Transport of Herbicides in Streams and Shallow Ground Water -- Cedar River, Iowa

This project investigated the movement of selected agricultural chemicals between surface water in the Cedar River and ground water in an adjacent alluvial aquifer in east-central Iowa. The selected chemicals include nitrate, alachlor, ametryn, atrazine, cyanazine, deethylatrazine, deisopropylatrazine, metolachlor, metribuzin, prometon, prometryn, propazine, simazine, and terbutryn. On a small scale, USGS scientists investigated chemical movements by installing observation wells in the alluvial aquifer adjacent to the river at an unfarmed site (an area where agricultural chemicals were not used) 15 kilometers downstream of Cedar Rapids, Iowa. Ground-water samples from these wells, as well as river and other surface-water samples, were collected during base-flow conditions and selected periods of runoff from May 1989 through July 1991. The results of this project showed that nonpoint source contaminants could migrate from surface water into ground water. This was demonstrated by modeling the water-quality data with a two-dimensional ground-water flow model. The model described quantitatively the movement of bank-storage water into and out of the alluvial aquifer in response to rising and falling river stages. On a large scale, the movement of agricultural chemicals from ground water to surface water was quantified for two periods of time in 1989 and 1990 along a 117-kilometer reach of the Cedar River.

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