Molinate 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
Molinate (CAS 2212-67-1) is a thiocarbamate herbicide originally produced by Stauffer Chemical (now Zeneca) (Tomlin 1994). Its IUPAC name is S-ethyl azepane-1-carbothioate; S-ethyl perhydroazepin-1-carbothioate. Its formula is C9H17NOS and molecular weight is 187.3. It is soluble in water to 88 mg/L at 20°C and its log Kow is 2.88. The current analytical practical quantitation limit (PQL) for molinate in water is 0.5 mg/L (NSW EPA 2000).
Uses and environmental fate
Molinate is a selective systemic herbicide, which inhibits germination after rapid uptake by roots (Tomlin 1994). Molinate is used primarily in rice, particularly in the irrigation areas of south-western New South Wales. It has been measured at concentrations up to 3.6 µg/L in irrigation supply water and over 200 µg/L in drainage water from rice farms (Korth et al. 1995).
Molinate persists in aerobic, acidic soil for 8 to 25 days (DT50) but persists up to 160 days in flooded soil (Tomlin 1994).
Aquatic toxicology
Freshwater fish: nine species, 48 to 96-hour LC50 of 43 to 39,500 µg/L. The low figure was for Cyprinus carpio but another publication reported a figure of 29 mg/L for this species.
Freshwater crustaceans: 10 species, 48 to 96-hour LC50 or EC50 (immobilisation) of 180 to 33,200 µg/L. Ostracods and cladocerans were most sensitive and crayfish and prawns, least. No observed effect concentration (NOEC) (8 days) for Australian Moina australiensis (reproduction) was 110 µg/L, giving an acute-to-chronic ratio (ACR) of 22 (Julli & Krassoi 1995). The 8-day LC50 was 150 µg/L. The acute toxicity (48-hour EC50) to M. australiensis was 2400 µg/L and to the Australian Ceriodaphnia dubia was 430 µg/L.
Freshwater insects: one species, 96-hour LC50 of 340 to 370 µg/L.
Freshwater algae: no data available.
Freshwater amphibians: two species, 96-hour LC50, 14,000 to 34,000 µg/L.
Marine crustaceans: one species, 96-hour LC50 of 1300 to 9910 µg/L. Chronic NOEC (42 to 56 days) for shrimp Neomysis mercedis (growth and reproduction) of 25.6 µg/L giving an ACR of 354.
Marine molluscs: one species, 96-hour LC50 of 197,000 µg/L.
Australian and New Zealand data
These are listed above under freshwater crustaceans for two waterfleas.
Factors that modify toxicity
None were reported.
Guidelines
A freshwater moderate reliability trigger value for molinate of 3.4 µg/L was derived using the statistical distribution method (95% protection) and application of a freshwater ACR of 21.8.
A marine low reliability trigger value of 3.4 µg/L was derived by adopting the freshwater figure. This 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.
Julli M & Krassoi FR 1995. Acute and chronic toxicity of the thiocarbamate herbicide, molinate, to the cladoceran Moina australiensis Sars. Bulletin of Environmental Contamination and Toxicology 54, 690-694.
Korth W, Thomas M, Foster S, McCorkelle G & Bowmer KH 1995. Toxicity of rice and maize pesticides to Ceriodaphnia sp: Implications for management of irrigation drainage water in Australia. Australasian Journal of Ecotoxicology 1,
55–62.
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.
