Abstract: Fresh Groundwater Beneath Continental Shelf: Findings of Atlantic Continental-Margin Drilling Program

F. A. Kohout, M. H. Bothner, F. T. Manheim

The drilling of 22 test holes on the Atlantic outer continental shelf has for the first time provided a semi-quantitative picture of fresh groundwater in offshore areas. Salinity of subseafloor sediments was determined from 175 samples of interstitial water squeezed from sediment cores with a hydraulic press.

Data from 5 of the test holes on a transect across the shelf from the New Jersey coast permitted construction of a cross section through the lens of relatively fresh water and the transition zone to seawater. The cross section indicates that low-salinity water (less than 2 parts per thousand) extends more than 16 km offshore and that salinities below 6 parts per thousand extend more than 100 km from shore. The lens is essentially flat and does not appear to be strongly controlled by lithologic units of Miocene to Pleistocene age which dip seaward. The extent of low-salinity water offshore helps to explain why saltwater encroachment into municipal well fields on the coastline has not taken place so rapidly as might be predicted on the basis of the Ghyben-Herzberg principle.

The general pattern of decreasing salinity with depth below the seafloor off the Maryland-Delaware-New Jersey coast also was observed on the landward side of Georges Bank about 250 km off the Massachussetts coast. Here, caving of Pleistocene glacial-outwash sand and gravel stopped the drill at relatively shallow depths. However, minimum salinity values were about 23 parts per thousand and were decreasing sharply; the trend suggests that low-salinity water also might be present under Georges Bank.

Drilling sites near the shelf edge (100 to 200 km from shore) characteristically showed no change of salinity with depth; values reflected average seawater salinity for the area of 30 to 33 parts per thousand. At two locations about 100 km off the Georgia-Carolina coast, the salinity of the interstitial water increased with depth to values greater than the normal salinity of seawater in contrast to other areas farther north. The increase to about 41 parts per thousand suggests that evaporite deposits may be present in deeper strata.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC