Impact
of Anthropogenic Development on
Coastal Ground-Water Hydrology in Southeastern Florida,
1900-2000
Collegiality and Courtesy are the magic words to success
for this list.
By Robert A. Renken, Joann Dixon, John Koehmstedt,
Scott Ishman, A.C.
Lietz, Richard L. Marella, Pamela Telis, Jeff Rodgers,
and Steven Memberg
Prepared as part of the U.S. Geological Survey
Greater Everglades Priority Ecosystems Science Program
Circular 1275
http://fl.water.usgs.gov/PDF_files/cir1275_renken.pdf
(PDF, 87 pages)
Summary (Pages 70-71)
The urban and agricultural corridor of southern Florida
lies between the Everglades and water-conservation
areas to the west and the Atlantic Ocean to the east.
The area, which includes eastern Miami-Dade, Broward,
and Palm Beach Counties, has experienced explosive
population growth (from less than 4,000 residents
in 1900 to 5 million residents in 2000), and as such
is subject to widely conflicting stresses on the environment.
A highly controlled water-management system evolved
during the 20th century largely to provide drained
land for a rapidly expanding population. Reclamation
of Everglades wetland areas provided the opportunity
for westward expansion of agricultural, mining, and
urban activities. Surface water is impounded in water-conservation
areas that lie west of the protective levee system,
partly to: (1) sustain an Everglades ecosystem, (2)
keep overland sheetflow from moving eastward and flooding
urban and agricultural areas, and (3) use for water
supply. Parallel environmental interests exist in
coastal areas of the urbanagricultural corridor. Coastal
residential and urban areas must be drained for flood
control, whereas the underlying surficial aquifer
system must simultaneously serve as the principal
source for water supply, and ground-water heads must
be maintained to prevent saltwater intrusion. Changes
in predevelopment ground-water flow patterns and the
associated reduction in ground-water discharge to
coastal bays have altered salinity and affected the
local ecology. Active since the early 1920s, extractive
mining has increased considerably, largely to satisfy
urban construction
needs. The limited availability of limestone that
meets construction requirements and simul- taneous
competition for land by both industries have placed
both in direct conflict.
Surface- and ground-water systems were altered considerably
by the construction of a complex system of canals
and levees as well as by heavy municipal withdrawals.
Between 1900 and 1948, uncontrolled canal drainage
increased the rate of flow from the Everglades, reduced
the extent of inundated land, and lowered ground-water
levels. These canals failed, however, to transport
the load imposed during flood events and worsened
drought conditions through overdrainage of the surficial
aquifer system.
Drought and hurricane-related flooding provided the
impetus for the 1949 establishment of the Central
and Southern Florida Flood Control Project and District;
canals were enlarged, conveyance structures and controls
were installed, and protective levees were constructed.
The southern Dade conveyance system was completed
as the final phase of the project during the 1980s,
and involved redirecting surface-water flow and controlling
ground-water levels in southeastern Miami-Dade County.
Ground water represents the principal source of water
for municipal supply in the tri-county area of Miami-Dade,
Broward, and Palm Beach Counties and has increased
from three well fields producing about 66 Mgal/d in
1930 to about 65 well fields producing 770 Mgal/d
in 1995. West Palm Beach is the only large municipality
that uses surface water for supply purposes. Miami-
Dade County uses a centralized well-field infrastructure
in which five large-capacity well fields withdraw
the majority of the supply. A decentralized well-field
infrastructure has been constructed in Broward and
Palm Beach Counties, in which municipalities have
relied on developing their own source of supply to
meet local demand. On the basis of temporal analysis
of well-field locations and production levels, there
has been a historic shift from large well fields near
the coast to more western facilities partly designed
to mitigate saltwater intrusion.
Surface water is the primary source of water for cultivation
of sugar, field, and row crops in much of Palm Beach
County, particularly in the Everglades Agricultural
Area; conversely Miami-Dade agricultural growers primarily
rely on ground water withdrawn from shallow wells
and conveyed using truck-mounted pump and spray irrigation
systems. The agricultural industry of Broward County
has been largely displaced by residential and urban
development, despite once having been the Nation’s
primary winter producer of tomato, pepper, and bean
crops, between the 1920s and 1940s. Broward County
producers mostly relied on surface-water supplies
until about 1960, when they converted to the use of
ground water. Where as agricultural activities in
Broward County had become a minor factor in the county
economy, cultivated lands expanded considerably in
Miami-Dade and Palm Beach Counties between 1953 and
1988. Damage caused by Hurricane Andrew, which resulted
in an agricultural financial loss exceeding $1 billion,
appears to have contributed to the decline in cultivated
lands in Miami-Dade County since 1992. Agricultural
water use in the tri-county area increased from 505
Mgal/d in 1953 to almost 1,150 Mgal/d in 1988, declining
to 764 Mgal/d in 1995.
The surficial aquifer system, the principal source
of ground water in southeastern Florida, is a wedge-shaped,
eastward thickening sequence of limestone, quartz
sand, shell, and terrigeneous mudstone of Pliocene
to Holocene age. The prolific Biscayne aquifer, a
sole-source aquifer, is the most transmissive of three
separate aquifers that comprise the surficial aquifer
system. Transmissivity of limestone-rich areas is
greater than 1,600,000 ft2/d but decreases to 54,000
ft2/d where the surficial aquifer system mostly consists
of sand; yields of 1,000 to 7,000 gal/min are reported
from some wells completed in the cavernous part of
the surficial
aquifer system.
