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).  However, the contribution from on-site wastewater systems (OWSs) is difficult to estimate because of uncertainty in the percentage of N lost to denitrification. The objective of our study was to develop an OWS model of N fate and transport and calibrate it using data from an experimental site. We developed a two-dimensional N chain model using HYDRUS. Transformations from ammonium to nitrite, nitrate, and nitrogen gas were modeled using first-order kinetics. Two years of measured pressure head and suction sampler data from the experimental site were used in the calibration process. Conditions near the trench were close to saturation throughout the two- year experiment, but there were distinct wet and dry periods in response to weather and to daily dosing of the trench. The model currently predicts about one half of the observed annual N load to groundwater. We intend to incorporate a degree-of-saturation effect on the transformation coefficients.