Nonequilibrium Transport of Reactive Solutes through Layered Soil Profiles with Depth-Dependent Adsorption

Abstract

The pesticide atrazine was used as a reactive tracer in a series of miscible displacement experiments that were conducted with soil columns containing the same amount of sludge or manure, but distributed either as a layer or uniformly in depth. Travel time probability density function (pdf) of Cl- and atrazine measured from both types of columns was analyzed based upon a transfer function model that assumes a two-site nonequilibrium advection−dispersion equation (ADE). Sorption sites contributed by both organic materials were primarily rate-limited. Travel time of atrazine, estimated by temporal moment analysis on the measured travel time pdf, was consistently increased in organic material-amended columns for both distributions. When flux concentration of atrazine was fitted to the nonequilibrium ADE, either a higher overall distribution coefficient (Kd) or a higher fraction of instantaneous adsorption sites (f) or both were found for layered columns, indicating an enhanced accessibility to sorption sites of organic amendments probably due to less shielding by soil minerals.