## 2 Materials and Methods  
### Study Area and Site Selection  
The study area consisted of 7 watersheds located in Durham County that drain to Falls Lake in the Piedmont geologic region of North Carolina (Figure 1). Each watershed was grouped and classified according to the dominant wastewater treatment approach (Table 1). Watersheds served by septic systems are numbered in order of increasing septic system density: 1 being the lowest (approximately 0.2 systems ha\({}^{-1}\)) and 3 being the highest (approximately 0.4 systems ha\({}^{-1}\)). Watershed boundaries were delineated using _StreamStats 4.0_, which is a web-based tool developed by the United States Geological Survey ([[https://streamstats.usgs.gov/ss/](https://streamstats.usgs.gov/ss/)]([https://streamstats.usgs.gov/ss/](https://streamstats.usgs.gov/ss/))), and land cover data were also complied from _StreamStats_ for each watershed (Table 1). The CS1, CS3, and SEW watersheds are tributaries or segments of Lick Creek; the CS2 watershed drains the headwaters of Laurel Creek; and the SF watersheds discharge to Little Lick Creek. Laurel Creek is considered a tributary of Lick Creek for watershed management purposes. Each of these watersheds were analyzed independently based on septic system densities (Table 1). Most of the septic systems in the studied watersheds were originally permitted between the 1950s and 1970s based on public record via City of Durham's interactive Go Maps ([[https://maps.roktech.net/durhamnc_gomaps4/](https://maps.roktech.net/durhamnc_gomaps4/)]([https://maps.roktech.net/durhamnc_gomaps4/](https://maps.roktech.net/durhamnc_gomaps4/))). Falls Lake serves as the main water supply for the City of Raleigh and provides primary and secondary water-based recreational activities for the region. Raleigh serves as the county seat for Wake County, which ranks among the top 50 counties in the United States for population growth and it gained approximately 65 new residents each day from 2010 to 2017 [25]. Sustainable management of water resources is vital for continued growth and economic development in the region. Falls Lake and many of its tributaries, including Lick Creek and Little Lick Creek, are designated as impaired waters [26]. Lick Creek and Little Lick Creek have watershed restoration plans designed to improve water quality within their respective watersheds. These plans suggested that failing septic systems and surface discharging systems (i.e., SF-type systems) could be significant sources of wastewater constituents (e.g., nutrients, bacteria, oxygen-demanding substances) and contributors to aquatic habitat degradation in Lick Creek and Little Lick Creek [27; 28]. The technology used for wastewater treatment (e.g., SF, CS, SEW) varied in different sub-watersheds of Falls Lake. Some areas were served mostly by CS, some mostly by SF, and others by SEW. Water quality monitoring data were not available for many of the sub-watersheds served by these different technologies, so more information was needed to determine if the wastewater treatment approach and/or septic system density affected water quality. Figure 1: Watershed delineation map for the 3 conventional septic (CS), 3 sand filter (SF), and sewered (SEW) watershed. LC = Lick Creek; LLC = Little Lick Creek; and LaC = Laurel Creek.