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Alafia River Minimum Flows and Levels - Southwest Florida Water ...

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The Upper <strong>Florida</strong>n aquifer (UFA) system consists of a thick series of limestone <strong>and</strong><br />

dolomite units that were deposited during the Eocene, Oligocene <strong>and</strong> early Miocene<br />

Epochs starting 53 million years ago. The UFA includes the Suwannee Limestone,<br />

Ocala Limestone, Avon Park Formation <strong>and</strong> permeable sections of the lower Tampa<br />

Member of the Hawthorn Group that are in direct hydraulic connection with the<br />

underlying carbonates. Average thickness of the UFA in the area ranges from 1000<br />

to 1200 ft (Miller 1982). The base of the UFA is marked by a regionally extensive<br />

unit of low-permeability evaporites, generally referred to as the Middle Confining Unit<br />

(MCU) (Ryder 1985). Lithologic logs for the area show that the top of the UFA<br />

generally dips southeastward <strong>and</strong> depth to the top of the UFA (limestone) can range<br />

from 30 to 180 feet below l<strong>and</strong> surface (bls).<br />

Within the UFA, there are two principal water-producing zones. These flow zones<br />

are typically associated with the Tampa Member <strong>and</strong> Suwannee Limestone (the<br />

upper flow zone), <strong>and</strong> highly permeable zones within the Avon Park Formation (the<br />

lower flow zone). These water-bearing zones are separated by relatively lower<br />

permeable portions of the Ocala Limestone. Although the Ocala Limestone is often<br />

regarded as a semi-confining unit, there is usually sufficient hydraulic connection<br />

across the Ocala such that the Upper <strong>Florida</strong>n aquifer essentially acts as a single<br />

hydrologic unit.<br />

2.1.4.4 Br<strong>and</strong>on Karst Terrain<br />

The Br<strong>and</strong>on Karst Terrain (BKT) is an area of approximately 40 square miles,<br />

located to the north of the <strong>Alafia</strong> <strong>River</strong> <strong>and</strong> west of Lithia Springs (Figure 2-3). The<br />

limestone in this area has been heavily weathered by chemical dissolution <strong>and</strong> the<br />

area is dominated by karst topography including a high density of ancient <strong>and</strong><br />

modern sinkholes, internal drainage, springs, <strong>and</strong> significantly increased<br />

transmissivities in the limestone.<br />

The BKT was formed in an area where the IAS pinches out <strong>and</strong> the confining units of<br />

the Hawthorn begin to thin. This has allowed large quantities of highly aggressive<br />

(acidic) surface waters to leak through <strong>and</strong> dissolve the underlying limestone to form<br />

sinkholes. These sinkholes further facilitated the downward movement of surfacewater<br />

<strong>and</strong> dissolution of the underlying limestones to create a well-developed<br />

underground drainage system capable of moving large quantities of groundwater<br />

through a network of conduits <strong>and</strong> voids (Jones <strong>and</strong> Upchurch 1993).<br />

The enhanced underground flow conditions of the BKT is reflected in the reduced<br />

gradient or 'flattening' of the potentiometric surface observed in the UFA overlying<br />

the BKT. As a result, regional groundwater flow in the southern portion of the terrain<br />

is diverted to the south <strong>and</strong> southeast, towards the springs <strong>and</strong> the <strong>Alafia</strong> <strong>River</strong>.<br />

Fracture trace analysis in the BKT area also indicates the presence of at least two<br />

2-8

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