Column tracer studies in surface and subsurface horizons of two typic argiudolls

Soil properties that vary in space and/or time within a soil profile introduce heterogeneity to solute transport. The objectives of this work were to (i) study transport of bromide through the horizons of two soils under no-tillage but differing in soil structure and organic carbon content, and (ii) determine the nature of the process by fitting breakthrough curves (BTC) with a convection-dispersion model assuming equilibrium (CDEeq) and nonequilibrium (CDEneq). Three sets of undisturbed soil cores were collected from the A, B, and C horizons of two soils and used to independently measure BTC of bromide, water retention (Tempe cells and pressure extractors), and saturated hydraulic conductivity (constant head). Data were fitted with the deterministic models CDEeq and CDEneq as implemented in CXTFIT version 2.1 and used to estimate pore water velocity (v) and hydrodynamic dispersion (D). The shape of BTC of horizons A and B exhibited tailing, asymmetry, and early appearance of the tracer, but there was no statistical difference between the goodness-of-fit of the CDEeq and CDEneq models. Values of v, D, and dispersivity (lambda), indicated that flow in the A and B horizons were dominated by convection and dispersion, whereas the prevalent transport mechanism in C horizons was convection. Values of X were significantly correlated (r = 0.83; P < 0.05) to clay content mainly because of the correlation between clay and the dispersion coefficients D estimated with either model. Transport properties of soil horizons should be considered when modeling nonreactive solute transport for assessing risks of groundwater contamination.