Fosnrs 7: Development of an Analytical Groundwater Model for Fate and Transport of Nitrogen from Onsite Wastewater Systems.

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.  The objective of this task is to provide a user friendly and practical tool to assess the fate and transport of nitrogen.  STUMOD-FL described in the previous paper is a simple to use tool designed to model the physical processes that control the movement of wastewater constituents through the vadose zone.  This work focuses on the development of the groundwater module that will accept nitrogen inputs from STUMOD-FL and evaluate subsequent nitrogen fate and transport in shallow groundwater.  The current version of the groundwater module is based on an analytical solution to the groundwater flow equation (Galya 1987).  This module in conjunction with STUMOD-FL allows the user to model the effects of OWS on water quality down gradient of the STU.  The groundwater module is capable of calculating nitrate concentrations along a flow path incorporating advection, diffusion and first order degradation kinetics.  An alternate version of the groundwater module, based on the Dominico & Robbins (1985) solution, has also been evaluated. 

As part of the validation process outputs from the groundwater module have been compared to field data gathered at the GCREC field site in Florida.  Groundwater module outputs have also been compared to a MODFLOW model of an OWS and results indicate that the analytical model outputs are in agreement.  Output uncertainty analysis has been performed based on a Monte Carlo approach using parameters representative of Florida hydrogeology 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.  Methods to accurately determine parameter values for the groundwater module have also been proposed and evaluated.  Preliminary results from the integrated vadose zone and saturated zone modules indicate that it is a robust and user friendly tool that does not require extensive modeling experience to operate, while still providing outputs that are in line with field observations and industry accepted numerical models.  The model provides OWS professionals an opportunity to rapidly evaluate several scenarios in a relatively short time and aids in the selection of nitrogen reduction strategies for OWS in a given watershed.