Trifluralin in freshwater and marine water
Toxicant default guideline values for protecting aquatic ecosystems
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
Trifluralin (CAS 1582-09-8) is a 2,6-dinitroaniline herbicide, first introduced by Eli-Lilly and Co (now DowElanco). It is a selective soil herbicide, which disrupts cell division and inhibits root development after uptake in the hypocotyl region (Tomlin 1994). Its IUPAC name is α,α,α-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine, molecular formula is C13H16F3N3O4 and molecular weight is 335.3. It has low water solubility to 0.395 mg/L at pH 7 (technical) and a Kow of 5.3. The current analytical practical quantitation limit (PQL) for trifluralin in water is 0.1 µg/L (NSW EPA 2000).
Uses and environmental fate
Trifluralin is usually applied as a pre-emergence herbicide for grasses and broad-leaved weeds in a variety of vegetables, fruit, winter cereals and cotton. In Australia, trifluralin has around 2700 uses in about 35 food crops, as well as cotton, flowers and improved pasture.
Trifluralin is stable to hydrolysis but may be decomposed by ultraviolet (UV) light. It is strongly adsorbed by soil and resistant to leaching. Trifluralin has a moderate tendency to bioaccumulate (Kamrin 1997).
Freshwater fish: 12 species, 48 to 96-hour LC50 of 8.4 to 2200 µg/L. Most figures were < 500 µg/L. The most sensitive species, Lepomis macrochirus, had other data between 18 and 190 µg/L and the geometric mean was 48 µg/L. Similarly Oncorhynchus mykiss figures varied from 10 µg/L to 210 µg/L with a geometric mean of 41 µg/L.
Freshwater amphibians: one species, 48 to 96-hour LC50, 100 to 170 µg/L.
Freshwater crustaceans: 12 species, 48 to 96-hour EC50 (immobilisation) or LC50 of 37 to 2200 µg/L. Shrimp, ostracods and copepods were among the most sensitive, and Gammarus the least, although toxicity to two crayfish exceeded the water solubility by > 2 times.
Freshwater insects: two species, 48 to 96-hour LC50, 1000 to 4200 µg/L.
Freshwater molluscs: one species, 48-hour LC50 of 8000 µg/L (toxicity to three other species exceeded the water solubility by ≥2 times).
Freshwater algae: no data.
Marine crustacean: one species; 96-hour LC50, 300 to 330 µg/L.
Australian and New Zealand data
Data were reported for the mosquitofish, Gambusia holbrooki, introduced in Australia, of 1100 µg/L and for the gudgeon, Hypseleotris gallii, of 270 µg/L.
Factors that modify toxicity
Variations in temperature, hardness and pH did not appear to alter the toxicity of trifluralin to fish (Johnson & Finley 1980). Early life-stages of fish were more sensitive than either yolk-sac fry or fingerlings (Johnson & Finley 1980).
A moderate reliability freshwater guideline figure of 4.4 µg/L was derived for trifluralin using the statistical distribution method with 95% protection and the default acute-to-chronic ratio (ACR) of 10. The 99% protection level was 2.6 µg/L and is recommended as the trigger value for slightly to moderately disturbed systems, both because of its potential to bioaccumulate (few data available) and the proximity of the 95% figure to the lowest fish acute toxicity figure.
Trifluralin has a moderate tendency to bioaccumulate. The 99% protection level is recommended for slightly to moderately disturbed systems if there are no data on bioaccumulation for the specific site.
In the absence of sufficient marine data this figure (2.6 µg/L) was adopted as a low reliability trigger value for use only 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.
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.
Kamrin MA 1997. Pesticide profiles: Toxicity environmental impact and fate. CRC Press, Lewis Publishers, Boca Raton
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.