Nitrogen cycling in clay-textured soils with onsite wastewater treatment systems (OWTS) is studied and modeled much less often than sand- and loam-textured soils because there is little data on OWTS performance in these soils. Information on the nitrogen loads from these systems is needed for quantification of total maximum daily loads (TMDLs). The objective of this study was to calibrate a 2D HYDRUS model using experimental soil pressure head and vadose zone nitrogen (N) and chloride (Cl) data from a conventional OWTS that was installed in a clay soil in the Piedmont region of Georgia. An N chain model with water-content dependent first-order transformation rates for nitrification and denitrification was developed. The overall predicted soil pressure heads and solute concentrations were similar to data collected from the field experiment. The calibrated model made it possible to estimate water and solute fluxes in the drainfield and N losses from the OWTS. The estimated annual N loss from leaching at the lower boundary of the experimental drainfield was 3.8 kg yr−1. Scaled up to an OWTS size typical for GA and a zoning density of 5 homes ha−1, the N load to groundwater would be 57.4 kg ha−1 yr−1, which is comparable to agricultural production losses to groundwater. The model predicted 52% of the N removal in the system was from denitrification, whereas plant uptake and change in N storage accounted for ≤5% of the N loss. These estimates were specific to clay-textured soils and should be valuable to TMDL developers who need to predict load allocations for nonpoint sources in the Piedmont.