| HIGHLIGHTS OF |
| THE WATER STORY |
| As Related to |
| THE CROSS FLORIDA BARGE CANAL |
| Prepared by: Philip E. LaMoreaux, Consulting Hydrogeologist, |
| Tuscaloosa, Alabama |
THE WATER STORY:
CROSS FLORIDA BARGE CANAL
HIGHLIGHTS
The Cross-Florida Barge Canal will function in the natural water system of Florida much like natural features such as rivers, sinkholes, and lakes. A detailed monitoring network and numerous technical studies have provided comprehensive information on both the surface and the ground water (sub surface) conditions in the vicinity of the canal. Data already accumulated on geology and hydrology provide adequate background for criteria to govern general design measures and to justify the feasibility of initiating construction now. The general design of the canal emphasizes maintaining the quantity and quality of the existing natural water regime. The proposed operating water levels of the canal will essentially remain in balance with the natural ground water levels.
Natural ground water levels will change slightly in the immediate vicinity of the Summit Pool of the Canal; however, these changes will be localized within a narrow zone two or three miles to either side of the Canal. The vertical adjustments in water levels will vary gradually from zero at the outer limits, to a very few feet at the banks of the Canal channel. In the Eureka Pool just upstream (south) of Eureka Lock and Dam, ground water levels will rise around 15 feet at the edge of the lake. This rise will taper off to zero about 5 miles out from the impoundment. The net result of this higher water level will be a thicker layer of sub-surface water available to supply local shallow wells.
Accumulated data, the basis for any scientific conclusion, indicate that there is ample water available to safely operate the Canal during all seasons of the year, wet or dry, and that there will be no effect on the ground water of southern Florida. Geologically and hydrologically, the Cross-Florida Barge Canal Project is one of the most extensively studied construction projects in history.
Measurements of yearly rainfall help determine how much water enters the hydrologic system. Along the canal route, rainfall averages about 53 inches per year.
Measurements of stream levels and stream flow determine the amount of surface water available to operate the canal.
Measurements of ground water levels help determine the quantity of ground water available for use in canal operations. Recharge and discharge areas and direction of ground water
flow are also determined. Analyses of water samples provide Information on the quality of water in the ground and in rivers, springs and lakes along the course of the canal.
The Canal in Perspective:
The elevation to which water will rise in wells is known as the potentiometric surface. The potentiometric surface in Florida can be represented by a contour map which shows the configuration of this surface. Ground water movement is at right angles to the contour, from areas where the potentiometric surface is high toward areas where the surface is low.
The canal route is in a trough (an area with a low potentiometric surface) between potentiometric highs. The trough extends across the drainage areas of Rainbow and Silver Springs and reflects the large quantities of water discharging from the springs. regionally, ground water moves toward the canal from the north and from the south as shown by the arrows. Operating water levels in the canal will be approximately at the potentiometric surface along most of the proposed route and there will be little exchange of water between the canal and the Floridan Aquifer.
It is readily apparent on the map that the canal will in no way affect the ground water in north or south Florida, nor will it have any effect on the Okefenoke swamp or the Everglades. The impact of the canal will be confined to the immediate vicinity of the canal channel and the impounded reservoirs.
General Information About The Canal:
GEOLOGY
To fully understand the hydrology of an area, one must understand the physical character of the rocks which contain the water.
West of Ocala, the canal will be excavated into the upper surface of the Floridan Aquifer to a depth ranging from 12 to 27 feet below the water table. The aquifer in this area Is from 1000 to 1200 feet thick and it is comprised primarily of limestone and dolomite. The canal design is such that the operational water level will be as close to the natural ground water levels as possible to minimize the interchange of water between the canal and the aquifer.
East of Ocala, the canal will be excavated into or follow a course over fine material comprised of sand, clay, sandy limestone and shell beds which overlie the Floridan Aquifer. These materials will impede, filter, or, in some cases, serve as a barrier to the downward movement of water from the canal Into the aquifer.
