DEGRADATION OF CARBON-14-ATRAZINE AND CARBON-14-METOLACHLOR IN SOIL FROM 4 DEPTHS

Degradation of C-14-atrazine -chloro-4-ethylamino-6-isopropyl-amino-s- triazine] and C-14-metolachlor -ethyl-6-methylphenyl)-N-(2-methoxy-1-m ethylethyl) acetamide] was monitored for 6 and 2 mo, respectively, usi ng sterile and nonsterile soil microcosms. Both chemical and biologica l degradation were observed for atrazine, metolachlor degraded only bi ologically. The calculated half-life of atrazine was 3.6 wk in nonster ile surface samples (0-5 cm). At the surface, after 22 wk, bound resid ues accounted for almost 60% of the recovered radioactivity while 36% was recovered as (CO2)-C-14, indicating significant cleavage of the tr iazine ring. For sterilized surface samples, atrazine degraded chemica lly with bound residues accounting for 63% of the recovered label and had a calculated half-life of 6.2 wk. Degradation and binding were som ewhat lower in soil samples from 20 to 25 cm and deeper subsurface sam ples (45 and 75 cm) showed almost no degradation and very little bindi ng. Metolachlor degraded only in the surface nonsterile samples; no de gradation was observed in subsurface samples or in sterile samples fro m any depth. Bound residues occurred in high amounts in the surface so il (31%) but declined rapidly with depth, indicating that organic matt er is the primary binding site for metolachlor. Very little (CO2)-C-14 (<1.6%) was produced from metolachlor in any sample. This study showe d that both herbicides degraded slower and sorbed less to the soil wit h increasing soil depth, especially below 25 cm. Quantifying degradati on rates of agricultural chemicals in the vadose zone is important for predicting and preventing groundwater contamination as well as for su ccessful implementation of in-situ bioremediation of contaminated subs oils.