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LEAFLET NO. 5


WATER



















STATE OF FLORIDA
STATE BOARD OF CONSERVATION
DIVISION OF GEOLOGY


FLORIDA GEOLOGICAL SURVEY
Robert O. Vernon, Director



LEAFLET NO. 5


WATER CONTROL VS. SEA-WATER
INTRUSION, BROWARD COUNTY,
FLORIDA

By
C. B. Sherwood and R. G. Grantham



Prepared by the
UNITED STATES GEOLOGICAL SURVEY
in cooperation with
BROWARD COUNTY
and the
FLORIDA GEOLOGICAL SURVEY


TALLAHASSEE
1965

















PREFACE


Residents in coastal areas are aware of
the perpetual battle between man and the sea.
Reports of beach erosion, destruction of pro-
perty, damage to ships, and loss of cargos are
of common knowledge. However, one battle
between man and the sea is a silent struggle
that receives little publicity because it is
invisible and not spectacular. This invisible
struggle is waged to protect fresh-water sup-
plies in coastal areas from inroads by the sea.
This leaflet tells in general terms how the
problem of sea-water intrusion in the aquifer
in Broward County came about, what has been
done to control the intrusion, and what must
be done to conserve the fresh water and keep
the ocean where it belongs.
Although the leaflet deals specifically with
conditions in Broward County, the principles
described are valid in any coastal area having
a similar hydrology. For this reason, this leaf-
let is of interest to people living in other coas-
tal areas of Florida.





WATER CONTROL VS. SEA-WATER
INTRUSION, BROWARD COUNTY, FLORIDA

By
C.B. Sherwood and R.G. Grantham

Broward County abounds in water--both
fresh and salt. With careful management, the
supply of fresh water is adequate for the present
and future needs of the area. The wealth of
salt water is a major asset--good for shipping
and recreation, such as swimming, boating,
and fishing. However, the ever present problem
is keeping salt water in its place. In fact, the
primary threat to the invaluable fresh-water
resources of the county is intrusion of sea water
into coastal streams and into subsurface water-
bearing materials. Sea-water intrusion is a
silent menace. It can spread without raising
alarm; it can contaminate domestic wells and
destroy city water supplies; and it can kill
crops and render soils unusable for agriculture.
It not only can, but has. However, studies
indicate that with proper detection and control
measures, sea-water intrusion can be halted,
and with time the sea water can be flushed out.
Broward County is underlain by the Bis-
cayne aquifer, a highly productive water-bearing
system of limestone, sandstone, and sand that
extends from land surface to depths of as much
as 200 feet near the coast. The aquifer yields
copious supplies of fresh water to municipal
well fields in the area, but the porous nature
of the materials exposed to the sea make it
also especially vulnerable to sea-water intru-
sion.
The inland extent of sea-water and salty-
water intrusion in Broward County in 1964 is
shown by red and pink shading on the map and
cross section in figure 1. Salty water as dis-
tinguished from sea water is a mixture of fresh
and sea water. The salty water body in the
aquifer is wedge-shaped, being thickest at the
coast, and thinning inland to an edge where it
underlies the fresh ground water at depths from
160 to 200 feet below the land surface.







EXPLANATION
M Sea water
gp Sally ground water
C Fresh waer
- Canal and control
Well field
GENERALIZED
CROSS SECTION
A


0 I 2 1 : ,


Figure 1. Map showing extent of sea-water intrusion,
Broward County, Florida.

Because sea water contains large amounts
of dissolved salts, it is slightly heavier than
fresh water. A 41-foot column of fresh water
is required to balance 40 feet of sea water.
Thus, sea water moves inland unless fresh-water
levels are appreciably higher than sea level.
In coastal streams and in porous subsurface
materials, there is a constant balancing between
the two. If fresh-water levels are high, sea water
is held near the coast. If fresh-water levels are
low, sea water moves up the tidal streams and
inland in the aquifer beneath the fresh ground
water. Theoretically, in a coastal aquifer, each
foot of fresh water above sea level would indi-
cate 40 feet of fresh water below sea level.
Prior to drainage improvements in Broward
County the existing streams were shallow and
relatively ineffective as drainage channels.
Therefore, fresh-water levels were high and
little or no salt water intruded. In fact, old-time
residents reported flowing wells in salt-water





