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As the population of the Triangle area in central North Carolina increases, the demand for good quality drinking water from streams and lakes within the upper Neuse and upper Cape Fear River Basins also increases. The Triangle area includes Raleigh, Cary, Research Triangle Park, Durham, Chapel Hill, and the surrounding communities. The U.S. Geological Survey examined temporal trends in water quality for 13 stream and 8 reservoir sites in the two basins on the basis of data collected during 1989–2013. Trends were analyzed by using a fitted time-series model that accommodated for shifting trends and variations in streamflow at multiple time scales. Seventeen water-quality properties and constituents were evaluated, including specific conductance and major ions, nutrients, and organic carbon. Suspended solids and suspended sediment were examined at stream sites; chlorophyll a and Secchi transparency were examined at lake sites. The investigation identified considerable changes in population, land cover, streamflow, and selected water-quality characteristics in the study area over the 25-year period. Specific conductance and concentrations of calcium, magnesium, potassium, sodium, and chloride tended to increase throughout the study area. Area-wide increases were also observed for organic nitrogen. Trends for other water-quality constituents varied on a more site-specific basis because of local watershed influences such as changes to wastewatertreatment processes and substantial shifts from rural to urban land use. Water quality is influenced by multiple, often confounding factors, and thus may change in a manner that is not uniform over time. Long-term monitoring is critical for tracking these trends and ensuring resiliency of water supplies for the future. Results from this study may promote the understanding of water-quality response to a growing population and to land-cover changes and can assist waterresource managers in the Triangle area in tracking progress toward water-quality goals. Introduction The Triangle area of North Carolina includes the cities of Raleigh, Cary, Durham, and Chapel Hill; Research Triangle Park; and the surrounding communities. Ongoing growth in the Triangle area continues to increase the demand for local drinking-water supplies. At the same time, urbanization brings landscape changes that are likely to alter area hydrology and inputs of nutrients, sediment, and other water-quality constituents to lakes and rivers used as sources of drinking water. Several streams and lakes in the Triangle area are considered impaired because of degraded water quality (North Carolina Department of Environmental Quality, 2014), and all surface waters in the study area are designated “nutrient sensitive” (North Carolina Department of Environmental Quality, 2018). Raw water quality determines the type and level of water treatment needed to meet drinking-water standards. McDonald and others (2016) estimated that watershed degradation has led to increased water-treatment costs at 29 percent of cities globally, increasing costs, on average, by 53 percent for operation and maintenance and by 44 percent for replacement capital costs. Recognizing the potential for ongoing population growth and landscape change to affect water-supply quality and quantity, State and local governments have committed to long-term monitoring and periodic assessments to protect these valuable resources. In 1988, several local governments joined to form the Triangle Area Water Supply Monitoring Project (TAWSMP) to systematically evaluate the quality of water-supply sources in the region. In partnership with the TAWSMP, the U.S. Geological Survey (USGS) has collected and analyzed water-quality samples from reservoirs and streams and has collected continuous records of streamflow in the study area for nearly 30 years. Data collected by USGS, State, and local monitoring programs form a valuable, longterm database for streamflow and water quality in the Triangle area. To date, however, the number of regional assessments conducted for long-term water-quality trends for area water supplies is limited. 2 Trends in Water Quality of Streams and Reservoirs Used for Water Supply in the Triangle Area of NC, 1989–2013 Understanding temporal changes is critical for documenting progress toward water-quality goals and informing water-supply planning and land-management decisions. Therefore, the USGS compiled publicly available streamflow and water-quality data and analyzed trends during the 25-year period of 1989–2013 for streams and reservoirs in the Triangle area. Stream sites with long-term increases or decreases in flow were identified, and long-term increases or decreases in concentrations of selected water-quality constituents were identified for 13 stream sites and 8 reservoir sites. Previous Studies This study supplements a previous analysis of waterquality trends in the Triangle area of North Carolina for the period 1983–95 (Childress and Bathala, 1997). At a national