Nitrogen Dynamics in a Piedmont Onsite Wastewater Treatment System

Abstract

Total Maximum Daily Load (TMDL) analyses of lakes and reservoirs with nutrient impairments commonly identify onsite wastewater treatment systems (i.e. septic systems) as an important potential source of nitrogen (N). In most cases, however, the contribution from on-site wastewater systems (OWSs) is difficult to estimate because of uncertainty about how much of the N is lost due to denitrification. The objective of this study was to quantify wastewater N concentrations in the soil and the extent of denitrification in an OWS commonly used in the Piedmont region. An OWS was installed in Griffin, GA and vadose zone N concentrations were monitored at different depths in the drainfield. Nitrate (NO3-N) concentrations remained low for the first four months after wastewater dosing began and then increased monthly for the next 11 months for all depths beneath the drainfield. The average NO3-N concentration at 90 cm on the last sampling date was 20 mg L-1. Denitrification was characterized in the drainfield by using Cl as a conservative tracer and calculating N/Cl ratios. We estimated that denitrification may account for up to 70% of N removal in OWSs. Currently, we are calibrating a model of the N dynamics using data from our experimental site. Once the model is calibrated, it will be used to estimate denitrification losses by OWSs in other regions by varying the soils and climate data