Malathion is a contact- and stomach-acting organophosphorus pesticide which inhibits acetylcholinesterase (AChE) activity, thus disrupting the nervous system and making it toxic to all innervated animals. In higher vertebrates, it is readily hydrolysed to inactive derivatives. Malathion's broad-spectrum action has led to widespread use against insect and other invertebrate pests. In Europe, most malathion is used in agriculture, principally against mite and insect pests of oilseed rape, root crops, vegetables and fruits. Other uses include domestic, horticultural and glasshouse pest control (mostly against aphids), and application against ectoparasites of humans and animals.

Malathion may enter the aquatic environment, either directly in industrial effluents, or indirectly in run-off from land or by deposition from the atmosphere (following spray drift or in precipitation).

80 river and estuary sites were sampled twice for malathion over the winter of 1988-89 (SAC Scientific 1989 cited in Hedgecott (1991)). At 11 sites, malathion was detected once, with a mean concentration of 1.5 ng l^-1 (range 1.0 to 3.0 ng l^-1). At a 12th, malathion concentrations of 5.1 and 7.8 ng l^-1 were recorded. At all other sites, malathion was below the detection limit of 1 ng l^-1 (there is no further information as to whether malathion was detected at any of the estuary sites).

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. No water column concentration was found to exceed the EQS value (see Appendix D). Monitoring data were not available for sediments or biota.

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

The available data indicate that biodegradation is an important removal process for malathion in the aquatic environment, with various species of bacteria and fungi able to utilise it as a source of either carbon or phosphorus. Paris et al (1975a, 1975b) estimated half-lives for degradation by freshwater bacteria of 28 and 41 hours (initial concentrations approximately 1.5 and 1.1 mg l^-1). Efficient biodegradation has also been observed in saline waters (Bourquin 1977). The Marine Biological Association (MBA) (1979) report biodegradation rates for estuarine and saltwater micro-organisms corresponding to half-lives of 0.5 to 9 days and 3 to 12 days or more, respectively.

Malathion has a low vapour pressure and does not readily volatilise. Sorption to suspended solids and sediments will account for some removal from water. The major abiotic removal process in water is hydrolysis. The MBA (MBA 1979) provides estimates of half-lives in sea water, ranging from 7 to 40 days at 10 °C, and from 2 to 9 days in sterile sea water at 25°C.

An exhaustive literature review on the toxicity of malathion 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 (Hedgecott 1991). The most sensitive groups of organisms have been identified.

The most sensitive saltwater organisms tested are certain crustaceans, with lowest acute LC50s or EC50s of 0.4 to 3.1 µg l^-1 for Dungeness crabs Cancer magister and mysid shrimps Mysidopsis bahia (Caldwell 1977, Goodman et al 1988), and retarded development of Dungeness crab eggs and larvae at 0.3 µg l^-1 (reported by MBA 1979). Hedgecott (1991) concluded that early life stages of crustaceans may be more sensitive than adult stages.

Fish species appear to be less sensitive with a maximum acceptable concentration of 6 µg l^-1 reported for a full life cycle test with sheepshead minnows Cyprinodon variegatus (reported by Suter and Rosen 1988). An acute LC50 of 300 µg l^-1 has been reported by the MBA (1979) for adult sheepshead minnows.

Hedgecott (1991) found no bioaccumulation data for saltwater species, but concluded it would not expected to differ greatly from that found for freshwater organisms (i.e. low).

Potential effects include:

• acute toxicity to invertebrate (particularly crustacea) and fish to concentrations above the EQS of 0.02µg l^-1 (annual average) and 0.5 µg l^-1 (maximum allowable concentration) in the water column.

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