Thiobencarb 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
Thiobencarb (CAS 28249-77-6) is a selective thiocarbamate herbicide, absorbed by foliage and roots, first introduced by Kumai Chemical Industry Co. Its IUPAC name is S-4-chlorobenzyl diethylthiocarbamate, formula is C12H16ClNOS and molecular weight is 257.8. Thiobencarb is soluble in water to 30 mg/L (at 20°C) and has a log Kow of 3.4 (Tomlin 1994). The current analytical practical quantitation limit (PQL) for thiobencarb in water is 5 µg/L (NSW EPA 2000).
Uses and environmental fate
Thiobencarb is used as a rice field herbicide against a selected range of weeds. Thiobencarb is stable in water of pH 5 to 9 for 30 days at 21°C (Tomlin 1994) and is stable to light. It is readily adsorbed by soil and bound effectively. Half-life in soil is 2 to 3 weeks under aerobic conditions but much longer (6 to 8 months) under anaerobic conditions (Tomlin 1994).
Freshwater fish: 14 species, 48 to 96-hour LC50, 110 to 2950 µg/L.
Freshwater crustaceans: six species, 48 to 96-hour LC50, 200 to 9240 µg/L.
Freshwater molluscs: five species, 48 to 96-hour LC50, 5000 to 82,600 µg/L.
Freshwater algae: three species, 72-hour EC50 (population growth), 17 to 3790 µg/L (Selenastrum was much more sensitive than Chlorella: Sabater & Carasco 1996). A 96-hour NOEC growth of 5 µg/L for Scenedesmus ac
Marine algae: one species (diatom), 96-hour EC50 (growth and population growth), 377 to 650 µg/L.
Marine fish: one species, 96-hour LC50, 658 to 1400 µg/L.
Marine crustaceans: two species, 96-hour LC50, 150 to 350 µg/L. Chronic NOEC (42 to 56 days, growth and mortality) of 3.2 µg/L, giving an acute-to-chronic ratio (ACR) of 95.
Factors that modify toxicity
None were identified.
A freshwater moderate reliability freshwater trigger value for thiobencarb of 2.8 µg/L was derived using the statistical distribution method (95% protection) and applying an ACR of 95. Although there are marine data on fish, crustaceans and algae — the Organisation for Economic Co-operation and Development (OECD) minimum pre-marketing set of data — it was considered preferable to adopt the freshwater figure as a marine low reliability trigger value of 2.8 µg/L. This should only be used 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.
NSW EPA 2000. Analytical Chemistry Section, Table of Trigger Values 20 March 2000, LD33/11, Lidcombe, NSW.
Sabater C & Carrasco JM 1996. Effects of thiobencarb on the growth of three species of phytoplankton. Bulletin of Environmental Contamination and Toxicology 56, 977-984.
Tomlin C 1994. The pesticide manual: A world compendium. 10th edn, British Crop Protection Council & Royal Society of Chemistry, Bath, UK.