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

Introduction

The Triangle area (fig. 1), including Raleigh, Cary, Research Triangle Park, Durham, Chapel Hill, and their surrounding communities, is one of the most rapidly developing areas of North Carolina. According to the 1990 census, 77 percent of the households in the area depend on public drinking-water supplies withdrawn from area streams and lakes. Among the most important of these water supplies are Falls of the Neuse Reservoir (hereafter referred to as Falls Lake), B. Everett Jordan Reservoir (hereafter referred to as Jordan Lake), Lake Michie, Little River Reservoir, Cane Creek Reservoir, University Lake, Eno River, Neuse River, Deep River, and the Cape Fear River. Because of heavy reliance on surface water for public water supply and recognition of the importance of protecting area surface-water resources, local governments established the Triangle Area Water Supply Monitoring Project in 1988 with the assistance of the Triangle J Council of Governments.

The goals of the Project are to:

Supplement existing data on major ions, nutrients, and trace elements to enable determination of long-term trends;

Examine differences in water quality among water supplies within the region, especially differences among smaller upland sources, large multipurpose reservoirs, and run-of-river supplies;

Provide tributary loading and in-lake data for predictive modeling of Falls and Jordan Lakes; and

Establish a data base for the presence of synthetic organic compounds in surface water in the region.

With cooperative assistance from the U.S. Geological Survey (USGS), the Project has funded collection and analysis of water-quality samples from reservoirs and streams, and collection of continuous discharge records from streams in the study area (fig. 1). In October 1988, the USGS began water-quality sampling and streamflow monitoring, in cooperation with the Triangle Area Water Supply Monitoring Project Steering Committee, at sites located at area run-of-river and reservoir water supplies and tributaries (Garrett and others, 1994; Childress and Treece, 1996). These sites and sites monitored by the North Carolina Division of Water Quality (DWQ; formerly known as the Division of Environmental Management) form the Triangle Area Water Supply Monitoring Network.

Description of the Study Area

The study area is located in the Piedmont Province of North Carolina. It includes part of a seven-county area within the upper part of the Neuse and Cape Fear River Basins (fig. 1).

The headwaters of the Neuse River are formed by the Flat River, the major tributary to Lake Michie which was filled in 1926; the Little River, the major tributary to Little River Reservoir which was filled in 1988; and the Eno River. These rivers drain to Falls Lake, a reservoir filled in 1983. Lake Michie and Little River Reservoir provide water supply to Durham, the Eno River provides water supply to Hillsborough, and Falls Lake provides water supply to Raleigh. From Falls Lake, the Neuse River flows south through the eastern part of Wake County receiving runoff from Raleigh and its suburbs and wastewater effluent from the City of Raleigh Wastewater Treatment Plant. Farther south, the Neuse River serves as a water supply for Smithfield, Johnston County, and several municipalities served by Johnston County.

The headwaters of the Cape Fear River Basin are formed by the Haw River and its tributaries. Jordan Lake, which was filled in 1982, impounds the Haw River and New Hope Creek. Although the Haw River supplies 80 percent of the inflow to Jordan Lake, most of the storage is in the New Hope arm (Moreau and Challa, 1985). Jordan Lake is a water-supply source for the towns of Cary and Apex, for Chatham County, and potentially for other jurisdictions in and around the Triangle area. Other tributaries to Jordan Lake are Northeast Creek and Morgan Creek.

University Lake, a water-supply reservoir filled in 1932, impounds water from about 30 square miles (mi2) in the upper Cape Fear River Basin. The lake was formed from Morgan Creek and two tributaries to provide drinking water for Chapel Hill, Carrboro, and the University of North Carolina at Chapel Hill. Cane Creek Reservoir, filled in 1989, is a public water supply for Carrboro and Chapel Hill and impounds water from about 31 mi2 in the upper Cape Fear River Basin. A more detailed description of the study area is provided in Childress and Treece (1996).

Current Land Cover

The Piedmont part of the Neuse River Basin (the upper Neuse River Basin) is the most densely populated and industrialized part of the basin and has the greatest density of waste dischargers (North Carolina Department of Environment, Health, and Natural Resources, 1993). Falls Lake receives inflow from a 771-mi2 watershed of combined forested and agricultural lands, and urban and residential areas. Based on data from 1988, land use for 53 percent of the watershed is forest (including wetlands) and 29 percent is agriculture (fig. 2C). Developed areas account for about 18 percent. The Lake Michie drainage area (167 mi2) accounts for approximately 22 percent of the drainage to Falls Lake and is 52 percent forest and wetlands, 40 percent agriculture, and only 7 percent developed. The Little River Basin (98 mi2) accounts for approximately 13 percent of the drainage to Falls Lake and is about 55 percent forested, 38 percent agriculture, and only 7 percent developed.

