Relationships between N transport and hydraulic function of onsite wastewater treatment systems (OWTSs) in clay-textured soils are largely unknown. The objectives of this study were to quantify N concentrations in a clay soil and estimate denitrification in the vadose zone beneath a conventional OWTS. An OWTS installed in the Piedmont region of Georgia was dosed with residential strength wastewater three times per day for 2 yr. Soil near trench bottoms cycled between saturated and unsaturated conditions under the dosing schedule we used. Mean NH4–N concentrations decreased to <1 mg L−1 within 90 cm of trench bottoms due to adsorption, immobilization, and nitrification. Mean NO3–N concentrations increased as the drainfield matured and ranged from 10 to 25 mg L−1 near the end of the two year study period. Mean pressure heads and Cl− concentrations measured at 3.3 m and 6.6 m from trench inlets indicated that significantly more water infiltrated the drainfield at the 3.3 m position. Wet conditions at the 3.3 m position may have restricted nitrification, while dry conditions at the 6.6 m caused rapid nitrification. Nitrogen:chloride ratios suggested that 61% of N entering the drainfield was lost, potentially as volatile N-species. The 30-cm zone directly beneath the trenches had high biomass N and C contents (15 mg kg−1 N and 60 mg kg−1 C) and was presumably the most biologically active zone. The fate and transport of N was dynamic in this system due to variable moisture conditions near the trench-soil interface which may have intermittently stimulated denitrification.