Under islands and near sea coasts, the freshwater extends to a considerable depth in the aquifer below sea level. As a result, the underground boundary between the fresh water and saltwater of the sea is inclined as shown in the following figure.
If withdrawal by pumping is not excessive, the recharge of fresh water on the land is enough to keep the water table elevated above sea level and maintain a reservoir of fresh water, which bulges into the seawater at depth. But if the withdrawals exceed recharge, the water table is lowered and the pressure of fresh water on the bulge decreases thereby allowing the seawater to move in. There is no solution to the intrusion of seawater except to reduce the pumping or artificially increase the recharge.
On an island, the freshwater occurs in a lens-shaped area the major part of which extends below sea level. By the Herzberg formula, it is possible to determine the amount of freshwater present in this lens below sea level. In the above figure, we can see that the level of the water table is higher than the sea level, as well as the depth of the water table, is also deeper than the sea level, thus making the freshwater distinct from the seawater. The seawater intrudes only in case if a very large amount of freshwater is yielded. The saline seawater only percolates when the aquifer is emptied by pumping out the freshwater. If we show the thickness of the freshwater aquifer with h, and width with l, and the total thickness of the freshwater is H. The total thickness of the freshwater would be equal to the width and height of the aquifer.
But H is also equal to hg, where ”g” is the specific gravity of saltwater, then,
The average specific gravity of seawater is 1.025. By substituting this value in the above formula, we have: