Natural attenuation of nitrogen loading from septic effluents: Spatial and environmental controls

We assess the role of septic systems as potential nitrogen (N) sources to coastal open water bodies. To quantify the potential role of septic tanks, we document the distribution pattern and functionality of septic tanks in McIntosh County in Georgia, USA, and examine factors governing the mitigation of septic N loading in coastal groundwater. Employing a field calibrated 2D variable-density reaction-transport model, we focus on the role of setback distance of a leaky septic source from the receiving surface waters, on transport and biogeochemical characteristics of the subsurface environment, and on leachate composition and reactivity. We conclude that the removal of bioavailable nitrogen via denitrification may be increased by increasing the septic system setback distance, in particular in brackish and saline coastal settings where sulfide produced in sulfate reduction can limit N2 production.