The salt wedge moves slowly. In 34 years (1910 to 194i) it has
moved about 8pO00 feet inland at Silver Bluff, and possibly more depending .upon its original position which is here assumed to have been about at the shore line. Old residents of this area, however, tell of a well that formerly was sunk in Biscayne Bay to an unknown but fairly shallow depth (30 to 50 feet, possibly) and that potable water was pumped from it by boatmen. If the water obtained was fresh, then the fresh water-salt water boundary must have lain somewhat east of the shore line, out under the Bayp and the total movement of salt water inland may be nearer 10,000 than 8,O00 feet. Using 8#000 feet as a base figure the average rate of movement inland has been 235 feet per year, and at this rate, if conditions remain about as they now are, it would be many more years before the salt wedge moved in as far as the present well field. But the salt wedge does not present an even front. Tongues of salty water from the Bay follow under and along the drainage canals and extend from the top of the aquifer to the very bottom. In cross section these tongues have a trapozoidal shape, narrow at the top and wide at the base.
The most threatening of these tongues is that of the Miami Canal with ito spearhead now almost dormant about one-half mile west of N. We 36th Street Bridge. This tongue may be moving at the same or a similar rate as the salt wedge along the shore. If it is, then it will be only a matter of about 43 years before the well field is reached inasmuch as the distance to be traveled is about 2 miles. Only observations carried out over a much longer period of time will definitely establish this rate of movement.
The above considerations are based on conditions as they have existed