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

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<strong>Water</strong> in the surficial aquifer is recharged primarily by rainfall. Depth to water can<br />

range from zero to up to fifty feet below l<strong>and</strong> surface. Fluctuations in water level<br />

occur seasonally with higher water levels occurring during the summer rainy months<br />

<strong>and</strong> lowered water levels during the late spring dry season.<br />

2.1.4.2 Intermediate Aquifer System<br />

The intermediate aquifer system (IAS) is generally comprised of siliciclastic<br />

sediments, limestone <strong>and</strong> dolomite beds, collectively known as the Hawthorn Group,<br />

that were deposited during the Miocene <strong>and</strong> Pliocene Epochs. The Hawthorn Group<br />

is further subdivided into the Peace <strong>River</strong> (siliciclastic deposits) <strong>and</strong> Arcadia<br />

Formations (carbonate deposits). Within the District, the thickness of IAS sediments<br />

ranges from completely absent in the north to greater than 700 feet in the south.<br />

These stratigraphic units generally thicken <strong>and</strong> dip from north to south.<br />

The IAS consists of three principal components: the upper intermediate confining<br />

unit (ICU), an aquifer or water-bearing formation (IA), <strong>and</strong> the lower intermediate<br />

confining unit. In the southern portions of the District, all three units of the IAS are<br />

typically present, limiting flow between the overlying surficial aquifer <strong>and</strong> the<br />

underlying Upper <strong>Florida</strong>n aquifer. However, in areas north of the <strong>Alafia</strong> <strong>River</strong>, the<br />

IAS begins to thin out, <strong>and</strong> transitions from an aquifer system (with confining units)<br />

to simply a confining unit (the ICU). These areas become more susceptible to<br />

sinkhole development as the clay units thin <strong>and</strong> allow for organic-rich surface water<br />

to leak through <strong>and</strong> dissolve underlying carbonates. The approximate northern limit<br />

of the IAS extends in a line from southwest Hillsborough County to north central Polk<br />

County, although a thin discontinuous intermediate aquifer may exist locally north of<br />

this line (SWFWMD 1993). In areas north of the <strong>Alafia</strong> <strong>River</strong> <strong>and</strong> west of Lithia<br />

Springs, the thick confining units have also been compromised by intense karst<br />

activity. This area, known also as the Br<strong>and</strong>on Karst Terrain (BKT) (Upchurch &<br />

Littlefield 1988; Jones & Upchurch 1993), provides a direct connection for<br />

groundwater flow between the surficial aquifer <strong>and</strong> the underlying Upper <strong>Florida</strong>n<br />

aquifer (see Figure 2-3).<br />

2-6

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