Report No DWI0121

Jul 1990

To derive provisional environmental quality standards (PEQSs)) for malathion for the protection of the different water uses, based on readily-available information on environmental fate and toxicity.

This report is one of a series aimed at proposing PEQSs for those 'Red List' (DoE 1989a) substances for which no environmental quality standards (EQSs) currently exist.

Malathion is an organophosphorus insecticide and acaricide. It may persist in the aquatic environment, with half-lives from about one to 40 days, depending on conditions. Freshwater and saltwater organisms show varying sensitivity to the presence of malathion. Invertebrates, especially certain crustaceans and insects, are the most sensitive. A PEQS of 10 ng/l is proposed for the protection of freshwater life, derived by applying an arbitrary safety factor of 100 to the acute LC50s of around 1 µg/l obtained for various invertebrates, in particular Daphnia and Gammarus. The proposed PEQS is supported by the available chronic toxicity data, in particular the 'no effect' concentration of 8 ng/l reported for Gammarus pseudolimnaeus (Bell 1971). For the protection of saltwater life a higher PEQS of 20 ng/l is proposed, based on the 'no effect' concentration reported for early life-stages of the Dungeness crab (Caldwell 1977) and bearing in mind that other crustacean species may be more sensitive. These PEQSs should be considered as annual averages because of the large safety margins between them and the acute toxicity data. There are insufficient data to enable any maximum acceptable concentrations to be proposed.

Analytical techniques currently available to the water industry need some development before they will be adequate for monitoring the proposed PEQSs. Bioaccumulation data are limited but suggest only a low-to-moderate tendency to accumulate in aquatic organisms. In higher vertebrates metabolism and elimination is rapid. The likely safe concentration of malathion in drinking water of 7 µg/l based on the ADI derived by FAO/WHO (1973) is much higher than the maximum allowable concentration of 0.1 µg/l specified for individual pesticides in the EC Drinking Water Directive (CEC 1980).

This is only a brief review of the available data, but the suggested PEQS values are considered to be adequate as guide values to protect freshwater and saltwater life. A more detailed study is recommended for a fuller assessment of the impact of malathion on the different water uses. Further investigations should be carried out on the effects of malathion on one or more of the UK species of Gammarus (eg G. pulex or G. lacustris).

This report reviews the more readily-available information on the fate, persistence, toxicity and bioaccumulation of the insecticide malathion in the aquatic environment, in order to derive provisional environmental quality standards for the protection of freshwater and saltwater life. Brief consideration is also given to the presence of malathion in drinking water.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.