Parathion 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

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).

Parathion (CAS 56-38-2), otherwise known as ethyl parathion, is a phosphorothioate OP pesticide, introduced by American Cyanamid Co, ICI Plant Protection Ltd and Monsanto Chemical Co (Tomlin 1994). Parathion-methyl (CAS 298-00-0) is a related chemical. Parathion is a non-systemic insecticide and acaricide which acts by contact, ingestion and inhalation (Tomlin 1994). Its IUPAC name is O,O-diethyl O-4-nitrophenyl phosphorothioate, formula is C10H14NO5PS and molecular weight is 291.3. Parathion is only soluble in water to 11 mg/L at 20°C and its log Kow is 3.83 (Tomlin 1994). The current analytical practical quantitation limit (PQL) for parathion in water is 0.1 mg/L (NSW EPA 2000).

Uses and environmental fate

Parathion is used for control of sucking and chewing insects and mites in a variety of crops. In Australia, parathion has over 80 uses and parathion-methyl a similar number and variety (NRA 1997a). They are used on fruit, grapes, and vegetables, as well as potatoes, clover, cotton and tobacco.

Parathion hydrolyses only slowly in acidic pH up to 7 but more rapidly in alkaline media. Its DT50 was 260 days at pH 7 and 130 days at pH 8 (Tomlin 1994). It has low mobility in soil and undergoes rapid degradation in biologically active soils to short lived metabolites (e.g. paraoxon, amino parathion and 4-nitrophenol).

Aquatic toxicology

Parathion has high toxicity to fish and very high toxicity to crustaceans and insects. Its toxicity to algae and other invertebrates is moderate.

Freshwater fish: 14 species, 48 to 96-hour LC50 of 18 to 3600 µg/L. Figures for Oncorhynchus mykiss varied from 750 to 10,000 µg/L and outlying figures were reported for Gambusia holbrooki (2700 to 26,500; Australian data) and Heteropneustes fossilis (26,000 to 31,000).

Freshwater crustaceans: seven species 48 to 96-hour LC50 or EC50 (immobilisation) of 0.04 (Orconectes nais) to 5.1 µg/L. Outlying results were reported for additional species Asellus aquaticus (12 to 23 µg/L), A. brevicaudatus (213 to 2130 µg/L) and Gammarus lacustrus (3.5 to 12.8 µg/L). Tomlin (1994) reported a 48-hour EC50 to D. magna of 0.0025 mg/L but this could not be verified. A 21-day chronic no observed effect concentration (NOEC) for reproduction of D. magna was 0.002 µg/L.

Freshwater insects: 15 species, 0.3 to 32 µg/L. Chaoborus sp was most sensitive and a stonefly, Pteronarcys sp., least. Some outlying nominal figures between 32 and 100 µg/L were reported. A 21 days chronic NOEC for mortality of Choeon dipterum was 0.15 µg/L.

Freshwater molluscs: three species, 96-hour LC50, 6570 to 17,000 µg/L.

Freshwater annelid: one species, 96-hour LC50 of 7100 µg/L.

Freshwater algae and ciliates: three species, 48 to 96 hour EC50 (growth) of 2900 to 15,000 µg/L.

Freshwater mesocosms: two experiments on ponds treated with methyl parathion were reported but neither had sufficient number of treatments (Stephenson & Kane 1984, Crossland 1984).

Marine crustaceans: one species, 96-hour LC50 of 360 to 740 µg/L although a measured figure of 0.57 µg/L was reported (AQUIRE 1994).

Marine annelids: one species, 48-hour LC50 of 2700 µg/L.

Australian and New Zealand data

Data (96-hour LC50) were available for two fish, the introduced mosquitofish Gambusia holbrooki (2700 to 21,650 µg/L) and the firetail gudgeon Hypseleotris gallii (1300 µg/L). The mosquitofish were exceptionally tolerant but the gudgeon data were within the range for overseas species.

Factors that affect toxicity

Toxicity of parathion was affected little by changes in hardness (Johnson & Finley 1980). Toxicity was increased by 3 to 10 times for Asellus when temperature was increased from 15°C to 21°C, and similarly for bluegills P. promelas between 7°C and 29°C (Johnson & Finley 1980).


A freshwater moderate reliability trigger value of 0.004 µg/L was derived for parathion using the statistical distribution method with 95% protection. An acute-to-chronic ratio (ACR) of 71.2 was used. The 99% protection level was 0.0007 mg/L. The 95% protection level was below all acute toxicity figures but was above the chronic NOEC for D. magna. However, the 95% figure should provide adequate protection for most slightly to moderately disturbed systems. These figures are below the analytical detection limit and any reliable detection of parathion may indicate exceedance of the trigger value.

The marine data were even more limited. A marine low reliability trigger value of 0.004 µg/L was adopted for parathion from the freshwater figure should only be used as an indicative interim working level only.


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.

AQUIRE (Aquatic Toxicity Information Retrieval Database) 1994. AQUIRE standard operating procedures. USEPA, Washington, DC.

Crossland NO 1984. Fate and biological effects of methyl parathion in outdoor ponds and laboratory aquaria; II. Effects. Ecotoxicology and Environmental Safety 8, 482–495.

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.

Stephenson RR & Kane DF 1984. Persistence of effects of chemicals in small enclosures in ponds. Archives of Environmental Contamination and Toxicology 13, 313–326.

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

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