22 FLORIDA GEOLOGICAL SURVEY
The specific gravity of ground water is, for practical purposes, 1.000, and the specific gravity of sea water is ordinarily about 1.025. If the specific gravity of sea water is 1.025, the above equation shows that h = 40 t. In other words, for every foot of fresh water above sea level, there is 40 feet of fresh water below sea level.
This relationship is modified somewhat by the conditions at the Venice well field. The bottom of the water-table aquifer is about 18 feet below sea level. Assuming the stage of the waterway remains at sea level, the above relation shows that the salt-water fresh-water interface will be at the bottom of the aquifer below a point where the water table is 0.45 foot above sea level. Where the water table is below 0.45 foot above sea level, the depth to the interface may be determined by h = 40 t; and where the water table is higher than 0.45 foot above sea level, the aquifer will be filled with fresh water. Thus, the distance from the edge of the waterway to the point where elevation of the water table is 0.45 foot determines the width of the base of the salt-water wedge.
The theoretical position that the water table would take if the waterway were constructed may be determined mathematically. It can be shown (Jacob, 1950, p. 378) that the following formula, based on the assumptions of Dupuit, describes the steady-state profile of the water table between two completely penetrating streams:
h ho = 2 W/P (ax x2/2)
where h is the height of the water table above the base of the aquifer, h,, is the height of the stream stage above the base of the aquifer, W is the rate of accretion (rainfall penetration) to the water table, P is the permeability of the material composing the aquifer, a is the distance from the stream to the ground-water divide, and x is the distance from the edge of the stream to any point h on the water table.
The distance from the waterway to the point where the water table is 0.45 foot above sea level can be estimated by use of this formula. The bottom of the water-table aquifer is about 18 feet below sea level so that ho = 18 feet; W at the waterway is estimated to average about 1 foot per year after taking into account leakage from the water-table aquifer. The permeability P of the material composing the water-table aquifer is estimated to be about 250 gpd per ft2. And if the proposed waterway simulates one stream, and Hatchett Creek simulates the other, the distance to