Hydrologic conditions for a two-aquifer system consisting of the Gordo and Eutaw aquifers of Cretaceous age in west-central Alabama were simulated using a three-dimensional finite difference digital model. The model was calibrated to observed heads in the aquifers using a least squares method for obtaining the values for hydraulic parameters which provided the best fit to 28 calibration points in the model. The standard error of estimate for computed heads in the model is 3.46 feet.
Data collected during the period of 1900 to 1970 indicate that the modeled aquifer system is in a steady-state condition. The results of steady-state model simulations show that about 56 percent of the discharge from the system is by upward vertical leakage through confining beds to rivers, about 18 percent is to unregulated flowing wells, 10 percent is to pumped wells, and 16 percent is to boundaries. The areal distribution of water in the aquifer containing chloride concentrations greater than 1,000 mg/L appears to be related to flow patterns in the system.
Areas of greatest head loss in the system are those where there is natural discharge and pumpage. More than 80 percent of the head loss in the system can be attributed to natural discharge by upward leakage through the confining beds.
