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Origins and delineation of saltwater intrusion in the Biscayne aquifer and changes in the distribution of saltwater in Miami-Dade County, Florida

Technical Report

Origins and delineation of saltwater intrusion in the Biscayne aquifer and changes in the distribution of saltwater in Miami-Dade County, Florida

Abstract and Figures

Intrusion of saltwater into parts of the shallow karst Biscayne aquifer is a major concern for the 2.5 million residents of Miami-Dade County that rely on this aquifer as their primary drinking water supply. Saltwater intrusion of this aquifer began when the Everglades were drained to provide dry land for urban development and agriculture. The reduction in water levels caused by this drainage, combined with periodic droughts, allowed saltwater to flow inland along the base of the aquifer and to seep directly into the aquifer from the canals. The approximate inland extent of saltwater was last mapped in 1995. An examination of the inland extent of saltwater and the sources of saltwater in the aquifer was completed during 2008–2011 by using (1) all available salinity information, (2) time-series electromagnetic induction log datasets from 35 wells, (3) time-domain electromagnetic soundings collected at 79 locations, (4) a helicopter electromagnetic survey done during 2001 that was processed, calibrated, and published during the study, (5) cores and geophysical logs collected from 8 sites for stratigraphic analysis, (6) 8 new water-quality monitoring wells, and (7) analyses of 69 geochemical samples. The results of the study indicate that as of 2011 approximately 1,200 square kilometers (km2) of the mainland part of the Biscayne aquifer were intruded by saltwater. The saltwater front was mapped farther inland than it was in 1995 in eight areas totaling about 24.1 km2. In many of these areas, analyses indicated that saltwater had encroached along the base of the aquifer. The saltwater front was mapped closer to the coast than it was in 1995 in four areas totaling approximately 6.2 km2. The changes in the mapped extent of saltwater resulted from improved spatial information, actual movement of the saltwater front, or a combination of both. Salinity monitoring in some of the canals in Miami-Dade County between 1988 and 2010 indicated influxes of saltwater, with maximum salinities ranging from 1.4 to 32 practical salinity units (PSU) upstream of the salinity control structures. Time-series electromagnetic induction log data from monitoring wells G–3601, G–3608, and G–3701, located adjacent to the Biscayne, Snapper Creek, and Black Creek Canals, respectively, and upstream of the salinity control structures, indicated shallow influxes of conductive water in the aquifer that likely resulted from leakage of brackish water or saltwater from these canals. The determination that saltwater influxes were recent is supported by the similarity in the oxygen and hydrogen stable isotope composition in samples from the Snapper Creek Canal, 1.6 kilometers (km) inland of a salinity control structure, and in samples from well G–3608, which is adjacent to the canal, as well as by the relative ages of the water sampled from well G–3608 and other wells open to the aquifer below the saltwater interface. Historical and recent salinity information from the Card Sound Road Canal, monitoring well FKS8 located adjacent to the canal, and the 2001 helicopter electromagnetic survey indicated that saltwater may occasionally leak from this canal as far inland as 15 km. This leakage may be prevented or reduced by a salinity control structure that was installed in May 2010. Saltwater also may have leaked from the Princeton Canal. Results of geochemical sampling and analysis indicate a close correspondence between droughts and saltwater intrusion. Tritium/helium-3 apparent (piston-flow) ages determined from samples of saltwater with chloride concentrations of about 1,000 milligrams per liter (mg/L) or greater generally corresponded to a period during which droughts were frequent. Comparison of average daily air temperatures in Miami, Florida, with estimates of recharge temperatures determined from the dissolved gas composition in water samples indicated that saltwater likely entered the aquifer in April or early May when water levels are typically at their lowest during the year. Conversely, most of the samples of freshwater with chloride concentrations less than about 1,000 mg/L indicate recharge temperatures corresponding to air temperatures in mid to late May when rainfall and water levels in the aquifer increase, and the piston-flow ages of these samples correspond to wet years. The piston-flow ages of freshwater samples generally were younger than ages of samples of saltwater. Saltwater samples that were depleted in boron, magnesium, potassium, sodium, and sulfate, and enriched in calcium relative to the concentrations theoretically produced by freshwater/seawater mixing, generally were found to be associated with areas where saltwater had recently intruded. The calcium to (bicarbonate + sulfate) molar ratios (Ca/(HCO3+SO4)) of these samples generally were greater than 1. Saltwater samples from some of the monitoring wells, however, indicated little or no enrichment or depletion of these ions relative to the theoretical freshwater/seawater mixing line, and the Ca/(HCO3+SO4) molar ratios of these samples generally were less than 1. Results indicated that aquifer materials are approaching equilibrium with seawater at these well locations.
