Effect of concentration on persistence of alachlor in soil

To determine the behavior of alachlor [2-chloro-N-(methoxymethyl) -N-(2,6-diethylphenyl)-acetamide] at concentrations common with spill and waste disposal sites, alachlor degradation was determined over a concentration range of 10 to 10 000 mg kg^-1 in a Webster clay loam (fine loamy, mixed, mesic Typic Haplaquoll) and an Estherville sandy loam (sandy, mixed, mesic Typic Hapludoll) in laboratory incubation experiments. Effect of concentration on the overall behavior of alachlor was similar in both soils. Based on percent of applied chemical, persistence of alachlor increased with increasing concentration. Mineralization and formation of degradation products and bound residues decreased at higher concentrations. At 10 000 mg kg^-1, alachlor was extremely persistent, with estimated 50% dissipation times (DT₅₀) of 12.6 and 13.5 yr in the Webster and Estherville soil, respectively. Although the DT₅₀ increased with increasing concentration, significant amounts of alachlor, in absolute mass, degraded at higher concentration. However, mineralization at 1000 and 10 000 mg kg^-1 was similar to that at 100 mg kg^-1. Specific alachlor biodegradation mechanisms, limited water solubility, and kinetics of dissolution of precipitated alachlor and alachlor desorption are postulated as the rate-limiting factors for the degradation of alachlor at elevated levels. A field experiment also indicated alachlor dissipation at high concentration was extremely slow over the first 6 mo after application. Degradation and leaching increased by the following spring. It would appear that an effective remediation means to detoxify alachlor-contaminated soil would be to land-spread the soil, thereby diluting the concentration of alachlor in the soil to <100 mg kg^-1.