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 mi scible 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 fun ction (pdf) of Cl- and atrazine measured from both types of columns wa s analyzed based upon a transfer function model that assumes a two-sit e nonequilibrium advection-dispersion equation (ADE). Sorption sites c ontributed by both organic materials were primarily rate-limited. Trav el time of atrazine, estimated by temporal moment analysis on the meas ured travel time pdf, was consistently increased in organic material-a mended columns for both distributions. When flux concentration of atra zine was fitted to the nonequilibrium ADE, either a higher overall dis tribution coefficient (Kd) or a higher fraction of instantaneous adsor ption sites (f) or both were found for layered columns, indicating an enhanced accessibility to sorption sites of organic amendments probabl y due to less shielding by soil minerals.