William H. Graham | USGS Science

USGS1999

Metolachlor and alachlor breakdown product formation patterns in aquatic field mesocosms

The transformation of metolachlor [2-chloro- N -(2-ethyl-6-methylphenyl)- N -(2-methoxy-1-methylethyl)acetamide] and alachlor [2-chloro- N -(2,6-diethylphenyl)- N -(methoxymethyl)acetamide] in aquatic systems was investigated using outdoor tank mesocosms. Metolachlor and alachlor levels and their ethane sulfonic acid (ESA) and oxanillic acid breakdown products were monitored over time under five experimental treatments (each in quadruplicate). Background water conditions were identical in all treatments with each treatment differing based on the level and type(s) of herbicide present. Treatments included a no-herbicide control, 10 μg/L metolachlor, 25 μg/L metolachlor, 25 μg/L alachlor, and 25 μg/L alachlor plus 25 μg/L metolachlor in combination. The experiment was initiated by adding herbicide(s) to the units to the target concentrations; herbicide and breakdown product levels and other chemical parameters were then monitored for 85 days. In general, metolachlor half-lives were longer than alachlor half-lives under all treatments, although the differences were not statistically significant. Metolachlor half-lives (±95% confidence limits) ranged from 33.0 d (±14.1 d) to 46.2 d (±40.0 d), whereas alachlor half-lives ranged from 18.7 d (±3.5 d) to 21.0 d (±6.5 d) for different treatments. Formation patterns of ESA were similar in all treatments, whereas oxanillic acid formation differed for the two herbicides. Alachlor oxanillic acid was produced in larger quantities than metolachlor oxanillic acid and either ESA under equivalent conditions. Our results suggest that the transformation pathways for alachlor and metolachlor in aquatic systems are similar and resemble the acetochlor pathway in soils proposed by Feng ( Pestic. Biochem. Physiol . 1991 , 34 , 136); however, the oxanillic acid branch of the pathway is favored for alachlor as compared with metolachlor.

Environmental Science & Technology↗

USGS1999

Natural attenuation of chloroacetinilide herbicides in aquatic systems

Chloroacetanilide herbicides (e.g. alachlor and metolachlor) and their breakdown products are frequently found in agriculturally impacted and other natural waters. Although these compounds are usually present at low concentrations in aquatic systems, defining their fate and attenuation rate is important because they can be toxic at comparatively low exposure levels. This paper summarizes results from a four-year research program assessing factors that affect herbicide attenuation rates and breakdown product formation patterns in aquatic systems. Over 70 different field microcosms (11.3 m 3 cylindrical fiberglass tanks) were selectively manipulated (in replicate) to create nine unique aquatic ecosystems for this study. Alachlor and/or metolachlor were added to these units and transformations were observed over time. Alachlor half-lives ranged from 6.0 days to more than 100 days depending upon the temperature, oxygen conditions, and the nutrient conditions in the microcosm waters. The highest rates of alachlor decay were observed in warm, nutrient-rich, anaerobic water columns, whereas the lowest alachlor decay rates were observed under cooler, aerobic, oligotrophic water conditions. Similar relationships were observed for metolachlor, although metolachlor decay rates were about 40% slower. Our data suggest that herbicides will decay rapidly in waters that are high in nutrients possibly resulting from human activities. Alternately, low nutrient or pristine waters will have the lowest rates of herbicide decay. On a policy level, all efforts should be made to prevent exposure of comparatively pristine waters to herbicide inputs.

Conference Paper↗