Diazinon in freshwater and marine water

​​Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Extracted from Section 8.3.7 ‘De​tailed 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

Organophosphorus pesticides are derivatives of phosphoric, phosphonic, phosphorothioic, or phosphonothioic acids, comprising many chemicals with a wide range of uses (WHO 1986). They exert their acute effects in insects, fish, birds and mammals by inhibiting the acetylcholinesterase (AChE) enzyme, but may also have a direct toxic effect (WHO 1986).

Diazinon (CAS 333-41-5) is a phosphorothioate OP pesticide, first introduced by Ciba-Geigy AG. It is a non-systemic pesticide and acaricide which acts by contact, ingestion and breathing vapour (Tomlin 1994). Its IUPAC name is O,O-diethyl O-2-isopropyl-6-methylpyrimidin-4-yl phosphorothioate, molecular formula is C12H21N2O3PS and molecular weight is 304.3. It is soluble in water to 60 mg/L at 20°C and its log Kow is 3.3 (Tomlin 1994). Kamrin (1997) states that ‘diazinon does not bioconcentrate significantly in fish’. The current analytical practical quantitation limit (PQL) for diazinon in water is 0.1 mg/L (NSW EPA 2000).

Uses and environmental fate

Diazinon is used for controlling sucking and chewing insects and mites on a wide variety of crops, for fruit flies on harvested fruit as well as flies, cockroaches and other household pests (Tomlin 1994). In Australia, diazinon has almost 450 registered uses (NRA 1997a) including over 50 food crops such as fruit, root and leaf vegetables, mushrooms, rice, nuts, cereal, and non-food crops such as cotton, turf, trees and nursery plants. Diazinon is commonly used on farm and pet animals against ectoparasites (NRA 1997a). It is also used for pest control in domestic, industrial and agricultural buildings, boats, trains and other vehicles, food processing areas, food stored animal hides, on garbage tips and on ponds against mosquitoes (NRA 1997a). Diazinon was a common toxicant in sewage treatment plant effluents in the USA and its source was traced to household insecticide use (D Mount pers. comm. 1997).

Diazinon hydrolyses more rapidly in acid than in alkaline conditions; its DT50 at pH 7.4 is 185 days and at pH 10.4, it is 6 days (Tomlin 1994). Hence it is relatively persistent in water at neutral pH. Diazinon is strongly adsorbed to soil with a Kom of 332 mg/g (Tomlin 1994), and adsorption to sediments is expected. It is degraded by initial oxidation to the diazoxon and hydrolysis with a laboratory DT50 of around 11 to 21 days (Tomlin 1994). The oxon is more toxic than the parent.

Aquatic toxicology

Toxicity of organisms to diazinon varied widely, even within the same group, but many species were extremely sensitive to diazinon. Algae and molluscs were generally least sensitive and cladocerans most sensitive.

Freshwater fish: 23 species, 48 to 96-hour LC50 figures varied widely for different species, from 22,000 to 24,000 µg/L. It was difficult to classify any particular species as outliers. The most sensitive species were Anguilla anguilla, Lepomis macrochirus and Oncorhynchus mykiss, while the least sensitive were two Cyprinus spp. and Pimephales promelas. Chronic no observed effect concentrations (NOECs) for early life-stage P. promelas (growth) were 86 to 160 µg/L.

Freshwater crustaceans: 11 species, 48 to 96-hour LC50 or EC50 (immobilisation) of 0.2 to 22.0 µg/L. Cladocerans were generally among the more sensitive species. Outlying figures were reported for Asellus hilgendorfi (250 µg/L); Gammarus lacustris (170 to 230 µg/L; other Gammarus spp. were very sensitive) and the crab Orconectes propinquus (537 µg/L). A chronic NOEC for Daphnia magna (immobilisation and reproduction) of 0.2 µg/L gave an acute-to-chronic ratio (ACR) of 4.

Freshwater insects: eight species, 48 to 96-hour LC50 of 25 to 140 µg/L, although low figures of 0.03 to 10.7 µg/L were reported for an additional species, Chironomus tentans (geometric meanof 0.22 mg/L).

Freshwater molluscs: seven species, 48 to 96-hour LC50 of 2500 to 20,000 µg/L although a relatively low figure of 48 µg/L was reported for Physagyrina.

Other freshwater invertebrates: two species, 48 to 96-hour LC50 of 1500 to 6160 µg/L for annelids, although an additional species (rotifer) showed figures of 11,000 to 31,000 µg/L.

Freshwater algae: seven species, 48 to 96-hour EC50 (growth) of 2500 to 20,000 µg/L.

Freshwater mesocosms: Giddings et al. (1996) tested 18 outdoor microcosms with diazinon but used only two treatments. A 70-day NOEC of 4.3 µg/L was calculated. Experiments in streams with diazinon were unreplicated (Arthur et al. 1983). Neither of these experiments could be used for deriving guidelines.

Marine crustaceans: two species, 96-hour LC50, 4.2 to 21.0 µg/L.

Australian and New Zealand data

The only Australian data were for three waterfleas: Ceriodaphnia dubia had a 24-hour EC50 (immobilisation) of 2.3 to 4.9 µg/L; Daphnia carinata, 1.2 µg/L and Moina australiensis with 5.2 µg/L. These were excluded because of the short duration but they were within the range for overseas species.

Factors that modify toxicity

The toxicity of diazinon is significantly increased at higher temperatures. The 48-hour LC50 to Aplocheilus latipes decreased from 24,000 µg/L at 10°C, to 11,000 µg/L at 20°C and 600 µg/L at 30°C, an overall increase of 40-fold (Tsuji et al. 1986).


Amoderate reliability freshwater trigger value of 0.01 mg/L was derived for diazinon using the statistical distribution method with 95% protection and an ACR of 17.5. With very limited marine data, 0.01 mg/L was adopted as a marine low reliability trigger value. This figure should only be used as an indicative interim working level.


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.

Arthur JW, Zischke JA, Allen KN & Roger O 1983. Effects of diazinon on macroinvertebrates and insect emergence in outdoor experimental channels. Aquatic Toxicology 4, 283-301.

Giddings JM, Biever RC, Annunziato MF & Hosmen AJ 1996. Effects of diazinon on large outdoor pond microcosms. Environmental Toxicology and Chemistry 15, 618-629.

Kamrin MA 1997. Pesticide profiles: Toxicity environmental impact and fate. CRC Press, Lewis Publishers, Boca Raton Fl.

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.

Tsuji S, Tonogai Y, Ito Y & Kanoh S 1986. The influence of rearing temperatures on the toxicity of various environmental pollutants for killifish (Oryzias latipes). Eisei Kagaku 32, 46-53.

WHO 1986. Environmental health criteria 63.Organophosphorus insecticides: A general introduction. World Health Organization, Geneva.