Dicamba
(endorsed 2011)
Guideline
Based on human health concerns, dicamba in drinking water should not exceed 0.1 mg/L.
Related chemicals
Dicamba (CAS 1918-00-9) belongs to the chlorophenoxy chemical class. Other pesticides in this class include 2,4-D, MCPA and mecoprop (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, dicamba would not be a health concern unless the concentration exceeded 0.1 mg/L. Excursions above this level even for a short period are of concern as the health-based guideline is based on short-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: Dicamba is used as a herbicide for the control of annual and perennial broad-leaf weeds in home garden lawns, turf and agricultural crops.
There are currently products registered in Australia that contain dicamba, its acid derivative, its dimethylamine salt or its sodium salt. Dicamba products are intended for professional or home garden use. Products are available as concentrated solutions to be applied diluted and undiluted using boom spray, aerial spray, and hand-held sprays or watering cans. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main sources of public exposure to dicamba and its metabolites are the use of home garden products, and residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of dicamba 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
Dicamba is a very polar herbicide and results of its analysis in water have traditionally been negative. This may be a factor in the absence of reliable data on the presence of dicamba in Australian waters. A survey of pesticides including dicamba in New South Wales drinking water reported in 1989 (Ang et al. 1989) did not find detectable levels of dicamba in any sample. A recent published paper (Tran et al. 2006) reported use of state-of-the-art analytical techniques to determine the presence of a range of herbicides including dicamba in agricultural drainage waters in Australia. While a range of herbicides was detected, no dicamba was found in the samples.
One of the main studies on dicamba incidence in water was from Canada, to derive Canadian Water Quality Guidelines (Caux et al. 1993). Dicamba was detected in less than 8% of surface water samples, with a maximum concentration of 13 µg/L; and in groundwater, it was detected in 2% of samples but at concentrations to 517 µg/L.
Treatment of drinking water
Dicamba has been shown to be completely removed when water undergoes advanced oxidation with iron-catalysed ultraviolet irradiation and peroxide (Fenton reaction) (Huston and Pignatello 1999).
Measurement
Dicamba is a very polar herbicide that is not amenable to direct analysis by gas chromatographic techniques. It is commonly extracted from water via solid phase extraction techniques and determined by high performance liquid chromatography using ultraviolet detection. This approach leads to limits of detection of approximately 1 µg/L (Krzyszowska and Vance 1994). This process has recently been refined in a study investigating a range of hydrophilic solid phase extraction techniques (Tran et al. 2006).
History of the health-based guideline
The current acceptable daily intake (ADI) for dicamba is 0.03 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 3 mg/kg bw/day from a short-term (developmental toxicity) study. The NOEL is based on maternal toxicity (decreased bodyweight) in rabbits. The ADI incorporates a safety factor of 100 and was established in 1991.
The previous ADI for dicamba was 0.05 mg/kg bw/day. This ADI was established in 1986; however, there is no information indicating how it was derived.
The previous NHMRC health-based guideline value was 0.1 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Dicamba is readily and extensively absorbed via the gastrointestinal tract in all species studied. It does not undergo significant metabolism and is excreted rapidly, predominantly in urine, largely unchanged.
Acute effects: Dicamba has low acute oral and dermal toxicity. Dicamba is a moderate skin sensitiser in guinea-pigs.
Short-term effects: A 13-week dietary study in rats reported decreased bodyweight gain and elevated liver enzyme levels at 500 mg/kg bw/day.
Long-term effects: A 2-year dietary study in mice reported increased male mortality and reduced female bodyweight gain at the high dose (450 mg/kg bw/day). The NOEL for this study was 150 mg/kg bw/day.
Carcinogenicity: Based on long-term dietary studies in mice, there is no evidence of carcinogenicity for dicamba.
Genotoxicity: There is evidence of genotoxicity but only at dose levels well in excess of the likely level of human exposure.
Reproductive and developmental effects: A 3-generation reproduction study in rats did not report any evidence of toxicity in adult rats or offspring at doses up to 25 mg/kg bw/day. In developmental studies in rats and rabbits, there was no evidence of delayed development or teratogenicity. Maternotoxicity (decreased bodyweight gain in rats and neurotoxicity in rabbits) was reported. The most sensitive NOEL for maternotoxicity was 3 mg/kg bw/day in rats, which was used to establish the current ADI.
Poisons Schedule: Dicamba is included in Schedule 5 and 6 of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010), depending on its concentration and use. Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline
The health-based guideline of 0.1 mg/L for dicamba was determined as follows:
where:
3 mg/kg bw/day is the NOEL based on a short-term (developmental) study in rabbits.
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.
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.
Ang C, Meleady K, Wallace L (1989). Pesticide residues in drinking water in the north coast region of New South Wales, Australia, 1986-87. Bulletin of Environmental Contamination and Toxicology, 42:595-602.
Caux PY, Kent RA, Tache M, Fan, GT, MacDonald DD (1993). Environmental fate and effects of Dicamba: A Canadian perspective. Reviews of Environmental Contamination and Toxicology, 133:1-58.
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.
Huston PL, Pignatello JJ (1999). Degradation of selected pesticide active ingredients and commercial formulations in water by the photo-assisted Fenton reaction. Water Research, 33(5):1238-1246.
Krzyszowska A, Vance G (1994). Solid phase extraction of dicamba and picloram from water and soil samples for HPLC analysis. Journal of Agricultural and Food Chemistry, 42:1693-1696.
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.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
Tran ATK, Hyne RV, Doble P (2006). Determination of commonly used polar herbicides in agricultural drainage waters in Australia by HPLC. Chemosphere, 67(5):944-953.
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