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 with suction lysimeters installed at different depths in the drainfield.  Nitrate concentrations in the drainfield remained low for the first four months after wastewater dosing began and then increased monthly for the next 14 months. The average NO₃-N concentration at 90 cm below the drainfield ranged from 10-30 mg L^-1 in the last 6 months of the study. Denitrification was characterized in the drainfield by using chloride as a conservative tracer and calculating N/Cl ratios. The N/Cl ratio in the effluent was approximately 1 mg N mg Cl^-1, and decreased to less than 0.5 mg N mg Cl^-1 in the drainfield.  Based on the N/Cl ratios calculated in this study, denitrification may account for >50% of the nitrogen removed in the drainfield.