Bentazone is a contact herbicide absorbed by the leaves and is used to control broad leafed weeds in winter and spring cereals. In 1992, 85 tonnes of bentazone were used in Great Britain, which declined to 73 tonnes in 1994. In 1994, it was the 40th most commonly used active substance on arable crops (PSD Data, Pers. Comm., Dr Marsden, DoE 1996).

The main mode of action of bentazone is by inhibition of photosynthesis, causing depletion of carbohydrate reserves and loss of chloroplast membrane integrity (Fletcher and Kirkwood 1982).

The major diffuse sources of bentazone into the aquatic environment are likely to be run-off from soils or accidental over-spray as a result of its use on agricultural land. Bentazone has a low affinity for particulate or organic carbon material, log Kow of 0.35, (RSC 1991); log Koc of 2.42 (Donigian and Carsel 1987).

Murgatroyd et al (1996) reported that no exceedances to the EQS were measured in estuarine or coastal waters by the NRA in 1993. In other parts of Western Europe, up to 1 µg l^-1 (Leistra and Boestsen 1989) of bentazone has been recorded. However, in estuarine and coastal waters, bentazone concentrations are generally below detection limits (Readman et al. 1993).

The available data suggest that concentrations of bentazone in UK coastal and estuarine waters are unlikely to exceed relevant quality standards derived for the protection of saltwater life.

Murgatroyd et al (1996) reviewed the fate and behaviour and aquatic toxicity of bentazone. However, little information was available.

Photolysis appears to be the main abiotic degradation process for bentazone (80% degradation of bentazone in a 24 hour period when exposed to artificial sunlight, Nilles and Zabik 1975), as in water, no hydrolysis of 1 mg l^-1 of bentazone was observed after 122 days at 22 °C (US EPA 1987).

An exhaustive literature review on the toxicity of bentazone to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Murgatroyd et al 1996). The most sensitive groups of organisms have been identified.

Murgatroyd et al (1996) found no reliable data on the toxicity of bentazone to marine organisms, but concluded that bentazone was of low to moderate toxicity to freshwater organisms, with acute effects ranging from 10-6,232 mg l^-1 (saltwater organisms are likely to exhibit similar sensitivity).

Algae appear to be the most sensitive species. Bentazone affects algae through inhibition of photosynthesis and, although no data are available, aquatic macrophytes are likely to be of equal or greater sensitivity. The majority of reliable toxicity data indicates that invertebrates and fish are much less sensitive to bentazone than algae.

No data could be located.

Murgatroyd et al (1996) found no data on bioaccumulation in marine orgasms, but concluded, based on evidence for freshwater organisms, that the bioaccumulation potential for bentazone was low.

Potential effects include:

• toxic effects on algae and macrophytes at concentrations above the EQS of 500 mg l^-1 (annual average) and 5,000 mg l^-1 (maximum allowable concentration) in the water column.

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