Detailed studies of the surface and subsurface rocks in the vicinity of the canal have produced large volumes of data that have been used to determine the physical characteristics of the rocks as they are today and also to determine what the environmental conditions were when the rocks were deposited many millions of years ago. The studies also reveal the distribution of the various rock units, the age of the rocks, and a geologic record of the movements in the earth's crust since the rocks were deposited. A basic knowledge of all these factors helps in gaining insight into how present geologic and hydrologic conditions were brought about and makes possible a better understanding of the existing conditions.
The Floridan Aquifer, one of the most extensive limestone aquifers in the United States, is the principle artesian aquifer in Florida and is the most important source of ground water. The Floridan Aquifer is comprised of formations that vary considerably in their capacity to hold and transmit water. For practical reasons, in Florida and parts of adjacent states, the formations In the Floridan Aquifer are usually treated as a single continuous hydraulic unit in the subsurface. In the canal area, the Floridan Aquifer consists primarily of limestone and dolomite formations that are from 20 million to 50 million years old.
SOURCE OF WATER:
THE HYDROLOGIC CYCLE
Water comes from the ocean and returns to the ocean in a never ending cycle. Initially, water evaporates from the ocean and becomes vapor in the air. Carried overland by winds, it condenses and falls as rain, part of which seeps into the ground, part evaporates, and part is used by trees and plants. The remainder returns to the ocean through streams that drain the land.
Water seeps into the ground from direct rainfall and from streams and lakes. Some is retained near the surface as soil moisture. Some seeps downward to a zone saturated with water. Water in this saturated zone is called ground water. The upper surface of the saturated zone is the water table.
In the area west of Ocala where the Floridan Aquifer is at or near the surface and no impermeable bed exists between the aquifer and the land surface, the aquifer is referred to as "unconfined" and under water-table conditions. The water table in this region is very responsive to precipitation since replenishment is from infiltration of local rainfall and recharge from streams. The water level in wells in this region coincides with the level of the water table.
East of Ocala, the Floridan Aquifer is overlain by sand, clay, sandy limestone and slitstone. Some of the clay beds are impermeable and serve as a quicludes or confining beds which confine the water In the underlying Floridan Aquifer, causing a pressure build-up from the weight of the water In the aquifer at higher elevations. If a well is drilled through the upper confining bed or a quiclude, the water will rise in the well to a level made possible by the pressure. Such a well is called an "artesian well." The level or elevation to which water will rise in tightly cased artesian wells is known as the potentiometric surface. Where the land surface is lower than the potentiometric surface, water flows freely at the land surface from "flowing artesian wells."
WATER MOVEMENT
The quantity of ground water in a formation and the direction and rate at which the water moves is determined by the physical characteristics of the formation and the geologic structure.
Ground water flows just as the water in rivers except that it flows much more slowly since it has to move through the pores and openings in the rock. The flow of streams is measured in feet per second, but movement of ground water is usually measured in feet per year. Water in limestone generally moves more freely in fractures or solution channels than it does through pore spaces.
In limestone formations such as those comprising the Floridan Aquifer, solution openings occur. Some of these openings become large channels or cavities many feet in diameter, capable of holding and transmitting large quantities of water.
In fine-grained material like clay, the openings are numerous but small, and incapable of permitting water to flow freely.
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The direction of movement of ground water is indicated by the shape of the water table or the potentiometric surface. The low areas on the potentiometric surface occur at points where water is discharged from the aquifer to springs, wells, streams or the oceans. Water moves from the potentiometric high areas to the low areas, thus causing springs to flow and wells to yield water. Water discharged from Rainbow and Silver Springs is drained from the areas indicated on the map.
