21
man-made reservoir, impounded behind Hoover Dam on the Colorado River. But only a fraction of the water stored can be claimed for use.
Neither do we know how much replenishment the aquifer receives each year, although the total discharge of the large springs indicates that it, too, is large. The aggregate discharge of the springs represented in figure 11 is about 6,000 cubic feet a second. But this is only a small fraction of the whole. Probably most of the discharge goes directly into the sea, through countless springs and widespread seepage. This submarine discharge cannot be observed or measured, but we believe that it exceeds by several times the discharge of the terrestrial springs. Thus, we conclude that the discharge from the aquifer, and hence the replenishment to it, must be reckoned in the tens of thousands of cubic feet a second, and that the replenishment easily exceeds the flow into Lake Mead. The areas of recharge aggregate about 1.3,000 square miles in- i central and northern Florida. Full development and use of the ar-/
V/
A i nf~rIf*;
Gulf S' r Augustin
of __s
Mexco u tr U_ 
Floridan a uifer
Yt t L
Rejected recharge. Maximum recharge Recharge through Artesian flow:
Aquifer exposed but Aquifer exposed or covered sink holes Piezometric surface
full of water with porous material above ground Geology Vertical relief greatly exaggerated
FIGURE 12. controls recharge and movement of the water.
The Floridan aquifer is a thick section of limestone which crops out at the land surface in some areas, but which is blanketed by watertight material in other areas. Recharge occurs most readily where the aquifer crops out, but a substantial amount of water enters through sinkholes which breach the watertight blanket.
Flowing wells may be obtained wherever the piezometric surface (see fig. 13) is above the land surface, as in the right-hand part of the section shown. Here the water is confined beneath the watertight blanket, and the aquifer is essentially a conduit. Where the aquifer is unconfined (left half of the section) it functions as a reservoir. At the far left of the section the aquifer is full and, therefore, is rejecting recharge.