ELUTION OF AGED AND FRESHLY ADDED HERBICIDES FROM A SOIL

Elution of atrazine and metolachlor residues from a long-contaminated soil under saturated flow was compared to elution of freshly-injected compounds from the same soil. The mobility of the injected herbicide was far greater than the native. A two-compartment diffusion sorption model-having a fast compartment S1 in rapid exchange with water and a slow compartment S2 with exchange by radial diffusion kinetics-gave good simultaneous fits to native and injected elution curves and predicted flow rate effects and postleaching soil herbicide profiles. An analogous model with first-order kinetics was less successful. The diffusion model parameters indicated that (i) at apparent equilibrium, the bulk (82-92%) of the sorbate was in S2; (ii) the short-term (24-h) batch partition coefficient greatly underestimates the apparent true value and instead reflects partitioning into S1; and (iii) the time scale for sorption is many months. The absence of particle-size size effects on desorption rates suggests that the diffusive medium of S2 is microparticles or microstructures (less-than-or-equal-to 1 mum) distributed among all particle-size fractions.