The Relationship between Land Use and Vulnerability to Nitrogen and Phosphorus Pollution in an Urban Watershed

Characterization of the vulnerability of water bodies to pollution from natural and anthropogenic sources requires understanding the relationship between land use and water quality. This study aims (i) to explore the influence of upstream land use on annual stream water concentrations and loads of total nitrogen (TN) and phosphorus (TP) and (ii) to characterize the vulnerability of water bodies to TN and TP pollution as a function of land use under varying climatic conditions. Multiple linear regression models were used across 23 stream locations within the Jordan Lake watershed in North Carolina between 1992 and 2012 to explore land use–water quality relationships. The percentage of urban land use and wastewater treatment plant capacity were the most important factors with strong ( R 2 > 0.7) and significant ( p < 0.01) positive correlations with annual TN and TP concentrations and loads. Percent agricultural land was negatively correlated with TN in 18 out of 21 yr of the study period. Using analysis of covariance, significant ( p < 0.01) differences were determined between models developed for urban land use with TN and TP loads based on annual precipitation. Using concentrations instead of loads resulted in a nonsignificant difference between models for average and wet years. Finally, a procedure was developed to characterize the vulnerability to TN and TP pollution, computed as the probability of exceeding the nutrient standard limits. Results indicated that the vulnerability to TN and TP was controlled primarily by urban land use, with higher values in dry years than normal and wet years. Core Ideas Strong and significant relationships were found between land use and water quality. Urban land/ wastewater treatment capacity were dominant factors in regression models. Significant differences found between land use–water quality models for wet/dry years. The impact of rainfall on land use–water quality models was better captured by loads. Vulnerability to TN and TP were controlled by urban land use in dry years.