248 D15. Coastal Hydrogeology in an Age of Rising Seas and Other Advances in Hydrogeology

Wednesday, 12 October 2022
1:30 PM - 5:30 PM
407

PARTICLE TRACKING AND ADVECTION/DISPERSION/REACTION SIMULATION OF SEPTIC EFFLUENT IN MIAMI-DADE COUNTY FLORIDA

Paper number: 248-7
Program: Technical Sessions
Time: 2:50 PM – 3:05 PM

Authors

Abstract

A 30-year particle tracking simulation of septic effluent from Miami-Dade County Florida’s 100,000 septic systems was created using MODPATH with results from the Urban Miami-Dade Groundwater Model, published by the USGS in 2016. The model covers an area of 4,772 km2 in Southeast Florida. Simulations yielded an overall fate of septic return flows of 70% to surface water, 19% to wellfields, and 7% to the Biscayne Bay.

A nutrient concentration data set from the Miami-Dade County Synoptic Groundwater Analysis Report was post-processed and used to test for statistically significant correlation between spatially paired groundwater nutrient concentrations and the septic return flows at their recharge and discharge points in the MODPATH simulation. Variograms were computed and modeled to create Kriging estimates for nutrients on uniform 500 × 500 m grids equivalent to the model grid using SGeMS software; this enabled correlation of the Kriging estimates of concentration for various analytes with the septic return fluxes at both the input and the discharge cells for each pathline. Both flows and concentrations were transformed to their base-10 logarithms for the analysis. The focus was on Total Nitrogen and Phosphorus (TN, TP), Total Kjeldahl Nitrogen (TKN), and nitrate and nitrite (NO3−, NO2−). Regression and correlation analyses were performed on the paired nutrient and septic return flow grid point estimates. p-values were used to determine significance of the correlations.

The strongest correlation among the data sets was seen between initial discharge points and TP, with a weak positive correlation value of 0.16 and a highly significant p-value of effectively zero. The second strongest correlation was seen with the ending discharge locations and TP, with 0.12 for the correlation also with p-value of effectively zero. Significant p-values were also seen for the correlation between initial points and TKN (4.x10-5); between discharge points and TN (7.17x10-2).

Subsequent work is focused on the attenuation, transformation, and degradation of water constituents from septic effluents. The MT3DMS package is used to simulate three-dimensional multispecies dispersion, advection, and chemical reactions during transport from septic effluent recharge to discharge sites.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-383008

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