Nitrate removal capacity of constructed wetlands

Abstract

Constructed or restored wetlands have great potential for reducing nonpoint source contamination of surface and ground waters by agricultural chemical contaminants. The work reported here combines field and experimental studies of factors affecting nitrogen loss in the Des Plaines River Experimental Wetlands, Northeastern Illinois, USA. These wetlands receive approximately 5-36 cm/week of pumped river water with significant but seasonally variable loads of nitrate and organic nitrogen. On an annual basis, the wetlands removed 7895% of the nitrate and 54-75% of the total nitrogen received. At the low hydrologic loading rate, organic nitrogen exports approximately equaled imports. However at the higher hydrologic loading rate, the wetlands exported 22-31% more organic nitrogen than received. Seasonal variation in nitrate and organic nitrogen loads had significant effects on the effectiveness of the wetlands as sinks for total nitrogen. The wetlands were nitrogen sinks during periods of high nitrate loading and nitrogen sources during periods of low nitrate loading. Experimental studies demonstrated the effects of nitrate concentration, temperature, and location on rates of nitrate loss. Results suggest that nitrate loading rates might influence not only nitrate loss rates but also loss rate coefficients. INTRODUCTION Studies suggest that wetlands may act as sinks for a variety of compounds (HowardWilliams, 1985; Nixon and Lee, 1986), and there is considerable interest in die use of wetlands as sinks for nonpoint nitrogen loads firom rivers draining agricultural watersheds.