development might cause the salty water to move up and contaminate the fresh-water resource.
The hardness of the artesian water varies considerably from one place to another (see fig. 20). It is less than 100 parts per million in an area -west of Gainesville, where recharge enters the aquifer through sinkholes, but increases'progressively as the water moves away from this area. The hardness is derived mainly by solution of the limestone and dolomite that make up the Floridan aquifer. As the water enters the. aquifer, it is quite soft but contains carbon dioxide and organic acids that enable it to dissolve the rocks more readily. During its long journey from the area of recharge, always in intimate contact with the rocks, the water picks up several hundred parts per' million of hardness.
Where the artesian water is too salty for use, the shallow ground water and the streams, lakes, and ponds constitute the principal sources of supply. In some such areas, as around Miami, the shallow aquifers are highly productive and will very likely supply the local needs for many years to come. On the other hand, in areas such as those along the middle east coast, the shallow formations and surface sources do not yield an adequate supply, and eventually the municipalities must pipe water from sources in adjoining areas to supply their steadily growing requirements.
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FI(URE 21.-Artesian pressures have diminished at Jacksonville.
Lines connect points of equal artesian head, in feet above sea level. When the first wells were drilled, about 1880, the artesian pressure in the vicinity of Jacksonville was sufficient to raise water 601 feet above sea level (map at left). Now, owing to a draft of 80 mgd, the pressure is substantially less (map at right). Large additional supplies of artesian water may be developed, but at the cost of a further decrease in pressure.