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Geochemistry of the Floridan Aquifer System in Florida and in Parts of Georgia, South Carolina, and AlabamaBy Craig L. Sprinkle Professional Paper 1403-I
The chemical quality of the ground water in the Floridan aquifer system is determined primarily by mineral-water interaction. However, some changes in water quality have been imposed by development, particularly near coastal pumping centers. A total of 601 chemical analyses, all from different wells, most completed in the upper part of the aquifer system, were used to describe the variations in water chemistry and to study the processes responsible for observed changes. The Floridan aquifer system is a vertically continuous sequence of Tertiary carbonate rocks that are of generally high permeability and are hydraulically connected in varying degrees. The rocks are principally limestone and dolomite, but they grade into limy sands and clays near the aquifer system's updip limits. Major minerals in the aquifer system are calcite, dolomite, and, locally, gypsum or quartz; minor minerals include apatite, glauconite, and clay minerals such as kaolinite and montmorillonite. Trace amounts of metallic oxides or sulfides are present in some areas. The aquifer system consists of the Upper and Lower Floridan aquifers, separated in most places by a less permeable confining unit that has highly variable hydraulic properties. Only the Upper Floridan aquifer is present throughout the study area. Freshwater enters the aquifer system in outcrop areas located primarily in central Georgia and north-central Florida. Discharge occurs chiefly to streams and springs and, to a lesser extent, directly into the sea. Most of the flow into and out of the system takes place where it is unconfined or where the upper confining unit is thin. Secondary permeability developed by dissolution of aquifer material is most prominent in these areas of dynamic flow. Dissolved-solids concentrations in water from the Upper Floridan aquifer generally range from less than 25 milligrams per liter near outcrops to more than 25,000 milligrams per liter along the coasts. The dominant cations in the ground water are Ca2+, Mg2+, and Na+; the dominant anions are HCO3-, Cl-, and SO42-, The concentration of Ca2+ is controlled primarily by calcite saturation. Concentrations of Mg2+, NA+, and Cl- are highest where mixing of freshwater and saltwater occurs. Concentrations of HCO3- reflect the control of calcite solubility. The concentration of SO42- is highest where gypsiferous rock units are present in the aquifer system. The major geochemical processes that occur in the Upper Floridan aquifer, based on water-quality maps and computations using a geochemical model, are (1) dissolution of aquifer minerals toward equilibrium, (2) mixing of ground water with recharge, leakage, or seawater, (3) sulfate reduction, and (4) cation exchange between water and aquifer minerals. Similar processes apparently control minor dissolved constituents, although quantification is difficult with the available data. Statistical tests of available nutrient data indicate that concentrations of N (nitrogen) species in unconfined recharge areas may be increasing over time; more detailed studies of all N species are needed to test this hypothesis, however. Data on trace metals, radionuclides, and man-made organic contaminants are limited. Available data indicate that most freshwater within the Upper Floridan is potable, but detection of pesticides in a few samples indicates that the system is susceptible to contamination from the land surface in some areas, particularly where its upper confining unit is thin or absent. Geochemical models
were used to examine changes in major chemical elements along selected ground-water
paths within the Upper Floridan aquifer. Water in the Upper Floridan aquifer
can be categorized into four hydrochemical facies, whose exact distribution
is determined by confined or unconfined conditions of the aquifer and by
chloride concentrations. The reaction models are considered plausible based
on available chemical, isotopic, and hydrologic information, and they clearly
reveal the dominant effects of aquifer confinement on ground-water chemistry.
In unconfined areas, large volumes of dilute water recharge the system and
dissolve minerals contained in the aquifer, thus enhancing the aquifer's
permeability. In coastal areas, geochemical-model results indicate that
the mixing of freshwater and seawater leads to carbonate dissolution or
precipitation, depending in part on the ratio of freshwater to seawater.
The geochemical models also indicate that significant sulfate reduction
may occur in confined areas; this suggests that there is some flux of carbon
into the aquifer system, a factor that must be considered when using 14C measurements to determine ground-water-flow velocities. (The entire paper is available below) These reports and documents have been scanned from the original hard-copy materials and are made available on the internet in both HTML and PDF formats. Because these are scanned documents, we are unable to provide fully-accessible versions of these reports. If you cannot fully access the information in these documents, please contact Heather S. Henkel at hhenkel@usgs.gov. The HTML versions of these documents have been created to provide the information in a format that is quickly and easily readable over the Internet. Selective pages and images can be printed from this HTML version by placing the cursor inside the right-hand frame and selecting the print option from the browser. The PDF version of the reports are also provided, and are the recommended format to use for the best printer format and resolution. Please note - some PDF files are very large (over 2 MB) and may take some time to download, depending on your system. ![]() ![]() |
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03 December, 2004 @ 02:25 PM
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