THE SOIL TREATMENT UNIT MODEL (STUMOD); A USER FRIENDLY TOOL FOR EVALUATING AND DESIGNING ONSITE WASTEWATER TREATMENT SYSTEMS

Onsite wastewater treatment systems (OWS) are used by about 25% of U.S. households. Treatment of wastewater occurs primarily in the soil receiving effluent from the septic tank, before it reaches the water table. Additional attenuation of contaminants from wastewater may occur in the saturated zone however it is desirable to design OWS to prevent the movement of contaminants off-site and minimize the contaminant load to groundwater. In some locations the use of OWS has resulted in contamination of groundwater and surface water resources due to poor design. Mathematical models are useful tools in the design of OWS because they can easily be manipulated to represent a variety of field conditions and designs. The Soil Treatment Unit Model (STUMOD) is a user friendly and practical tool designed to model the physical processes that control the movement of wastewater constituents through the vadose zone into the saturated zone. Currently this model is capable of calculating nitrogen speciation, total nitrogen, dissolved organic carbon, contaminants of emerging concern, and pathogens for a vertical profile in the vadose zone.

As part of the validation process STUMOD outputs have been compared to laboratory and field data gathered from a number of studies. Default parameter values in STUMOD were identified through an extensive literature review and laboratory experiments. STUMOD has also been compared to a HYDRUS model of an OWS and results indicate that STUMOD outputs are in agreement. Output uncertainty analysis was performed based on a Monte Carlo approach using parameters ranges obtained from literature for the 12 USDA soil textural classes to gain an understanding of the distribution of model outputs. Critical parameters were identified through a parameter sensitivity analyses to inform users which parameters to adjust from default values. Results from the validation process show that STUMOD is a robust but user friendly tool that does not require extensive modeling experience to operate, while still providing outputs that are in line with laboratory and field observations as well as industry accepted numerical models. STUMOD provides OWS professionals an opportunity to rapidly evaluate several scenarios in a relatively short time and aids in the design of OWS.