Well fields have been constructed farther inland during
the latter part of the 20th century because of coastal
saltwater intrusion. Competition between agriculture,
the Everglades ecosystem, and mining interests ultimately
limits construction of new well fields along the western
margin of the urban corridor. The underlying Floridan
aquifer system is viewed as an alternative source
of water for municipal supply either by treatment
of saline water through reverse-osmosis processes,
or by storage of freshwater by use of aquifer storage
and recovery (ASR). A large network of ASR wells is
being evaluated for Everglades restoration purposes
if regional hydrologic and local geotechnical, hydraulic,
and water-quality issues can be resolved. The lower
part of the Floridan aquifer system is used for disposal
of liquid waste by deep well injection for municipal
industrial wastewater and reverse-osmosis concentrate.
More than 20 Class I injection wells were operating
in the tri-county area by 2000, injecting treated
wastewater into the Boulder Zone at depths of 2,000
to 3,000 ft below NGVD 1929. Deep well injection is
under greater scrutiny in recent years largely because
of a growing need to use the aquifer for purposes
other than for waste disposal.
Few data are available to accurately document the
predevelopment conditions within the surficial aquifer
system; the water table probably subtly reflected
the Atlantic Coastal Ridge topography. Peat and muck
deposits, an important predevelopment component of
Everglades surface- and ground-water hydrology, functioned
as a storage reservoir to a water column that extended
upward from the underlying aquifer and maintained
a higher water table that prolonged the hydroperiod
and restricted movement of a coastal saltwater interface.
Surface-water stage within the adjoining Everglades
was sufficient to allow water to discharge through
traverse glades areas, and shoreline and submarine
springs discharged freshwater. Uncontrolled canal
drainage and a lengthy drought in 1945-46 caused water
levels to reach their lowest recorded levels, exacerbating
municipal well-field saltwater intrusion problems.
The modern-day water table largely reflects the hydrologic
influence of numerous engineering features, including
primary and secondary canal systems, gated control
structures, levees, impoundments, pump systems, and
the drawdown effects of the larger well fields. Ground-water
movement is largely coastward and water levels are
highest near the water-conservation areas, except
locally in southeastern Palm Beach County and northeastern
Broward County, where surface water is pumped from
the Hillsboro Canal into secondary canals to artificially
maintain water levels. Regional water-level comparison
maps of the difference in “average conditions”
show that improved drainage systems built during the
1950s lowered inland ground-water levels and increased
land areas for urban and agricultural development.
Gated coastal canal structures are used to retard
landward movement of saline water during the dry season
through maintenance of stage higher than local water
levels, inducing seepage into the aquifer. Management
of canal stage has helped to increase ground-water
levels in some coastal areas. Long-term canal coastal
discharge appears to have declined, but coastal canal
stage has been maintained gradually at higher levels,
presumably to impede saltwater intrusion. Diminished
coastal discharge is attributed to the rerouting of
surface water to secondary canals, and induced recharge
to the aquifer caused by increased municipal withdrawals.
Calcium bicarbonate water is dominant in shallow parts
of the surficial aquifer system, whereas sodium bicarbonate
and sodium chloride water are dominant in more deeply
buried parts of the aquifer system or along the coast.
Chloride concentrations generally are less than 100
mg/L at depths shallower than 50 ft, except in coastal
areas and southeast of Lake Okeechobee. Chloride concentrations
are less than 100 mg/L at the 150-ft depth in eastern
Palm Beach County, eastern Broward County, and much
of central and northwestern Dade County.
A broad zone of diffusion characterizes the saltwater
interface in southeastern Florida in which the position
of the interface is a consequence of three principal
mechanisms: westward lateral movement of seawater
within the surficial aquifer system, seepage from
tidal canals, and upconing of relict seawater. Prior
to 1945, uncontrolled drainage contributed considerably
to lowering the water table of the surficial aquifer
system along the Miami Canal. Water levels were lowered
further by heavy municipal withdrawals, inducing tidal
seepage into the aquifer system. Canal drainage contributed
greatly to intrusion of the saltwater interface in
Broward County, lowering ground-water levels with
the subsequent landward movement of saltwater in the
surficial aquifer system from the Atlantic Ocean.
Well-field withdrawals and tidal seepage are an important,
but less important, source of saltwater intrusion.
Predevelopment freshwater spring discharge in Biscayne
Bay diminished considerably following the emplacement
of canal drainage networks and the loss and compaction
of inland peat deposits that formerly maintained higher
water levels in the ecosystem, and stored excess surface
water that helped to recharge the underlying aquifer.
Changes in land use and water-management practices
have greatly impacted the marine ecosystem of Biscayne
Bay, resulting in increased nutrient loads and other
pollutants, and increased turbidity. Prior to construction
of the major canals, the salinity of the southernmost
part of Biscayne Bay was much lower than normal marine
salinity, especially near the coastline from Manatee
Bay to possibly as far north as the Coral Gables Canal.
The increase in salinity interpreted for both Biscayne
and Florida Bays in the early 1900s through the 1970s
is likely related to the increased development of
the canal system and modifications in surface-water
drainage. This is consistent with the progressive
inland saltwater intrusion. Post-1940 water-management
practices to control water discharge greatly affected
the Biscayne Bay ecosystem by increasing the frequency,
and particularly the magnitude, of salinity fluctuations.
By altering the natural variability in freshwater
discharge to Biscayne Bay, the natural cycles of the
nearshore marine organisms were disrupted, resulting
in biotic fluctuations similar to the frequency and
magnitude of the salinity changes.
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