The part of the study area that lies in the upper Neuse River Basin above Smithfield is about 25 percent developed (fig. 2C). The part from the Falls Lake dam to Smithfield is 39 percent developed and includes the city of Raleigh and its suburbs. From 1970 to 1990, the population in the upper Neuse River Basin grew steadily and increased by approximately 70 percent (North Carolina Department of Environment, Health, and Natural Resources, 1993). Lake Benson, just south of Raleigh, receives drainage from the 65-mi2 Swift Creek watershed, of which about 50 percent is forested. However, urban and residential land uses in the watershed are increasing as Raleigh, Cary, and surrounding communities continue to develop. For example, the Swift Creek Basin near Apex was about 75 percent developed in 1988 (fig. 2C), a 40-percent increase in developed land cover since 1975 (fig. 2B).

Municipal wastewater-treatment plants (WWTP's) that discharged to tributaries of Falls Lake prior to 1995 include Durham's Northside WWTP, which discharged to Ellerbe Creek; Little Lick WWTP, which discharged to Little Lick Creek; Butner WWTP, which discharged to Knap of Reeds Creek; and Durham's Eno River WWTP, which discharged to the Eno River (fig. 3). In November 1994, Durham's Northside WWTP was upgraded to tertiary treatment and renamed North Durham Water Reclamation Facility. This facility receives wastewater previously sent to the Eno River and Little Lick WWTP's. The Eno River and Little Lick WWTP's ceased operation in June and November 1994, respectively.

In the Cape Fear River Basin, most of the population and industry are located near the headwaters of the Haw and Deep Rivers between Burlington and Greensboro. Drainage from the Haw River Basin, a 1,300-mi2 watershed of mixed forested and agricultural lands and urban and residential areas, is to Jordan Lake. Jordan Lake also receives drainage from the New Hope Creek Basin, a 400-mi2 watershed that is mostly developed (including Durham, Chapel Hill, and Research Triangle Park) but includes forested and agricultural land uses, much of which are undergoing industrial and residential development (fig. 2C).

Cane Creek Reservoir is in the Haw River Basin and receives drainage from 31 mi2 of mostly forested and some agricultural lands (fig. 2C). University Lake is in the Morgan Creek Basin and receives drainage from an approximately 30-mi2 area mostly west of Carrboro. More than half of the area is forested with some agricultural and residential development.

Municipal WWTP's that discharge to tributaries of the New Hope Creek arm of Jordan Lake include Durham County Triangle WWTP, which discharges to Northeast Creek; South Durham Water Reclamation Facility, which discharges to New Hope Creek; and Orange Water and Sewer Authority (OWASA) Mason Farm WWTP, which discharges to Morgan Creek near Farrington (fig. 4).

Previous Studies

Previous studies of water-quality trends in the Neuse and Cape Fear Rivers have been for earlier time periods and generally were for sites in the downstream parts of the watershed. Crawford (1985) tested for trends in major ions, nutrients, and physical properties in the Cape Fear River near Lock 1 for the period 1955-80. Increases in specific conductance and concentrations of dissolved manganese, sodium, potassium, sulfate, solids, and total nitrite plus nitrate were reported and were related to increases in population and manufacturing operations. Decreases in dissolved silica and pH levels also were reported.

Harned and Davenport (1990) reported trends for nutrients, chlorophyll a, and other water-quality constituents in the lower Neuse River for the period 1945-88. These trends were related to general changes in land use and basin characteristics. For the lower Neuse River, declines in total phosphorus, total ammonia plus organic nitrogen, nitrate, and orthophosphorus were reported. Increases in concentrations were reported for chlorophyll a. The decrease in nitrogen and phosphorus species was correlated with changes in agricultural land-use factors, such as tobacco and soybean acreages, quantities of fertilizers sold, and numbers of chickens raised.

Harned and others (1995) used seasonal Kendall trend analysis to detect trends in water quality in the Neuse River from its headwaters to Kinston for the period 1980-90. Several sites analyzed for this report coincide with sites analyzed by Harned and others (1995). Decreasing trends in total phosphorus in North Carolina streams were attributed to the 1988 phosphate-detergent ban. A decreasing trend in total nitrogen below Falls Lake was probably due to the 1983 impoundment that formed Falls Lake. An increasing trend in total nitrogen was detected in the middle part of the Neuse River Basin and was probably due to increased development and changing agricultural practices.

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Last modified: Wed Jun 10 10:13:12 1998