Dale M. Johnson James M. Gerhart Paul D. Ryder 1980 The Morris Bridge well field in west-central Florida, which is being developed may have a maximum well-field withdrawal of 40 million gallons per day. The water will be pumped from the Floridan aquifer--a sequence of carbonate rocks about 1,100 feet thick underlying surficial sand and clay deposits. A highly fractured and transmissive zone about 500 feet below National Geodetic Vertical Datum of 1929 will supply a large proportion of the water. Two-dimensional and three-dimensional digital flow models were used to evaluate the hydrogeology of the area. The model-derived leakance distribution (a property of the confining bed) for a 285-square-mile area ranged from 0.00002 to 0.008 per day. Model-derived transmissivity values for the Floridan aquifer ranged from 37,000 to 600,000 feet squared per day. Model-derived specific yield values for the surficial aquifer ranged from 0.05 to 0.30. The three-dimensional model was used to predict drawdowns in both the Floridan and surficial aquifers in response to a 40 million gallon per day stress. Mass-balance data from a 30-day simulation with no recharge from rainfall show percentage of withdrawn water that is derived from: (1) aquifer storage, (2) the Hillsborough River, and (3) reduction of evapotranspiration losses. (USGS) application/pdf 10.3133/wri8029 en U.S. Geological Survey Model evaluation of the hydrogeology of the Morris Bridge well field and vicinity in West-Central Florida reports