Metolachlor/s-Metolachlor
(endorsed 2011)
Guideline
Based on human health concerns, metolachlor in drinking water should not exceed 0.3 mg/L.
Related chemicals
Metolachlor (CAS 51218-45-2) belongs to the chloroacetamide class of chemicals. Other pesticides in this class include dimethenamid and propachlor. Metolachlor is a racemic mixture of the R- and S-isomers; S-metolachlor is the purified S-isomer (Tomlin 2006).
Human risk statement
With good water quality management practices, the exposure of the general population is expected to be well below levels that may cause health concerns.
If present in drinking water as a result of a spillage or through misuse, metolachlor would not be a health concern unless the concentration exceeded 0.3 mg/L. Minor excursions above this level would need to occur over a relatively long period to be a health concern, as the health-based guideline is based on medium-term effects.
With good water quality management practices, pesticides should not be detected in source waters used for drinking water supplies. Persistent detection of pesticides may indicate inappropriate use or accidental spillage, and investigation is required in line with established procedures in the risk management plan for the particular water source.
General description
Uses: Metolachlor is a pre-emergent herbicide for the control of grasses and broad-leaf weeds in agricultural crops.
There are registered products that contain metolachlor or its isomer s-metolachlor in Australia. These products are intended for professional use and are available as concentrated solutions to be applied in diluted form using boom and hand-held ground sprays. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to metolachlor and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of metolachlor may potentially lead to contamination of source waters through processes such as run-off, spray drift or entry into groundwater.
Typical values in Australian drinking water
No reports of metolachlor in Australian drinking waters have been identified.
Treatment of drinking water
Metolachlor can be a relatively difficult pesticide to treat in drinking water. Oxidation by chlorine or ozone are only partially effective at typical doses, and adsorption to activated carbon can be incomplete (Ormad et al. 2008). However, a combination of ozone, activated carbon and coagulation-flocculation can be effective (Ormad et al. 2008).
Measurement
Metolachlor can be measured in drinking waters by gas chromatography with nitrogen–phosphorus detection. The limit of detetion for this method is 0.75–0.01 µg/L (WHO 2004).
History of the health values
The current acceptable daily intake (ADI) for metolachlor is 0.08 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 7.5 mg/kg bw/day from a medium-term (6-month) dietary study in dogs. The NOEL is based on decreased bodyweight gain. The ADI incorporates a safety factor of 100, and was established in 1987.
The previous health value was 0.3 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metolachlor and its isomer S-metolachlor demonstrate similar toxicological profiles. Therefore, the toxicological endpoints that occur at the lowest dose level for either chemical (both referred to as metolachlor) have been reported below.
Metabolism: Metolachlor is readily and extensively absorbed in the gastrointestinal tract of rats. It is extensively metabolised and excreted in urine and faeces. Metabolites have not been identified.
Acute effects: Metolachlor has low acute oral and dermal toxicity. It is a skin sensitiser in guinea-pigs.
Short-term effects: Medium-term dietary studies conducted in rats and dogs reported biochemical changes and changes in bodyweight and organ weight as the most sensitive toxicological effects. In a 13-week study in rats, there were effects indicative of mild liver toxicity, reduced bodyweight gain and increases in relative and absolute organ weights at 200 mg/kg bw/day. A 6-month study in dogs reported decreased bodyweight gain, food consumption and absolute liver weights at 25 mg/kg bw/day. The NOEL of 7.5 mg/kg bw/day from this study is the basis for the current ADI.
Long-term effects: Long-term dietary studies have been conducted in mice, rats and dogs. In a 2-year dietary study in mice there were changes in spleen weight at 150 mg/kg bw/day. In a 2-year rat study there was decreased bodyweight gain and food consumption, and an increase in pituitary carcinomas and liver neoplastic nodules at 150 mg/kg bw/day. In a 1-year dietary study in dogs, there was decreased bodyweight gain at 32 mg/kg bw/day.
Carcinogenicity: There was some evidence of carcinogenicity in rats at high dose levels, but the threshold noted for this effect is well in excess of the likely level of human exposure.
Genotoxicity: Metolachlor is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: A 2-generation reproduction study in rats and developmental studies in rats and rabbits did not produce any evidence of effects on reproductive parameters or foetal development.
Poisons Schedule: Metolachlor is included in Schedule 5 of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010). Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline
The health-based guideline of 0.3 mg/L for metolachlor was determined as follows:
where:
7.5 mg/kg bw/day is the NOEL based on a medium-term (6-month) study in dogs.
70 kg is taken as the average weight of an adult.
0.1 is a proportionality factor based on the assumption that 10% of the ADI will arise from the consumption of drinking water.
2 L/day is the estimated maximum amount of water consumed by an adult.
100 is the safety factor applied to the NOEL derived from animal studies. This safety factor incorporates a factor of 10 for interspecies extrapolation and 10 for intraspecies variation.
The World Health Organization has a health-based guideline value of 0.01 mg/L for metolachlor (WHO 2004).
References
NOTE: The toxicological information used in developing this fact sheet is from reports and data held by the Department of Health, Office of Chemical Safety.
DoHA (2010) The Poisons Standard; Schedule 1-Standard for the Uniform Scheduling of Medicines and Poisons, Department of Health and Ageing, Commonwealth of Australia, Canberra.
NHMRC (National Health and Medical Research Council), NRMMC (Natural Resources Management Ministerial Council) (2004). Australian Drinking Water Guidelines. National Water Quality Management Strategy, Paper 6. NHMRC and NRMMC.
Ormad MP, Miguel N, Claver A, Matesanz JM, Ovelleiro JL (2008). Pesticides removal in the process of drinking water production. Chemosphere, 71: 97-106.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th edition, British Crop Production Council, UK.
WHO (World Health Organization) (2004). Guidelines for Drinking-water Quality. 3rd Edition, WHO, Geneva, Switzerland.
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