Degradation of carbon-14-atrazine and carbon-14-metolachlor in soil from four depths

Degradation of 14C-atrazine [2-chloro -4-ethylamino-6-isopropyl-amino-s-triazine] and 14C-metolachlor [2-chloro-N-(2-ethyl-6 methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] was monitored for 6 and 2 mo, respectively, using sterile and nonsterile soil microcosms. Both chemical and biological degradation were observed for atrazine, metolachlor degraded only biologically. The calculated half-life of atrazine was 3.6 wk in nonsterile surface samples (0-5 cm). At the surface, after 22 wk, bound residues accounted for almost 60% of the recovered radioactivity while 36% was recovered as 14CO2, indicating significant cleavage of the triazine ring. For sterilized surface samples, atrazine degraded chemically with bound residues accounting for 63% of the recovered label and had a calculated half-life of 62 wk. Degradation and binding were somewhat lower in soil samples from 20 to 25 cm and deeper subsurface samples (45 and 75 cm) showed almost no degradation and very little binding. Metolachlor degraded only in the surface nonsterile samples; no degradation was observed in subsurface samples or in sterile samples from any depth. Bound residues occurred in high amounts in the surface soil (31%) but declined rapidly with depth, indicating that organic matter is the primary binding site for metolachlor. Very little 14CO2 ( 1.6%) was produced from metolachlor in any sample. This study showed that both herbicides degraded slower and sorbed less to the soil with increasing soil depth, especially below 25 cm. Quantifying degradation rates of agricultural chemicals in the vadose zone is important for predicting and preventing groundwater contamination as well as for successful implementation of in-situ bioremediation of contaminated subsoils