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U.S. Department of the Interior
U.S. Geological Survey
Scientific Investigations Report 20145025
Prepared in cooperation with Miami-Dade County
Origins and Delineation of Saltwater Intrusion in the
Biscayne Aquifer and Changes in the Distribution of
Saltwater in Miami-Dade County, Florida
SiO
2
Freshwater
Freshwater
inflow
Leakage from
unprotected
canals
Residual
saltwater
Well field
Infiltration
from tidal
marshes
Encroachment
from ocean
along the base
of the aquifer
Leading edge
of saltwater
front
Ion exchange
front
Freshwater/saltwater
interface
NaCl, B, Br,
K, Li, SO
4
Little or no ion exchange
seaward of the ion
exchange front
Limestone
+
Quartz Sand
CaCO
3
SiO
2
+
Biscayne Aquifer
Pinecrest sand member
Quartz sand
Depleted
Enriched
Saltwater
Cl, Br = Conservative
Cl/Br ratio = 300 to 800
Sewage
B
K
Li
Na
Ca
HCO
3
SO
4
Mg
Cover. Conceptual diagram of sources and mechanisms of the saltwater that has intruded parts of aquifers in southeast
Florida, and changes in water chemistry that may result from this intrusion.
Origins and Delineation of Saltwater
Intrusion in the Biscayne Aquifer and
Changes in the Distribution of Saltwater in
Miami-Dade County, Florida
By Scott T. Prinos, Michael A. Wacker, Kevin J. Cunningham,
and David V. Fitterman
Prepared in cooperation with Miami-Dade County
Scientific Investigations Report 2014–5025
U.S. Department of the Interior
U.S. Geological Survey
U.S. Department of the Interior
SALLY JEWELL, Secretary
U.S. Geological Survey
Suzette M. Kimball, Acting Director
U.S. Geological Survey, Reston, Virginia: 2014
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Suggested citation:
Prinos, S.T., Wacker, M.A., Cunningham, K.J., and Fitterman, D.V., 2014, Origins and delineation of saltwater intrusion
in the Biscayne aquifer and changes in the distribution of saltwater in Miami-Dade County, Florida: U.S. Geological
Survey Scientific Investigations Report 2014–5025, 101 p., http://dx.doi.org/10.3133/sir20145025.
ISSN 2328-0328 (online)
iii
Acknowledgments
The authors would like to acknowledge the contributions of information provided by
EAS Engineering, Inc., the Florida Department of Environmental Protection, the Florida Keys
Aqueduct Authority, Miami-Dade County Department of Environmental Resource Manage-
ment, and the South Florida Water Management District. We greatly appreciate the cooperation
of the following property owners who allowed us to make measurements on their lands: the AA
Baker Group, LTD, the Archdiocese of Miami, Alamo Rent A Car, Barry University, FRS Hold-
ings, Inc., the City of Homestead, Homestead Miami Speedway, Hallandale Beach Elementary
School, the Miami-Dade County Department of Public Schools, Miami-Dade County Parks
and Recreation Department, the South Florida Water Management District, and Palmer Trinity
School. Bob Brown of the Archdiocese of Miami; Edward Swakon of EAS Engineering, Inc.;
Dr. Claudius Carnegie, Jorge Corrales, and Greg Mohr of the Miami-Dade County Department
of Public Schools; Maria Idia Macfarlane, Sonia Villamil, and Virginia Walsh of the Miami-
Dade Water and Sewer Department; and Steve Krupa of the South Florida Water Management
District were instrumental in helping us gain access to numerous properties where the time-
domain electromagnetic soundings were completed. Maria Idia Macfarlane of the Miami-Dade
County Water and Sewer Department helped to oversee much of the well installation. Jorge
Corrales helped collect many of the surface geophysical measurements that were made on lands
owned by the Miami-Dade County Department of Public Schools. Michael J. Alexander, proj-
ect manager of the Homestead Miami Speedway, not only provided necessary access to speed-
way properties for measurements but also helped make necessary contacts with other properties
owners in the area. Jim Happell of the University of Miami, Rosenstiel School of Marine and
Atmospheric Science, John Stowell of Mount Sopris Instrument Company, and Roy Sonenshein
of the National Park Service, Everglades National Park provided substantive reviews.
