FRESH GROUND WATER STORED IN AQUIFERS UNDER THE CONTINENTAL SHELF: IMPLICATIONS FROM A DEEP TEST, NANTUCKET ISLAND, MASSACHUSETTS¹

Abstract

A deep water‐resource and stratigraphic test well near the center of Nantucket Island, about 40 miles (64 km) off the New England Coast, has encountered freshwater at greater depth than predicted by the Ghyben‐Herzberg principle. An uppermost lens of fresh‐water, which occupies relatively permeable glacial‐outwash sand and gravel to a depth of 520 ft. (158 m), is probably in hydrodynamic equilibrium with the present level of the sea and the height of the water table. However, two zones of freshwater between 730‐820 ft. (222‐250 m) and 900‐930 ft. (274‐283 m) are anomalously deep. A third zone extending from 1150‐1500 ft. (350‐457 m) contains slightly salty ground water (2 to 3 parts per thousand dissolved solids). Several explanations are possible, but the most likely is that large areas of the Continental Shelf were 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 8000 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 positions of hydrodynamic equilibrium. The implications are far reaching because all continental shelfs were exposed to similar hydrologic influences during Pleistocene time.

Pleistocene hydrology of North America: The role of ice sheets in reorganizing groundwater flow systems

“… Kohout et al [1977] proposed that the presence of unusually fresh water within the permeable units of the Atlantic continental shelf could be attributed to meteoric recharge during Pleistocene sea level lowstands. During sea level lowstands, large portions of the continental shelf were exposed to meteoric recharge.…”

“…The lowering of sea level associated with the buildup of continental ice exposed large portions of the continental shelf to meteoric recharge [ Groen et al , 2000; Person et al , 2003]. Sea level lowstands and continental shelf sub–ice sheet recharge help to explain well‐documented occurrences of freshwater to brackish water plumes up to 100 km off the coast of New England [ Kohout et al , 1977; Person et al , 2003] and more typically 20–40 km offshore [ Meisler et al , 1984; Essaid , 1990; Voss and Andersson , 1993]. With dwindling supplies of potable groundwater in coastal areas, offshore freshwater plumes associated with Pleistocene sea level lowstands and ice sheet meltwaters may represent important freshwater resources (or brackish water for desalinization facilities) in arid regions of the world such as Saudi Arabia as well as near large urban centers in more temperate climates.…”

Geological processes and the management of groundwater resources in coastal areas

“…Age dating of groundwater in the coastal plains near Paramaribo and Jakarta shows that they are indeed of Late Weichselian age . Moreover, observations of groundwater heads of the deeper aquifers at Nantucket Island (Kohout et al, 1977) and Suriname (Groen, 1998) indicate stagnant conditions or even landward gradients, providing additional evidence that these offshore meteoric groundwater wedges are not related to active flow systems with onshore recharge.…”

Fresh-water and salt-water distribution in passive margin sediments: Insights from Integrated Ocean Drilling Program Expedition 313 on the New Jersey Margin

“…Due to their architecture and sediment composition, these margins are capable of accommodating good reservoirs, generally studied offshore for hydrocarbon purposes. It has been known for some time that such margins can also contain a large amount of fresh water (Cohen et al, 2010;Hathaway et al, 1979;Kohout et al, 1977), part of which can be released to the ocean via submarine groundwater discharge (Taniguchi et al, 2002). With more than 30% of the world population living in coastal areas, such systems are clearly of high interest for determining and managing viable water resources (Barlow, 2003;Bear et al, 1999).…”

“…Offshore, they form complex systems and their dynamics remain poorly known. Brackish to fresh waters have been found in boreholes several kilometers away from the coastline on the continental shelves and upper slopes (Austin et al, 1998;Hathaway et al, 1979; Scientifi c Committee on Oceanic Research, Land-Ocean Interactions in the Coastal Zone, 2004;Kohout et al, 1977;Malone and Martin, 2000;Manheim and Paull, 1981). In carbonate environments, offshore exploratory drillings have identifi ed fresh waters (salinity < 1 g/L) in deep karstic cavities (Scientifi c Committee on Oceanic Research, Land-Ocean Interactions in the Coastal Zone, 2004;Vernet, 2000).…”