Factors Affecting Nitrogen and Phosphorus Loads in Nutrient-Sensitive Water of Eastern North Carolina

The proposed work will include computation of nutrient (total nitrogen, total nitrate, and total phosphorus) loads in various streams throughout eastern North Carolina. Most stream sites selected for study are located in the Tar-Pamlico River basin, Neuse River Basin, and Cape Fear River basin; a few sites from the Lumber, Chowan, and Roanoke River basins also will be included (map and data table). Statistical analyses will be conducted to examine the influence of particular watershed variables on total nitrogen, total nitrate, and total phosphorus loads at the study locations. The intent of the data analysis is to determine which variables may be a useful indicator for identifying watersheds in NSW with a high potential for exporting nutrients.

The stream sites selected for study, based on existing USGS streamflow gages and available nutrient data, will include locations in both the Piedmont and Coastal Plain Provinces. Available streamflow data from the USGS database and water-quality data from NCDENR and USGS databases will be compiled and processed for use in computing loads at each site. Information on land use cover, density of riparian buffers, streamflow characteristics, groundwater discharge, point sources, and nonpoint sources will be compiled for each watershed. Statistical analyses will be performed to determine which environmental variables may significantly influence total nitrogen, total nitrate, and total phosphorus export in the watersheds selected for study.

Approach

The following tasks will be performed to meet the project objectives.

Task 1: Develop list of stream sites for potential evaluation

Potential stream study sites will be based on those that have both a USGS continuous streamflow gage and also are sampled through the NCDENR DWQ ambient water-quality program or USGS project activities. The focus will be on active USGS and NC DWQ stream sites that have sufficient data during the past 5 to 10 years for use in determining streamflow characteristics, percent base flows, and nutrient loadings. A list of potential sites to be included in the study is presented in table 1.

Task 2: Compile and process streamflow data

For each study site selected in Task 1, stream-flow data for the 5-10 year study period will be compiled from the USGS National Water Information System (NWIS) database and processed to determine annual streamflow characteristics for use in statistical correlations with nutrient loadings. Hydrograph separations will be performed on the data to determine the amount of base flow, or percent contribution of the annual stream flow derived from groundwater discharge.

Task 3: Compile and process water-quality data

DWQ will provide nutrient data for each of the ambient monitoring stations paired with a USGS stream-gage site. Monthly water-quality data for the past 5 to 10 years will be combined with the streamflow data (task 2) to compute total nitrogen, total nitrate, and total phosphorus loads at each stream study site. At selected sites, part of the analysis will be to determine if the total in-stream nutrient loads can be separated into that derived from both baseflow (groundwater discharge) and from overland runoff.

Task 4: Compile watershed characteristics

Geographic Information System (GIS) information will be used to determine the types and extent of land use, based on 2001 data, within the drainage area of each stream study site. The density of riparian buffers bounding the streams in each study area also will be determined using GIS methods. The numbers and types of point sources (such as waste-water treatment plant discharges) and nonpoint sources (such as livestock feeding operations) will be compiled for each watershed and incorporated in the GIS datasets. These watershed variables will be used in statistical correlations with the stream nutrient loads to better understand those factors that most significantly influence nutrient export in NSW of eastern North Carolina.

Task 5: Statistical data analysis

All data (tasks 2-4) will be compiled into a database for processing and analysis. The streamflow and watershed data (tasks 2 and 4) will be compared statistically to the nutrient load data (task 3) to determine if there are similarities or differences in loadings based on such things as basin size (drainage area), stream runoff, groundwater discharge, land cover, point sources (such as waste-water treatment plant discharges), and nonpoint sources (such as types and amounts of animal production facilities). Results of the statistical analyses will be used to determine which watershed variables may be useful indicators of watershed potential for exporting or retaining nutrients.

Task 6: Prepare final report describing study results

The USGS will work in collaboration with NCDENR to complete a USGS Scientific Investigations Report that presents and discusses the study results and findings. The focus of the report will be to document the watershed characteristics and nutrient loadings for each streamflow and water-quality monitoring site and to discuss the results of the statistical evaluation of the factors that may influence nutrient loads from the watersheds.