WATER FOR THE CANAL
The Canal will be supplied with water from streams and ground water. In a state of equilibrium, with both locks closed, the water level in the Summit Pool will stabilize and ground-water inflow will equal outflow. As water is lost from the Summit Pool by lockage and evaporation, ground water will flow naturally into the Pool in areas where the operating level of the canal becomes lower than the potentiometric surface in the aquifer adjacent to the canal. The operating level of the Summit Pool will be determined by the water level in the aquifer which will be highest during the wet season and lowest during the dry season. The water level fluctuation between seasons is demonstrated by maps showing areas with water levels 42.5 feet or higher above mean sea level for May (low-water period) and September (high-water period), 1968.
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Lockage water and water lost from the Summit Pool by evaporation will be replaced by pumpage from Silver River. The Silver River water to be back pumped into the Summit Pool is more mineralized than the Summit Pool water and it will not accelerate the dissolving of limestone along the reach of the Summit Pool.
Water pumped from Silver River into the Summit Pool normally would flow into the Ocklawaha River. The pumping operation will have no effect on Silver Run upstream from the pump in-take, nor will it affect Silver Springs. Waters released during Dosh Lock operations will reenter the Ocklawaha River system at a location slightly removed from the pump intake off Silver Run.
SEASONAL WATER LEVEL FLUCTUATIONS
ZONES OF OUTFLOW
Knowledge of the proposed operating level of the Canal makes it possible to predict the effect of the canal on the potentiometric surface in the adjacent aquifer. Adjacent to the Summit Pool where the Canal is to be excavated into the Floridan Aquifer, water levels will be lowered on both sides of the Canal where ground-water inflow occurs, and raised on both sides of the Canal where outflow occurs. The Summit Pool operating level will be 44 feet or higher 60 percent of the time.
When the Eureka Lock and Dam are completed, the impoundment will cause ground water levels in the areas adjacent to the dam to rise, thus, making available more fresh ground water that can be developed for local use.
There are two comparatively narrow zones (approximately 4 miles and 1 1/4 miles long) in the Summit Pool where ground water will possibly flow from the Canal into the aquifer. Sufficient information is available from completed studies to adequately determine the general impact of these outflow zones, but continuing studies are needed to determine details related to design and construction across the zones. As construction progresses, more detailed information will evolve that will provide precise design and operational guidelines. As precise geologic and hydrologic conditions are established during channel excavation, proven engineering and construction techniques will be used to ensure sufficient protection of the subsurface area from pollution or other undesirable consequences.
METHODS OF POLLUTION CONTROL
Varied methods and techniques are available for water pollution control. They include: adequate monitoring facilities; Implementation of operational procedures designed to minimize accidental spills of contaminants; advance preparation of contingency plans for instant application in the event of accidental spills; and preventative construction measures such as grouting, or sealant lining of outflow areas. It should be recognized that a spill which might occur in the canal under controlled conditions would be less of a problem than one which might result from the wreck of a chemical truck or tank car at some culvert or bridge adjacent to streams or sinkholes in the Ocala area.
As an example, zones of outflow might be lined to prevent outflow into the aquifer. Replenishment to the aquifer of the quantity of ground water that would be intercepted by sealing off the zones of outflow could be accomplished by installing injection wells on the downgradient side of the canal and injecting part of the water pumped up from Silver River back into the aquifer. Observation wells can be strategically located to monitor the water in the aquifer, that is moving away from the canal. Pump intake locations in Silver River also can be monitored to prevent pumping contaminated water into the Summit Pool.
SALT WATER ENCROACHMENT
Rivers that empty into the Gulf of Mexico or the Atlantic normally contain salt water for a distance Inland, depending upon the steepness of the gradient of the stream, the volume and rate of fresh water discharge and the tidal influence which interact to form a natural balance in the location of the fresh water-salt water boundary.
