SPATIAL VARIABILITY OF TRIAZINE HERBICIDES IN THE LOWER MISSISSIPPI RIVER

During May 15-17, 1990, an intense rainstorm moved across Iowa, Illino is, Indiana, and Ohio where triazine herbicides are heavily used for g rowing agricultural crops. Following the storm, the peak concentration s of triazine herbicides in some secondary tributaries to the Upper Mi ssissippi and Ohio Rivers were as high as 36 mug/L. This runoff water was funneled into the Lower Mississippi River at the Upper Mississippi -Ohio River confluence at Cairo, IL. The spatial variability of this r unoff event was measured by collecting midchannel water samples for tr iazine herbicide analysis from 1 to 2 m below the surface of the Missi ssippi River every approximately 16 km from Baton Rouge, LA, upriver t o the Mississippi-Ohio River confluence during May 26-29, 1990. All sa mples were analyzed for triazine herbicides by using an enzyme-linked immunosorbant assay. The results showed a background level of approxim ately 2.7 mug/L, an upriver gradient of 0.2 mug/L per 100 km, and long itudinal spatial variability that is hypothesized to be the result of cross-channel gradients and ''slugs'' of water from various upriver tr ibutaries with length scales of 100-150 km and amplitudes of approxima tely 1 mug/L.