Water-Quality Trends for Streams and Reservoirs in the Research Triangle Area of North Carolina, 1983-95

Prepared in cooperation with the Triangle Area Water Supply Monitoring Project Steering Committee

By Carolyn J. Oblinger Childress and Neeti Bathala
Jordan Lake
This report presents the results of analyses of trends in concentrations of nitrogen, phosphorus, suspended sediment, suspended solids, sodium, chloride, iron, zinc, manganese, and chlorophyll a at 34 stream and reservoir sites that are part of the Triangle Area Water Supply Monitoring Project. Trend results are discussed in relation to broad-scale changes in land cover that have occurred in the area since the mid-1970's.

Data used for trend analysis of stream sites are from samples collected by the U.S. Geological Survey (USGS) from 1982 through 1987 for a study for the U.S. Army Corps of Engineers (Garrett, 1990a and b), by the North Carolina Division of Water Quality (DWQ) from 1982 through 1995 for the statewide ambient water-quality monitoring network, and by the USGS from 1988 through 1995 for the Triangle Area Water Supply Monitoring Project (Childress and Treece, 1996; Ragland and others, 1996). These three sources provide stream water-quality data, collected monthly, which are ideally suited for trend analysis. Using continuous streamflow data from USGS gages, stream water-quality data were statistically adjusted to eliminate the effects of changing streamflow.

Lake samples were collected from 1988 to 1995 by the DWQ for the North Carolina Lake Assessment Program and by the USGS for the Triangle Area Water Supply Monitoring Project. Generally, these data were collected only between April and October and were collected at variable time intervals.

BLUE DOT Abstract
BLUE DOT Introduction
BLUE DOT Methods of Land-Cover Estimation
BLUE DOT Trends in Land Cover and Water Quality
BLUE DOT Sample Collection and Analysis Methods
BLUE DOT Trend Analysis Methods
BLUE DOT Significant Findings
BLUE DOT References

Triangle Area Water Supply Monitoring Steering Committee

Chatham County
Orange County
Town of Apex
Town of Carrboro
Town of Cary
Town of Chapel Hill
City of Durham
Town of Hillsborough
City of Sanford
Orange Water and Sewer Authority


The Division of Water Quality of the North Carolina Department of Environment, Health, and Natural Resources maintains a Surface Water Ambient Monitoring System with approximately 380 sampling locations throughout the State. The USGS wishes to acknowledge the assistance of the Division in collecting water-quality samples and providing data from ambient monitoring sites located within the study area.

Copies of this report can be purchased from:

U.S. Geological Survey
Branch of Information Services
Federal Center, Box 25286
Denver, CO 80225-0286
Water-Resources Investigations Report 97-4061

For Further Information Contact:

District Chief
U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, NC 27607
Phone: (919) 571-4000
Home Page:

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Last modified: Thu Jun 18 15:50:48 1998