Digital simulation of the regional effects of subsurface injection of liquid waste near Pensacola, Florida

Industrial, organic, liquid waste has been injected into a part of the lower limestone of the Floridan aquifer at one site since 1963 and at another site since 1975, raising water levels in the injection zone throughout a large region. The hydrogeologic conceptual model of the injection zone is a layer tightly confined above by a thick layer of clay and in which lateral hydraulic conductivity decreases rapidly below the upper 60 feet. Recharge areas are to the north and east, where the confining layer pinches out. There appear to be permeability barriers to the northwest, west, and southwest due to facies changes, faults, or pinchouts. Measured and reconstructed preinjection water levels suggested that flow in the aquifer is from the northern recharge areas toward the southeast. A steady-state model simulation incorporating the cited boundary assumptions approximately simulated this pattern. A two-dimensional flow model and the subsurface waste injection program (SWIP) were calibrated to simulate the water level increases at various monitor wells since 1963. Sensitivity analyses showed the simulations to be quite sensitive to moderate errors in either transmissivity or storage parameter specifications. The predictive use of the hydraulic model is understood to be restricted to the geographical locations of data used for model calibration. (USGS)