U.S. Department of the Interior
U.S. Geological Survey

| Background| Geologic Framework of the Aquifer System| Geophysical Investigations| Research Plan| Collaboration and Partnerships



Restoration and management of the south Florida ecosystem will be guided by hydrologic models that simulate water flowing through the wetlands and shallow subsurface aquifers beneath them. The restoration of the ecosystem is, essentially, the restoration of the natural hydrologic system. As surface water is re-diverted from manmade canals to its more natural state as overland flow, several changes are predicted to occur. First, because water flowing over land moves more slowly than in canals, overland flow should remain in the wetland ecosystem for a longer period each year. Second, as the flowing water spreads out over the wetlands, recharge to the shallow aquifers should increase as more of that water infiltrates into the ground. The U.S. Corps of Engineers and the South Florida Water Management District (SFWMD) will use hydrologic models to anticipate the consequences of these proposed restoration plans. This research program is designed to provide essential subsurface data to improve hydrologic models for land and water managers in southwest Florida where subsurface information is lacking. Obtaining hydrogeological data requires core drilling, corehole testing, and rock and sediment analysis.


PROJECT GOAL

The objective of this project is to provide to hydrologic modelers a three-dimensional database of the geologic and hydrologic properties of the sediments and rocks of the surficial aquifer system in southwest Florida, in Collier and Monroe Counties. Emphasis will be placed on the geologic framework of the aquifer.


BACKGROUND

Nearly all sediment and rock in the subsurface of south Florida contains ground water; however, much of that water is not accessible for use. Zones from which economically significant quantities of water can be withdrawn are called aquifers. The most important properties of an aquifer are porosity and permeability. Porosity is the amount of void space (pores) in sediment or rock that water can occupy, and permeability is the rate at which water can flow between those pores. The quantity and flow rate of ground water in aquifers are determined primarily by differences in porosity and permeability. These important sediment and rock properties were determined in the geologic past by a combination of original depositional processes, by past and present dissolution of the rock, and by the precipitation of new minerals. In other words, the rocks of an aquifer system have a history, and understanding that history will enable us to construct more realistic hydrologic models and to approximate aquifer properties between sampling sites. In southwest Florida, the surficial aquifer system (from the water table down to a depth of about 200 feet) is the primary source of freshwater. This system includes the water-table aquifer, the lower Tamiami aquifer, and the confining unit (rock and sediment of low permeability) that separates them. Surface water that flows through the wetlands is in the water-table aquifer.



GEOLOGIC FRAMEWORK OF THE AQUIFER SYSTEM

Understanding the geologic history of the sediments and rocks of the aquifer system is necessary to place the hydrologic properties of that system into a geologic framework. Most of the sediments in the surficial aquifer system were deposited in the last 5 million years, when quartz silt and sand, clay, carbonate minerals, and shells of invertebrate organisms accumulated in the shallow marine environment that covered south Florida. During that period, the sea level rose to cover south Florida numerous times. Each time the sea level rose, more carbonate minerals, shells, quartz sand, and clay accumulated. Each time the sea level fell, quartz sand and clay were transported into the marine environment, and newly exposed carbonate sediments were subjected to weathering. Some of those sediment surfaces exposed to weathering in the past may have developed into highly porous zones forming conduits for ground-water flow today, or they may have become tightly cemented to form confining units that separate aquifers. Two independent methods will be used in this study to estimate the age of the aquifer rocks and sediments. Samples from cores will be examined for fossil dinoflagellate cysts, pollen, mollusks, foraminifers, and ostracodes, and their age determined by correlation to other distant sites that have been dated isotopically. Age also will be estimated by the isotopic composition of strontium in unaltered shells. The ratio of the stable isotopes of strontium in the oceans has varied over geologic time such that, in the last 40 million years, there has been a unique relation between age and isotopic composition. Marine invertebrates incorporate the strontium isotopic ratio of the ocean into their shells as they grow, thereby preserving evidence of their age.


GEOPHYSICAL INVESTIGATIONS

Geophysical logs provide a continuous downhole record of the properties of the rocks that form the aquifer. They are especially valuable in providing physical and chemical properties of the corehole where particular intervals of core recovery are poor. Also, they allow extension of hydrologic test data from discrete samples to the rest of the core. Geophysical logs, combined with aquifer water properties and flow measurements, will be used to relate large-scale ground-water circulation to the distribution of hydrologic properties of the aquifer. For example, flowmeter logs can confirm that the most permeable intervals, as inferred from core measurements, coincide with the intervals that conduct the most flow in the vicinity of test wells. Geophysical logs also will indicate which confining units act to separate the aquifer system into discrete aquifers having different water quality and hydraulic head.


RESEARCH PLAN

Approximately 40 coreholes will be drilled through the surficial aquifer system, averaging 200 feet in depth.
• Geophysical logs will be run after the holes are drilled. Geophysical probes will be placed into the coreholes to determine water and rock composition, rock porosity, water-flow rates, hole diameter, and roughness. A flowmeter will be used to measure direction (up or down) and magnitude of flow in the aquifer at specific depths.
• Cores will be examined for mineralogy, texture, sedimentary structures, and fossils to determine rock type, age, mineralization, and porosity and to differentiate the zones that form aquifers from the confining units that separate them.
• Stratigraphic units will be identified and correlated between coreholes and their properties estimated where core data are absent.
• Permeability will be measured on core samples, and the data integrated with hydrologic and geophysical logs, to estimate the rate at which ground water moves through the aquifer.
• Aquifer tests will be performed on selected coreholes to measure permeability of the aquifer system.

COLLABORATION AND PARTNERSHIPS

U.S. Geological Survey (USGS) scientists in this project have formed a partnership with the Florida Geological Survey (FGS). The FGS and USGS will share responsibility for drilling the coreholes, describing the core, and performing permeability testing. The FGS will curate the cores and make them available to all scientists at their warehouse facility in Tallahassee after analyses are completed. The FGS maintains an electronic lithologic data base on the World Wide Web and will include descriptions of all cores drilled in that data base. Mapping the spatial distribution of hydraulic conductivity in the aquifer system will require the integration of all data collected by both agencies. Members of this project are coordinating with another USGS/SFWMD cooperative study, in an area where the two studies overlap, to assess the gray limestone aquifer of the surficial aquifer system of central and southeast Florida as a source ofg public-supply water. The two projects will share the same corehole sites in the Big Cypress National Preserve.




For More Information, Please Contact:
Suzanne D. Weedman
U.S. Geological Survey, MS 926A
Reston, VA 20192
(703) 648-6379
sweedman@usgs.gov
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