Diuron and linuron are herbicides which are members of what is usually known as the phenylurea group. Diruon is also increasingly being used as a booster biocide in antifoulant paints. Because of their similar mode of action, a consideration of the effects to the marine environment of both linuron and diuron is discussed here.

These compounds are stable to hydrolysis in the normal pH range and exist as the undissociated form in natural waters. The vapour pressures at about 20 °C indicate negligible losses through vaporisation in the environment, and sunlight-induced photochemical decomposition is of relatively minor importance.

Phenylurea herbicides are easily taken up from soil solution by the root systems of plants and rapidly translocated into stems and leaves by the transpiration system, moving primarily via the xylem. Linuron and isoproturon, and to a lesser extent other phenylurea derivatives, can also be taken up through application to the leaves, especially in the presence of surfactants.

The principal mode of action of these substances is through disruption of photosynthesis, probably by inhibition of an enzyme involved in the Hill reaction.

Lewis and Gardiner (1996) have reviewed data on the use and fate and behaviour of these herbicides.

Diuron is used extensively on industrial land and to a limited extent in agriculture. A major user of diuron in the UK was British Rail to control weed growth on railway tracks. Its 32,000 km of track is sprayed annually, with 22 tonnes of diuron used in 1991. This amount probably increased as the triazine herbicides, atrazine and simazine, were banned from August 1993 for non-agricultural uses. Lewis and Gardiner (1996) were unable to estimate the total amount of diuron used on industrial land and around domestic buildings in the UK.

Little information is available on the quantities of linuron used. However, Lewis and Gardiner (1996) estimated that approximately 100 tonnes were used on crops in 1992.

Linuron is mainly used as an agricultural herbicide and, therefore, its main routes of entry into the aquatic environment are likely to be run-off, leaching, spray drift during application, washing down of machinery used during application and accidental spillage.

Monitoring data from the National Rivers Authority (NRA) and the National Monitoring Programme Survey of the Quality of UK Coastal Waters are presented in Appendix D Monitoring data for diuron and linuron were only available for the NRA survey. No water column concentration was found to exceed the EQS value (see Appendix D). Monitoring data were not available for sediments or biota.

The available data suggest that concentrations of diuron and linuron in UK coastal and estuarine water do not exceed relevant quality standards derived for the protection of saltwater life.

Lewis and Gardiner (1996) found few data on the fate and behaviour of these herbicides in water. However, based on the information available, the authors concluded that removal from water was likely to be by biodegradation and adsorption, with half-lives in the range of a few weeks to a few months reported.

An exhaustive literature review on the toxicity of diuron and linuron 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 (Lewis and Gardiner 1996). The most sensitive groups of organisms have been identified.

Lewis and Gardiner (1996) reviewed the limited data on the toxicity of linuron and diuron to saltwater organisms. The authors concluded (although only acute toxicity data were available) that the results of Mayer (1987) for diuron and Kemp et al (1985) for linuron suggested that these herbicides may have similar toxicities to saltwater organisms.

In freshwaters, algae and macrophytes appear to be the species most sensitive to these herbicides. The lowest effect concentrations reported for these species are approximately 20 µg l^-1 and higher for all four herbicides.

Based on data for freshwater organisms, Lewis and Gardiner (1996) concluded that because of a similar mode of action, and because it is likely that they could occur together in the aquatic environment, it was possible that they could exert an additive toxic effect on aquatic life when present together.

No data could be located.

Lewis and Gardiner (1996) found no data on the bioaccumulation of these herbicides in saltwater organisms. However, data for freshwater organisms suggest bioaccumulation is unlikely to be significant (reliable reported BCFs were <100), and therefore likely to be the same for saltwater organisms.

Potential effects include:

• toxic effects on algae and macrophytes at concentrations above the EQS of 2 mg l^-1 in the water column.

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