2,4-D in freshwater and marine water

​​Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Description of chemical

2,4-D (2,4-dichlorophenoxyacetic acid) (CAS 94-75-7) and its various salt and ester formulations is one of the most commonly used herbicides in Australia (NRA 1997a). It is a strong acid and forms water-soluble salts with sodium, potassium, calcium and other alkaline metals. Sequestering agents are added to formulations to prevent precipitation of calcium and magnesium salts (Tomlin 1994). Its formula is C₈H₆Cl₂O₃ and molecular weight is 221. The acid has log K_ow between 2.6 and 2.9, a pKa of 2.64 at pH 1 and water solubility of 900 mg/L at 25°C. The current analytical practical quantitation limit (PQL) for 2,4-D in water is 0.6 µg/L (NSW EPA 2000).

In Australia, the parent acid is registered as a herbicide, as well as the sodium, potassium, diethanolamine, dimethylamine, isopropylamine and tri-isopropanolamine salts, as well as ethyl, butoxyethyl, n-butyl and isobutyl esters (NRA 1997a). The salt formulations are highly soluble in water but the esters have very low solubility.

Uses and environmental fate

2,4-D is a selective systemic herbicide, which is taken up by roots (acid and salts) or leaves (esters), and it acts as a growth inhibitor (Tomlin 1994). The acid and its salts and esters have over 17,000 registered uses in Australia (NRA 1997a), commonly for control for broad-leaved weeds in cereals, pasture, turf, sugar cane, rice, cotton, orchards and plantations, and rights-of-way. It is an important herbicide in conservation tillage. Typical weeds include Patterson’s curse, castor oil bush, nutgrass, nettle, thistle, boxthorn, blackberry, banana suckers, lantana, boneseed and many others (NRA 1997a). The ethyl ester has aquatic uses against water hyacinth.

Environmental fate is reviewed more fully by WHO (1989). It is rapidly degraded by microbes in soil with a half-life <7 days and K_oc around 60 (Tomlin 1994). Higher sediment or nutrient loads in water and higher temperatures, were associated with more rapid degradation (Nesbitt & Watson 1980). 2,4-D does not bioaccumulate appreciably.

Aquatic toxicology

Aquatic toxicology of 2,4-D acid formulation: figures are in mg/L (i.e. x 1000 µg/L).

Freshwater fish: 23 species, 48-96 h LC50, 1.4-4800 mg/L.

Freshwater crustaceans: eight species, 48-96 h LC50 or EC50 (immobilisation), 1.8-144 mg/L.

Freshwater insects: two species, 48-96 h LC50, 1.6-44 mg/L.

Freshwater rotifer: one species, 48-h LC50, 117 mg/L.

Freshwater ciliate: one species, 72-h EC50 (growth) 104-485 mg/L.

Marine fish: two species, 48-96 h LC50, 1.5-3.5 mg/L.

Marine crustaceans: one species, 48-96 h LC50, 3370-6730 mg/L (crab Chasmagnathus granulata). A 24-h figure of 402 mg/L was reported for Artemia sp.

Marine mollusc: one species, 96-h LC50, 259 mg/L.

Marine algae: no data available.

Factors that modify toxicity

Toxicity of 2,4-D to Cyprinus carpio doubled for every 10°C increase in temperature: LC50 decreased from 40.8 mg/L at 17°C to 5.6 mg/L at 39°C (WHO 1989). This would imply a doubling of toxicity for 10°C increase in temperature.

Variations in toxicity with pH depended on the formulation (Johnson & Finley 1980). The acid, butyl ester and diethyl amine salt were less toxic to fish at pH 8.5 than at pH 6.5 by a factor of about 2. The dodecyl/tetradodecyl amine was 3.5 times more toxic to P. promelas at the higher pH 8.5.

Ageing of test solutions for 21 days reduced toxicity of the butyl ester and propylene glycol butyl ester by a factor of 2 but did not affect toxicity of amine salts (Johnson & Finley 1980). Eggs of fish were considerably less sensitive to 2,4-D than fingerlings or fry.

There appears to be some effect of hardness on toxicity but this has not been specifically studied (WHO 1989). It would probably be greater for salts and acids than for the esters. Johnson & Finley (1980) reported no change in toxicity with hardness between 44 and 300 mg/L.

Guideline

Although no algal data were available for guideline derivation, data on over 25 freshwater algal species were available from AQUIRE (1994). The screening process removed most of these data, mainly because end-points were not reported or test duration did not fit the criteria. Nevertheless, these data indicated toxicity to alga less than or similar to toxicity to species listed above. For example, the 20-d EC50 and LOEC to Scenedesmus quadricauda was 98 and 70 mg/L respectively.

Afreshwater moderate reliability trigger value of 280 µg/L was calculated for 2,4-D acid using the statistical distribution method with 95% protection and an AF of 10.2.

As there are limited marine data a marine low reliability trigger value of 280 µg/L for 2,4-D acid was adopted from the freshwater figure. 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.

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

Nesbitt HJ & Watson JR 1980. Degradation of the herbicide 2,4-D in river water. The role of suspended sediment, nutrients and water temperature. Water Research 14, 1689–1694.

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.

WHO 1989. Environmental health criteria — 84: 2,4-dichlorophenoxyacetic acid (2,4-D) — Environmental aspects, World Health Organization, Geneva.