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1 Fj ' diameter wells within this aquifer are generally 300 to 500 gallons per minute (gpm). ' The confining unit between the shallow and Floridan aquifers is made of deposits of the sandy clay, clay, and consolidated zones of the Hawthorn ' Formation (Miocene), perhaps containing some of the Tamiami Formation overlying the Hawthorn Formation. This unit is hydrogeologically important ' in that it prevents contamination of the fresh water within the surficial aquifer with the brackish water of the Floridan aquifer. The Hawthorn ' Formation extends to a depth of roughly 450 feet bls at the study site. The Floridan aquifer is the second aquifer underlying the study site. The ' water within this zone is brackish, and is under artesian pressure. The lithology of this unit is generally a limestone with good permeability. ' Yields from 10 -inch diameter wells installed into the Floridan aquifer can be up to 3,000 gpm. tThe United States Geological Survey (USGS) maintains a series of observa- tion wells in the study area to monitor the water table levels. The USGS uses water levels measured from these wells during wet and dry seasons to prepare contour maps to show the high and low levels of the water table. ' The shape of the water table, the hydraulic gradients, and the general direction of groundwater movement can be determined from the contours. The ' direction groundwater moves is generally downgradient, perpendicular to the contour lines. Maximum and minimum water levels within the aquifer usually ' occur at the end of the wet and dry seasons, respectively. Water level maps by the USGS will be provided in the immediate future. ' The grade elevation of the study site ranges from 2.9 to 6.5 feet msl, with the average grade about 4.5 feet msl. According to Masteller & Moler, the ' water table is at depths of 10 to 30 inches for about 2 to 6 months per year, and within a depth of 10 inches below ground level during the wet ' season. r ' IRC.VB4 6/4/87 4-2