HIGH FRESH-WATER LEVELS


LW FRESH-WATER LEVELS


Figure 2. Mechanics of sea-water intrusion.
bays and inlets. Later, as deep, ettective drain-
age canals were cut far inland to reduce flooding
of farms and urban areas, coastal water levels
were lowered greatly and salt-water intrusion
began almost unnoticed. In addition, the rapid
urbanization of the area brought an increased
demand for drainage and for coastal canals to
create attractive waterfront property. The de-
sirable aspects of these developments were
clearly apparent; the undesirable aspect--salt
intrusion--was difficult to detect until domestic,
industrial or irrigation supplies began to be
contaminated. Uncontrolled tidal canals influ-
enced the position of salt-fresh water contact
in two ways--they lower fresh ground-water
levels, thus reducing the opposition to inland
movement of salt water and they provide a
channel for sea water to move inland. The inland
penetration of the salt front in the New River
area of Fort Lauderdale was caused chiefly by
extensive construction of canals. Fortunately,
this type of intrusion can be corrected if a
salinity control structure is built near the coast
to raise the level of fresh water and to prevent
the upstream movement of salt water. Salinity
control structures have been constructed in
coastal reaches of all primary canals of the
Central and Southern Florida Flood Control
Project to combat sea-water intrusion and to
control water levels on an area-wide basis. In
the older canals the controls were located as
far seaward as the existing land use and marine

















































Figure 3. Effects of construction of tidal canals.


Figure 4. Salinity control structure.






















2-~- -~


Figure 5. Photographs of salinity control structure

interests permitted--in the newer canals they
are placed very close to the coast.
In Broward County the inland movement of
the salt front is accelerated by the lowering of
fresh-water levels near the coast as a result of
large withdrawals of ground water. When esta-
blished, the municipal well fields generally
were located an appreciable distance inland.
However, the westward expansion of urbaniza-
tion necessitated drainage of larger areas by
canals which in many instances passed within
the area where ground-water levels were being


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Figure 6. Well field and uncontrolled canal.

lowered by well fields. In an area of heavy
pumpage the ground-water flow is toward the
well field, thus the combined effects of the
canal and the pumpage can induce salt water
to move into the well field. In contrast, a con-
trolled canal can provide a perennial source of
fresh water to replenish the well field and to
prevent salt intrusion by bringing in additional
fresh water from outside the area. Water-control
structures as correction measures are presently
being constructed in some critical areas where
municipal supplies are threatened.


Figure 7. Well field and controlled canal.









































































































Figure 8. Map showing adjustment in salt. front..


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CANAL AND CONTROL
MUNICIPAL SUPPLY WELL





















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I .. L L N... ..... ...... ..
EXPLANATION Ns.
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1 ot -- -^0
MUNICIPAL sUPPLy WELL









CANAL C/4 POMPANO











PROSPECT WELL FIELD
10 MGO








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N N. 4 u
M OlC (





The map sequence below shows successive
adjustments to the salt front pattern, which have
occurred since 1941 in response to canal con-
struction, large scale pumping, and salinity
control works in the Middle River Prospect
well field area, near Fort Lauderdale.
In the early 1940's pumpage of ground
water was negligible and existing streams were
shallow and drained very little water; conse-
quently, fresh-water levels were high and salt-
water intrusion was confined to areas adjacent
to natural tidal channels. By the mid 1950's the
primary canals had been constructed, the Pros-
pect well field had been established, and exca-
vation of an extensive secondary canal system
by land developers was underway. Coastal
water levels were being lowered and a very
significant inland adjustment of the salt-front
pattern resulted.
In 1963 the effects of the construction of
the canal and control which integrated Cypress
Creek into the flood control system are shown.
Although pumpage had increased threefold,
ground-water levels remained high and the salt
front was essentially stabilized in the area
north of the well field. In contrast, south of the
well field the salt front adjacent to the tidal
portion of the North Fork of the Middle River
moved steadily inland into the well field. The
feeder canal shown in the fourth map has been
proposed to provide higher fresh-water levels
for the control of salt intrusion and for recharge
to the well-field area.
Although the salinity-control structures in
major canals have retarded intrusion in some
areas, the rapidly increasing urbanization and
water use create an urgent need for legislation
to provide salinity control area wide. During
1963, the increased threat to ground-water
supplies, accentuated by the contamination of
a major well field, resulted in legislation to
prevent the construction of additional salt-water
canals and to require salinity control structures
where needed in existing canals.
Expansion of the secondary drainage sys-
tem and increased water use to keep pace with





