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The Effect of Restored and Native Oxbows on Hydraulic Loads of Nutrients and Stream Water Quality
Citation:
Kalkolff, S., L. Hubbard, AND J. Schubauer-Berigan. The Effect of Restored and Native Oxbows on Hydraulic Loads of Nutrients and Stream Water Quality. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-16/297, 2016.
Impact/Purpose:
The primary objectives of this report are to (1) document the methods used to collect data that estimate and characterize the nutrient loads from the oxbow inlets and compare these loads with the nutrient loads at the oxbow outlets and the receiving streams and (2) to summarize flow, concentrations, and loads in oxbow wetland inflow and outflow; summarize water levels and water quality in groundwater in the vicinity of the oxbow wetlands; summarize concentrations, and loads in the streams receiving discharge from the wetlands.
Description:
The use of oxbow wetlands has been identified as a potential strategy to reduce nutrient transport from agricultural drainage tiles to streams in Iowa. In 2013 and 2014, a study was conducted in north central Iowa in a native oxbow in the Lyons Creek watershed and two reconstructed oxbows in the Prairie Creek watershed (Smeltzer west and Smeltzer east) to assess their effectiveness at reducing nitrogen and phosphorus loads. The tile line inlets carrying agricultural runoff to the oxbows, the outfall from the oxbows and the surface waters in the streams receiving the outfall water were monitored for discharge and nutrients from February 2013 to September 2015. Smeltzer west and east also had four monitoring wells each, two in the upland and two between the oxbow and Prairie Creek to monitor surface water groundwater interaction. The Smeltzer west and east oxbow sites also were instrumented to continuously measure the nitrate concentration. Rainfall was measured at one Lyons Creek and one Smeltzer site. Daily mean nitrate-N concentrations in Lyons Creek in 2013 ranged from 41 mg/L to 11.8 mg/L, the median daily mean nitrate-N concentration was 33 mg/L. Daily mean nitrate-N concentrations in Prairie Creek in 2013 ranged from 15.0 mg/L to 32 mg/L in June. The median daily mean nitrate-N concentration for the sampled period was 11.2 mg/L. In 2014, daily mean nitrate-N concentrations in Prairie Creek ranged from 0.17 mg/L to 26.7 mg/L in July; the daily mean nitrate-N concentration for the sampled period was 5.67 mg/L. Nutrient load reduction occurred in oxbow wetlands in Lyons and Prairie Creek watersheds in north-central Iowa but efficiency of reduction was variable. Little nutrient reduction occurred in the native Lyons Creek oxbow during 2013. Concentrations of all nutrient constituents were not significantly (P>0.05, Wilcoxon rank sum) different in water discharging from the tile line than in water leaving the Lyon Creek oxbow. A combination of physical features and flow conditions suggest that the residence time of water in the oxbow may not have been sufficient to allow for removal of substantial amounts of nutrients. Approximately 54 percent less nitrate-N was measured leaving the Smeltzer west oxbow than was measured entering from a small 6 inch field tile. The efficiency of nitrate-N removal in the oxbow was not able to be definitively quantified as other hydrologic factors such as overland and ground water flow into and through the oxbow were not addressed and may provide alternative routes for nutrient transport. Damage to the Smeltzer east oxbow outfall weir prevented analysis of its nutrient load reduction capability. The study provides important information to managers and land owners looking for strategies to reduce nutrient transport from fields. Additional research is needed to understand how increased discharge from larger field tiles and drainage district mains may influence the efficiency of nutrient reduction in relation to the size, type and landscape setting of a wetland.