Optional Enrichment Article

Wishing for Water - When Salt and Fresh Mix

Groundwater pumping can also 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 freshwater 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.

Diagram showing the effect of pumping too much groundwater at a place such as Cape Cod. The Cape has salt water on both sides, and this salt water extends inland beneath the sandy soils of the Cape. A lens of freshwater supplied by rain sits on top, and extends below sea level, much like an iceberg. If the upper surface of this water is lowered to sea level by pumping too much water from a well, the salt water will rise into the well.

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Before groundwater pumping, a lens of fresh groundwater extended above and below sea level, floating on deeper salt water. But if enough groundwater is pumped out to lower the water table to sea level, then salt water will rise to sea level, and the well will be pumping salt water.

Credit: R.B. Alley