Determining Nitrogen Loading Rates Based on Land Use in an Urban Watershed

Due to the role of nutrients as limiting agents for eutrophication in fresh water and marine estuaries and as a human health risk in drinking water, greater scrutiny of land use in urban areas is needed. Few studies have attempted to determine the contribution of individual land uses to water quality degradation in urban areas. A 40% urban, 332 ha watershed in Ithaca, NY was selected as the research site. Runoff collected from 98 precipitation events over a two-year period and three land uses was analyzed for ammonium (NH 4 + -N), and nitrate (NO 3 - -N) and mass losses were calculated. Monitored land uses included fertilized lawns (FL), urban barren (UB) areas and wooded (FR) areas. Stream gauges were installed at the stream entrance to the urban area and the watershed outlet to monitor the impact of the urban land uses on stream water quality. A multivariate analysis of the data revealed that the FL land use had significantly higher (p<0.05) nitrogen (N) loss than the other land uses in areas with shallow soil and high runoff potential. On the deeper soil where runoff was low, the FL land use had significantly lower (p<0.05) N loss than the UB or FR land uses. Precipitation derived N inputs had an influence on N lost from all land uses in the watershed, while N measured in throughfall under the FR land use canopy was of the same order of magnitude as the N lost in runoff from these areas. As the stream flowed through the urban area there was a significant increase (p<0.05) in stream flow rates under storm conditions and a significant decrease (p<0.05) in stream flow rates under dry conditions when compared to the undeveloped upper watershed. Areally weighted N loads in the stream were as high, and in many cases significantly higher (p<0.05), from the undeveloped upper watershed than following streamflow through the urban area. This indicated that the urban area may be a N sink. The results demonstrate that it is imperative to assess land use performance both spatially and under varying climatic conditions in order to reduce surface water contamination.