EVALUATING POTENTIAL IMPACTS OF ON-SITE WASTEWATER TREATMENT SYSTEMS ON THE NITROGEN LOAD AND BASEFLOW IN STREAMS OF WATERSHEDS IN METROPOLITAN ATLANTA, GEORGIA

On-site wastewater treatment systems (OWTSs) are widely used in the Piedmont and Blue Ridge of the Southeastern U.S. for domestic wastewater treatment. OWTSs often are considered consumptive water use and can be potential sources of nitrogen (N) pollution for streams. This region heavily depends on surface waters for its water supply; therefore, the impact of OWTSs on surface water quality and quantity must be determined. The overall objective of this study was to determine the impact of OWTSs on the N load and baseflow in streams of watersheds in metropolitan Atlanta, Georgia. This article presents results of the differences in the N load and baseflow as well as other water quality indicators, such as specific conductance (SC) and chloride (Cl-), in streams of watersheds impacted by high-density (HDS) and low-density (LDS) OWTSs. Synoptic samples and discharge measurements of 24 watersheds were taken under baseflow conditions in November 2011, March 2012, July 2012, and November 2012. Mean baseflow measurements in November 2011, March 2012, and November 2012 were not statistically different between watersheds and showed no relationship with OWTS density within the watershed, but July 2012 measurements were significantly higher in the HDS watersheds and increased linearly with increasing OWTS density. SC and Cl- concentrations increased linearly with increasing OWTS density within the watershed, and nitrate (NO3-) concentrations increased linearly with increasing OWTS density above a threshold of about 100 OWTS per km(2). Results suggest a potential increase in baseflow because of the presence of OWTS effluent, which may offset the effects of impervious surfaces and maintain baseflow during drought conditions. Results also indicate a positive correlation between NO3- concentration and OWTS density within the watershed above a density of about 100 OWTSs per km(2). This study showed that OWTSs have potential positive and negative impacts on the water quality and quantity in watersheds of the Piedmont and Blue Ridge. It provides data that may be used to inform users as well as watershed planners about the potential influence of OWTSs on the N load and baseflow in streams based on the OWTS density within a watershed.