A TEST OF THE LOCAL EQUILIBRIUM ASSUMPTION FOR ADSORPTION AND TRANSPORT OF PICLORAM

The inability of transport models that use a local equilibrium assumption (LEA) to describe contaminant transport under certain conditions has led to the development of various criteria for assessing LEA applicability. Our objectives were to determine the influence of pore water velocity on the adsorption and transport of picloram (4-amino-3-5,6-trichloropicolinic acid) and evaluate the use of dimensionless Damkohler numbers for identifying experimental conditions conducive to nonequilibrium transport. Three sets of transport experiments were conducted to study the movement of Br- and picloram through an Amsterdam silt loam soil (fine-silty, mixed Typic Haploboroll). Experiments were performed by displacing a 100-mL Br-C-14-labeled picloram pulse through disturbed soil columns (5.1-cm diam, 30-cm length) at pore water velocities of 4.1, 40.6, and 200.1 cm d-1. The kinetic parameters of picloram adsorption and desorption were obtained independently. Results indicated that Br- breakthrough curves (BTCs) were symmetrical at each pore water velocity. In contrast, picloram BTCs were shifted to the left on a pore-volume basis and demonstrated increased tailing with increasing pore water velocities. The use of LEA with a batch-determined distribution coefficient (K(d)) in the convection-dispersion equation adequately described picloram BTCs at the 4.1 cm d-1 pore water velocity, but overestimated the elution time at faster pore water velocities. These observations were consistent with conclusions derived from criteria used to assess LEA applicability and indicate that knowledge of reaction kinetics and convective velocities can be easily used to identify conditions conducive to nonequilibrium transport.