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The Verna well field, in the northeastern corner of Sarasota County about 17 miles east of the city of Sarasota, has been in operation since 1966. About 8 million gallons of water per day is pumped from the 3-square-mile well field to supply the city of Sarasota. This amounted to only about 8 percent of the total pumpage from Manatee and Sarasota Counties during 1980. Irrigation water use is far greater than other uses combined. Hydrogeologic units that consist of the surficial aquifer, intermediate aquifers and confining beds, and the Floridan aquifer total about 1,700 feet thick. The Verna well field contains 39 production wells, one of which taps the upper third of the 1,300-foot thick, highly transmissive Floridan aquifer. The other 38 wells tap the lower two-thirds of the moderately transmissive intermediate aquifers and the upper 100 to 200 feet of the Floridan aquifer. About 3 million gallons per day is derived from the intermediate aquifers and 5 million gallons per day from the Floridan aquifer. Sulfate-rich water moves laterally and upward within the Floridan aquifer toward the production wells. Pumping has resulted in water-level declines and changes in water quality. Drawdowns of 20 to 30 feet have occurred in the producing zone at the well-field boundaries. Under average pumping conditions, drawdowns in the intermediate aquifers were computed to be more than 1 foot over a 90-square-iaile area and more than 20 feet over a 6-square-mile area. A rising trend in sulfate concentrations has occurred in water from the production wells due to upward movement from deep in the Floridan aquifer. Concentrations have increased from about 200 milligrams per liter in 1966 to about 400 milligrams per liter in 1982 in the western part of the well field. In the eastern part, sulfate concentrations have increased from about 300 to 350 milligrams per liter during this period. The increases roughly coincide with increases in pumping rates. Some feasible water-management alternatives to improve yield and water quality from the producing zone include installation of a network of connector wells, development of computer models of ground-water flow or solute transport, and maintenance of a monitor-well network. If an estimated 1.4 million gallons per day could be recharged to the intermediate aquifers from the surficial aquifer by gravity drainage through a network of connector wells, the size of the cone of depression around the well field would be reduced, and the dissolvedsolids concentration of water in the producing zone possibly would be reduced from about 820 milligrams per liter to about 700 milligrams per liter. Models would be useful for optimizing distributions of pumping so that maximum yield is attained and mineralization is minimized. Models could also be used for assessing the effects of nearby irrigation pumpage on the well field. The outlook for using a model as a tool for developing additional quantities of water on the existing property is not very promising. A monitoring network would be useful in detecting vertical, areal, and temporal changes in water levels and water quality in each aquifer. Information gained through the network could be used to develop models for management of ground-water resources.

Hydrogeology of the Verna well-field area and management alternatives for improving yield and quality of water, Sarasota County, Florida