Paraquat 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

Paraquat (CAS 4685-14-7) is a bipyridylium herbicide, usually used as the dichloride salt (CAS 1910-42-5). It was introduced by ICI Plant Protection Division (now Zeneca) (Tomlin 1994). It is a non-selective herbicide, which acts by contact and absorption by foliage (Tomlin 1994). Its IUPAC name is 1,1'-dimethyl-4,4'-bipyridinium, formula is C12H14N2 (add Cl2 for dichloride) and molecular weight is 186.3 (257.2 for dichloride). The dichloride is soluble in water to around 700 g/L and has a very low log Kow.

Uses and environmental fate

Paraquat is used for control of a wide variety of broad-leaved weeds and grasses in orchards, vineyards, plantations and some root vegetable crops and in forestry. It is also used in pastures, in rights-of-way, as a desiccant for some food crops and against aquatic weeds. Paraquat has around 750 registered uses in Australia (NRA 1997a) as outlined above.

Paraquat is readily hydrolysed in alkaline conditions but not in acidic or neutral water, and it undergoes photodecomposition (Tomlin 1994).

Aquatic toxicology

Freshwater algae: no data for paraquat.

Freshwater fish: 10 species, 48 to 96-hour LC50, 5200 to 156,000 µg/L (48 h figures as high as 200, 269 and 570 mg/L were reported for two species of fish).

Freshwater crustaceans: seven species, 48 to 96-hour LC50 or EC50 (immobilisation) 1300 to 11,000 µg/L.

Freshwater amphibians: five species, 96-hour LC50, 500 to 28,000 µg/L.

No marine data were available for paraquat.

Factors that modify toxicity

No data were available.


A freshwater low reliability trigger value of 0.5 µg/L was derived for paraquat using an assessment factor (AF) of 1000. In absence of marine data, this was adopted as a marine low reliability trigger value. These figures should only be used for use as indicative interim working levels.


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.

NRA 1997a. Database extraction of selected pesticides: Registered uses in Australia, National Registration Authority, July 1997, Canberra.

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