MCPA 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
MCPA (monochlorophenoxyacetic acid) (CAS 94-74-6), is a chlorophenoxyacetic acid herbicide, similar to 2,4-D. It is a strong acid and acts by uptake by roots and leaves. Its IUPAC name is (4-chloro-2-methylphenoxy)acetic acid. Its formula is C9H9ClO3, and molecular weight is 200.6. It is soluble in water to 734 mg/L at 25°C. The acid has a log Kow of 2.75 at pH 1 and 0.46 at pH 5, and a pKa of 3.07.
Uses and environmental fate
MCPA and its salts and esters is used widely as a selective systemic herbicide, with hormone-like action, against a large variety of weeds in cereals any many other food crops, pasture, rights of way, forests, turf and lawn. It has almost 9000 registered uses in Australia (NRA 1997a).
Freshwater fish: five species, 48 to 96-h LC50 of 25,000 to 621,000 µg/L (outlying figures in different references: 1500 µg/L for Cyprinus carpio compared to 97,000 to 163,500 µg/L; and 1440 to 2460 µg/L, compared to 59,000 µg/L).
Freshwater amphibian: one species, 24-hour LC50 of 10,000 µg/L. This could not be used.
Freshwater crustaceans: one species: 96-hour LC50 of 11,000 µg/L.
Freshwater insects: one species, 96-hour LC50 of 335,000 µg/L.
Freshwater algae: no reliable data could be screened, although CCME (1991, Appendix XVIII) report a range of data on various ester and salt formulations under different conditions. The most sensitive figure was a 5-day lowest observed effect concentration (LOEC), for the MCPA acid to the diatom, Navicula pelliculosa, of 26 µg/L with an LC50 (cell density) of 630 µg/L. Higher LOECs were reported with the dimethylamine salt (73 µg/L) and the 2-ethylhexyl ester (77 µg/L). Low no observed effect concentrations (NOECs) and LOECs were reported for Anabaena flosaquae for the acid (470 and 1200 µg/L), the 2-ethylhexyl ester (17 and 53 µg/L) and the dimethylamine salt (NOEC 61 µg/L).
For the duckweed Lemma gibba, a NOEC of 130 µg/L and LOEC of 260 µg/L were reported for the MCPA acid, a LOEC of 24 µg/L for the dimethylamine salt and a NOEC of 24 µg/L for the 2-ethylhexyl ester (CCME 1991).
No marine data were available. CCME (1991, Appendix XVIII) report an oyster abnormality EC50 of 155,000 µg/L.
Factors that modify toxicity
No specific data are available but readers are referred to ‘2,4-D in Freshwater and Marine Water’ for some indications of trends in toxicity, particularly with regard to ester and salt formulations. CCME (1991, Appendix XVIII) refers to varying toxicity of ester formulations, many of which were non-toxic up to water solubility.
A freshwater low reliability trigger value of 1.4 µg/L was calculated for MCPA acid using an assessment factor (AF) of 1000 on the low carp figure. Although algal toxicity appears high (from CCME 1991), this figure appears sufficiently protective and is similar to the Interim Canadian guideline (2.6 µg/L). In the absence of marine data, it is recommended that 1.4 µg/L be adopted as a marine low reliability trigger value. Both figures should be used only as indicative interim working levels.
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.
CCME 1991. Canadian water quality guidelines, updates. Canadian Council of Ministers for the Environment, Ottawa.
NRA 1997a. Database extraction of selected pesticides: Registered uses in Australia, National Registration Authority, July 1997, Canberra.