|SQUILLACE PAUL J|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/1996
Publication Date: N/A
Interpretive Summary: Walnut Creek is a tributary to the Skunk River, a major stream in central Iowa. The creek passes over a sand and gravel aquifer before flowing into the Skunk River. The movement of water through the creek bed and into the underlying aquifer was confirmed through a series of measurements of water levels in the aquifer and flow in the stream. Chemical analyses of water samples show that the water entering the aquifer contains nitrate and herbicides, including atrazine. It was estimated that the creek could contribute as much as 11,000 times more atrazine to the aquifer than leaching through a field with an area equal to that of the stream bed. The process of contaminating an aquifer with agricultural chemicals through a stream bed had not been well documented before this study. The stream and aquifer conditions discussed in this paper are common in the corn belt. Alluvial aquifers, sometimes called buried valley aquifers, are found adjacent to most major rivers in the cornbelt. This type of aquifer has been found to contain the greatest contamination of all ground water resources in the corn belt. Consequently, this process may be very important in explaining contamination of important ground water resources used by communities and farmers.
Technical Abstract: In the midwestern United States, a tributary stream, as it transects the flood plain of the main stem river, can lose water and dissolved agrichemicals to the alluvial aquifer. The flux of detectable herbicides and metabolites from Walnut Creek, a tributary stream, into the alluvial aquifer adjacent to the South Skunk River in Iowa ranged from less than 10 to about 3,000 ug/d/m**2 on April 7 and June 8, 1994. While nitrate (as nitrogen) was greater than 2,000,000 ug/d/m**2 for both sampling dates. By way of comparison, on these two sampling dates, the flux of atrazine beneath Walnut Creek was 200 and 11,000 times an estimated flux of atrazine to alluvial groundwater due to leaching from a field, and the flux of water from Walnut Creek to alluvial groundwater was about 1,000 and 2,000 times the average daily recharge to the alluvial groundwater due to precipitation. On the basis of water-level measurements, the flux of agrichemicals from Walnut Creek to the aquifer probably occurs throughout the year. Calculations show that the average vertical hydraulic conductivity through the streambed was about 30 and 90 m/d for the two sampling dates and that estimated groundwater velocities ranged from 1 to 5 m/d.