Occurrence, distributions, and transport of herbicides and their degradation products in the Lower Mississippi River and its tributaries

Studies at 17 sites along the lower Mississippi river and its major tributaries confirmed the presence of several triazine and chloroacetanilide herbicides and their degradation products. Loads of these compounds were determined under various seasonal and hydrological conditions between July 1987 and June 1989. Stream loads were relatively low during the drought of 1987 and 1988, but much greater during 1989, which was a relatively wet year. Suspended sediments also contained trace levels of atrazine, cyanazine and metolachlor. The distribution coefficients of these compounds showed a considerable variation among sites and were much larger than values reported in the literature. Less than 2 per cent of the quantity of atrazine applied annually in the midwest was transported into the Gulf of Mexico.