Fosnrs 6: Stumod-FL - a Tool for Predicting Fate and Transport of Nitrogen in Soil Treatment Units in Florida.

A practical modeling tool to evaluate the fate and transport of nitrogen in the OWS subsurface is being developed by the Colorado School of Mines (CSM) in collaboration with Hazen and Sawyer and FDOH as part of the FOSNRS project.  The modeling includes fate and transport of nitrogen both in the saturated and unsaturated zones building off of the Soil Treatment Unit Model (STUMOD) developed at CSM through support from the Water Environment Research Foundation.  This new version, STUMOD-FL developed for the FOSNRS project, is a simple to use spreadsheet tool designed to simulate the physical processes that control the movement of wastewater constituents through the vadose zone.  STUMOD-FL is a spreadsheet based user-friendly tool requiring minimal skill from the user and only a basic understanding of soil hydraulic and treatment mechanisms.  Numerical model experience is not required. STUMOD-FL is based on fundamental principles of water movement and contaminant transport using an analytical solution to calculate suction and soil moisture profiles in the vadose zone and a simplification of the general advection dispersion equation.  STUMOD-FL, is tailored to Florida-specific soil and climate conditions and includes default model parameters representing dominant soil properties found in Florida.  However, STUMOD-FL allows user-specified input and can be calibrated to site-specific data.

STUMOD-FL accounts for evapotranspiration and the effect of high/seasonally variable water tables on nitrogen removal in soil and incorporates up to four soil layers including the biomat. The effects of ET are expressed in two primary ways: root water uptake and root nutrient uptake.   The potential evapotranspiration is calculated using Hargreaves Method. The actual evapo-transpiration is calculated from the potential evapotranspiration with adjustment factors to account for soil moisture content and root distribution.  The nutrient uptake is driven primarily by plant nutrient demand but also the actual ET and concentration of nutrients in the soil water.  The root water uptake has a flexible formulation that considers a maximum allowable uptake, plant nutrient demand, and soil water nutrient availability uptake mechanisms through ET. STUMOD-FL estimates the water table fluctuations in response to precipitation based on one-dimensional flow in unconfined aquifers.  Assumptions in the model development include that the major cause of water table fluctuation is precipitation and that the time lag from precipitation to water table response is negligible.

Nitrogen species concentrations and the fraction of total nitrogen reaching the aquifer and mass flux is estimated by STUMOD-FL at user specified soil depths based on user input data including hydraulic loading rate, sorption, nitrification and denitrification rates.  The effect of soil moisture content (a surrogate for redox conditions) is accounted for in nitrification and denitrification reactions.  STUMOD-FL predictions are being compared to controlled pilot-scale test site data on nitrogen fate and transport under a variety of typical operating conditions. Model outputs provide insight into the behavior of soil treatment and quantitative estimations of nitrogen removal as affected by a range of conditions.