Minimum Lot Size Estimates for Nitrogen Assimilation in Onsite Wastewater Treatment Systems.

State regulatory agencies set standards for minimum lot size for homes with 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 estimate the minimum lot sizes that would be required for all 12 soil textural classes. The model was run for each soil textural class for two years using weather data for  April 2009 to April 2011.The minimum lot size was calculated using an equation in the Georgia OWTS Manual. Denitrification losses varied widely among soils, ranging 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. We found that 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 0.27 to 1.12 ha and were largest for medium-textured soils where denitrification and recharge were intermediate.