###### Abstract  
Wastewater contains elevated concentrations of fecal indicator bacteria (FIB). The type of wastewater treatment technology and septic system density may influence the FIB concentration and exports at the watershed scale. The goal of this study was to gain a better understanding of FIB concentrations and exports from watersheds served by conventional septic (CS) systems, sand filter (SF) septic systems, and a municipal sewer (SEW) system. Seven watersheds (3 CS, 3 SF, and 1 SEW) were monitored to quantify FIB concentration and export monthly from April 2015 to March 2016. The type of wastewater treatment did not yield significant differences in FIB concentration or exports when pooling watersheds using similar wastewater treatment. Watersheds with the highest septic densities (approximately 0.4 systems ha\({}^{-1}\)) contained greater FIB concentrations and exports than watersheds with the lowest (approximately 0.1-0.2 systems ha\({}^{-1}\)), but only FIB concentrations significantly differed. These findings suggest that when the septic system density exceeds 0.4 systems ha\({}^{-1}\), water quality degradation from septic leachate may be observable at the watershed scale, especially in watersheds dominated by residential development. More research is recommended to determine if this density threshold is similar for other water pollutants and/or in watersheds with differing hydrogeological, land use, and wastewater characteristics. bacteria; transport; wastewater; sand filter; septic; fecal contamination +
Footnote †: journal: Medicalwater. Furthermore, no more than 10% of samples should exceed statistical threshold values (STVs) of 410 and 130 MPN 100 mL\({}^{-1}\) for _E. coli_ and enterococci, respectively. Prior studies have shown that FIB in surface waters can be linked to waste material from a variety of sources, including wildlife [6], pets [7], livestock [8], and improperly treated domestic wastewater from cities and rural areas. In North Carolina, regulations have been enacted to reduce non-point source pollution via implementation of agricultural best management practices, stormwater control measures, and improvements in centralized wastewater infrastructure [9]. Efforts to improve water quality via improvement of septic system treatment efficiency were not initiated, partly due to a lack of information regarding their watershed contributions. Septic systems are commonly used in rural or suburban communities where it may not be economically feasible to extend municipal sewer lines. In the United States, approximately 25% of Americans rely on septic systems for their primary means of wastewater treatment and 30% of new construction uses septic systems [10; 11]. Previous studies have found greater concentrations of FIB in waters downgradient from septic systems relative to water downgradient of municipal sewers [12; 13; 14; 15; 16]. Water quality and/or aquatic habitat degradation from septic-derived FIB may be exacerbated during periods of septic system malfunction, which release partially treated or untreated wastewater directly to groundwater or surface water [17]. Most of these studies have focused on conventional septic (CS) systems that collect, treat, and discharge wastewater effluent to subsoils beneath drainfield trenches. Alternative septic technologies exist that are generally used at sites that fail to meet the minimum soil requirements for CS outlined in state regulations. Sand filter (SF) systems are one example of commonly used alternative septic systems. SFs have a septic tank and effluent from the tank is piped to the SF where aerobic treatment occurs, and the SF effluent is discharged directly to drainageways or streams. Other research showed SF effluent contained FIB concentrations of up to 5000 MPN 100 mL\({}^{-1}\)[18; 19]. These studies suggest that wastewater treatment efficiency and water quality adjacent to septic systems may be influenced by system type, but more information is needed to determine if differences in water quality are observed at larger spatial scales.