76-8 Predicting Nutrient Concentrations in Northern Miami-Dade County Canals from Septic Effluent Flows in Groundwater Based on Particle Tracking Simulation Results

Session: Contaminants Near Groundwater-Surface Water Interfaces

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Authors:

Abstract:

Miami-Dade County (MDC) has over 100,000 septic systems that act as sources of nutrient loading, affecting regional surface and groundwater quality. This study quantifies nutrient loading from septic system effluent flows in groundwater into four major canals (Miami River/C-6, Little River/C-7, Biscayne Canal/C-8, and Snake Creek Canal/C-9) in northern Miami-Dade County, Florida, to assess their impact on surface water quality. This investigation leverages canal flow data and our previous particle tracking simulations of septic effluent flows in MDC groundwater to develop a model to predict concentrations of total nitrogen (TN) and total phosphorus (TP) as a function of distance along a canal.

Simulated septic effluent groundwater discharge within 500 meters of main canal segments was projected onto canals, and mass loading was estimated from the modeled effluent flows using both literature-derived median effluent concentrations (TN: 60 mg−N/L; TP: 10 mg−P/L) and optimized best-fit values determined by inverse modeling via a sum of squares minimization between the model and observed water quality data from canal sampling stations.

Our results show that literature-derived effluent concentrations underestimated TN in upstream canal sections and showed variable accuracy for TP. The optimized effluent concentrations significantly improved the fit of modeled canal concentrations to the observations, particularly for the C-7 canal. Underestimation persisted in the upstream reaches of other canals, however. Dynamic flow conditions were not used; instead, annual average canal flows were used for modeled concentrations, which may represent a limitation for the study. This investigation highlights the need for more refined septic source characterization and the addition of tributary inflows and discharges to improve predictive models for nutrient concentrations in complex canal systems. These types of studies are important for effective water quality management in coastal urban environments.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10821

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