continued rapid development of coastal Broward
County will lower water levels and will increase
the danger of salt intrusion in the future. By the
year 2000 water use for municipal supplies
alone is predicted to exceed one-half billion
gallons per day or more than ten times the
present use. Hydrologic studies indicate that
these water needs can be met by preventing
the construction of new avenues for salt intru-
sion and by making the maximum use of the
regional water-management system (see below).
The primary water-control system of the
Central and Southern Florida Flood Control
District is designed to alleviate the effects of
both flood and drought. This is accomplished
by draining a part of the flood waters to the
sea and storing a part in conservation areas
for release (luring droughts. This system, sup-
plemented by a controlled secondary drainage
network and by proper water-management can
provide the solution to Broward County's salt-
intrusion problem and can assure the county of
a long-term water supply.
The hydrologic data which made possible
the long-term delineation and monitoring of the
salt front were collected in cooperation with


Figure 9. Map showing water-management system.




the City of Fort Lauderdale. The collection of
additional data in northern Broward County was
begun in 1960 in cooperation with the City of
Pompano Beach and in 1963 with the City of
Deerfield Beach.
The cooperative investigation of the water
resources of Broward County by the U.S. Geolo-
gical Survey includes several salinity and hydro-
logic studies designed to aid in dPucting and
countering salt intrusion. Among these are: (1)
a continuing program of water level and salinity
data for observation wells and sampling points
on canals to monitor the movement of salt in
the aquifer and canals; (2) a test drilling pro-
gram to determine the extent of intrusion in the
aquifer; (3) electrical analog model studies to
determine the effects of proposed changes in
the canal system and increase in pumpage; and
(4) hydrologic studies to determine the water
levels required at salinity controls in canals
and the amount of fresh water flow required in
canals to stop salt intrusion.
The results of these cooperative studies
will be published by the Florida Geological
Survey and the U.S. Geological Survey. Reports
and data are currently available from the U.S.
Geological Survey, 51 S.W. First Avenue, Miami,
Florida. Further information on the mechanics
of salt intrusion in the southeastern coastal
area of Florida may be found in the references
listed below.








REFERENCES


Cooper, H.H., Jr.
1964 (and Kohout, F.A., Henry, II.R., and
Glover, R.E.) Sea water in coastal aqui-
fers: U.S. Geol. Survey Water-Supply
Paper 1613-C.

Klein, Howard
1957 Interim report on salt-water encroachment V
in Dade County, Florida: Fla. Geol. Sur-
vey Inf. Circ. 9.

Kohout, F.A.
1960 Flow pattern of fresh water and salt
water in the Biscayne aquifer of the
Miami area, Florida: Internat. Assoc.
Sci. Hydro., no. 52.

1961 A case history of salt-water encroach-
ment caused by a storm sewer in the
Miami area, F lorida: Am. Water Works
Assoc. Jour., v. 53, no. 11.

Parker, G.G.
1955 (and others) Water resources of south-
eastern Florida, with special reference
to the geology and ground water of the
Miami area: U.S. Geol. Survey Water-
Supply Paper 1255.

Sherwood, C.B.
1959 Ground-water resources of the Oakland
Park area of eastern Broward County,
Florida: Fla. Geol. Survey Rept. of Inv.
20.

1963 .(and Klein, Howard) Use of analog plotter
in water-control problems: National Water
Well Assoc., Ground Water Journal, v.
1, no. 1.

Tarver, G.R.
1964 Hydrology of the Biscayne aquifer in
the Pompano Beach area, Broward County,
Florida: Fla. Geol. Survey Rept. of Inv.
36.

Vorhis, R.C.
1948 Geology and ground water of the Fort
Lauderdale area, Florida: Fla. Geol.
Survey Rept. of Inv. 6.