In a deep water-resources and stratigraphic test well near the center of Nantucket Island, about 30 miles (48 kilometres) off the New England coast, freshwater has been found at greater depths than predicted by the Ghyben-Herzberg principle. An uppermost lens of freshwater, which occupies relatively permeable glacial-outwash sand and gravel to a depth of 520 feet (158 metres), is believed to be in hydrodynamic equilibrium with the present level of the sea and the height of the water table. However, two zones of freshwater at 730 to 820 ft (222-249 m) and 900 to 930 ft (274-283 m) are anomalously deep. Although several explanations are possible, the most likely is that the entire surface of the Continental Shelf was exposed to recharge by precipitation during long periods of low sea level in Pleistocene time. After the last retreat of glacial ice, seawater rapidly drowned the shelf around Nantucket Island. Since then, about 8,000 years ago, the deep freshwater zones which underlie dense clay layers have not had time to adjust to a new equilibrium. Under similar circumstances, freshwater may remain trapped under extensive areas of the Continental Shelf wherever clay confining beds have not permitted saltwater to intrude rapidly to new hydrodynamic equilibria positions. The implications are far reaching because all continental shelves worldwide were exposed to similar hydrologic influences during Pleistocene time.
