Use of a Gammarus pulex bioassay to measure the effects of transient carbofuran runoff from farmland.

There is now much information on the presence of pesticides in surface waters, but very little about their actual effects on aquatic life. This paper reports on the transient concentrations of a carbamate insecticide carbofuran, which were observed in a head-water stream draining treated farmland, and describes the resulting effects on a bioassay organism. One month after an application of 3 kg carbofuran/ha as broadcast granules to an oilseed rape crop, carbofuran concentrations of up to 26 micrograms/liter were measured in a nearby headwater stream after heavy rainfall. The majority of the carbofuran was translocated via field drains (where concentrations up to 264 micrograms/liter were detected), although the possibility of some surface runoff cannot be ruled out. Peak carbofuran concentrations only persisted for about 24 hr after the rainfall event, although measurable levels could be detected for at least 4 days. An in situ bioassay of streamwater which monitored the feeding rate of the gammarid amphipod crustacean Gammarus pulex showed that feeding stopped completely during the rainfall event, and was rapidly followed by death of all the caged organisms. Subsequent laboratory studies of toxicity showed that the peak concentrations of carbofuran had exceeded the G. pulex 24-hr LC50 of 21 micrograms/liter, and that concentrations as low as 4 micrograms/liter could reduce feeding rate. These findings are discussed with reference to regulatory risk evaluation procedures.