The authors also would like to acknowledge the contributions of staff members of the
U.S. Geological Survey. Michele Markovits helped with many aspects of this study, including
the geochemical sampling, geophysical data collection, data entry, and sample preparation and
analysis. Adam Foster provided substantive geochemical advice and helped to compile sample
results in the database. Robert Valderrama completed most of the processing of time series elec-
tromagnetic induction log sets, and Brian Banks collected the most recent logs for these data-
sets. Ronald Bruce Irvin provided quality assurance of these log sets and partially automated
the application of necessary adjustments. Lee Massey and Corey Whittaker helped to collect
water samples. Brian Banks, Adrian Castillo, Eric Carlson, and Jeff Robinson helped to collect
time-domain electromagnetic soundings. Jeff Robinson also collected many of the borehole
logs as the new monitoring wells were being installed and helped to oversee some of the well
installation. Ronald Bruce Irvin created the necessary scripts, software, and les to convert the
static “Manual Water-Level Measurements in South Florida” website into a dynamic website
that serves routinely collected salinity information as well as most of the salinity information
collected during this study. The new website has proven to be valuable to local water manag-
ers as they seek greater understanding of the changes in the extent and magnitude of saltwater
intrusion in the Biscayne aquifer. Ronald Bruce Irvin also oversaw quality assurance for the
time-series electromagnetic induction logs used during this study. Michael W. Bradley, Ed
Busenberg, Richard H. Coupe, Kim H. Haag, Steven Hinkle, Carole Johnson, Robert Renken,
Dorothy Sifuentes, Rick M. Spechler, David Sumner, Kimberly A. Swidarski, Kim Walten-
baugh, and Peggy Widman provided invaluable support during the review, approval, and publi-
cation process of this report.
iv
Contents
Abstract ...........................................................................................................................................................1
Introduction.....................................................................................................................................................2
Definition of Terms ................................................................................................................................2
Purpose and Scope ..............................................................................................................................6
Approach ................................................................................................................................................6
Description of Study Area ...................................................................................................................6
Hydrologic Controls .....................................................................................................................6
Topography and Physiography ..................................................................................................7
Climate ...........................................................................................................................................7
Water Use in Miami-Dade County ............................................................................................7
Hydrogeology/Hydrostratigraphic Framework .....................................................................10
Previous Studies .................................................................................................................................11
Origin of Saltwater Intrusion in Southeastern Florida ...........................................................................12
Mechanisms of Saltwater Intrusion ................................................................................................12
Upconing of Relict Saltwater from Previous Sea-Level High Stands ...............................12
Encroachment of Seawater along the Base of the Biscayne Aquifer ..............................13
Infiltration of Saltwater from Saltwater Marshes ................................................................13
Seepage from Streams or Canals in which Saltwater has Migrated Inland ...................13
Effects of Urban and Hydrologic Development on Saltwater Intrusion ....................................16
Predevelopment Conditions .....................................................................................................17
Relation of Urban Development to Saltwater Intrusion ......................................................17
Data Collection and Analysis .....................................................................................................................20
Salinity Measurements ......................................................................................................................21
Time-Series Electromagnetic Induction Logging ..........................................................................21
Comparison of Bulk Resistivity to Specific Conductance of Water and Chloride
Concentration ........................................................................................................................22
Time-Domain Electromagnetic Soundings .....................................................................................22
HEM Survey Analysis .........................................................................................................................22
Approximate Inland Extent of Saltwater in the Biscayne Aquifer .......................................................25
Improved Spatial Coverage and Precision of the Inland Extent of Saltwater ..........................25
Changes between 1995 and 2011 .....................................................................................................25
Characterization and Distribution of Saltwater in the Biscayne Aquifer ..........................................28
Major and Trace Ion Geochemistry .................................................................................................28
Strontium Isotope Age Dating ..........................................................................................................34
Oxygen and Hydrogen Stable Isotopes ...........................................................................................35
Tritium and Uranium Concentration .................................................................................................38
Tritium/Helium-3 Age Dating .............................................................................................................38
Sulfur Hexafluoride Age Dating ........................................................................................................39
Estimates of Recharge Temperature Based on Dissolved Gases ..............................................39
Sources of Saltwater in the Biscayne Aquifer .......................................................................................41
Relict Seawater from Previous Sea-Level High Stands ...............................................................41
Historical Leakage of Saltwater from Canals ................................................................................42
The Card Sound Road Canal and FEC Railway Borrow Ditches ........................................43