Dichlorvos is a contact and stomach-acting insecticide and is used as a household and public health insecticide. Dichlorvos, as the active ingredient of Nuvan 500 EC (also known as Nuvan 50 EC and Aquaguard, 50% wt/vol or 500 g ai l^-1), was used extensively in salmon farming to control the salmon louse Lepeophtheirus salmonis (although it appears its use in this industry is diminishing).

The principal, direct routes of entry for dichlorvos into waters include industrial effluents and accidental discharges (e.g. from pesticide manufacturing plants, formulation plants and marketing outlets), use in salmon fisheries, disposal of unused insecticide and the cleaning of application and mixing equipment. Dichlorvos may also indirectly enter the aquatic environment via spray drift during application and in land run-off.

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. Only one water column concentration was found to approach the EQS value (see Appendix D). Monitoring data were not available for sediments or biota.

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

Dichlorvos is rapidly degraded in water both chemically and biologically. The main degradation process is hydrolysis. Persistence in water is low (days) and depends on the pH and temperature (Jones and Stewart 1996).

The vapour pressure of dichlorvos is relatively low so it is therefore unlikely that volatilisation is a significant removal process from the aquatic environment. Adsorption is not expected to be a significant removal pathway due to the low octanol-water (log Kow = 1.4) and soil organic carbon-water (log Koc = 1.34) partition coefficients.

An exhaustive literature review on the toxicity of dichlorvos 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 (Jones and Stewart 1996). The most sensitive groups of organisms have been identified.

The principal source of dichlorvos in the marine environment is the direct application to salmon farms located in estuaries and coastal waters.

Only two studies investigating the toxicity of dichlorvos to marine algae have been reported and the results indicate a relatively high tolerance compared to other phyla.

Jones and Stewart (1996) reviewed data on the toxicity of dichlorvos to marine organisms. The authors concluded that crustaceans were the most sensitive class. The lowest acute 96 hour LC50 values for adult crustacean species include 4 µg l^-1 for the sand shrimp Crangon septemspinosa, 15 µg l^-1 for the grass shrimp Palaemonetes vulgaris (Eisler 1969), 4.4 µg l^-1 for the shrimp Crangon crangon and 9 µg l^-1 for the amphipod Hyale nilssoni (Thain et al 1990, McHenery et al 1990a).

Early life stages of the common lobster Homarus gammarus show similar sensitivity to the effects of dichlorvos as the adult sand shrimp. The 12, 24, 48 and 96 hour LC50s for stage 5 lobster larvae were 53, 28, 11 and 5.7 µg l^-1, respectively (Ciba-Geigy SP 3560.25 cites in Jones and Stewart 1996) and the corresponding calculated 12 and 96 hour NOEC values were 6.25 and 1.56 µg l^-1, respectively (McHenery et al 1990b).

Marine fish species appear to have similar sensitivities to the toxic effects of dichlorvos as freshwater fish species. Reported acute LC50 values for adult fish range from 200 µg l^-1 for striped mullet (Eisler 1970) to 5 mg l^-1 for goby (Hirose and Kitsukawa 1976). Early life stages of herring appear to be only marginally more sensitive with a 96 hour LC50 of 122 µg l^-1 (Ciba-Geigy SP 3560.25 cited in Jones and Stewart 1996).

Jones and Stewart (1996) concluded that bioaccumulation in marine organisms was likely to be low.

Potential effects include:

• toxicity to invertebrates (particularly crustacea) and fish to concentrations above the EQS of 0.04 mg l^-1 (annual average) and 0.6 mg l^-1 (maximum allowable concentration) in the water column.

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