Daily water use in the Withlacoochee River region in 1977 averaged about 2,005 million gallons per day, 94 percent of which was saline surface water used in thermoelectric power-generation cooling. Industrial and irrigation uses required 73 percent of the freshwater. The largest user of freshwater was Hernando County, using 43.0 million gallons per day.

The ground-water system is comprised of up to three different aquifers--the surficial, the secondary artesian, and the Floridan. Little is known about the surficial and secondary artesian aquifers.

The Floridan aquifer consists mostly of limestones and dolomites, and is as much as 1,500 feet thick. Transmissivities are known to be as high as 25 million feet squared per day. Yields of 2,000 gallons per minute from 12-inch wells are possible. Although the range in fluctuations of the potentiometric surface is as great as 20 feet, no significant change has occurred since the 1930's when data were first collected.

The quality of water within the Floridan aquifer is generally excellent except near the Gulf Coast and in extreme east Marion County, near the St. Johns River where saltwater is present in the aquifer. Iron and hydrogen sulfide are sometimes a problem, but they can usually be con-trolled by proper well design and aeration of the water. Concentrations of sulfate do not exceed 250 milligrams per liter in the study area, and only in a small part of the area do dissolved-solids concentrations exceed 250 milligrams per liter.

Summaries were compiled of more than 1,000 wells, 43 continuous-record gaging stations, 21 lakes, and 46 springs. The predominant chemical type for both streams and springs is calcium and magnesium bicarbonate due to the dissolution process of the carbonate rocks. Along the coastal areas and near the St. Johns River, water is commonly of the sodium chloride type. The majority of the streams have average dissolved-solids concentrations between 100 and 200 milligrams per liter, maximum-observed specific conductance between 250 and 750 micro-mhos per centimeter, and average total nitrogen concentrations of less than 1.2 milligrams per liter.

Data for six lakes showed that the range of stage between the 90 and 10 percent exceedance stages is as great as 4.5 feet and as small as 2.2 feet. Little water-quality data for lakes are available, especially for the important constituents such as biochemical oxygen demand, total nitrogen, total phosphorus, and total carbon.

Flow-duration data for springs show small ranges in discharge. The differences between the 10 and 90 percent exceedance discharges are 350 cubic feet per second for Silver Springs and 280 cubic feet per second for Rainbow Springs, the two largest springs in the area. Water quality of the springs is relatively constant with time because of the water's long residence time within the carbonate rocks.