During May 15-17, 1990, an intense rainstorm moved across Iowa, Illinois, Indiana, and Ohio where triazine herbicides are heavily used for growing agricultural crops. Following the storm, the peak concentrations of triazine herbicides in some secondary tributaries to the Upper Mississippi and Ohio Rivers were as high as 36 ??g/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 runoff event was measured by collecting midchannel water samples for triazine herbicide analysis from 1 to 2 m below the surface of the Mississippi River every ???16 km from Baton Rouge, LA, upriver to the Mississippi-Ohio River confluence during May 26-29, 1990. All samples were analyzed for triazine herbicides by using an enzyme-linked immunosorbant assay. The results showed a background level of ???2.7 ??g/L, an upriver gradient of 0.2 ??g/L per 100 km, and longitudinal spatial variability that is hypothesized to be the result of cross-channel gradients and "slugs" of water from various upriver tributaries with length scales of 100-150 km and amplitudes of ???1 ??g/L.
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Spatial variability of triazine herbicides in the Lower Mississippi River