Subsurface injection of treated sewage into a saline-water aquifer at St. Petersburg, Florida - Aquifer pressure buildup

The 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 nonpotable reuse. Treated sweage that had a mean chloride concentration of 170 milligrams per liter (mg/l) was injected through a single well for 12 months at a mean rate of 4. 7 multiplied by 10**5 cubic feet per day (ft**3/d). The volume of water injected during the year was 1. 7 multiplied by 10**8 cubic 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/in**2) 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.