Solute Transport through An Argiudol of the “Argentinian Humid Pampas”.

ABSTRACT

Solutes dispersion in soil has received considerable attention because of the concern about ground water pollution by agricultural chemicals. We studied solute dispersion by applying pulses of bromide (Br) tracer at the surface of Mar del Plata fine illitic loam soil during constant, unsaturated water infiltration at a flow rate of 11 mm h^-1. Water was applied, using a net of micro-sprinklers over the surface of 12 plots of 30 m². Ceramic soil–water suction samplers were installed at each plot at three depths: 1, 1.5 and 2m. Rain gauges were placed at the same position where the ceramic cups were installed to measure the water flow rate. Solute concentrations measured at the determined depths and times allowed estimation of average pore-water velocity and dispersion (D). The Br^- concentrations were fitted to the steady-sate convection-dispersion analytical model en the CXTFIT package. The bromide breakthrough curves (BTCs) were analyzed with the classical equilibrium convection–dispersion equation CDE and a non-equilibrium two-region mobile–immobile model. Pore water velocity determined with CDE for equilibrium at 1, 1.5 and 2m, were 6.5, 8.7, and 7.8 respectively and D were 413, 797, and 647. Pore water velocity determined with CDE for non-equilibrium at 1, 1.5 and 2m, were 1.6, 2.7, and 4.7 respectively and D were 413, 797, and 647. The relative fractions of mobile and immobile water (b) for non-equilibrium at 1, 1.5 and 2m were 0.22, 0.33, and 0.31 respectively. The non-equilibrium CD model proved to be better than the equilibrium in interpreting asymmetric BTCs. The lower values of b are an evidence of the degree of preferential flow for this soil.