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| Ground-water conditions in southern Florida |
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| Ground-water conditions in southern Florida |
This report has been reformatted for presentation on the World Wide Web. The official text of WRIR 01-4275 (6.3 MB download) is available in PDF format. The Adobe PDF Reader program is available, at no cost, from Adobe.
In recent decades, southern Florida has experienced a rapid population growth that is expected to continue into the next millennium. Because of the increasing demand on water supply, timely and in-depth analytical information is needed by water managers to assess current and long-term ground-water conditions in the region. This information is critical to management of the water supply and to avoid potential adverse effects on the hydrologic system including saltwater intrusion, loss of pumpage in residential water-supply wells, and aquifer compaction.
The U.S. Geological Survey (USGS) operates a ground-water monitoring network that presently consists of 476 wells and spans 10 counties and 3 aquifer systems in southern Florida (as of 2000). This existing network provides water managers with a reasonably comprehensive coverage of data reflecting changes that affect the aquifers. In most instances, water-level data from network wells are collected and analyzed monthly, and therefore, are not available for assessment on a near real-time basis. Although water managers generally make decisions regarding withdrawals from aquifers on a weekly or monthly basis, sometimes these decisions must be based on changes in water levels that occur over just a few days. A subset of the ground-water level monitoring network that has been equipped with satellite telemetry could provide the real-time water-level information that is needed. While this subset would be unable to provide the same spatial coverage of the complete network, it could still give insight into changes that occur during those intervals when data from the complete network are unavailable.
The USGS, in cooperation with the South Florida Water Management District (SFWMD), recently conducted a study to: (1) design a real-time ground-water level monitoring network that consists of representative wells from the existing larger network in southern Florida, (2) develop portrayal techniques as a tool for water managers to rapidly assess ground-water conditions, and (3) create a page on the World Wide Web to transmit the hydrologic information to water managers and the public as it is received. This timely information will enable water managers to plan and make decisions in advance of (and during) droughts, water shortages, and other severe hydrologic events.
The purpose of this report is to document the design of a real-time ground-water level monitoring network that provides in-depth, analytical information on the current state of hydrologic conditions in southern Florida with the information accessible on the World Wide Web. The Seasonal Kendall trend test is used to assess long-term water-level trends in various aquifers. A frequency analysis is made to compare rainfall deficiencies and water levels in monitoring wells. A regression analysis helps to identify those wells most representative of the existing continuous monitoring network in each aquifer. A correlation analysis of instantaneous water levels and chloride concentrations and an analysis of trends in chloride concentration are used to identify areas where a real-time ground-water level monitoring network well could aid in assessing saltwater intrusion.
The study area encompasses all of southern Florida, except for Monroe County (fig. 1 (48K)). Collier, Lee, and Hendry Counties are in southwestern Florida; Miami-Dade, Broward, Palm Beach, Martin, and St. Lucie Counties are in southeastern Florida. In the study area, the network also includes one monitoring well in Glades County that was considered in the analysis. The principal hydrologic units used for municipal and private water supply in southern Florida are the surficial aquifer system, intermediate aquifer system, and the uppermost part of the Floridan aquifer system (fig. 2 (50K)).
The three principal aquifer systems in southern Florida include locally named aquifers (fig. 2 (50K)). In southwestern Florida, the surficial aquifer system includes the water-table and lower Tamiami aquifers; the intermediate aquifer system includes the sandstone and mid-Hawthorn aquifers; and the uppermost part of the Floridan aquifer system includes the lower Hawthorn producing zone. In southeastern Florida, the surficial aquifer system includes the Biscayne aquifer and gray limestone aquifer in Miami-Dade and Broward Counties. Of these two aquifers, the gray limestone aquifer is not used extensively for municipal water supply. In Martin, Palm Beach, and St. Lucie Counties, the three principal aquifers are not differentiated into locally named aquifers.
Because these aquifers are composed of different types of rocks and unconsolidated sediments, the rate that water can flow through them varies. The Biscayne aquifer is exceptionally permeable, a large part of the aquifer has transmissivities that are greater than 1,000,000 ft2/d, and in some areas transmissivity can be as much as 2,900,000 ft2/d (Fish and Stewart, 1991). The other aquifers in southern Florida are not as permeable, with parts of the lower Tamiami, mid-Hawthorn and sandstone aquifers that have transmissivities of only about 134,000, 8,000, and 5,000 ft2/d respectively (Wedderburn and others, 1982; Knapp and others, 1986).
Transmissivities for the most permeable parts of the lower Hawthorn producing zone and gray limestone aquifer are about 47,000 and 300,000 ft2/d, respectively (Knapp and others, 1984; Reese and Cunningham, 2000). There are no wells in the gray limestone aquifer and only two wells in the lower Hawthorn producing zone that had data of the type required for the analyses described in the subsequent sections of this report. As a result, these two aquifers were not considered for this study.
Several studies have been undertaken to examine the spatial coverage of parts of the USGS ground-water monitoring network in southern Florida. Burns and Shih (1984) used semiannual water-level data collected over a 5-year period to define optimal coverage for the water-table and lower Tamiami aquifers in Collier County, southwestern Florida. Time-series analyses were also used to predict optimal sampling frequency for several monitoring wells using daily maximums recorded every 5 days. Burns and Shih (1984) also performed a qualitative assessment by mapping the effects of well-field withdrawals on monitoring wells to define areas of uncertainty. The ground-water monitoring network in Collier, Hendry, and Lee Counties, southwestern Florida, was examined by Hosung Ahn (South Florida Water Management District, written commun., 1996) using monthly water-level data from 342 wells collected over a 3-year period. Ahn used the Auto Regressive Integrated Moving Average (ARIMA) model and kriging to determine the optimal well network and sampling frequency. Swain and Sonenshein (1994) documented statistical techniques developed for analysis of the spatial coverage of a well network, redundancy of a well network, and optimal water-level measurement intervals for numerous wells completed in the Biscayne aquifer in Broward County, southeastern Florida.
The authors would like to acknowledge the considerable contribution made by Curtis Lamar Sanders of the USGS in Columbia, S.C. Mr. Sanders graciously provided many ideas regarding the types of statistical analyses and procedures that could be performed for this study. His suggestions and contributions proved to be invaluable. Michael Deacon and Richard Verdi of the USGS in Miami, Fla., contributed to the quality of the report by helping in the verification process.
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Funding for the USGS to design and maintain this site has been provided through a cooperative agreement with the South Florida Water Management District (SFWMD). Water-level conditions are monitored by the USGS with support from Federal, State, and local cooperators.
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