Report No DWI0118

Jun 1990

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

Following the Second Ministerial Conference on the Protection of the North Sea the UK announced tighter controls on the most dangerous substances entering the aquatic environment ("The Red List") (DoE 1987) by applying limit values and environmental quality standards (EQSs). "The Red List" has now been finalised and includes 23 substances (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.

Trifluralin is a herbicide used for pre-emergence weed control for a wide variety of vegetable crops. Application is normally by incorporation into the soil, although some mixtures are sprayed. Entry into the aquatic environment most likely occurs via direct industrial discharges, run-off from treated agricultural land or, occasionally, spray drift. Most of the trifluralin in natural aquatic systems is adsorbed onto the sediment whereas trifluralin present in the aqueous phase is rapidly removed by photodegradation.

For the protection of freshwater life a PEQS of 100 ng/l, expressed as an annual average concentration of dissolved trifluralin, is proposed by applying a safety factor of 100 to the 96 hour LC50 of 10 µg/l for rainbow trout obtained in clean water in the absence of suspended solids (Mayer and Ellersieck 1986). In order to protect the aquatic environment from episodic inputs of high trifluralin concentration, a maximum concentration for dissolved trifluralin of 1 µg/l is also proposed for freshwaters based on applying a safety factor of 10 to the 96 hour LC50 of 10 µg/l for rainbow trout to prevent short term acute effects. In addition a PEQS of 20 µg/l for total trifluralin expressed as a maximum concentration is proposed by applying a safety factor of approximately 100 (a factor of 10 to protect against acute effects and an additional factor of 10 to take into account the higher sensitivity of other species) to the acute 96 hour LC50 of 2.8 mg/l obtained for bluegill in the presence of suspended solids (Parka and Worth 1965).

For the protection of marine life a PEQS of 100 ng/l expressed as dissolved trifluralin and as an annual average concentration is proposed by applying a safety factor of 10 to the MATC obtained in a chronic test for sheepshead minnow (Parrish et al 1978). The safety factor makes allowance that other species may be more sensitive. No maximum PEQS is proposed for dissolved trifluralin as the inputs to marine waters are predominantly via rivers which undergo high dilution in estuarine and coastal areas and are therefore unlikely to be significantly affected by episodic events. In the absence of data on the bioavailability of trifluralin adsorbed in marine sediment it is proposed that the same PEQS of 20 µg/l for total trifluralin expressed as maximum concentration suggested for the protection of fresh water life should also be applied for the protection of marine life.

WH0 (1987) have derived a guideline value of 170 µg/l for trifluralin in drinking water. This value is much greater than the MAC of 0.1 µg/l laid down in the EC Drinking Water Directive (CEC 1980) for individual pesticides.

The analytical limits of detection which can currently be achieved by the water industry for trifluralin (approximately 10 ng/l in river water, Standing Committee of Analysts (1988)) are adequate for monitoring the proposed EQSs in natural waters. For the determination of dissolved trifluralin the sample should be allowed to settle for 1 hour and the supernatant should be analysed without filtration.

The PEQS values proposed in this report are based on a brief review of readily available literature and should, therefore, be regarded only as guideline values until a comprehensive assessment of all relevant literature can be undertaken.

This report uses available information on the toxicity and environmental fate of trifluralin 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.