REPORT OF INVESTIGATioNs No. 41


to about 50 ppm. In areas near the coast -these sands are in contact with salt water. All water from the water-table aquifer which contained more than 50 ppm dissolved minerals was from wells located vAthin a few hundred feet of salt-water bodies.
   Water in the secondary artesian aquifer is slightly more mineralized than water in the water-table aquifer. In some areas near the coast the water from this aquifer may be saline. Where there is no contamination by saline water, the water generally contains from 80 to 150 ppm dissolved minerals and is principally a calcium bicarbonate water. Most of the water samples contained hydrogen sulfide and some samples contained sulfate.
   The mineral concentration of water from the Floridan aquifer is higher than that from the secondary artesian or the water-table aquifers. In the northern part of the basin the higher mineral content is due entirely to higher concentrations of calcium and bicarbonate resulting from solution of the limestone. There are definite trends in mineral concentrations in water in the Floridan aquifer. These trends have been mapped (Toler and Shampine, 1964) and generally show increases in all constituents toward the southwest. An adaptation of the map of dissolved solids is shown on page 14 and indicates the trend of all the constituents. Sulfate, sodium, and hydrogen sulfide show little trend, but are found in significant quantities in. the southern half of the basin. In this area, sulfate ranged from 0 to 81 ppm, sodium from 2 to 164 ppm, and the odor of hydrogen sulfide was detected in water from most wells.

            CONTAMINATION BY SALINE WATER
    The large bays in the southern half of the Econfina Creek basin provide an access for salt water. several miles inland. Along the shoreline of these bays and along the Gulf, the salt water is in contact with the sands which form the water-table aquifer. During droughts, when the water levels in the sands are low, salt water may enter the aquifer and be pumped from shallow wells near the shore. Salt water is more dense than fresh water and it moves into the aquifer in the form of a wedge below a lens of fresh water. Fresh water will generally suppress the salt water about 40 feet below sea, level for every foot of elevation of fresh water above sea level. If the water level in the aquifer is lowered by pumping, the saline wedge adjusts to the new water levels and salt water may rise to contaminate a well. An interzone of water of intermediate composition is normally present instead of a sharp fresh-water salt-water interface.