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.
Highlights
Discharge of excess nutrients from upstream river watersheds into an estuary and its coastal area can reduce dissolved oxygen levels, thereby killing marine organisms (Mallin, 2000)
Stream flows were calibrated by comparing simulated stream flows and observed stream flows at the USGS station 02108000 for the study period spanning from 1999-2005
It should be noted that there were some daily flow inconsistencies associated with localized rainfall distributions in the Northeast Cape Fear River (NECFR) watershed
Summary
Discharge of excess nutrients from upstream river watersheds into an estuary and its coastal area can reduce dissolved oxygen levels, thereby killing marine organisms (Mallin, 2000). Several studies have indicated that anthropogenic loads and the resulting eutrophication are the two major sources of nutrient loads in the blackwater river watersheds located in the Southeastern coastal plain of the United States, from Virginia south to northern Florida and in areas of the Gulf Coast (Mallin et al, 1997; Cahoon et al, 1999; Moran et al, 1999; Seitzinger et al, 2002; Graves et al, 2004; Hendrickson et al, 2007). Owing to major shifts in land uses in some blackwater watersheds, a large portion of Nitrogen (N) and Phosphorus (P) are retained within the colored decomposed organic matter (Graves et al, 2004)
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