Tuesday, June 5, 2007 - 10:45 AM
130

Whole stream response to NO3 loading in three streams draining agricultural landscapes

John H. Duff1, Anthony J. Tesoriero1, William B., Richardson2, Eric A. Strauss3, and Mark D. Munn1. (1) Water Resources, US Geological Survey, 345 Middlefield Road, Mail Stop 439, Menlo Park, CA 94025, (2) U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 Fanta Reed Rd., La Crosse, WI 54603, (3) Department of Biological Sciences, Fort Hays State University, 600 Park St., Hays, KS 67601

Stream and groundwater discharge, transient storage, NO3- mass balance, and denitrification were measured in three agricultural streams draining orchard/dairy and row crop to identify NO3- transport-retention mechanisms.  Stream channel characteristics ranged from a riparian-covered heterotrophic stream in Maryland with high sediment C (Morgan Creek) to a broad, open-canopied autotrophic stream in Nebraska with low sediment C (Maple Creek) to a narrow, grass riparian channelized drain in Washington (DR2 Drain). In all streams, As was a small percentage of A (As/A=0.03-0.11), Fmed200 was low (0.003-0.018), and the modeled storage depth was < 4 cm. Groundwater accounted for 5-11% of discharge and 6-42% of total NO3- inputs. Un-amended denitrification potentials extrapolated to reach-scale NO3- dynamics accounted for 4 to >100% of the NO3- losses. Nitrate retention in the stream with the highest denitrification potentials was equivalent to only 2% of the whole-stream NO3- load. Nitrate retention as a percentage of the whole-stream NO3- loads was highest in the open-canopied stream (30%), which had the lowest denitrification potentials but highest PAR, P/R ratio, pH, DO saturation, and hyporheic exchange flux.  Although NO3- retention mechanisms varied, solute-biota interactions were insufficient to cause a high level of permanent NO3- removal in these low-gradient agricultural streams.

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