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Nonhazardous, secondarily treated, domestic wastewater (effluent) has been injected about 1 kilometer below land surface into the Boulder Zone of the Floridan aquifer system at the North District Wastewater Treatment Plant in southeastern Florida. The Boulder Zone contains saline, nonpotable water. Effluent transport out of the injection zone is a risk of underground effluent injection. At the North District Wastewater Treatment Plant, injected effluent was detected outside the Boulder Zone. The U.S. Geological Survey, in cooperation with the Miami-Dade Water and Sewer Department, investigated effluent transport from the Boulder Zone to overlying permeable zones in the Floridan aquifer system. One conceptual model is presented to explain the presence of effluent outside of the injection zone in which effluent injected into the Boulder Zone was transported to the Avon Park permeable zone, forced by buoyancy and injection pressure. In this conceptual model, effluent injected primarily into the Boulder Zone reaches a naturally occurring feature (a karst-collapse structure) near an injection well, through which the effluent is transported vertically upward to the uppermost major permeable zone of the Lower Floridan aquifer. The effluent is then transported laterally through the uppermost major permeable zone of the Lower Floridan aquifer to another naturally occurring feature northwest of the North District Wastewater Treatment Plant, through which it is then transported vertically upward into the Avon Park permeable zone. In addition, a leak within a monitoring well, between monitoring zones, allowed interflow between the Avon Park permeable zone and the Upper Floridan aquifer. A groundwater flow and effluent transport simulation of the hydrogeologic system at the North District Wastewater Treatment Plant, based on the hypothesized and non-unique conceptualization of the subsurface hydrogeology and flow system, generally replicated measured effluent constituent concentration trends. The model was calibrated to match observed concentration trends for total ammonium (NH4 +) and total dissolved solids. The investigation qualitatively indicates that fractures, karst-collapse structures, faults, or other hydrogeologic features may permit effluent injected into the Boulder Zone to be transported to overlying permeable zones in the Floridan aquifer system. These findings, however, are qualitative because the locations of transport pathways that might exist from the Boulder Zone to the Avon Park permeable zone are largely unknown. Introduction Historically, most nonhazardous, secondarily treated, domestic wastewater (effluent) in southeastern Florida was discharged to the Atlantic Ocean before the North District Wastewater Treatment Plant (figs. 1, 2) was constructed in 1970. Effluent from the North District Wastewater Treatment Plant (hereafter referred to as the treatment plant) was discharged to the Atlantic Ocean through an outfall until the mid-1990s. In 2008, environmental impacts of effluent discharge to the ocean led the State of Florida to determine that the elimination of ocean discharge was in the public interest. As a result, the primary means of effluent discharge at the treatment plant is shifting from ocean outfall to underground injection. The Florida Department of Environmental Protection (FDEP) permits effluent from the treatment plant to be injected into the Boulder Zone through injection wells IW–1 (G–3950), IW–2 (G–3952), IW–3 (G–3805), and IW–4 (G–3954) (figs. 2 and 3; table 1; Cunningham, 2015, pl. 2). Construction of the injection wells began in 1994 for IW–2 and IW–3 and in 1997 for IW–1 and IW–4. Effluent injection to the Boulder Zone commenced in 1997 and monthly average injection rates from June 1997 to April 2012 at the treatment plant ranged from zero (no reported injection) to 2.3 cubic meters per second (m3/s) (fig. 4). The Boulder Zone is a highly transmissive hydrogeologic unit near the base of the Floridan aquifer system in the early Eocene Oldsmar Formation (fig. 5). At the treatment plant, the Boulder Zone is about -870 to -1,050 meters (m) Distribution of Effluent Injected Into the Boulder Zone of the Floridan Aquifer System at the North District Wastewater Treatment Plant, Southeastern Florida, 1997–2011 By Jeffrey N. King and Jeremy D. Decker 2 Distribution of Effluent Injected Into the Boulder Zone of the Floridan Aquifer System at the North District . . . 80°00' 80°10' 80°20'

scientific investigations report

Distribution of effluent injected into the Boulder Zone of the Floridan aquifer system at the North District Wastewater Treatment Plant, southeastern Florida, 1997–2011