Rate-limited transport of hydroxyatrazine in an unsaturated soil

Unsaturated column experiments on an intact soil core were conducted under two different soil pressure heads, -8 and -19 cm, to characterize chemical sorption and desorption for hydroxyatrazine. Breakthrough curves for (H2O)-H-3 were similar under the different experimental conditions. The advection-dispersion transport model described (H2O)-H-3 transport well with Peclet numbers of 0.17 and 0.21, respectively. The transport of hydroxyatrazine, a common degradation product of atrazine, was slightly enhanced at the -8 cm head relative to the -19 cm head (84% vs 79% mass recovery). A model that describes sorption with a single rate coefficient and desorption as a distribution of rate coefficients fits the hydroxyatrazine breakthrough curve well. The strong similarities in transport properties at different soil-water tensions suggests that water and contaminants can be transported through macroporous soils across a range of moisture conditions in the vadose zone in agricultural fields.