Case History: City of Cape Coral, Florida
Contents:
Background
Location of a Site for a Horizontal Well
Design and Installation of the Pilot Horizontal Well System
Installation of the Horizontal Well System
Operational Testing of the Horizontal Well System
Background
The City of Cape Coral is located in Lee County, Florida. In the early 1960’s nearly the entire area of the present city (approximately 150 square miles) was purchased and developed by a private corporation. An extensive man-made canal system was constructed. A series of weirs and locks separates freshwater portions of the canal system from the saltwater portions. At the end of the year 2000, the City had an estimated population of approximately 100,000 persons.
Municipal water supply and wastewater treatment is provided by the City of Cape Coral Utility Department. The source of the raw water for the City’s municipal supply is brackish water from the Upper Floridan aquifer at depths between 500 and 800 feet. This water is treated by reverse osmosis (RO) to provide potable water. In order to more efficiently use its available water resources, the City undertook the construction of an extensive dual water system to provide irrigation water to its residents. The benefits of this system are twofold: (1) it significantly reduces or eliminates wet weather discharges and (2) it curtails new and eventually will reduce withdrawals for irrigation purposes from aquifers overlying the City’s source of potable supply, thus helping to reduce the rate of water quality deterioration in that aquifer.
By the end of the year 2000, the average day demand from the City’s reuse system was about 11 million gallons (MG) and during the dry season (October through May) peak day demands exceeding 35 MG had been recorded. Because wastewater generation does not meet average day demands, the City uses withdrawals from its fresh water canal system to supplement treated wastewater. During the dry season, withdrawals from the canal system provide about 70% of the total reuse water. Since excessive withdrawals from the freshwater canal system can present aesthetic impacts and lead to resident complaints, the City has evaluated several potential means to provide additional supplemental sources of water for its reuse system. The most promising of these additional means are aquifer storage and recovery (ASR) and horizontal well systems.
An ASR system would operate by withdrawing water from the canal system during the wet season, when it would otherwise discharge to tidal waters, and injecting it into a subsurface storage zone, most likely below and well isolated from the aquifer used to supply raw water to the City’s RO plant. The injected water would be stored and later recovered during the dry season as needed.
Horizontal wells could provide supplemental water for the reuse system if installed in the water-table aquifer in areas where the canals are widely spaced. By withdrawing water in these areas, which would not otherwise be drained by the canals, additional supplemental water could be provided for the reuse system during the dry season.
A cost comparison of expected ASR to horizontal well systems by the City indicated that a horizontal well installed in Cape Coral would cost about 30% that of an ASR well. Therefore, the City determined to proceed with the design and permitting of a pilot horizontal well system. In addition, the City applied for and was awarded a grant for matching funds from the South Florida Water Management District’s Alternative Water Supply Program for installation and operational testing of a pilot horizontal well system.
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Location of a Site for a Horizontal Well
A review of available subsurface geological data was performed and this information was integrated with the location of City-owned parcels, potential impacts from withdrawals to other users or wetlands, and the canal configuration to prioritize parcels for testing.
Two sites were selected for initial testing. At each site three borings were installed to a depth of 25 feet. At the first site the lithology encountered was fine grained sand and silty sand. At the second location the material encountered to a depth of about 9 feet was similar to that encountered at the first site, however below that depth a shell bed with well sorted medium to coarse grained sand was encountered. The borings were continuously cored and the core samples were analyzed by sieve analysis. Based upon the sieve analysis, a zone at each site was selected to be screened, screen slot size and gravel packs were specified, and two four-inch diameter screened test wells were installed at each site.
Thereafter, a constant-rate, extended-duration pumping test was conducted at each site. Based upon analysis of the data collected during the pumping tests, it was determined that the second site had a transmissivity (i.e. the ability of an aquifer to transmit water, which is essentially the permeabilty multiplied by saturated thickness) about 25 times greater than the first site. Therefore, the second site was selected for the installation of a pilot horizontal well system.
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Design and Installation of the Pilot Horizontal Well System
Based upon computer modeling of various length and configured horizontal wells at the site, it was determined that a well with a horizontal length of approximately 800 feet and an eight inch diameter would optimize the yield at the site. The interval selected for installation was between the depths of 20 and 21 feet. High density polyethylene (HDPE) pipe was selected for the horizontal portion of the well to be encased in a nonseamed, fine-meshed filter fabric (often referred to as a sock) to prevent sediment ingress into the horizontal well. A gravel pack and depth of emplacement of the gravel pack was also designed.
Installation of the well was to be accomplished by chain trencher method. Initially a single, centrally located vertical riser was contemplated, however, after considering cost and logistics for this design, it was determined that two vertical risers, each with a 400 foot run of horizontal pipe, and each equipped with submersible pump would be more cost effective. Surface facilities were designed and a pipeline from the horizontal welll system to the discharge location was installed. This pipeline discharges into a canal system which routes water to the City’s surface water withdrawal pumping stations.
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Installation of the Horizontal Well System
The contractor selected to construct the horizontal well system was Hydrotech/HydroRock Inc. of Ft. Myers, Florida. Equipment was mobilized to the site January 2001 and the well was fully constructed, including surface facilities, pumps, and electrical connections, and put into service on March 19, 2001. One of the important considerations in the construction of the system at the selected site was the presence of a very hard limestone layer between the depths of about 9 and 14 feet. Track mounted rock-cutting equipment was used to cut a path for the chain trenchers which installed the horizontal portions of the system. The chain trenchers dug to the required depth and laid the horizontal HDPE slotted pipe and gravel pack in a single pass (i.e. sequentially the trench was cut seconds before the pipe and gravel pack were emplaced with the same machine).
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Operational Testing of the Horizontal Well System
The horizontal well system has been tested for a period of approximately six weeks of continuous pumping at 800,000 gallons per day.
The City is currently evaluating locations for installation of a second horizontal well system.
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