Report No DWI0119

Jul 1990

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

The UK Government has announced the adoption of a more precautionary approach to the control of the most dangerous substances entering the aquatic environment, 'The Red List' (DoE 1987), by applying limit values and environmental quality standards (EQSs). The Department of the Environment has drawn up a list of 23 substances to be included in the 'Red List' (DoE 1989). This report is one of a series aimed at proposing PEQSs for those 'Red List' substances for which no EQSs have so far been derived.

Dichlorvos is a contact and stomach-acting insecticide. Entry into the aquatic environment may occur from diffuse inputs via land run-off and spray drift, by direct application (eg in salmon farms) or from point discharges. Dichlorvos is not very persistent in natural waters. Chemical and biological degradation result in half-lives of a few days.

For the protection of freshwater life a PEQS of 1 ng/l is proposed based on applying an arbitrary safety factor of approximately 100 to the lowest acute 48 hour LC50 of 70 ng/l for Daphnia pulex {Sanders and Cope l966) and the 96 hour LC50 of 100 ng/l for stonefly naiads (Sanders and Cope 1968). These laboratory tests were conducted under static conditions without analysis of toxicant concentrations and the PEQS must, therefore, be regarded as tentative until more relevant data are available. For the protection of marine life a PEQS of 40 ng/l is proposed, based on the application of a safety factor of 100 to the acute 96 hour LC50 of 4 µg/l for the sand shrimp (Eisler 1969). Both these PEQSs should be expressed as annual average concentrations because of the large margin of safety between acutely toxic levels and the PEQSs. These values apply outside a defined mixing zone.

In addition, if dichlorvos is applied directly to the marine environment (ie in salmon farms), it is suggested that 1 hour after its release the maximum concentration outside a defined mixing zone should not exceed 25 µg/l, a concentration which did not result in mortalities or cumulative effects in lobster larvae exposed for repeated 6 hour periods (Ciba-Geigy SP 3560.49-V1597). Furthermore, the dichlorvos concentration outside the mixing zone must decrease to below 6 µg/l, the 12 hour no observed effect concentration (NOEC) for lobster larvae, before the next release of dichlorvos takes place (Ciba-Geigy 3560.55). For areas designated to require special protection it is proposed that the dichlorvos concentration should not exceed 1.5 µg/l the 96 hour NOEC for lobster larvae, at any time.

Insufficient environmental data are available to verify the proposed standards for the protection of aquatic life.

Using the ADI of 0.004 mg/kg body weight for dichlorvos (WHO/FAO 1977) and allowing that 10% of the ADI may be derived from drinking water, the resulting acceptable concentration in drinking water is 12 µg/l, for an adult drinking 2 l water per day. This value is much greater than the MAC of 0.1 µg/l laid down in the EC drinking water directive for individual pesticides. Because of the uncertainties concerning the removal of dichlorvos in water treatment processes and during storage, no PEQS can be proposed for the protection of waters used for the abstraction to potable supply.

The analytical limits of detection which can currently be achieved by the water industry for dichlorvos (eg, approximately 40 ng/l in river water, Standing Committee of Analysts 1988) are inadequate for monitoring the proposed annual average PEQSs for the protection of fresh and marine waters.

The PEQS values proposed in this report are based on a review of the readily available literature. Indications are that several studies are currently being carried out on the toxicity of dichlorvos particularly to marine species and these data should be included in any future review. The values proposed should therefore be regarded as tentative. Attempts should be made to improve the detection limits of the analytical methods for dichlorvos.

This report uses available information on the toxicity and environmental fate of dichlorvos to derive PEQSs for the protection of freshwater and marine life, and considers likely safe levels for waters used for abstraction to potable supply. Insufficient data were available to consider other water uses.

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