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An Isotopic Study of Northeast Everglades National Park and Adjacent Urban Areas

Third Interim Technical Report for Isotope Study, Phase I

Prepared for Everglades National Park as per scope of services dated June 12, 1997

Submitted to ENP on June 14, 2000
Finalized September 18, 2000

prepared by:

Helena Solo-Gabriele, Assistant Professor, and Walter Wilcox, Graduate Student
University of Miami
Dept. of Civil, Arch., and Environmental Engineering
P.O. Box 249294
Coral Gables, FL 33124-0630
Phone: (305)284-3489
Fax: (305)284-3492

> Abstract
Background
Site Description
Collection &
Analysis
Results
Box Models
Discussion &
Conclusions
References
Tables & Figures
Appendix

ABSTRACT

As part of a comprehensive project evaluating the hydrology of the West Wellfield/L-31N site, an isotopic study was initiated to evaluate the sources of water to the wellfield and the potential impacts of rock mining lakes on water flows within the area. The site incorporates northeast Everglades National Park, including the eastern edge of Shark River Slough, as well as the western extent of sub-urban Miami. As per scope of services with four different funding agencies (SFWMD, USGS, ENP, and Kendall Properties and Investments), a total of 580 water samples were collected from January 1996 through December 1998. Samples were analyzed for their isotopic composition (δ18O and δD) using standardized methods which are based upon the conversion of oxygen and hydrogen into a form that can be measured by a mass spectrometer. Data collected early during this study confirmed that the isotopic composition of water was a suitable tracer of Everglades water given that differences were observed between water found deep within the Everglades and recharge within urban areas.

Results from this study indicate that the majority of water within the study site originates from local rainfall, with additional inputs from Water Conservation Areas located immediately to the northwest of the site. Surface waters within the Everglades side of the study site are generally evaporated as evidenced by their high isotopic values (δ18O > 1.0 and δD > 6). Heavy Everglades surface water is the source of shallow and deep groundwater in the Everglades. At one location adjacent to Levee 31N, it was found that surface water infuses very rapidly presumably due to a local geologic disturbance. This rapid infusion of water results in a "conduit" of isotopically light water (after large rain events) that travels along with the southeasterly groundwater flow pattern, gradually mixing with the surrounding groundwater. Upon nearing Levee 31N, Everglades groundwater flows in a more easterly direction within a series of geologic layers within the Biscayne aquifer which appear to be semi-confined. Groundwater within the urban side is generally very light (similar to that of un-evaporated rainfall) at shallow depths and relatively heavy (similar to that of Everglades water) at large depths. Lakes within the study site serve as "breaks" between the geologic layers causing deep groundwaters (which originate from the Everglades) to mix with shallow groundwater (which originate from recharge within the urban side of the site). Water from both the lakes and the deep groundwater continue to migrate toward the east until the operations of the municipal pumping wells at the West Wellfield causes this water to be drawn to the intakes.

Two box models were developed through this study to quantify flows between different regions within the study site. The models, one called the "simple" model and another called the "complex" model, were based upon water and isotopic balances. The simple model indicates that more than 60 percent of the water pumped by the West Wellfield, ultimately originates from within the Everglades; the proportion of Everglades water to the West Wellfield increases during drier weather conditions. Results from the complex model indicate that Everglades waters move preferentially through deeper groundwater layers within the Biscayne aquifer. Water within these deeper groundwater layers move east until they reach the rock mining lakes where the majority of the deep groundwater flows through the lakes. Water from both the lake and the lowest groundwater layers then migrate eastward and a portion of this water is drawn by the municipal wellfield.

Project Status and Focus of this Report

The University of Miami project investigating the West Wellfield/L-31N site is separated into two phases: an isotope study (phase I) and a hydrologic modeling effort (phase II). This report presents the results from the isotope study (phase I) which includes analysis of: surface water, groundwater and rainwater collected from within Everglades National Park, groundwater and rainwater from the West Wellfield, and canal water from L-31N. Two earlier reports on the isotope portion of the study have been prepared. The first one titled, "Sources of Water to the West Wellfield," and dated July 10, 1998 summarizes the isotope data corresponding to the first 150 samples analyzed. A second report titled, "An Isotopic Study of Two Rock Mining Lakes," dated March 8, 2000 focused on establishing the physical, physico-chemical, and isotopic characteristics of two rock mining lakes located between the West Wellfield and northeast Everglades National Park and corresponds to another 110 samples that were collected from the two rock mining lakes. It was found during the analysis of the lake data that negligible variations in isotopic composition were observed with depth. Therefore, in the report included herein, the lake data corresponds to the depth average.

The hydrologic modeling effort (phase II) is funded by the U.S. Geological Survey (USGS). The focus of the hydrologic model was to estimate seepage below Levee 31N and to develop an algorithm for estimating real-time seepage rates. The final report documenting these results has been approved for publication by the USGS as Water Resources Investigations Report 00-4066. Copies of the USGS publication will be distributed to interested parties once it is available.

Funding for the isotope portion of the West Wellfield/L-31N study was received from four different agencies: the South Florida Water Management District (SFWMD), Kendall Properties and Investments Inc., the USGS, and Everglades National Park (ENP). ENP and the USGS funded the analysis of the last 320 samples and the results presented herein includes an analysis of all the isotopic data (580 samples) collected. Earlier reports (Solo-Gabriele 1998; Solo-Gabriele and Sternberg 1998) included a more thorough description of the motivation for the West Wellfield/L-31N study, more details concerning the characteristics of the study site, justification for use of stable isotopes as tracers for water flow in the area, and a description of the quality control program for the isotope analysis. Please refer to the earlier reports for these details.

Organization of Report

This report is separated into two chapters: administrative background (chapter I) and technical results (chapter II). A chapter on administrative background was considered necessary due to the fact that four different funding agencies provided financial support for this study, each with its own contractual requirements. A summary of agency funding as well as contractual obligations associated with sample analysis are included within the first chapter. For additional details concerning the technical content of this study please refer to Wilcox 2000.

Acknowledgments

We thank Tom Van Lent of ENP for facilitating funding for this portion of the project.

Disclaimer

Opinions and recommendations provided within this report do not necessarily represent the views shared by the agencies that funded this project.



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