scale, Sprague and others (2009) evaluated trends in nutrients in major rivers of the United States during 1993–2003, including two sites that were evaluated in the current study. Oelsner and others (2017) investigated trends in nutrients, sediment, ions, carbon, and other water-quality constituents in U.S. rivers and streams during 1972–2012, including five stream sites in the current study, by using the Weighted Regressions on Time, Discharge, and Season (WRTDS) method (Hirsch and others, 2010; Hirsch and De Cicco, 2015). Spruill and others (2006) studied trends of suspended sediment and nutrients in the upper Cape Fear River Basin during 1976–2004, including one site in the current study. Water-quality trends in selected Triangle area streams have also been evaluated periodically by the North Carolina Department of Environmental Quality (NCDEQ) as part of their river basin assessment program. The methods used for trend analysis in the current study differ from the methods used in previous investigations as discussed later in the report. Purpose and Scope This report presents a regional comparison of trends in water quality at selected water-supply streams and reservoirs in the Triangle area of North Carolina. Trends in water quality were related to trends in population density, land cover, and other relevant watershed characteristics. Watershed and streamflow characteristics, trend analysis methods, and factors potentially affecting trends are discussed. Twenty-one sites (13 stream sites and 8 lake sites) were evaluated for water-quality trends during the 25-year period of 1989–2013, hereafter referred to as “the trend period.” The period of analysis was less than 25 years for some sites and (or) constituents because sufficient data were unavailable. The evaluation of trends generally involved a nested comparison of a 1-period trend model for the entire period with a 2-period trend model for “early” (1989–2001) and “late” (2002–13) subperiods to assess whether trend rates and directions shifted over time. Step-change functions were included in the trend models when known events contributed to abrupt changes in water quality. The trend models that best fit the data were selected for use in the regional evaluation. The magnitude of uptrends and downtrends was expressed as a percent change in concentration; the model-fitted median concentrations for beginning and ending years of the trend period also were reported. Seventeen water-quality properties and constituents were examined for trends, although not all constituents were analyzed at all sites. Water-quality characteristics included specific conductance, dissolved ions, nutrients, total organic carbon, suspended sediment and (or) solids, chlorophyll a, and water clarity. Water-quality data were obtained from publicly available sources; specifically, the USGS National Water Information System (NWIS) and the U.S. Environmental Protection Agency (EPA) STOrage and RETrieval (STORET) database. Water-quality datasets that were used as input for the trend analyses are available from Cain and others (2018). Streamflow data from USGS streamgage-monitoring stations used in the trend analyses also were obtained from NWIS. The trend-analysis approach used in this study allowed site-to-site comparisons throughout the study area during consistent time periods. A full discussion of site-specific trends was beyond the scope of this report; however, site-specific trends are presented when especially noteworthy. Study Area The study area is in the upper Neuse and upper Cape Fear River Basins in the Piedmont Physiographic Province of North Carolina (fig. 1). Characterized by low, rolling hills, the Piedmont extends westward to the Blue Ridge escarpment and eastward to the Coastal Plain. The geology of the region is complex, composed mainly of metamorphic and igneous rocks—primarily gneiss and schist—with granitic intrusions. The western part of the study area is generally located in the Carolina Slate Belt, which consists of heated and deformed volcanic and sedimentary rocks, including slate, gneiss, and metamudstone. The eastern part of the study area is in the Sanford-Durham Triassic Subbasin, which is dominated by claystones, siltstones, and other fine-grained sedimentary rocks (Bain and Brown, 1981; Daniel and Dahlen, 2002). Sites selected for trend analysis were in Chatham, Durham, Orange, and Wake Counties. From 1990 to 2010, the population of these four counties increased 85 percent— from 737,825 to 1,365,886 people (Forstall, 1996; U.S. Census Bureau, 2017). The population of the same four counties is projected to increase by another 44 percent by the year 2030 (North Carolina Office of State Budget and Management, 2017). Streams and reservoirs in the Triangle area are important water-supply sources and also support recreation, aquatic life, and other uses. Numerous efforts have been made through the years to protect these water sup

Trends in water quality of selected streams and reservoirs used for water supply in the Triangle area of North Carolina, 1989–2013