Augmentation Plan, Aquifer, Well Depths, Connectivity

Sent by Jackie Host on October 1, 2011:

INTERMEDIATE AQUIFER SYSTEM
The top of the Hawthorn Group serves as the upper confining unit that separates the surficial and intermediate aquifer systems. The degree of hydraulic connection between the two systems varies throughout the UECB as the formation thickness ranges from 25 feet to complete absence. Discontinuous limestone, dolomite, shell, and sand beds make up the intermediate aquifer system. Due to breaches in the lower confining unit, which can allow significant vertical leakage, the base of the intermediate aquifer is connected hydraulically to the upper Floridan aquifer.

FLORIDAN AQUIFER SYSTEM

Four major components make up the Floridan aquifer system: an upper zone of a relatively high permeability, a middle confining zone of low permeability, a lower zone of low-to-high permeability, and a lower confining unit. The karst limestone formations that constitute the Floridan aquifer system are of Eocene age. The upper Floridan aquifer is a zone of high permeability, and its thickness generally increases from west to east within the study area.

The middle confining unit exhibits lower permeability, and its thickness ranges from 50 to 200 feet. Little is known of the lower Floridan aquifer because few wells have penetrated its depths. Its  thickness ranges from about 1,100 to 1,200 feet. The lower confining unit, which has a low hydraulic conductivity, is considered to be the bottom of the Floridan aquifer system.

RECHARGE AND DISCHARGE

Surficial Aquifer System
Precipitation and discharge from lakes are the primary means of recharge to the surficial aquifer system. Although precipitation is the main source, some lakes contribute recharge during 23 periods of low or negligible rainfall. This phenomenon is a consequence of declining ground water levels in the surficial aquifer surrounding the lakes. Ground water discharges from the surficial aquifer include vertical leakage into the intermediate and upper Floridan aquifers, flux into lakes within the UECB, evapotranspiration, and pumping from domestic wells.

Intermediate Aquifer System
Through vertical leakage, the overlying surficial aquifer supplies most of the recharge to the intermediate aquifer. Recharge can occur from lakes directly on the upper confining unit of the Hawthorn Group. Ground water moves through direct hydraulic connections in the confining unit. The intermediate aquifer mainly discharges in the form of vertically downward leakage to the Floridan aquifer system through the lower confining unit in the Hawthorn Group. Leakage also occurs through breaches and cavities in the formation. Pumping from private wells in the region is another form of discharge.

Floridan Aquifer System

Recharge to the Floridan aquifer system is derived from the intermediate aquifer system and from areas where lakes and the surficial aquifer system have a direct hydraulic connection. Overall, rainfall has a significant effect on recharge to the Floridan aquifer system. The recharge from the intermediate aquifer occurs through the lower confining unit of the Hawthorn Group. In addition, dissolution cavities in the upper Floridan aquifer may create hydraulic connections to the overlying intermediate aquifer because the lower confining unit becomes discontinuous, and they can even form connections completely through the Hawthorn Group into the surficial aquifer 24 system. Water level changes in the upper Floridan aquifer due to precipitation may have a lagtime of approximately one to two months (Hoenstine and Lane, 1991).

Potentiometric maps prepared by the USGS and SJRWMD consistently delineate a major ground water mound in the upper Floridan aquifer in the western part of the UECB (Figs. 7 through 10). Centered in the Keystone Heights area, this mound, or potentiometric high, indicates that the lakes and surficial aquifer system are a major source of recharge to the underlying Floridan aquifer system.