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CAS Number: 51218-45-2
Synonyms: Bicep, Codal, Dual, Milocep
Contaminant Type: Chemical

Metolachlor is a synthetic organic compound of the chloroacetanilide class of herbicides. Its IUPAC name is 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide, with common trade names of Dual, Bicep, Codal and Milocep. [529] It is primarily used as a pre-plant, pre-emergence weed control for corn, cotton, peanuts, soybeans, and sorghum. Non-agricultural uses include ornamental plants and rights-of-way. An estimated 59 million pounds is applied annually in the United States. [529]

Metolachlor is not regulated by USEPA in drinking water, but is an unregulated chemical drinking water contaminant listed on the USEPA's Contaminant Candidate Lists 1, 2 and 3. [529, 620]

USEPA classified Metolachlor as a possible human carcinogen based on liver effects in animals.[529]

The following properties and phenomena suggest metolachlor would be present in water treatment plant intakes. [529, 530] Based on relatively low Henry's Law constant and vapor pressure, is considered nonvolatile under ambient environmental conditions. Photolysis of metolachlor is not expected to be significant. It is relatively resistant to hydrolysis at moderate pH values of 5 to 9. Reported half lives in aerobic surface waters were weeks to months. It has relatively low soil/water partitioning coefficient indicating it adsorbs moderately to soil, thus it would be expected to have lower concentrations in ground water than in surface water.

Its primary method of degradation is microbial activity. [529, 530] Therefore metolachlor degrades much faster in soil than in water, again implying lower concentrations in ground water than in surface water. Four degradates have been identified, but the most commonly detected are metolachlor ethane sulfonic acid (ESA) and metolachlor oxanilic acid (OA). [529] For information on metolachlor degradates, go to Find a Contaminant. ESA and OA are more soluble than the parent herbicide and more resistant to microbial degradation than the parent. [529] These factors imply ESA and OA would be detected more frequently and at higher concentrations than their parent. Other reported degradates are 2-ethyl-6-methylaniline and 2-[(2-ethyl-6-methylphenyl)-amino]-1-propanol. [558]

Metolachlor enters the water through accidental spills, spraying, leaching from fields via run off, and leaching of groundwater. It is often used in combination with other herbicides. [530] USEPA reported on three studies of metolachlor occurrence. [529] In the USGS National Water Quality Assessment (NAWQA) (1992-2001) for surface waters, median reported concentrations were 0.029 ug/L in agricultural areas and less than the reporting level of 0.013 ug/l in urban areas. In the NAWQA (1992-2001) for ground waters, reported concentrations of metolachlor were nondetects. In USEPA's Unregulated Contaminant Monitoring (1993-1999) of public water supplies where reporting levels varied, metolachlor was detected in 0.86 percent of samples with a median concentration of 1 ug/L. In a Wisconsin study (1999-2000) metolachlor was found in 36 percent of private drinking water wells averaging 1.4 ug/L.

Date of Literature Search: March 2007

529 USEPA; 2006; Regulatory Determinations Support Document for Selected Contaminants from the Second Drinking Water Contaminant Candidate List (CCL2); Regulatory Determinations Support Document for Selected Contaminants from the Second Drinking Water Contaminant Candidate List (CCL2); EPA Report 815-D-06-007. Chapter 13 pp. 1-29. Office of Ground Water and Drinking Water, U.S. Environmental Protection Agency, Washington, DC.
530 Spectrum Laboratories; 2007; Metolachlor Chemical Fact Sheet; http://www.speclab.com/compound/c5121845.htm; As posted on August 24, 2007. Spectrum Laboratories, Ft. Lauderdale, FL.
558 Pesticide Action Network North America; 2007; Pesticide Action Network (PAN) Pesticides Database; http://www.pesticideinfo.org/Index.html; As posted on August 24, 2007. Pesticide Action Network North America, San Francisco, CA.
620 USEPA Office of Ground Water and Drinking Water; 2008; Contaminant Candidate List 3 (CCL3); USEPA Office of Ground Water and Drinking Water; http://www/epa.gov/OGWDW/ccl/ccl3.html