The Buckman Lock (St. Johns Lock) on the canal near Palatka is approximately 80 miles up the St. Johns River from the Atlantic Ocean virtually eliminating any risk of salt water movement into the east end of the canal from the river. The St. Johns River at Palatka has only about a 1-foot range in tidal fluctuation; and U.S. Geological Survey records indicate salinity of about 700 parts per million for water in the St. Johns River at Palatka. Water with that salinity is considered only slightly brackish. The U.S. Geological Survey classifies waters with a salinity ranging from 1000 to 3000 parts per million as slightly saline. The stage differential of approximately 18 feet between Lake Ocklawaha (Rodman Pool) and the lower reach of the canal will minimize the possibility of locking any slightly brackish water into the pool. The sill on the Buckman Lock (St. John's Lock) will also serve as a partial barrier against brackish water entering the pool. Monitoring devices on the canal will record continually any changes in the concentration of brackish water. Lock gates and valves will permit locking any excessive concentration back into the lower channel.
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On the west end, the Gulf reach of the canal will behave similar to the Withlacoochee River where the fresh water-salt water contact constantly shifts upstream or downstream in response to natural tidal fluctuations and changes in stream discharge. Salinity studies indicate that salt water sometimes moves up the canal as far as the Inglis Lock. The Floridan Aquifer underlying the coastal swamps, which extend Inland one to two miles, contains chloride concentrations in excess of 2000 milligrams per liter. In most places, a mile or more inland from the coastal swamps, the water in wells is fresh. Because ground water levels generally become higher with distance Inland from the coast, the potential for salt-water intrusion of the aquifer adjacent to the Withlacoochee River and the canal diminishes with distance inland from the coast. If salt water is locked up into Inglis Pool, the monitoring system will detect it and it will be flushed out via the bypass channel. Continuing Geological Survey monitoring to date indicates no significant salt water encroachment despite the canal's excavation and existence for the past several years. The remote probability of salt water entering the Summit Pool or the Floridan Aquifer, therefore, is recognized fully; and engineering designs and operational plans will be formulated to prevent this problem arising.
RELATIONSHIP OF CANAL TO
RAINBOW SPRINGS AND SILVER SPRINGS
Rainbow Springs issue from points where the contact of a highly permeable zone and a zone of low permeability in the Floridan Aquifer approaches the land surface. The highly permeable zone wedges out and part of the large volume of water moving through it is diverted by the zone of low permeability to discharge at the surface through Rainbow Springs. The flow of this spring combines with that of the Withlacoochee River near Dunnellon to supply the Inglis Pool. The level of Rainbow Springs is about 3-4 feet higher than the normal level of the Inglis Pool, and the springs will not be affected by canal operations.
Faulting is one of the most important geologic factors causing Silver Springs. Faulting east of the springs has placed poorly permeable beds in position as a barrier to eastward flow of water in the Floridan Aquifer. Enough pressure is thus maintained at the spring site to cause overflow from open limestone caverns and sinkholes. Silver Springs is the most reliable and constant source of water for the Eureka Pool. Certain chemical characteristics of Silver Springs water indicate circulation deeper than that generally associated with water from the upper Floridan Aquifer. This may be a clue that Silver Springs receives a greater percentage of its water from depths in excess of those originally thought, and less from the upper part of the aquifer where the Summit Pool will be excavated. The detailed relationship between the Summit Pool and the spring will be known when excavation is accomplished, but on the basis of present knowledge it is safe to say that technology is available to allow the canal and Silver Springs to exist in safe harmony. The existing drainage wells at Ocala pose more of a threat to the quality of water at Silver Springs than will the canal.
EVALUATION OF EARTHQUAKE POTENTIAL
Earthquakes result from sudden movements along faults in the earth's crust. Strains in the crust are built up over a long period of time and are released suddenly. No damaging earthquakes have been recorded in Florida, and no known, active "earthquake" faults extend into the State.
Some concern has been expressed as to the possibility of earthquakes being caused by the "loading" effect resulting from filling the canal reservoirs. In this regard, the Inglis Dam and reservoir were constructed sixty years ago and there have been no associated earthquakes. Florida is inactive as far as earthquakes are concerned and it is beyond the realm of scientific probability to assume that impoundments the size of those proposed for the canal will trigger earthquakes.