Precipitation was sampled around the Great Lakes as part of the Integrated
Atmospheric Deposition Network (IADN) from 1991 to 1997. Many of the monthl
y volume weighted mean concentrations of the analytes have significantly de
creased during the last 6 years with first-order half-lives on the order of
3 +/- 2 years. There was little, if any, spatial variability in the observ
ed half-lives, suggesting similar loss processes for the various lakes and
suggesting that the decrease in precipitation concentrations reflects a reg
ional decrease in environmental levels. The first-order half-lives of organ
ochlorine pesticide concentrations in Great Lakes precipitation were not si
gnificantly different than the corresponding atmospheric gasphase, lake wat
er, and biota half-lives, suggesting that the entire system is at or near l
ong-term equilibrium.