EVALUATING THE USE OF FULL-GRID CONSTANT HEAD BOUNDARY CONDITIONS TO IMPLEMENT SOLUTE TRANSPORT MODEL SIMULATIONS FOR SEPTIC EFFLUENTS IN MIAMI-DADE COUNTY, FLORIDA

Miami-Dade County, a metropolitan area in south Florida with over 2.7 million residents, is surrounded by sensitive water bodies: the Everglades National Park to the west and Biscayne Bay to the east. The Biscayne Aquifer, the area's primary source of drinking water, faces water quality threats from sea level rise, a rapidly growing population, and over 120,000 septic systems. These systems negatively impact not only the aquifer but also Biscayne Bay.

Building on prior modeling efforts, we leveraged the Urban Miami-Dade Groundwater Model (UMD) from the USGS to conduct the first regional particle tracking simulations of septic effluents in southeastern Florida. These simulations estimated septic effluent discharge locations, with a distribution of 70% to canals, 19% to well fields, and 7% to the bay.

To further assess risks, we aimed to improve existing groundwater flow simulations by incorporating contaminant transport processes (e.g., dispersion, degradation). This is being achieved by coupling the UMD model with the Groundwater Solute Transport Simulator for MODFLOW (MT3D-USGS) using FloPy and ModelMuse. This will allow for a more comprehensive understanding of potential risks to drinking water quality and the Biscayne Bay health. However, challenges arose when coupling the UMD model with MT3D because the UMD model relies on the Surface-Water Routing Package, which is not supported by MT3D.

To overcome this limitation, we are exploring alternative simulation methods to investigate nitrogen species transformation and transport of other wastewater constituents. One approach involves using UMD's output heads as constant head boundaries in a simple steady-state flow model for subsequent integration with MT3D. We present test cases to assess the viability of using such constant head boundaries on the transport model. Somewhat unexpectedly, this strategy appears to work relatively well, and further simulations using the full model are explored.

Ultimately, this study emphasizes the importance of careful wastewater management in vulnerable areas like Miami-Dade County. By identifying and reporting "hotspots" for water quality sampling efforts and prioritizing areas for septic-to-sewer conversion, we can ultimately improve water quality throughout the County.