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

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 ( K d ) 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.