Nitrate removal capacity of constructed wetlands

Constructed wetlands at the Des Plaines Wetland Demonstration Project in Wadsworth, Illinois were examined for their capacity to remove nonpoint source loads of nitrate from an agricultural watershed. Three flow through wetlands received different loading rates of pumped river water with seasonably variable loads of nitrate and organic nitrogen. The nitrogen removal capacities of the wetlands were estimated using mass balance and sediment-water microcosms for the 1991 and 1992 field seasons. Concentrations of nitrogen and flow volumes were monitored at inlets and outlets and mass balances of nitrogen loads were calculated. The wetlands received two different loading rates in 1991, and three loading rates in 1992. Concentrations of nitrate were highest during high flow periods, and most of the annual nitrate load was associated with the spring time flows. All three wetlands were found to be net sinks for nitrate and two of the three wetlands were net exporters of organic nitrogen. The efficiency of nitrate removal was inversely related to hydrologic load, and with the mass of nitrate loaded into the wetlands. Microcosm studies showed that nitrate loss is primarily a function of nitrate concentration and temperature. At a constant temperature, nitrate loss could be described as a first order areal process. Nitrate loss rate coefficients (k{dollar}sb{lcub}rm a{rcub}{dollar}), determined from microcosms, allow the prediction of nitrate removal for a wetland under various nitrate loads. These coefficients were expected to vary between wetlands with different loading rates, and over the course of the season. In 1992, nitrate loss rate coefficients in microcosms exposed to three different loading rates showed no consistent differences. The range of k{dollar}sb{lcub}rm a{rcub}{dollar}'s was far less than the range of hydrologic load and nitrate concentrations. K{dollar}sb{lcub}rm a{rcub}{dollar}'s for all the wetlands over both field seasons were comparable. K{dollar}sb{lcub}rm a{rcub}{dollar}'s appear to provide a reasonably consistent parameter for estimation of nitrate assimilative capacity in the Des Plaines wetlands.