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Johnson Engineering > News & Community > Outlook Newsletter > Johnson Engineering Pioneers Model to Get Water Quality Credits

Johnson Engineering Pioneers Model to Get Water Quality Credits

Florida’s Department of Environmental Protection (FDEP) has a Total Maximum Daily Loads (TMDLs) program to improve water quality to State or federally-identified impaired waterbodies, including the Caloosahatchee Estuary and Imperial River. The FDEP requires local governments to participate in mandated Best Management Action Plans (BMAPs) to reduce TMDLs to these waterbodies.

The cities of Bonita Springs and Cape Coral participate in BMAPs, and selected Johnson Engineering to calculate how much nutrient load was removed from their impaired waterbodies by the elimination of septic systems in areas within their city limits. The cities plan to submit the findings to FDEP in order to gain credits towards meeting their TMDL goals.

Johnson Engineering used recently developed GIS-based ArcNLET software to evaluate nutrient loading from the cities’ septic systems, and is one of the first private organizations to use the software. In addition, this marks one of the first uses of the software in south Florida. Application of the model to areas characterized by low hydraulic gradients with numerous canals and surface water features posed a challenge. Flow rates, denitrification decay coefficients and initial nutrient loads are the primary factors for determining loads reaching a waterbody.

To create a model that better represented the study areas, Johnson Engineering recommended modification of selected default model parameters. Andy Tilton, Paul Lohr, and Terry Bengtsson worked closely with City, FDEP and software creators/authors to develop innovative approaches to model set-up. This included use of MODFLOW groundwater flow models to generate better estimates of hydraulic gradients, as opposed to approximation of a water surface based on a Digital Elevation Model (DEM). Groundwater velocities and flow directions driven by hydraulic gradients strongly influence the ultimate fate of nutrients from septic systems. Use of MODFLOW models, along with other model parameters more appropriate for areas with water tables near land surface, resulted in improved estimates of nutrient load reductions.

The City of Bonita Springs has received approval by the FDEP for a credit of 912 pounds of nitrogen removal against the needs of the BMAP for the freshwater portion of the Imperial River. The City has estimated nitrogen removal costs at about $650 per pound based on several previous projects. This equates to savings of more than a half a million dollars of taxpayer money based on work already funded by residents through the Bonita Springs Utilities construction program.

For more information, contact Andy Tilton, P.E. at mkt@johnsoneng.com.