Midwest Fish & Wildlife Conference 2018

AUTHORS. Keith Schilling, Iowa Geological Survey; Chris Jones, IIHR-Hydroscience and Engineering, University of Iowa; Bryce Haines, University of Iowa; Keegan Kult, Iowa Soybean Association; Karen Wilke, The Nature Conservancy

ABSTRACT. Conservation practices are needed to reduce the export of nitrate-nitrogen (nitrate) from agricultural regions of the United States. A new practice being investigated is the use of reconstructed oxbows to improve water quality and enhance ecosystem services in riverine corridors. Water quality benefits in two newly reconstructed oxbows have been recently monitored in Iowa. One oxbow (Frye site) is located along a tributary of the Boone River in the recently glaciated Des Moines Lobe region dominated by intense row crop agriculture and tile drainage. A second site is located in eastern Iowa along Morgan Creek that flows through an older glacial landscape dominated by hillslopes and integrated drainage. At the Frye site, the oxbow was engineered to receive inputs from two drainage tiles. Over a two-year period, water and nitrate concentrations and loads into the oxbow were dominated by tile drainage inputs compared to groundwater seepage. Nitrate concentrations were highest in tile drainage water (9 to 17 mg/l) and lowest in downgradient groundwater (0.2 mg/l). The nitrate retention efficiency from May to September ranged from 44% to 47% and greater retention efficiencies were measured in late summer and early fall. At the Morgan Creek site, the oxbow was reconstructed to receive inputs from groundwater seepage and overbank flooding. Over a one-year period, nitrate loading into the oxbow was dominated by flood pulses. Nitrate concentrations in the stream ranged from 7 to 13 mg/l whereas concentrations in the oxbow and downgradient groundwater were consistently low (< 0.1 to 5 mg/l). Following a spring flood event, an in-situ sensor measured nitrate concentrations decreasing in the oxbow from 5.4 to 0.7 mg/l over a 21-day period. Nitrate retention efficiency was estimated to be 0.30 g N m-2 d-1 or a 74.2% reduction efficiency for the event. Overall, both newly reconstructed oxbows exhibited similar nitrate reductions despite vastly different hydrologic pathways.