Alachlor transformation patterns in aquatic field mesocosms under variableoxygen and nutrient conditions

Alachlor is one of the most commonly used herbicides in both Europe and Nor th America. Because of its toxic properties, its fate and attenuation in na tural waters is practically important. This paper assesses factors that aff ect alachlor decay rate in aquatic systems using field-scale experimental u nits. In particular, we used field mesocosms (11.3 m(3) outdoor fiberglass tanks) to examine the affect of oxygen level and other factors on decay rat e in water columns, This is one of the first studies ever performed where d iverse water column conditions have been successfully simulated using commo n mesocosm-scale facilities. Four treatments were assessed, including aerob ic systems (aerobic); low nutrient, oxygen-stratified systems (stratified-L N); moderate nutrient, oxygen-stratified systems (stratified-HN); and anaer obic systems (anaerobic). The lowest half-lives were observed in the anaero bic units (9.7 days) followed by the aerobic (21 days), stratified-HN (22 d ays), and stratified-LN (46 days) units. Our results indicate that alachlor is transformed most rapidly under anaerobic conditions, although the ambie nt phosphorus level also appears to influence decay rate. In this study, tw o common alachlor breakdown products. ethane sulfonic acid (ESA) and oxanil ic acid, were also monitored. Oxanilic acid was produced in greater quantit ies than ESA under all treatments with the highest levels being produced in the stratified-HN units. In general, our results suggest that previous lab oratory data, which indicated that high rates of alachlor decay can occur u nder oxygen-free methanogenic conditions, is translatable to field-scale ap plications. (C) 2000 Elsevier Science Ltd. All rights reserved.