REPORT OF INVESTIGATIONs No. 41


   In the bay area and along the gulf coast in the basin, two artesian aquifers are associated with the aquiclude. Here the aquiclude is thicker than it is to the north and is overlain and in part underlain by some shell-hash beds which contain water. The sandy clay material which forms the base of the water-table aquifer is sufficiently impermeable to confine water in the shell-hash beds under artesian pressure. Water producing zones in the shell-hash beds above the aquiclude are termed the secondary artesian aquifer. The water producing zones in the shell-hash beds below the aquiclude are considered part of the Floridan aquifer.
   The Floridan aquifer underlies the entire basin below the aquidlude. It is composed of limestone formations that are as much as 1,200 feet thick. However, the usable part of the aquifer, the part producing potable water, is the upper 500 to 700 feet.

                     WATER MOVEMENT
    Rain, falling on the basin, is readily absorbed by the porous surface sands. The portion that runs off directly to the streams depends on the amount and intensity of the rainfall. The rain water and the surface water are relatively pure but contain some salts carried in the evaporate from the ocean and some gases dissolved from the atmosphere. The surface water becomes colored after contact with decayed organic matter but the mineral content changes very little.
    The water absorbed by the sands seeps downward to the water table, the level below which the sand is saturated. The sands are not very soluble in the rain water and consequently the mineral concentration in water from the water-table aquifer is low.
    Some of the water then moves from the water-table aquifer into the streams and maintains flow during periods of no rainfall. In the northern part of the basin where the sand and clay are breached by sinkholes, some of the runoff and seepage from the sands is temporarily ponded in lakes and then moves into the Floridan aquifer. In other areas the water from the sands may seep slowly into the limestone through the clay layer.
    The amount of water moving from the water-table aquifer to the Floridan aquifer diminishes toward the southwest because the aqmidlude is thicker. Water that moves downward into the limestone of the Floridan aquifer then moves in the down gradient direction shown by the piezometric map (see p. 23). The gases acquired from the at-miosphere and from the soil zone form a weak acid solution which disolves the limestone and thereby causes an increase in the mineral