So far we have been concerned with groundwater in inland areas well away from the seacoast. In coastal areas, what’s the interaction of groundwater with the sea? You know that fresh water is less dense than salt water, by a small but important few percent. So where groundwater is in contact with seawater, the groundwater should tend to float upon, or rise through, the seawater.
Perhaps you can get the clearest picture by considering a small, porous, rainy island in the ocean (Figure 4-26). (Nantucket and Martha’s Vineyard are good examples—although Chamber of Commerce members probably would not appreciate that description.) Assume that the seabed all around the island is saturated with seawater. Provided that the seabed consists of porous material, this is an excellent assumption. Then the body of fresh water that constitutes the groundwater under the island is itself floored by an even lower layer of denser seawater. The sharpness of the boundary between the groundwater and the seawater is determined by the relative importance of groundwater flow, which maintains the sharpness of the boundary, and diffusion of the salt from the seawater into the groundwater, which tends to blur the boundary. The boundary emerges at the seabed some distance from shore, and landward from that line groundwater flows out to the seabed and then rises and mixes with the water of the ocean.
How deep is the boundary between groundwater and seawater under the island? That depends on the elevation of the groundwater table above sea level within the island. The depth of the boundary below sea level is just about 40 times the height of the groundwater table above sea level—because the ratio of sea-water density to fresh-water density is about 41/40. It’s a simple problem of buoyancy. If you are confused, think about floating a balloon full of fresh water in a tub of seawater. The ratio of submergent depth to emergent height of the balloon is governed by exactly the same principle.
Now suppose you drilled a well somewhere on the island. At first you are tapping only freshwater. But now you have to worry about two cones: not just the cone of depression that affects the groundwater table, but also an inverted cone that represents what might be called the “drawup” of the bounding surface between the groundwater and the seawater (Figure 4-27). If you pump fresh water out of the well so vigorously that the boundary is drawn all the way up to the base of the well, you start to pump seawater as well as fresh water from the well. This effect is called salt-water intrusion. Salt-water intrusions of this kind are common in coastal areas underlain by porous materials, as on Cape Cod and Long Island. The problem is reversible, but there is only one remedy: reduce the rate of pumping.
Easterbrook, D.J., 1999, Surface Processes and Landforms, Second Edition. Prentice Hall, 546 p. (Chapter 7)