Modeling Nitrogen in On-site Wastewater Treatment Systems

State regulatory agencies set standards for minimum lot size for homes on onsite wastewater treatment systems (OWTS) based on the expected nitrogen (N) load to groundwater. However, the data to support these standards are sparse. In a recent field study on a clay soil, we developed a two-dimensional model for N treatment. Our objective was to use this model to predict the N treatment for 12 soil textural classes using two years of weather data from the field experiment. We found that soil texture had a strong effect on OWTS performance. Denitrification losses varied widely among soils, from 1% in the sand class to 75% in the sandy clay class. This was due to the effect of water content on denitrification. Leaching losses to groundwater ranged from 27% in the sandy clay class to 97% in the sand class. It was important to consider differences in recharge among soil textural classes in estimating the minimum lot size to protect groundwater. The lot sizes ranged from 0.26 to 1.13 ha and were largest for mediumtextured soils where denitrification and recharge were intermediate.