A review of non‐equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality

Abstract

This review discusses the causes and consequences of 'non-equilibrium' water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger than c. 0.3 mm in equivalent cylindrical diameter allow rapid non-equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non-equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non-equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non-equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.

A review of model applications for structured soils: a) Water flow and tracer transport

“…A sensitivity analysis showed that flow variability in macropores at higher rainfall intensities markedly reduces water transfer and increases bypass flow. This might give an additional, field-scale explanation for the frequently observed phenomenon of the small transfer rate coefficient calibrated in DPM to simulate non-equilibrium PF and transport (see discussion in Jarvis, 2007). IN 3 M predicted water balance, water content variation, and dye coverage with depth as observed in irrigation and dye tracer experiments at three field sites reasonably well (Weiler, 2005).…”

“…Mass transfer under dynamic conditions, with changing directions to and from matrix, might even compensate some of the bias of the first-order term introduced for steady-state conditions, although this potential effect has not been analyzed yet. At least in view of other uncertainties, the inherent inaccuracy of the first-order term may be tolerable from a practical point of view (Jarvis, 2007). However, improved shape factors (see below, Section 4.1.3) and secondorder terms (Köhne et al, 2004a) were also tested.…”

“…Overviews of the relevant processes and conditions triggering PF in soils are discussed in the classic paper by Beven and Germann (1982) and in the recent review by Jarvis (2007) which covers the stateof-the-art in our understanding of PF and (agro-) chemical transport, and the effects of soil and crop management practices. The latter is also treated in another recent review (Ma et al, 2007).…”

“…The PF model concepts are only briefly explained here. For details, the reader is referred to recent review articles by Šimůnek et al (2003), Gerke (2006), Jarvis (2007), and Šimůnek and van Genuchten (2008). Here, the emphasis is on the applications of such models in describing experimental observations.…”

On the scope and management of pesticide pollution of Swedish groundwater resources: The Scanian example

“…Likely, fracture flow increases pollution vulnerability via an overall reduction in both travel time and natural remediationpotential relative to matrix flow, thereby allowing for comparatively rapid transfer of infiltrating water and pesticides. This proposal is theoretically valid (Freeze and Cherry 1979) and well-established regarding fracture and macropore flow, and associated pollutant transport in e.g., clay tills (Jorgensen et al 1998;Jarvis 2007). However, few studies comparing pesticide pollution in aquifers dominated by matrix flow to those dominated by fracture flow have been able to support such a hypothesis (Neil et al 1989;Barbash and Resek 1996).…”