Wishing for Water - When Salt and Fresh Mix
Groundwater pumping also can cause saltwater intrusion. Fresh water has lower density than salt water, and so floats on salt water in the same way that an iceberg floats on water. (Salt and fresh water will mix, but if the fresh water is renewed by rainfall, the mixed waters will be forced out through the beach to the ocean, and there will continue to be nearly pure fresh water sitting on salty ocean water.) If the water table is lowered by pumping fresh water for human use, the interface between salt and fresh water will rise in the same way that the bottom of an iceberg or a mountain range rises if the top is eroded. The difference in density between salt and fresh water is small; an iceberg floating in the ocean has 9/10 of its thickness below the surface, but the fresh groundwater lens of Cape Cod floating on ocean water has 39/40 below sea level. So if enough water is pumped out of the well to lower the water table 1 m, the salt water will have risen 39 m! If the fresh water table is lowered to sea level, the salt water will rise to sea level, and there will be no fresh water left at that point. Many wells on the very low land of Cape Cod were drilled below sea level into fresh water, but are starting to pump up salt water, causing large problems.