Methoxychlor 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

Most organochlorine pesticides have been phased out of use in recent years, mainly because of their residual properties and potential for bioaccumulation. The guideline trigger values stated are for toxicity only and need to be adjusted for bioaccumulation where appropriate. Where the statistical distribution method was used, figures quoted are the 95% protection levels, usually applicable to slightly to moderately disturbed systems although 99% protection figures are recommended for chemicals that bioaccumulate.

Methoxychlor (CAS 72-42-5) is an organochlorine pesticide of similar basic structure to dichlorodiphenyltrichloroethane (DDT) but with lesser chlorine substitution and more reactive sites for breakdown. Its mode of action is by contact and ingestion. It was first introduced by Ciba-Geigy and DuPont. Its IUPAC name is 1,1,1-trichloro-2,2-bis(4-methoxyphenyl)ethane, molecular formula is C₁₆H₁₅CI₃PO₂ and molecular weight is 345.7. Methoxychlor has a very low solubility in water, 0.1 mg/L at 25°C (Tomlin 1994) and its log K_ow is 4.83 to 5.08 (Hansch et al. 1995). The current analytical practical quantitation limit (PQL) for methoxychlor in water is 0.05 µg/L (NSW EPA 2000).

Uses and environmental fate

Methoxychlor has been used against a wide range of chewing insects in crops, fruit and vegetables as well as against insect pests in animal houses, dairies and industrial premises (Tomlin 1994). It was once used with DDT in the cotton industry but its use has declined substantially.

Methoxychlor is degraded to photolysis and hydrolysis but biodegrades slowly. Daphnia sp., mayfly larvae and Oncorhynchus mykiss accumulated methoxychlor from 1000 to 3000 times when exposed at 50 ng/L (Johnson & Finley 1980). There appeared to be no food-chain biomagnification.

Aquatic toxicology

The toxicity of methoxychlor was high to very high for most species.

Freshwater fish: 16 species, 48 to 96-hour LC50, 1.2 to 75 µg/L. There were some outlying figures among several of these species, Ictalurus punctatus (1800 µg/L), Lepomis macrochirus (420 µg/L), Catastomus commersoni (260 µg/L), O. mykiss (132 µg/L), but these are mostly above the water solubility. The lowest figures (< 10 µg/L) were found for Salmo salar, Salvelinus fontinalis and Pimephales promelas.

Freshwater crustaceans: 11 species, 48 to 96-hour LC50 or EC50 (immobilisation), 0.5 to 34 µg/L.

Freshwater insects: two species, 96-hour LC50, 1.4 to 5 µg/L.

Marine fish: 14 species, 48 to 96-hour LC50, 12 to 150 µg/L.

Marine crustaceans: six species, 48 to 96-hour LC50, 0.42 to 25 µg/L. Two outlying figures of 44 and 130 µg/L were reported for the crab, Cancer magister (in the same study as the 0.4 µg/L figure).

Marine molluscs: one species, 96-hour EC50 (growth) of 97 µg/L.

Factors that modify toxicity

Increases in temperature over 12 to 16°C resulted in only a slight decrease in toxicity to trout and bluegills (Johnson & Finley 1980). Hardness did not appear to affect toxicity.

Guideline

Although freshwater algal data were not available, there was an abundance of acute data for methoxychlor on three families to allow application of a factor of 100, given also that it is an insecticide and high algal toxicity is not expected.

A freshwater low reliability trigger value for methoxychlor of 0.005 µg/L (5 ng/L) was derived using an assessment factor (AF) of 100. Similarly a marine low reliability trigger value of 0.004 µg/L (4 ng/L) was derived using an AF of 100. These should only be used as indicative interim working levels. These figures do not account for bioaccumulation.

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.

Hansch C, Leo A & Hoekman D 1995. Exploring QSAR. Hydrophobic, electronic, and steric constants. American Chemical Society, Washington, DC.

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.

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.