Preferential flow and solute transport in a large lysimeter, under controlled boundary conditions

Laboratory studies of solute transport in soils (soil columns) are not totally representative of field conditions (spatial variability, soil structure etc.). Field studies hardly allow quantification of fluxes and mechanisms. In this article, and intermediate approach is suggested, using a lysimeter (1.7 m3) of an almost undisturbed soil, with controlled boundary conditions, the aim being to be able to quantify fluxes and mechanisms at a scale closer to field conditions, thus yielding results that better depict reality. Two experiments, with constant water fluxes of 1.05 and 1.48 mm h(-1) were conducted. Solutes were introduced as concentration pulses. Species (2)H2O, Cl(-) and Br(-) were used as tracers, and K(+), NH4(+), NO3(-), atrazine as interactive and/or reactive solutes. Elution curves were analyzed by the method of moments. Results show that about 20% of the water are immobile. As a consequence of anion exclusion, anion tracers appear at the outlet with an advance of about 10% in time as compared to isotopic tracers. The added NH4(+) is mostly nitrified, and K(+) undergoes cation exchange with Ca2(+) and Mg2(+). Under our experimental conditions, leaching of atrazine is significant with low degradation. A third experiment was conducted, in which the flow was interrupted while the solute peak was within the lysimeter, and 400 soil samples were extracted from the lysimeter. Soil-water content distributions exhibit coefficients of variation within layers between 5% and 27%. Concentration distributions exhibit coefficients of variation within layers between 22% and 59%. There is no correlation between concentration and water content. The observed spatial variability suggests the occurrence of preferential flow. Concentrations in suction cups were 55%-136% of those measured in corresponding soil samples.