ABSTRACTThe city of St. Petersburg has been testing subsurface injection of treated sewage into the Floridan aquifer as a means of eliminating discharge of sewage to surface waters and as a means of storing treated sewage for future non‐potable reuse. The injection zone originally contained native saline ground water that was similar in composition to sea water. The zone has a transmissivity of about 1.2 X 106feet squared per day (ft2/d) and is within the lower part of the Floridan aquifer.Treated sewage that had a mean chloride concentration of 170 milligrams per liter (mg/1) was injected through a single well for 12 months at a mean rate of 4.7 X 105 cubic feet per day (ft3/d). The volume of water injected during the year was 1.7 X 108cubic feet.Pressure buildup at the end of one year ranged from less than 0.1 to as much as 2.4 pounds per square inch (lb/in2) in observation wells at the site. Pressure buildup in wells open to the upper part of the injection zone was related to buoyant lift acting on the mixed water in the injection zone in addition to subsurface injection through the injection well.Calculations of the vertical component of pore velocity in the semiconfining bed underlying the shallowest permeable zone of the Floridan aquifer indicate upward movement of native water. This is consistent with the 200‐ to 600‐mg/l increase in chloride concentration observed in water from the shallowest permeable zone during the