Quality of water recovered from a municipal effluent injection well in the Floridan aquifer system, Pompano Beach, Florida

Abstract

Approximately 69 million gallons of backflow from an injection well used for the disposal of secondary treated municipal effluent in the Floridan aquifer system near Pompano Beach, Florida, was periodically sampled for inorganic and organic quality from March 1975 through March 1977. Analyses of the backflowed effluent showed a concomitant increase in dissolved solids and a change in ionic composition as a function of cumulative volume of backflow. Both the increase in dissolved solids and the change in major ionic composition were directly related to the mixing of an estimated 6 to 7 percent of the moderately saline water in the Floridan aquifer system with the injected effluent. Although an estimated 3.5 billion gallons of effluent with an estimated chloride concentration of 84 milligrams per liter was injected into the aquifer system during the 16-year operation of the Collier Manor treatment plant, only 65 to 70 million gallons (about 2 percent of the total effluent) was recovered before the chloride concentration approached 250 milligrams per liter. Unlike the more conservative major inorganic constituents, the levels of nitrogen, phosphorus, and hydrogen sulfide decreased with cumulative backflow. The reduction in concentrations of these nonconservative variables in the final backflow samples exceeded that which was likely caused by a 6 to 7 percent mixing of the water in the Floridan aquifer system with injected effluent. The decreases of many nonconservative variables, therefore, were probably the result of both mixing and other processes such as ion exchange, sorption, bacterial activity, and chemical precipitation. Overall, the recovered effluent was of poor quality and was not suitable for most uses under existing State regulations. Additionally, the inorganic quality of the injected effluent was further degraded as a result of mixing with the moderately saline native water.