30 FLORIDA GEOLOGICAL SURVEY
where Q is the rate of salt-water leakage, P'/m' is the coefficient of leakage of the confining beds that separate the salt-water wedge from the first artesian aquifer, h' is the head differential across the confining beds, and A is the area of the interface between the saltwater wedge and the confining beds or the surficial area of the confining beds through which the salt water will leak.
Estimates were made of the rate of salt-water leakage and the resulting increase in the average chloride content of the pumped water. The increase will be greater than that estimated in some wells and less in others, depending on the location of the wells and the rate and pattern of pumping. Water from wells near the center of pumping and near the waterway will have the greatest increase in chlorides, and water from wells farthest from the center of pumping and the waterway will have the least increase in chlorides.
One set of estimates was based on a coefficient of leakage of 1.3 x 10- gpd per ft', and one set was based on a coefficient of leakage of T7 x 10-" gpd per ft3.
Based on a coefficient of leakage of 1.3 x 10-3 gpd per ft3, the rate of leakage of the salt water into the first artesian aquifer was computed to be about 10,000 gpd or about 300,000 gallons per month for a pumping rate of 17.4 million gallons per month drawn from the west, east, and south lines of wells. This is the rate and the pattern of pumping that was used to compute the drawdowns shown in figure 9. Assuming that the salt water contains 20,000 ppm chloride, about average for sea water, the chloride content of the pumped water would be increased by about:
300,000 x 20,000 =350 ppm
17,400,000
If it is assumed that the west line of wells is not pumped and that the pumping rate is 17.4 million gallons per month, the chloride content of the water would have increased on the average only 240 ppm. This is the rate and pattern of pumping that was used in computing the drawdowns shown in figure 10. But, if the amount of water drawn from each pump is reduced so that the pumping rate is only 11.4 million gallons per month, the chloride content of the water would increase on the average 80 ppm. This rate and pattern pumping was used in computed the drawdowns shown in figure 11.
In order to calculate the increase in the chloride content of the water based on a coefficient of leakage of 7 x 10-3 gpd per ft3, it is only necessary to multiply the estimates based on a coefficient of