U.S. Department of the Interior
U.S. Geological Survey
FS-158-96
Project Goal |
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.
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.
Figure 1. Project study area. |
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 Zones from which economically significant quantities of water can be withdrawn are called aquifers.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.
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.
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 of public-supply water. The two
projects will share the same corehole sites in the Big Cypress
National Preserve.
For more information contact:
Related information:
U.S. Department of the Interior, U.S. Geological Survey
- yearly reports of geophysical, stratigraphic, and hydrologic data
- report summarizing the geologic history of the surficial aquifer system in
southwest Florida
- regional geologic cross sections of the
surficial aquifer system of southwest
Florida
Suzanne D. Weedman
U.S. Geological Survey, MS 926A
Reston, VA 20192
(703) 648-6379
sweedman@usgs.gov
SOFIA Project: Hydrogeology of the Surficial Aquifer System in Southwest Florida
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Last updated: 05 November, 2004 @ 10:31 AM(TJE)