLaegdsmand et al, 2005;Schelde et al, 2002;Shang et al, 2008;Sharma et al, 2008 andZhuang et al, 2007). In these studies, average particle concentrations at the outlet of the soil column ranged from 0.3 mg L -1 to 263.2 mg L -1 , with peak concentrations ranging from 0.9 mg L -1 to 1200 mg L -1 .…”

Phosphorus mobilization in rewetted peat and sand at variable flow rate and redox regimes

“…Water and solute transport in soils is often characterized by nonequilibrium flow phenomena, where solutes are conducted relatively rapid through a small fraction of the pore volume, accompanied by diffusive mass transfer of solute between the mobile and immobile regions (e.g. Gardner and Brooks, 1956;Kjaergaard et al, 2004a;Seyfried and Rao, 1987). Such preferential flow of water and solutes is not restricted to structured soils, but also occur in structureless sandy soils (e.g.…”