ABSTRACT

An increased demand for fresh groundwater resources in South Florida has prompted Miami-Dade County to expand its water reclamation program and actively pursue reuse plans for aquifer recharge, irrigation, and wetland rehydration. The U.S. Geological Survey, in cooperation with the Miami-Dade Water and Sewer Department (WASD) and the Miami-Dade Department of Environmental Resources Management (DERM), initiated a study in 2008 to assess the presence of selected pharmaceuticals and other organic wastewater compounds in the influent and effluent at three regional wastewater-treatment plants (WWTPs) operated by the WASD and at one WWTP operated by the City of Homestead, Florida (HSWWTP).

To assess the range in concentrations of pharmaceuticals and other organic wastewater compounds in influent and effluent waters, 24-hour (hr) flow-weighted composite water samples were collected from the influent and effluent at each plant and analyzed for a broad range of pharmaceuticals and other organic wastewater compounds. These analyses included semivolatile organic compounds, pesticides and pesticide degradates, wastewater-indicator compounds, pharmaceuticals, antibiotics, and one hormone. Water samples were collected once in the wet season (high-flow conditions) and once in the dry season (low-flow conditions) to capture any seasonal variations in concentrations. Compounds detected in 24-hr flow-weighted influent composite samples included: 20 semivolatile organic compounds, 12 pesticides and pesticide degradates, 52 wastewater-indicator compounds, 5 pharmaceuticals, 14 antibiotics, and the hormone 17-beta-estradiol. Compounds detected in 24-hr flow-weighted effluent composite samples included: 19 semivolatile organic compounds, 13 pesticides, 49 wastewater-indicator compounds, 7 pharmaceuticals, 11 antibiotics, and 17-beta-estradiol. Percent reductions during treatment were calculated when possible. Among the various groups of compounds, semivolatile organic compounds and wastewater-indicator compounds generally had higher removal efficiencies than pharmaceuticals, antibiotics, and pesticides. Qualitatively, the total concentrations of pharmaceuticals and other organic wastewater compounds in the effluent at each plant were slightly higher in the dry season compared to the wet season; however, loads to the environment were generally larger in the wet season due to the higher flows through the WWTPs.

For decades, the HSWWTP has been discharging treated effluent directly to the water table using onsite soakage trenches. Water samples were collected from three monitoring wells at the HSWWTP to determine the occurrence of pharmaceuticals and other organic wastewater compounds in groundwater near the soakage trenches. Concentrations in the groundwater were generally below 0.5 microgram per liter (µg/L) with the exception of the fragrance galaxolide (estimated at 0.62–1.3 µg/L), the antibiotic sulfamethoxazole (0.14–0.57 μg/L), and the flame retardant tri(2-butoxyethyl)phosphate (estimated at 0.51 µg/L). Most pharmaceuticals and other organic wastewater compounds were attenuated in groundwater at this site; however, galaxolide, sulfamethoxazole, 3,4-dichloroaniline, 1,2-dichlorobenzene, and carbamazepine were detected in all groundwater samples, thus indicating that these compounds may be used as tracers of effluent at this site. Water samples were collected from monitoring wells once in the wet season and once in the dry season to determine any seasonal variations in concentrations. Results from seasonal sampling indicate that concentrations of pharmaceuticals and other organic wastewater compounds in groundwater are higher in the wet season, which is most likely related to the larger effluent loads to the groundwater during wet months.

Water and bed sediment samples were collected from two canal sites near the HSWWTP. These samples were analyzed for pharmaceuticals and other organic compounds to determine if compounds present in the effluent are being transported through the groundwater system and into the canal system. Water and bed sediment samples were also collected from one background canal site in Miami-Dade County, with no known sources of wastewater in the area, to compare with results from the canal near the HSWWTP. Results from these samples indicated that 51 compounds were detected in one or more canal samples with concentrations generally below 1.0 µg/L. A high estimated concentration of the plasticizer, diethylhexyl phthalate (DEHP, estimated at 11 µg/L), was detected in a canal water sample close to the HSWWTP; however, the source of this compound to the canal is most likely from surface runoff or the application of pesticides and not the HSWWTP. Results from seasonal sampling from the two canal sites indicated that concentrations of pharmaceuticals and other organic wastewater compounds were generally higher in the dry season. Water samples collected from the background canal site contained low levels (less than 0.76 µg/L) of 11 organic compounds in water samples, including 5 wastewater-indicator compounds and the non-prescription pharmaceutical acetaminophen; bed sediments contained detectable levels of 16 wastewater-indicator compounds. The presence of these compounds in water and bed sediment samples indicates that there is a probable nonpoint source of wastewater in the area.

First posted June 18, 2012