Preferential flow and solute transport in a large lysimeter, under controlled boundary conditions.
Introduction
Leaching of surface-applied fertilizers and pesticides beyond the root zone is of interest to agronomists, as they reduce the efficiency of the applied agrochemicals. The quality of the underlying groundwater is also of concern. Many studies of the transport of solutes through repacked laboratory soil columns were performed (Brigham, 1974; Gaudet et al., 1977; Kookana et al., 1993; Krzyszowska et al., 1994). However, these studies do not take into account the spatial variability of the soil properties, because of soil repacking and small column sizes. As a matter of fact, processes such as preferential flow might have a great influence on solute transport. However, experiments were run on large intact soil monoliths (Saffigna et al., 1977; Bergström, 1990; ECPA, 1974, but mostly submitted to natural (i.e. transient) boundary conditions for water fluxes, and to complex boundary conditions for solutes, e.g. solute formulation method (Ghodrati and Jury, 1992; Wietersen et al., 1995). However, it is difficult to use these experiments in predictive models of solute transport.
A few authors adopted an intermediate approach (Bowman et al., 1994; Phillips et al., 1995; Poletika et al., 1995). Using undisturbed soil blocks, larger than usual laboratory columns (lengths and widths between 30 and 80 cm), submitted to controlled boundary conditions for water (constant flux) and solutes (pulses or on-tine application). These studies, however were carried out with high water fluxes (5–20 mm.h−1) and without interactive solutes (except for Poletika et al., 1995). According to the soil types, these lysimeter sizes might be too low to depict preferential flow correctly (Netto et al., 1998). The aim of this article is therefore to use a larger undisturbed lysimeter. Lower water fluxes (closer to natural conditions) use simultaneously different types of tracers and reactive solutes. The scale of the lysimeter used here is the same as for a classical monitoring site in the field, equipped with a neutron probe access tube, tensiometers and soil solution samplers (suction cups) at different depths (Kengni et al., 1994).
Section snippets
The soil
The study was conducted on the Experimental Farm of the Lycee Agricole, La Cote Saint Andre, located 40 km northwest of Grenoble, France, The site is a typical glacial terrace, with a soil approximately 1 m thick overlying a layer of gravel and stones of high permeability of 10–20 m thickness. There is a water-table aquifer, which varies in depth between 9 and 15 m from the soil surface. It has a high sensitivity to nitrate pollution. Large number of wells reach a nitrate concentration close to or
Neutron probes
Volumetric water content was measured at depths between 10 and 120 cm by neutron probes, using 10 cm increments. Calibration was carried out according to Kengni (1993). Mean volumetric water contents were θ=0.253 for #1B, and θ=0.242 for #2.
Soil samples
Volumetric water contents form soil samples (#2), averaged in each layer, are compared to neutron probe water contents in Fig. 3. Water content values averaged on the 6 central sections (greyed in Fig. 2) are also shown. Because they are expected to be closer
Conclusions
The lysimeter used under controlled boundary conditions allows accurate quantification of transport processes for tracers and interactive/reactive solutes, through elution curve analysis and soil sampling. Such an accuracy is seldom reached at scales larger than laboratory soil columns. As a result of compaction problems near the edges, comparison with soil solution samplers (suction cups) was not good.
The moment method gave a preliminary estimation of the hydrodynamic and (bio) geochemical
Acknowledgements
The authors wish to thank the Lycée Agricole and the experimental farm of La Côte Saint André for technical support, the Service Central d’Ánalyses for chemical analyses. The Physiologie Cellulaire et Végétale laboratory of the Université Joseph Fourier, Grenoble, for immunoassay analyses of atrazine, the Section d’Application des Traceurs (CEA/DAMRI/SAR/SAT, Grenoble) for technical support, Michel Sardin for fruitful discussions and David E. Elrick for careful reading of the manuscript.
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