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Title: STREAM LEAKAGE AS AN ALTERNATIVE SOURCE OF AGRICHEMICALS IN AN ALLUVIAL AQUIFER

Author
item Burkart, Michael
item SIMPKINS, WILLIAM - IOWA STATE UNIVERSITY
item SQUILLACE, PAUL - U.S. GEOLOGICAL SURVEY

Submitted to: Geological Society of America Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 5/3/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Leaching of chemicals by precipitation or irrigation water through soils beneath cropped fields has been the most frequently studied process contributing to agrichemical contamination of aquifers. Results of this preliminary study show that infiltration of dissolved agrichemicals through a tributary stream bed is substantial and the process warrants further investigation. It was found that infiltration of dissolved agrichemicals occurred through the bed of Walnut Creek, a tributary draining 6500 hectares. The creek traverses about 1500 meters of the South Skunk River flood plain and underlying alluvial aquifer. Infiltration was estimated by measuring the loss of water from Walnut Creek and the concentrations of agrichemicals in the stream bed and alluvial aquifer. Infiltration was estimated from base-flow measurements in April 1994, before application of agrichemicals to the fields, and in June after application of agrichemicals during a small runoff event. In April, the flux of atrazine through the creek bed was estimated to be 60 ug/d/m**2, whereas in June, the flux of atrazine was 8,700 ug/d/m**2. The unit area-flux rate of nitrate-nitrogen was greater than 2,000 mg/d/m**2 for both sampling periods. The calculated fluxes of atrazine beneath Walnut Creek, for these two periods, were two to three orders of magnitude greater than the estimated flux of atrazine caused by in-field leaching. The large flux of agrichemicals from Walnut Creek to alluvial groundwater is due in part to the conductive sands and rather fast groundwater velocities. Average vertical hydraulic conductivity through the stream bed was calculated as 30 and 90 m/d, and estimated groundwater velocities were from 1 to 5 m/d.