PYRETHROID RESIDUES IN SEDIMENT AND WATER SAMPLES FROM MESOCOSM AND FARM POND STUDIES OF SIMULATED ACCIDENTAL AQUATIC EXPOSURE

This paper describes the residue analysis of water and hydrosoil sampl es taken from two separate large-scale aquatic ecotoxicology trials de signed to assess the environmental fate and effects of the pyrethroids lambda-cyhalothrin and cypermethrin. Comparison of the results demons trates the high degree of reproducibility of the chemical residue foun d the day after treatment using experimental mesocosms (lambda-cyhalot hrin) as opposed to an in-use farm pond (cypermethrin). Both studies s howed that pyrethroid residues were rapidly lost from the water column : residues of lambda-cyhalothrin were less than 2 ng litre-1 following the final application of a cumulative seasonal exposure equivalent to twelve 'drift' and six 'run-off' events, each delivering a dose equiv alent to that expected from a typical event under field conditions. Hy drosoil appeared to act as a sink for pyrethroid residues and, under t he stringent test conditions of the mesocosm study, lambda-cyhalothrin residues reached 3.2 mug kg-1 following the seasonal exposure describ ed above. The cypermethrin farm-pond study illustrated the localised p attern of exposure expected under natural field conditions, with site topography and cultivation practices which represent an average 'worst case'. Residues in hydrosoil reached a maximum level of approximately 25 mug kg-1 in one sampling zone at one interval, and thereafter decl ined to a level of < 9 mug kg within four months.