• Stanley, Jon G.
  Maine Cooperative Fishery Research Unit - Department of Zoology
  
• Trial, Joan G.
  Department of Entomology - University of Maine
  

Acute toxicity of chemicals to aquatic organisms is traditionally expressed as a concentration that kills a certain percentage of the organisms after a stated exposure. Fore persistent pesticides this procedure probably inadequately reflects the potential impact in nature, where the total exposure time may be longer than the duration of the tests. Because many organophosphate and carbamate pesticides break down rapidly, toxicity tests are carried out at a constant level of exposure  overestimate the effects in the field. After a week, jars filled with a solution of carbaryl in river water contained only 5% of the carbaryl initially present  (Eichelberger & Lichtenberg 1971). Carbaryl is broken down largely by alkaline hydrolyses (Aly & El-Dib 1971) and bacterial decomposition (Menzie 1978). In addition to being chemically degraded, pesticides also disperse in water currents. Toxicity tests would be more realistic if progressive declines in concentrations were taken into account. Fenitrothion disappeared over a several day period from water contaminated with spray applied to forests (Eidt 1975; Flannagan 1975; Hall et al. 1975; McLesse 1976) and rice fields (Kanazawa 1975). McLeese (1976) attempted to simulate stream conditions for toxicity tests on crayfish by adjusting levels of fenitrothion in static test containers. Accurate simulation of field concentrations requires knowledge of the disappearance rates of chemical from the environment. Eidt & Sunadaram (1975) and Lyons et al. (1976) mentioned a negative exponential decay for fenitrothion, and Rabeni & Stanley (1979) cited a disappearance rate for acephate; in general however, "decay" constants have not been published for non-persistent pesticides in natural waters.

The objective of this study was to determine disappearance constants of carbaryl from streams that received drift from spraying of nearby forests.