NITRATE REDUCTION FROM TILE DRAINAGE IN A RESTORED OXBOW WETLAND
A study was conducted in central Iowa to document the ability of oxbow wetlands to mitigate nitrate transported from field tiles to rivers and streams. Water entering from a field tile and water leaving a reconstructed oxbow wetland was monitored for both flow and nitrate concentrations during ice-free periods in 2013 and 2014. Flow was measured using a trapezoidal fume on the tile line and a drainage control structure at the outlet of the oxbow. Initially, only discrete water samples were collected from both the inflow and outflow of the oxbow with automatic samplers programed in increase sampling frequency during a rain or storm event. Automatic water samplers were replaced with optical nitrate sensors midway through the summer of 2013 to collect high frequency nitrate concentration data. Although the number of nutrient constituents was reduced through the use of the optical sensors, the 15-minute nitrate concentration data along with the continuous flow data allow for more accurate estimates of nitrate loads entering and leaving the oxbow wetland.
Data from the automatic samplers were not frequent enough, however, to sufficiently document water quality changes during rapidly changing flow conditions with the onset of a storm event as flow often peaked between programed samples. In contrast, nitrate data from the optical sensors that were collected at the same frequency as flow data were available to calculate loads at 15-minute intervals resulting in a much more accurate load estimate even during rapidly changing flow and concentrations that occur during storms. Nitrate concentrations in tile discharge entering the oxbow averaged 18.3 mg/l and 20.3 mg/l in 2013 and 2014, respectively, In contrast, nitrate concentrations in the water discharging from the oxbow into Prairie Creek were 11.1mg/L and 5.0 mg/L in 2013 and 2014, respectively. Although about 56 percent more water was measured leaving than entering the oxbow wetland, the mass of nitrate was 50 percent less in the oxbow discharge than entering from the tile line. Additional water that included overland flow and groundwater inflow are possible sources of additional input to the oxbow. Limited data also suggest that seepage from the oxbow to groundwater may be factor in nitrate transport through the oxbow wetland.