31
Current Draft of Water Has Made Only Minor
Subtractions From the Total Supply
Considered as a whole, the available supply of artesian water is scarcely touched by current draft. It is only in certain localities that the demands for water are approaching the capacity of the aquifer to supply it.
For example, about 100 million gallons a day is currently being drawn from wells in the vicinity of Jacksonville for municipal and industrial uses. As indicated in figure 21, this draft has caused substantial lessening of artesian pressures, especially around the town of Fernandina, north of Jacksonville. The decline of water levels does not, however, indicate a depletion of the reserve, as it would in some other areas. It merely indicates that the aquifer lacks sufficient capacity for transmitting water from the area of recharge. Around Jacksonville the aquifer is confined and is functioning principally as a conduit rather than a reservoir. Just as the size of a pipeline limits the quantity of water that will flow through it, so does the capacity of the aquifer to transmit water-its "transmissibility"-determine the rate of flow of the artesian water. But the rate of flow is determined also by the steepness of the piezometric surface; when the piezometric surface is steepened, water moves through the aquifer more rapidly. Thus, the lessening of artesian pressures in the Jacksonville area, by steepening the piezometric surface, has induced more water to move in from the recharge area. Each time the draft is increased, the pressures will be further lessened and the gradient steepened proportionately. The maximum yield of the wells will have been realized when the draft has grown to the extent that no further lowering is economically feasible. Other factors being equal, the maximum yield will be larger if the wells are distributed over a wide area. Also, more yield may be obtained by drilling wells closer to the recharge area, thereby shortening the distance the water must travel.
It appears unlikely that any alarming consequences will develop from heavy withdrawal in the Jacksonville area, providing wells are not drilled too deep. More likely economic expedience, rather than disastrous experience, will eventually call a halt to further development of the artesian water and motivate the development of a supplemental supply from other sources.
But not everywhere iq the outlook so happy. An excessive draft of ground water can be ruinous if it causes encroachment of salt water from the sea. A notable example of the places where salt-