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 w as far greater than the native. A two-compartment diffusion sorption m odel-having a fast compartment S1 in rapid exchange with water and a s low compartment S2 with exchange by radial diffusion kinetics-gave goo d simultaneous fits to native and injected elution curves and predicte d flow rate effects and postleaching soil herbicide profiles. An analo gous model with first-order kinetics was less successful. The diffusio n model parameters indicated that (i) at apparent equilibrium, the bul k (82-92%) of the sorbate was in S2; (ii) the short-term (24-h) batch partition coefficient greatly underestimates the apparent true value a nd 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 micropar ticles or microstructures (less-than-or-equal-to 1 mum) distributed am ong all particle-size fractions.