Modeling the Expected Performance of Wastewater Soil Treatment Units.

Onsite wastewater treatment systems (OWS) are an important part of water management infrastructure in the United States. The vast majority of OWS in the U.S. include a unit operation involving effluent dispersal into soil to achieve treatment. OWS are designed to hydraulically process and purify an effluent within the soil profile to the extent needed to protect public health and water quality.  A tool was developed for the purpose of predicting the fate and transport of nitrogen, organic matter, trace organic chemicals and pathogens in soil treatment units (STUMOD). The tool calculates fraction of contaminants reaching the aquifer. Input data include both operational and calibration parameters for hydraulics and chemical transport and transformation. The output is fraction of contaminants removed at selected soil depth or the water table. The chemical transport component is based on simplification of the general advection dispersion equation. STUMOD is relatively simple to use but detailed enough to account for important fate and transport processes. The model computes nitrogen removal via ammonium sorption, nitrification and denitrification using Monod-kinetics and plant uptake. The change in moisture content with depth is calculated using the Buckingham Darcy equation, thus, the effect of soil moisture content on nitrification and denitrification reactions is incorporated. The model has provisions to account for the effect of temperature and organic carbon content. The removal of organic matter and trace organic chemicals is calculated using first order biodegradation and sorption. Pathogen removal is calculated based on sorption and die-off rates.  Outputs generated based on default STUMOD parameters were compared to data from obtained based on extensive literature review, laboratory tests and field test sites. Both measured data and STUMOD outputs show a relatively higher removal in clayey soils compared to sandy soils. Consistent with literature data for most soils, STUMOD predicted ammonium conversion to nitrate within the first foot below the trench infiltrative surface.