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CASRN: 51218452

Update: August 1993

Current Massachusetts Regulatory Limit
ORSGL = 0.1 mg/L.

Federal Regulatory Limit
The U.S. EPA has not published an MCL for metolachlor. Listed in the January 1991 Drinking Water Priority List (Fed Reg., 1991).

Basis for Criteria
The ORSGL is derived based on the RfD presented below and assumes that a 70 kg adult ingests 2 L/day of water. The ORSGL incorporates a relative source contribution of 20% and a 10-fold uncertainty factor to account for potential carcinogenicity.

RfD: 1.5 x 10-1 mg/kg/day
UF: 100 (10 = interspecies; 10 = intraspecies) MF: 1

(U.S. EPA, 1993)

Critical Effects
In a 2-year study, Albino CD rats were fed 0, 30, 300 or 3,000 ppm (0, 1.5, 15 or 150 mg/kg/day) metolachlor. Effects noted included decreased body weight gain at the highest dose tested. A NOEL of 15 mg/kg/day was identified from this study. In a 2-generation reproductive study with Charles CD rats fed 1, 30, 300 or 1,000 ppm (0, 1.5, 15 or 50 mg/kg/day), a NOEL of 15 mg/kg/day was also identified based on reduced pup weight and reduced parental food consumption at the highest dose (Ciba Geigy Corporation, 1981; 1983).

Cancer Assessment: C
Metolachlor is currently classified as a class C Carcinogen. This classification is based on the appearance of liver lesions (combined neoplastic nodules and carcinomas) in female mice. No oral slope value is available (IBI, 1979).


Analytical Information
: 0.001 mg/L

Analytical Methods
524.2597; gas chromatography

PQLs and analytical methods may have been updated since this guidance value was last revised. Updated analytical methods for drinking water and their associated PQLs may be found at US EPA Drinking Water Analytical Methods .

Other Regulatory Data
Any Health Advisories, Reference Doses (RfDs), cancer assessments or Cancer Potency Factors (CPFs) referenced in this document pertain to the derivation of the current guidance value. Updated information may be obtained from the following sources:

Health Advisories - The U.S. EPA provides guidance for shorter-term exposures for chemicals based on their non-cancer effects. Current health advisories may be more current than those used to derive MCLs and may be found at US EPA Current Drinking Water Health Advisories .

RfDs, cancer assessments and CPFs - For specific information pertaining to derivation of drinking water criteria, consult the Federal Register notice that announces the availability of the most current guidance for that chemical. In addition, information on other current RfDs and CPFs as well as cancer assessments for specific chemicals may be found in the U.S. EPA Integrated Risk Information System (IRIS) at Please note that the information in IRIS may differ from that used in the derivation process as published in the Federal Register notice.

Ciba Geigy Corporation. 1983. MRID No. 00063398, 00084005, 00129377, 00144364, 00158924. Available from U.S. EPA.

Ciba Geigy Corporation. 1981. MRID No. 00080897. Available from U.S. EPA.

Federal Register. January 14, 1991. Drinking Water Priority List (56 FR 1470).

IBI. Industrial Biotest Laboratories. 1979. Two year chronic oncogenicity oral toxicity study with metolachlor in albino rats. Cited in U.S. EPA 1993.

U.S. EPA (U.S. Environmental Protection Agency). 1993. Integrated Risk Information System (IRIS). Washington, D.C..