Treated wastewater with high levels of ammonia has been injected, since March 1983 into the deep saline units of the Lower Floridan aquifer (LFA) from a treatment plant near the east coast of Miami-Dade County in southeastern Florida. Monitoring wells in the plant recorded ammonia concentrations above ambient levels at hydrogeologic units located about 1000 ft (304.8 m) above injection depths between 2500 and 2800 ft (762 and 853 m) below sea level. A solute-transport model was developed to assess the horizontal and vertical extent of the injected ammonia, with ammonia moving from the injected zone into the overlying units: the upper semiconfining unit, the uppermost permeable zone of the LFA, and the middle semiconfining units of the Avon Park Formation. Ammonia is assumed to be transported under the effects of local heterogeneity in a porous limestone aquifer with high-salinity ambient groundwater and via upward migration through quasi-vertical pathways. A flow model of the migration of the injected ammonia was calibrated with PEST using head, salinity, and ammonia concentration data measured from 1983 to 2013. Borehole geophysical data support the high permeability of the uppermost permeable zone in the LFA. Average simulated head, normalized salinity, and ammonia concentration residuals over all monitoring wells were −1.37 ft, 0.01, and −0.67 mg/L, respectively. Model results are consistent with undetectable ammonia concentrations in the Upper Floridan aquifer.