Diquat in freshwater and marine water

Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals’ of the ANZECC & ARMCANZ (2000) guidelines.

The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.

Description of chemical

Diquat (CAS 2764-72-9; or 85-00-7 for dibromide and 6385-62-2 for dibromide monohydrate) is a bipyridylium herbicide, introduced by ICI Plant Protection Division (now Zeneca) (Tomlin 1994). It is a non-selective herbicide with a contact mode of action and is absorbed by foliage (Tomlin 1994). One IUPAC name is 9,10-dihyro-8a,10a-diaziaphenanthrene. Its formula is C₁₂H₁₂N₂ (add Br2 for dibromide) and molecular weight is 184.2 (344.1 for dibromide). The dibromide salt is soluble in water to 700 g/L at 20°C and it has a very low log K_ow (-4.6). The current analytical practical quantitation limit (PQL) for diquat in water is 5 µg/L (NSW EPA 2000).

Uses and environmental fate

Diquat is used widely for control of annual broad-leaved weeds in many crops, vines, orchards, potatoes, sugarcane and ornamentals, for weed control in non-crop situations, such as pastures and rights-of-way, and for control of both emergent and submerged aquatic weeds (Tomlin 1994). It is also used for pre-harvest desiccation of foliage in many food crops and cotton.

In Australia, diquat has around 320 registered uses (NRA 1997a) as outlined above. It is often used in combination with other herbicides. Diquat is readily hydrolysed in alkaline conditions but not in acidic or neutral water. The DT50 at pH 7 was 74 d in simulated sunlight but it does undergo photodecomposition (Tomlin 1994).

Aquatic toxicology

Freshwater algae: four species, 72-h EC50 (growth), 19-73 , 72-h EC50 (growth), 19-73 µg/L.

Freshwater crustaceans: six species, 48-h LC50, 19,000 to 46,600 µg/L. Two copepods (Diaptomus mississipians and Eucyclops agilis) were most sensitive and an ostracod and a cyclopoid copepod the least.

Freshwater fish: 17 species, 48-96h LC50, 750 (Stizostedion vitreum) to 300,000 µg/L (Salmo trutta), although figures as high as 1718 and 2092 mg/L were reported for Ctenopharyngodon idella.

Freshwater amphibians: one species, 24-h LC50, 140,000 to 340,000 µg/L, but this could not be used.

No marine data were available for diquat.

Factors that modify toxicity

Toxicity to bluegills Pimephales promelas did not alter between 7°C and 22°C. An increase in pH to 9.5 increased toxicity to three fish species by a factor of 2. Diquat was 3 times more toxic at 40 ppm hardness than at 300 ppm (Johnson & Finley 1980).

Guideline

A freshwater moderate reliability trigger value of 1.4 µg/L was derived for diquat using the statistical distribution method at 95% protection and an assessment factor (AF) of 10. In absence of marine data, 1.4 µg/L was adopted as a low reliability trigger value for marine systems. This figure should only be used as an indicative interim working level.

References

ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.

Johnson WW & Finley MT 1980. Handbook of acute toxicity of chemicals to fish and aquatic invertebrates. US Department of the Interior, Fish and Wildlife Service, No 137, Washington DC.

NRA 1997a. Database extraction of selected pesticides: Registered uses in Australia, National Registration Authority, July 1997, Canberra.
NSW EPA 2000. Analytical Chemistry Section, Table of Trigger Values 20 March 2000, LD33/11, Lidcombe, NSW.

Tomlin C 1994. The pesticide manual: A world compendium. 10th edn, British Crop Protection Council & Royal Society of Chemistry, Bath, UK.