BUREAU OF GEOLOGY


Chloride concentrations in the water from this zone range from 1,000 to 4,000 mg/I (milligrams per liter).

      A 1,000-foot thick confining bed composed of limestone in terbedded with dolomite underlies the principal artesian water-bearing zone. The permeability of the bed varies with depth but generally is low in the vertical direction, according to Vernon (1970, p. 10). Chloride concentrations of water in the confining bed range from 4,000 to 18,000 mg/i; and the zone of transition between brackish and saline water (sea-water) occurs between 1,800 and 2,000 feet below sea level (Garcia-Bengochea, 1970).


      The Boulder Zone, consisting of cavernous strata, is at the bottom of the Peninsula well. The Bureau of Geology, Florida Department of Natural Resources, designated the Boulder Zone as the, most favorable place to inject treated liquid wastes (Vernon, 1970, p. 31). According to the Department of Natural Resources (Vernon, 1970, p. 23) the requisites for deep disposal are ". . . that the wells terminate in zones of high transmissivities that are filled by saline or unusable water. Such zones are located in the base or below the Floridan aquifer and are reasonably separated from usable waters by dense sediment with low or minimal vertical transmissivities". The terms "salie water" and "unusable water" need to be defined. Water containing more than 1,000 mg/i of dissolved solids is generally considered to be "saline" by the U. S. Geological Survey. However, a criteria based on 15,000 mg/i might be more appropriate if the saline water can be economically used, that is, economically converted to fresh water.


      An extension of the subsurface geology from the Everglades eastward to Bimnini, in the Bahamas, suggests that the Boulder Zone crops out in the Straits of Florida some 35 miles east of the well (fig. 3). Dredge hauls and seismic profiles indicate the principal artesian water-bearing zone in the Floridan aquifer system also crop out in the Straits of Florida (Malloy and Hurley, 1970, p. 1970). Malloy and Hurley (1970, p. 1947) indicate that karst features (sinkholes) found on the east slope of the Straits may be kept free of sediment by submarine flow from the Floridan aquifer.


      Analysis of a water sample from the bottom of the well indicates that locally the water in the Boulder Zone is chemically equivalent to sea water. Temperature data obtained during drilling (Garcia-Bengochea, 1970, plate 4-2) indicate that ground water becomes cooler with depth rather -than .warmer. The thermal gradient in ground water seems to be closely relat ed to the thermal gradient in the ocean in the adjacent Straits of Florida- (fig. 3).