English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

Sanibel and Captiva Islands, two of the barrier islands off the lower west coast of Florida, have a resident population of about 6,000 and an additional 6,000 visitors during the peak of the tourist season. Rapid growth and extensive development in recent years have imposed progressively greater stress on the water resources of the islands. Sanibel and Captiva Islands are underlain by a series of geologic formations containing some zones of relatively freshwater and other zones of more mineralized, or even very saline water. Two aquifers are the principal sources of water supply on the islands. The upper aquifer is in the upper part of the Hawthorn Formation. The lower aquifer is in the lower part of the Hawthorn Formation and the underlying upper part of the Tampa Limestone. Both aquifers are under sufficient artesian pressure to permit wells to flow at land surface. The potentiometric surface of the lower Hawthorn-upper Tampa aquifer has been lowered in the central part of Sanibel Island where the Island Water Association operates about 10 wells that tap the lower aquifer. In 1977, the average pumpage from these 10 wells was 1.4 million gallons per day. The water is processed through a desalination plant before distribution. In 1977, the total volume of water withdrawn from both the lower and upper aquifers for all purposes was 690 million gallons, an average of about 1.9 million gallons per day. Water from the upper Hawthorn aquifer during the period 1975-77 contained dissolved solids concentrations ranging from 721 to 2,390 milligrams per liter. Water from the lower Hawthorn-upper Tampa aquifer during the same period contained concentrations ranging from 1,700 to 4,130 milligrams per liter. Water from both aquifers is hard and contains relatively high concentrations of sodium, sulfate, and chloride. INTRODUCTION Sanibel and Captiva Islands form part of the barrier island chain off the lower west coast of Lee County, Florida (fig. 1). Both islands are resort areas and have a resident population of 6,000 and an additional 6,000 people during the peak of the tourist season (Sanibel Planning Council, 1977, written commun.). Rapid growth and extensive development in decent years have imposed progressively greater stress on the land and water resources of the islands. H 0 0 C o P H c r n > r t H C o P c x Historically, fresh ground water on Sanibel and Captiva Islands has been obtained from shallow wells, generally less than 15 feet deep. These wells provide only small supplies and frequently yield brackish or saline water during the dry season. A pipeline was constructed in 1966 which transports water purchased from the Greater Pine Island Water Association well field and treatment plant on the mainland to provide a dependable water supply to the islands. This pipeline extends beneath San Carlos Bay from St. James City to the northern end of Sanibel Island. The water-supply system on Sanibel and Captiva Islands at that time (1966) consisted of pumping stations and water distribution lines. Beginning in 1973, a number of deep (about 300 to 900 feet) artesian wells were drilled for public supply. The saline water from these wells (Boggess, 1974a, fig. 4) is processed by electrodialysis for domestic use. The pipeline and the desalination plant provided sufficient water for most needs, but recent increased demand has resulted in increased importation of water from the mainland. Purpose and Scope During the time that the desalination plant has been in operation, questions have been raised by water-management agencies concerning the ability of the deep artesian aquifers underlying Sanibel and Captiva Islands to continue supplying the water needed to meet the increasing demand with a salinity low enough for the desalination plant to operate effectively. The purpose of this investigation was to evaluate the watersupply potential of the deeper aquifers. During the 24-month investigation, October 1975-77, an important objective was to locate and inventory all existing wells that tapped the deep artesian aquifers. Except for a few, all known wells were found. Water levels and water samples were obtained at the wells. No test wells were drilled as part of this investigation, although one observation well was drilled on Sanibel Island as part of a countywide monitoring network. The data included in this report will provide a better understanding of the water quality of the deep aquifers underlying these offshore islands for the effective development of the water resources. Acknowledgments The authors gratefully acknowledge the cooperation and assistance of the residents of Sanibel and Captiva Islands in providing data included in this report. All well owners, including the Island Water Association of Sanibel, freely permitted access to their wells so that water-level data and water samples could be obtained. The cooperation and support of officials of the city of Sanibel and the Board of County Commissioners are also gratefully acknowledged. Well-Numbering System All wells listed in this report or for which data are cited are numbered as part of a countywide system. The wells were numbered in sequence as each well or test hole was inventoried. The locations of wells and test holes on which information was obtained during the investigation are shown in figure 2. GEOLOGY The surficial deposits on Sanibel and Captiva Islands are Holocene and Pleistocene in age and include sand, shells, clay, and limestone generally less than 120 feet thick. Below these deposits is the Tamiami Formation of Pliocene age which consists chiefly of gray and green clay and sandy clay. Thin beds of sandstone, sand, or limestone occur locally in the Tamiami, and phosphorite is a common accessory mineral. Thickness of the Tamiami ranges from about 200 feet beneath central Sanibel Island to less than 100 feet beneath other parts of the islands. The Hawthorn Formation and Tampa Limestone of Miocene age successively underlie the Tamiami Formation and range in thickness from about 300 feet beneath central Sanibel Island to about 400 feet beneath other parts of the islands. Both formations consist predominantly of gray and gray-white phosphatic limestone with interbedded marl or calcareous clay. Phosphorite is abundant with major concentrations in the lower part of the Hawthorn Formation. Below the Tampa Limestone, the Suwannee Limestone is usually penetrated 600 to 700 feet below land surface. This formation consists predominantly of tan limestone. The Suwannee Limstone may extend to 1,100 feet or more beneath Sanibel Island. The sequence of geologic units and water-bearing units are shown in figure 3 with lithologic and gamma-ray logs of well 2524 on the southwest shore of Sanibel. The gamma-ray activity of the phosphorite is high because it contains a minute quantity of uranium (Altschuler and others, 1958). The gamma-ray peaks in the lower part of the Hawthorn have been assigned numbers (4E, 5C, and 5D) and are used as markers for correlation purposes. They have been identified in nearly all the test holes and wells on Sanibel and Captiva Islands for which gamma-ray logs have been obtained. On the basis of gamma-ray correlation, the lithology is fairly consistent (fig. 4). The geologic section (fig. 4) is constructed from data of four test wells along an east-trending line across Sanibel Island (fig. 2). Comparable beds penetrated at greater depth in well 1704 than in well 2401 may indicate that some vertical displacement has occurred (fig. 4). For example, the gray-white, phosphatic limestone (which marks the top of the Hawthorn Formation) occurs at a depth of about 200 feet in well 2401, and at a depth of 350 feet in well 1704. These wells are 1.7 miles apart, so that the intevening slope of the top of the phosphatic limestone is about 90 ft/mi. Although the top of the limestone is an erosional surface, the beds lying at greater depths have similar slopes. Displacement of beds by folding is possible in this general area. Missimer and Gardner (1976)

Deep artesian aquifers of Sanibel and Captiva Islands, Lee County, Florida