Terbutryn
(endorsed 2011)
Guideline
Based on human health concerns, terbutryn in drinking water should not exceed 0.4 mg/L.
Related chemicals
Terbutryn (CAS 886-50-0) belongs to the triazene class of chemicals. There are many other pesticides in this class, which includes ametryn, propazine, prometon, and cyanazine (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, terbutryn would not be a health concern unless the concentration exceeded 0.4 mg/L. Excursions above this level even for a short period are of concern, as the health-based guideline is based on short- to 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: Terbutryn is a herbicide for the control of broad-leaf weeds in wheat, barley, triticale, peas, sugar cane and turf agricultural crops.
There are registered products that contain terbutryn in Australia. The products are intended for professional use. Terbutryn is available as concentrated solutions to be applied in diluted form using ground and aerial 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 terbutryn and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of terbutryn 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 specific reports of terbutryn in Australian drinking waters have been identified. However, a study of four small river systems in Germany revealed maximum concentrations of up to 5.6 µg/L (Quednow and Puttmann 2007). Terbutryn has also been reported in ground water in the UK at concentrations up to 0.13 µg/L (Lapworth et al. 2006).
Treatment of drinking water
Preoxidation of drinking water by chlorine is effective for the removal of terbutryn (Ormad et al. 2008). However, this approach will produce unidentified by-products, which will remain in the water. An alternative and equally effective approach is the preoxidation of terbutryn by ozone followed by activated carbon adsorption (Ormad et al. 2008).
Measurement
Numerous analytical methods are available for multiresidue analysis of triazene pesticides. For example, terbutryn may be measured in drinking waters by relatively common methods of solid phase extraction and gas chromatography–mass spectrometry, with a quantification limit of 30 ng/L (Ormad et al. 2008). More advanced methods, including gas chromatography coupled to high-resolution time-of-flight mass spectrometry, have also been reported (Hernandez et al. 2007). Terbutryn may also be measured in drinking waters along with other triazenes by ultra-performance liquid chromatography with tandem mass spectrometric detection. The limit of quantification for all analytes in this method was reported to be 5 ng/L (Drozdzynski and Folkman 2008).
History of the health values
The current acceptable daily intake (ADI) for terbutryn is 0.01 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 10 mg/kg bw/day from short-term dietary studies in rats (13-week) and dogs (6-months). The NOEL is based on clinical signs of toxicity in a 6-month dietary study in dogs, and evidence of liver and thyroid toxicity in a 16-week dietary study in rats. The ADI incorporates a safety factor of 100, and was established in 1986. An ADI for terbutryn was not set in Australia before 1986.
The previous health value was 0.3 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Terbutryn is readily and extensively absorbed via the gastrointestinal tract in rats. It is extensively metabolised, with the main metabolites in the form of polar metabolites and conjugates. It is excreted mainly in the urine and to a lesser extent in the faeces within 72 hours.
Acute effects: Terbutryn has low acute oral and dermal toxicity. It is a skin sensitiser in guinea pigs, although there are no reports of sensitisation in humans working with terbutryn.
Short-term effects: A 16-week dietary study in rats reported thyroid toxicity (follicular hypertrophy and follicular eosinophilic infiltration) and liver toxicity (increased serum alkaline phosphatase activity, cholesterol, alanine amino-transferase, and glucose) at doses of 50 mg/kg bw/day and above. A 6-month dietary study in dogs reported clinical signs of toxicity (increased salivation, hyper-responsiveness to sound, and timid behaviour) and intestinal epithelial necrosis at doses of 25 mg/kg bw/day and above. The NOEL in both of these studies was 10 mg/kg bw/day, and this is the basis for the current ADI.
Long-term effects: In long-term dietary studies in mice, there was no evidence of toxicity up to doses of 150 mg/kg bw/day. In long-term dietary studies in rats, there was decreased bodyweight gain and food consumption, follicular hyperplasia, and liver adenomas at 150 mg/kg bw/day. The lowest NOEL was 15 mg/kg bw/day (rats) in these studies.
Carcinogenicity: There was evidence of carcinogenicity but only at dose levels well in excess of the likely level of human exposure.
Genotoxicity: Terbutryn is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: A 3-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: Terbutryn 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 0.4 mg/L for terbutryn was determined as follows:
where:
10 mg/kg bw/day is the NOEL based on a medium-term (13-week) dietary study in rats.
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.
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.
Drozdzynski D, Folkman W (2008). Analysis of triazine herbicides in water using solid phase extraction followed by ultra-performance liquid chromatography with tandem mass detection (UPLC-MS/MS). Chemia Analityczna, 53(3):391-400.
Hernandez F, Portoles T, Pitarch E, Lopez FJ (2007). Target and nontarget screening of organic micropollutants in water by solid-phase microextraction combined with gas chromatography/high-resolution time-of-flight mass spectrometry. Analytical Chemistry, 79(24):9494-9504.
Lapworth DJ, Gooddy DC, Stuart ME, Chilton PJ, Cachandt G, Knapp M, Bishop S (2006). Pesticides in groundwater: some observations on temporal and spatial trends. Water and Environment Journal, 20(2):55-64.
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(1):97-106.
Quednow K, Puttmann W (2007). Monitoring terbutryn pollution in small rivers of Hesse, Germany. Journal of Environmental Monitoring, 9(12):1337-1343.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th edition, British Crop Production Council, UK.
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