Identifying Nutrient Contributors in North Carolina’s Coastal Plain Blackwater Rivers

Abstract

The goal of this study was to assess potential nitrogen and phosphorus contribution from point sources and non-point sources in North East Cape Fear River (NECFR) watershed in North Carolina. A watershed scale hydrologic model, Soil and Water Assessment Tool (SWAT), was used to quantify Total Nitrogen (TN) and Total Phosphorus (TP) loadings to North Carolina’s Coastal Plain Blackwater Rivers. The model was calibrated to measured flow rate, TN, TP and suspended sediment concentrations. The model results suggest that nonpoint sources contribute more than 95% of the nutrient loads from the NECFR to the Lower Cape Fear River (LCFR), where low dissolved oxygen is a prime concern. Among the nonpoint sources, developed open space, forested wetland and agricultural land were responsible for approximately 20 to 35% of the total annual sediment loads (68838.60 tons) and the Total Phosphorus (TP) loads (452.20 tons) delivered to the LCFR. In addition forested wetlands, agricultural lands and forested lands contributed approximately 20 to 33% of the TN loads (3496.79 tons) to the river. While the forested wetlands assimilated more than 70% of the nutrient inputs, they also acted as sources of nutrients in the watershed. The results demonstrate that these wetlands were not substantially capable of trapping nutrient loading coming from adjoining lands for a longer period. Owing to the presence of more ditches, these wetlands functioned as a passive nutrient source by transporting significant amount of TN and TP into the river. However, these findings need to be further justified by conducting a field study to understand assimilative capacity of wetlands in the watershed.