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