Dimethoate is extensively used as contact and systemic organophosphate insecticides on a wide range of crops.

Point sources for the contamination of surface waters is from the accidental spillage at manufacture sites, formulation and storage, and during transport and handling. Farms are another potential point source by either discharging directly into water courses or indirectly through the sewer network, or in surface run-off. Diffuse sources include run-off and leaching from agricultural land after pesticide application, spray drift during application and discharge of wastes from the domestic sector (either direct or via the sewer system) (Seager 1987).

However, the input of dimethoate to the marine environment is likely to be associated primarily with river outflow.

Monitoring data from the National Rivers Authority and the National Monitoring Programme Survey of the Quality of UK Coastal Waters are presented in Appendix D.

Dimethoate is soluble in water, and will not be expected to sorb onto sediment or bioconcentrate in aquatic organisms. Degradation of dimethoate is dependent on environmental conditions, such as temperature and pH. Heavy metal ions have been found to act as catalysts in the hydrolysis of dimethoate (Murgatroyd and Patel 1996).

Aqueous solutions of dimethoate are thought to be relatively stable. A half-life of 56 days (20 °C; pH 7.3-8.0) has been reported for the biological and chemical degradation of dimethoate in a natural river system (Eichelberger and Lichtenberg 1971). Degradation through hydrolysis may be significant, especially in alkaline waters; half-lives for hydrolysis of 3.7 and 118 days at pH 9 and 7 have been reported (Howard 1991). However, direct photolysis and evaporation of dimethoate from water are not expected to be important processes. A half-life of 8 weeks for biodegradation has been quoted (Howard 1991). However, this may be partially due to hydrolysis and oxidation.

An exhaustive literature review on the toxicity of dimethoate 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 and Patel 1996). The most sensitive groups of organisms have been identified.

There is some evidence that marine autotrophs are particularly sensitive to dimethoate exposure. Ramachandran et al (1984) noted significant reductions in respiration and photosynthesis in five out of six species of `seaweeds' exposed to nominal concentrations of 50 µg l^-1 dimethoate. Similar effects were found with the diatomic alga Concinodiscus concinnus, with a nominal EC50 of 40 µg l^-1 for growth and disruption of cell morphology at 50 µg l^-1 (Ramachandran et al. 1980). However, these values are much lower than those reported for algae by Ibrahim (1983 and 1984), with EC50s for growth ranging from 4.5 to 13 mg l^-1.

The data for marine animals vary widely, particularly for crustaceans. Portmann and Wilson (1971) reported a 48 hour LC50 between 0.3 and 1 µg l^-1 for the common shrimp Crangon crangon, but LC50s for two other crustaceans were both above the highest test concentrations of 33 µg l^-1 and 3.3 mg l^-1 respectively. In other tests, 1 mg l^-1 dimethoate caused 20% and 0% mortality in brown shrimps Pennies aztecus (Butler 1964) and prawns Pennies monodon (Vogt 1987) respectively. Crustaceans are probably the most sensitive organisms, but more studies are needed to confirm this assertion and to provide a more reliable set of data.

The same concentration of 1 mg l^-1 had no effect on longnose killifish Fundulus similis (Butler 1964), whereas for the fish Therapon jarbua, a 96 hour LC50 of 0.7 mg l^-1 was reported (Lingaraja and Venugopalan 1978).

Experimental evidence for bioaccumulation of dimethoate in aquatic organisms is limited to a very few tests, with no data for saltwater species. For the freshwater ciliate protozoan Tetrahymena pyriformis Bhatnagar et al. (1988) obtained a bioconcentration factor (BCF) of 3,547 based on dry weight following exposure for 12 hours to 1 mg l^-1 dimethoate. An equivalent wet weight bioconcentration factor is likely to be between one and two orders of magnitude lower. Kumar et al. (1988) found that the freshwater blue-greens Anabaena sp. and Aulosira fertilissima attained maximum bioconcentration factors based on dry weight of only 71 and 120 respectively when exposed to 1 to 10 mg l^-1 dimethoate over five days, indicating much lower accumulation than in Tetrahymena. The nature of chemical accumulation in unicellular organisms means that bioaccumulation tends to be much greater than in higher organisms, suggesting that bioaccumulation factors in higher organisms are likely to be lower than those indicated here.

Dimethoate has a high water solubility (25 g l^-1 at 21 °C), and a relatively low octanol-water partition coefficient (log Kow 2.71), both of which suggest that the substance will have a low tendency to accumulate in biotic tissues.

Potential effects include:

• acute toxicity to algae, invertebrates (particularly crustacea) and fish at concentrations above the EQS of 1 µg l^-1 (annual average) in the water column.

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