20 FLORIDA GEOLOGICAL SURVEY
probably the result of the intrusion of water from the Floridan aquifer through the water-plant wells into the second artesian aquifer. The next highest concentration of sulfate, 650 ppm, was in the sample taken from well 25. Well 25 was originally drilled to a depth of 185 feet but was later plugged back to 140 feet. The high concentration of sulfate in the sample from well 25 may result from water below 140 feet leaking upward past the plug. The sample having the lowest concentration of sulfate, 0.4 ppm, was taken from well 32, the easternmost well in the south line of wells. Well 32 taps only the first artesian aquifer.
SALT-WATER LEAKAGE FROM PROPOSED WATERWAY
The route of the proposed intracoastal waterway in the vicinity of the Venice well field is shown in figure 3. The proposed waterway parallels within a few hundred feet the west line of wells.
The waterway, when constructed, will be filled with salt water. The salt water, being heavier than fresh water, will displace the fresh water in the water-table aquifer and form a salt-water wedge whose base will rest on the upper confining beds. The salt water will leak downward into the first artesian aquifer when a downward hydraulic gradient exists across the upper confining beds. Such a gradient may be created by heavy pumping from the Venice well field.
Figure 7 shows how salt water leaking into the first artesian aquifer would contaminate the well field. If wells tapping the first artesian aquifer were pimped, the salt water in the first artesian aquifer would be drawn directly into the wells. On the other hand, if wells tapping only the second artesian aquifer were pumped, salt water in the first artesian aquifer would move downward through wells open to both aquifers into the second artesian aquifer. Or if the head in the\first artesian aquifer is greater than the head in the second artesian aquifer, salt water would seep downward through the confining bed separating the two.
In order to determine the effect of constructing the waterway on the well field, estimates were made of the rate of salt-water leakage into the first artesian aquifer for various patterns and rates of pumpage. The amount of the seepage was estimated by applying Darcy's law. To make the estimate, it was necessary to know the area of the interface between the salt-water wedge and the confining beds above the first artesian aquifer, the coefficient of leakage of the upper confining beds, and the head differential between the salt-water wedge and the first artesian aquifer.