NONEQUILIBRIUM TRANSPORT OF ATRAZINE THROUGH LARGE INTACT SOIL CORES

Preferential flow in heterogeneous soils may result in more rapid leac hing of pollutants through soils than would be predicted using transpo rt models based on the local equilibrium assumption (LEA). Our objecti ves were to evaluate nonequilibrium processes important to the transpo rt of tritiated water ((H2O)-H-3) and atrazine under varying pore wate r velocities and soil water contents, and to distinguish between trans port-related nonequilibrium (TNE) and sorption-related nonequilibrium (SNE). Column experiments were performed using a (H2O)-H-3-C-14-labele d atrazine pulse through intact soil cores (15.24-cm diam., 30-cm leng th) at pore water velocities of 0.12, 0.69, and 2.16 cm h(-1) (theta(v ) approximate to 0.39) and at 0.74 cm h(-1) (theta(v) approximate to 0 .44). The asymmetrical shape and the left-handed displacement of (H2O) -H-3 breakthrough curves (BTCs) as a function of pore water velocity a nd soil water content (theta(v)) indicated that (H2O)-H-3 was subject to TNE at only the 0.74 (theta(v) = 0.44) and 2.16 cm h(-1) pore water velocities. The asymmetrical shape and increased tailing of atrazine BTCs at all pore water velocities indicated that atrazine was influenc ed primarily by SNE at pore water velocities of 0.12 and 0.69 cm h(-1) , and a combination of both TNE and SNE at pore water velocities of 0. 74 (theta(v) = 0.44) and 2.16 cm h(-1). The convection-dispersion equa tion based on the LEA was unable to predict atrazine BTCs at any pore water velocity. Although the nonequilibrium bicontinuum (two-site/two- region) model provided excellent fit to all atrazine BTCs, fits to the model cannot be used to separate between TNE and SNE when both mechan isms are operative. Results of this study confirm that TNE and SNE are important transport processes in naturally structured soils under con ditions of relatively high pore water velocities and volumetric water contents.