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Title: TRIBUTARY STREAM INFILTRATION AS A SOURCE OF HERBICIDES IN AN ALLUVIAL AQUIFER

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

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Research as part of the Walnut Creek watershed project shows that water and dissolved herbicides in the creek can flow into an alluvial aquifer. Measurements and water samples were taken in the stream, the stream bed, and the aquifer in an area where the stream flows across a flood plain and the alluvial aquifer. Water level measurements and aquifer tests showed that water and dissolved chemicals can move from the stream to the aquifer within 6 days. Herbicide measurements showed concentrations increased in the aquifer about the same time as they increased in the stream. The process of stream-bed infiltration offers an alternative explanation for how this type of shallow aquifer can become contaminated by herbicides. It was shown that stream-bed infiltration can contribute as much as 1,000 times the rate of atrazine contamination to an aquifer as leaching beneath a field of similar size to the stream-bed area. One of the implications of these findings is that management of headwater cropland may be as important to controlling herbicide contamination of aquifers than managing land immediately overlying a shallow aquifer.

Technical Abstract: Where Walnut Creek flows across the South Skunk River alluvial aquifer, it provides a potential source of herbicides and nitrate for the aquifer. This reach of the creek lost water and dissolved contaminants to the alluvial aquifer through an intervening layer of fine-grained flood plain deposits. Estimates of potential flux of chemicals were based on measurements before and after herbicides were applied to the watershed. Relative head measurement in the creek, flood plain deposits, and aquifer confirmed the downward gradient of water movement. Hydraulic conductivity measurements from slug tests were used to calculate an average linear groundwater velocity of 6.3 x 10**-6 m/s in the flood plain deposits. The potential for atrazine flux to the aquifer from the creek was estimated to be between 60 and 3000 ug/d/m**2. This rate is one to three orders of magnitude greater than the estimated flux via leaching beneath a field. If the process of vertical stream leakage occurs in many hydraulic settings, it may constitute a substantial source of herbicides to shallow aquifers in many areas of the Midwest.