Model of Residence Time and Analysis of Nitrogen Removal for Two Constructed Wetlands at the Franklin Demonstration Farm in Lexington, Illinois

摘要

Pollution from nonpoint agricultural runoff has become a major problem facing our streams and rivers today. Not only are fish and aquatic life affected, but so is the quality of our drinking and recreational water resources. Studies have shown that wetlands have proven to be the most cost-effective and low maintenance method of removing nonpoint or diffused contaminate inputs. The biological processes and removal of nutrients in wetlands depend on the total surface area available for microbial activity in the soil and a certain period of water retention time. Since chemical processes take time, the measure of residence time is an important factor of the degree to which wetlands can change water chemistry. Knowing that nitrogen concentrations decrease as water residence time increases, a model of residence time will help interpret the mechanisms determining groundwater flow paths within and around the constructed wetlands. The main objectives of this research are to model wateru27s residence time, compare the size and gradient of two experimental wetlands, and determine the water flow paths within the site and how they relate to the areas of high denitrification rates. The two constructed wetlands chosen are the West and Gully located in Lexington, Illinois. Of the two, Gully is about half as small and has a higher gradient. Using MODFLOW to create a local-scale model that includes both wetlands and the tile drainage will help to determine how groundwater influences the fate of nitrogen and the effectiveness of wetland construction parameters. Since the outflows from groundwater to the wetlands are significantly greater than the inflows from the surface water, it can be assumed that the wetlands have zones where they are being recharged from groundwater. Franklin West had an average residence time of about 1.41 days (121,580 sec) which is much slower than Gullyu27s 0.011 day (918 sec) (Table 1). With a lower overall groundwater discharge into Franklin West, this may help to explain why the wetland system removes less N than Gully even though it has a longer surface water residence time. As the nutrients can also travel within the groundwater, they have the potential of contaminating the water supply, but with a long enough distance, denitrification and the process of diffusion within the subsurface can also remove excess N. With the 10 years of travel time from Franklin West to the Mackinaw River and the six years it takes water to travel from Gully to Turkey Creek, any nutrients from Franklin West will have more time, both within the wetland system itself and the groundwater, to denitrify and more effectively remove nutrients before the water reaches surface features. The results of this research will be beneficial when considering effective wetland design, monitoring procedures, and wetland management.